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1.	Nordström, Joakim, 1971- (författare) Deformation twinning in corrosion-resistant nickel alloys : with a rising nickel content 2024 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Sanicro 28 and Alloy 625 are corrosion-resistant nickel alloys with a fully austenitic structure and a very low carbon content, which means they are both well suited for cold working. Since the millennium shift deformation twinning has been a live research issue as it enhances strength and ductility simultaneously. As nickel has been pointed out as a high stacking fault energy element and deformation twinning should be promoted by a low stacking fault energy level they have been considered as opposite poles. Nonetheless, it is known since long that deformation twins can emerge in high stacking fault face centred cubic elements at low temperatures.In this thesis, we have investigated deformation twinning behaviour in corrosion-resistant nickel alloys. The objective is trying to distinguish between deformation twinning in TWIP steel and corrosion resistant nickel alloys regarding for instance size and bundles.Interrupted uniaxial tensile tests have been performed at several cold working temperatures for the alloys: Sanicro 28 (31% nickel) and Alloy 625 (61% nickel). The microstructure has been characterized in homogeneous deformation volume, by scanning electron microscopy electron backscattering diffraction and electron channelling contrast imaging, transmission electron microscopy and X-ray diffraction. In one investigation fracture behaviour has also been studied with secondary electrons. Ab initio calculations, crystal plasticity modelling and DAMASK simulations have been performed to support emphasizing active deformation mechanisms.It has been revealed that deformation twinning can occur in high Ni alloys. With increasing deformation twinning levels, the diffuse necking decreases. Ab initio calculations indicates that the initiation of deformation twins cannot be determined solely by the stacking fault energy. Distinct features were discovered at low strains that could be rejected from being neither deformation twins nor stacking faults. Level of texture increases with increasing strain and decreasing temperature and the texture modes are changed with decreasing temperature.
2.	Palmert, Frans, 1986- (författare) Crack growth in single crystal gas turbine blade alloys under service-like conditions 2022 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes, and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Crack growth testing has been performed, both using conventional isothermal testing methods and using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance-based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance for the understanding of the crack driving force under different testing conditions. The influence of hold time on crack growth behaviour was analysed, both in terms of creep crack growth and in terms of creep effects on the crack opening force. The transition between non-crystallographic and crystallographic crack growth was studied in detail and a criterion was developed to enable accurate predictions of this transition under a wide range of loading conditions representative for gas turbine blades.
3.	Pant, Prabhat, 1990- (författare) Residual Stress in Additive Manufacturing : Control using orientation and scan strategies 2022 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Components with complex features that are designed with their function as a core aspect often are not viable to be manufactured with traditional methods. This has been a bottleneck in the past, leading to heavier parts with various sub-assemblies and a significant waste of material. With the emergence of additive manufacturing (AM) technology manufacturing of complex components has now turned into reality. Within AM, the laser-based powder-based fusion (LPBF) method is one of the most widely adopted methods to manufacture near net shape complex metal components. However, to be implemented on a larger scale various hurdles must be mitigated first.One of the main persistent issues in LPBF is of residual stresses (RS), which are formed due to repeated sequences of heating and cooling, creating a high thermal gradient between the layers. These RS can play a significant role in the component’s functionality during service, but also can affect the manufacturing process. Therefore, a detailed investigation into the formation and control of RS is of foremost importance. This thesis aims at shedding light on various aspects of the RS formation especially, the effect of build orientations and different scan strategies. For this purpose, Inconel 718 (IN718) was selected as a material for investigation due to its wide use in gas turbine components and good weldability making it a good material for additive manufacturing processes.L-shaped components and test cubes were prepared for residual stress mapping and microstructure study. The RS were measured using neutron and X-ray diffraction methods where applicable. From the investigations, it was revealed that the L-shape components built in different orientations showed significant variation in RS magnitude, but a general trend of RS distribution with tensile stresses at the surface and compressive at the bulk for all the components. A simplified finite element model for RS prediction was established and validated based on the experimental results. Similarly, the use of different scan strategies can lead to a different magnitude of RS for the L-shape components. The remelting strategy with remelting done after every 3rd printed layer seems to decrease the RS magnitude in comparison to the counterparts printed without remelting. This has also been verified with a simplified finite element simulation. The microstructure study showed that crystallographic texture can also vary with the different scan strategies and no significant preferred orientations of the grains were found with the remelting done at every 3rd printed layer. However, with the total fill strategy, strong crystallographic texture was observed in the scan direction. Further investigations into the remelting scan strategies with different variables of remelting such as power, speed, and number of layers between the remelting scan revealed an effect of the laser power in the increment of texture intensity along the building direction. A combination of chess pattern and remelting every 3rd layer decreased the RS magnitude in comparison with other samples, where parameters for remelting strategies were changed. In addition, the crystallographic texture varied with different process parameters used for the remelting. For further reduction of RS without employing post-processing, investigations into novel scan strategies need to be undertaken and at the same time texture formation also needs to be investigated.
4.	Deng, Dunyong, 1989- (författare) On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718 2019 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM), also known as 3D printing, offers great design flexibility for manufacturing components with complex geometries, and has attracted significant interest in the aero and energy industries in the past decades. Among the commercial AM processes, selective laser melting (SLM) and electron beam melting (EBM) are the two most widely used ones for metallic materials. Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties, weldability and low cost. Due to its excellent weldability, IN718 has been intensively applied in the AM filed, to gain more understanding of the AM processes and fully realize AM’s potentials.The study objects in the present thesis include both EBM and SLM IN718. The solidification conditions in EBM and SLM are very different and are different to that of conventional cast, leading to unique microstructures mechanical properties. Therefore, this thesis aims to gain better understanding of the microstructures and anisotropic mechanical behaviours of both EBM and SLM IN718, by detailed characterizations and by comparisons with the forged counterpart.The as-built microstructure of EBM IN718 is spatially dependent: the periphery (contour) region has a mixture of equiaxed and columnar grains, while the bulk (hatch) region has columnar grains elongated along the building direction; the last solidified region close to the top sample surface shows segregation and Laves phases, otherwise the rest of the whole sample is well homogenized. Differently, the as-built microstructure of SLM IN718 is spatially homogeneous: the grains is rather equiaxed and with subgrain cell structures. These microstructures also respond differently to the standard heat treatment routines for the conventional counterparts.Anisotropic mechanical properties are evident in the room temperature tensile tests and high temperature dwell-fatigue tests. The anisotropic tensile properties of EBM IN718 at room temperature are more likely due to the directional alignment of porosities along the building direction rather than the strong crysiii tallographic texture of ⟨100⟩ _ building direction. While for SLM IN718, the anisotropy is more likely attributed to the different extents of ‘work-hardening’ or dislocations accumulated between the horizontally and vertically built specimens. The anisotropy mechanisms in dwell-fatigue crack propagations at 550 ◦C for EBM and SLM IN718 are identical: higher effective stress intensity factor when intergranular cracking path is perpendicular to the loading direction, but lower effective stress intensity factor when intergranular cracking path is parallel to or slightly deviated from the loading direction.The 2160s dwell-fatigue cracking behaviours at 550 ◦C are of significant interest for AM IN718, of which test condition is similar to that of real service for IN718 disk in turbine engine. Generally, after conventional or short-term heat treatments, EBM IN718 shows better dwell-fatigue cracking resistance than SLM IN718. The damage mechanism is different for EBM and SLM IN718: the intergranular cracking in EBM IN718 is due to environmentally assisted grain boundary attack, while creep damage is active for SLM IN718. The considerably ‘deformed’ microstructure, specifically the subgrain cell structures in SLM IN718 resulted from the manufacturing process, is believed to activate creep damage even at a low temperature of 550 ◦C. And for SLM IN718, heat treatment routine must be carefully established to alter the ‘deformed’ microstructure for better time dependent cracking resistance at elevated temperature.
5.	Kahlin, Magnus, 1982- (författare) 3D-printing for Aerospace : Fatigue Behaviour of Additively Manufactured Titanium 2021 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Laser powder bed fusion (L-PBF) and electron beam powder bed fusion (E-PBF) are two of the most common additive manufacturing (AM) methods which both provide the engineer with a great freedom of design.This means that parts with light weight, multifunctional applications and improved performance could be achieved through innovative design solutions which have attracted a lot of interest from the aerospace industry.This PhD project has focused on the following fatigue related areas forL-PBF and E-PBF Ti6Al4V material which all need to be addressed before AM can be fully introduced to critical aerospace applications: effect of geometry, roughness and loading on fatigue life, improved fatigue life through post processing, fatigue crack growth behaviour and fatigue prediction methods.The results show that the rough as-built surface is the single most severe factor for fatigue but that the fatigue strength of at least L-PBF material can be improved to levels similar to conventionally manufactured material using surface post processing. Furthermore, the results verify that acumulative damage approach gives good accuracy in predicting fatigue life for variable amplitude loading and that fatigue crack growth rates using material data from standard specimens can be used for damage tolerancean alysis independent of part geometry and stress level.The conclusion is therefore that the fatigue properties can be improved to acceptable levels and predicted using conventional methods. There are still some challenges to solve, however, especially within non-destructive testing before AM can be introduced to critical aerospace applications.
6.	Lundberg, Mattias, 1985- (författare) Residual stresses, fatigue and deformation in cast iron 2018 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The complex geometry of cylinder heads in heavy-duty diesel engines makes grey iron or compact graphite iron a preferred material choice due to its price, castability, thermal conductivity and damping capacity. Today’s strict emission laws have increased the demands on engine performance and engine efficiency. This means that material properties such as fatigue resistance need to be improved. Shot peening is often used to improve the fatigue resistance of components and the benefits of shot peening are associated with the induced compressive surface stresses and surface hardening. How different shot peening parameters can affect fatigue strength of grey and compact graphite iron has been investigated within the project underlying this thesis. To do this, X-ray diffraction (XRD) was utilized for residual stress measurements, scanning electron microscopy (SEM) for microstructural characterizations and mechanical fatigue testing for mechanical quantifications. The ultimate aim of this work has been to increase the fatigue resistance of cast iron by residual stress optimization.XRD measurements and SEM examinations revealed that the shot peening parameters shot size and peening intensity significantly influence residual stresses and surface deformation. Residual stress profiles, similar to the one general considered to improve the fatigue strength in steels, were obtained for both grey and compact graphite iron. Uniaxial push-pull fatigue testing on grey iron with these shot peening parameters reduced the fatigue strength with 15–20 %. The negative effect is likely related to surface damage associated with over peening and relatively high subsurface tensile residual stresses. With very gentle shot peening parameters, the uniaxial fatigue strength were unaltered from the base material but when subjected to bending fatigue an increase in fatigue strength were observed. An alternative way to increase the fatigue strength was to conduct a 30 min annealing heat treatment at 285 XC which increased the fatigue strength by almost 10 % in uniaxial loading. The improvement could be an effect of favourable precipitates forming during the annealing, which could hinder dislocation movement during fatigue.Measuring residual stresses using XRD and the sin2 -method demands accurate X-ray elastic constants (XEC) for meticulous stress analysis. The XEC referred to as 1~2s2 should therefore always be calibrated for the specific material used. The experiments conducted revealed that the XEC value is independent of the testing method used in this work. A small correction from the theoretical value should be applied when the material contains small amounts of residual stresses. The amount of residual stresses has a great impact on the XEC and thus on the stress analysis. Concluding that proper analysis of residual stresses in cast iron is not straight forward.
7.	Norman, Viktor, 1988- (författare) Fatigue of Heavy-Vehicle Engine Materials : Damage Mechanisms, Laboratory Experiments and Life Estimation 2018 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Due to increasing demands on sustainability exerted by end-costumers and policy makers, heavyvehicle manufacturers are urged to increase the engine efficiency in order to reduce the exhaust gas emission. However, increasing the efficiency is also associated with an elevated fatigue rate of the materials constituting the engine parts, which consequently reduces the engine service life. The aim of the present thesis is therefore to confront the expected increase by studying the fatigue behaviour and damage mechanisms of the materials typically employed in heavy-vehicle diesel engines. With this knowledge, this work seeks to guide the development of new heavy-vehicle engine materials, as well as to develop improved life estimation methods designated to assist the mechanical design of durable heavy-vehicle engines.In essence, a large set of thermo-mechanical fatigue (TMF) and combined thermomechanical and high-cycle fatigue (TMF-HCF) tests is conducted at engine load conditions on laboratory specimens of lamellar, compacted and spheroidal graphite iron. In this way, the fatigue performance and associated damage mechanisms are investigated. In particular, a new fatigue property is identified, the TMF-HCF threshold, which quantifies how resistant a material is to superimposed high-cycle fatigue.The damage mechanism at low temperatures (≲500°C) is confirmed to consist of the initiation, propagation and coalescence of numerous microcracks. Based on this, a successful fatigue life estimation model is formulated, allowing accurate estimations of TMF and TMF-HCF tests on smooth specimens, and TMF tests on notched specimens. In the latter case, the microcrack growth behaviour in non-uniform cyclic stress fields and its implications for life estimation are clarified. At elevated temperatures (≳500°C), surface oxidation is shown to govern the fatigue performance of cast iron grades intended for exhaust manifolds. It is observed that oxide intrusions are induced, from which surface fatigue cracks are initiated. Consequently, an optimal material at these conditions should have a low oxide growth rate and few casting defects at the surface, as these factors are found to stimulate the growth of intrusion.
8.	Palmert, Frans, 1986-, et al. (författare) Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions 2022 Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 163 Tidskriftsartikel (refereegranskat)abstract In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.
9.	Wärner, Hugo, 1988- (författare) High Temperature Fatigue Behaviour of Austenitic Stainless Steel : Microstructural Evolution during Dwell-Fatigue and Thermomechanical Fatigue 2021 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The global energy consumption is increasing and together with global warming from greenhouse gas emission, a need for more environmentally friendly energy production processes is created. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section, this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The generation of power must also be flexible to be able to follow the demands of the energy market and this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs.Because of the need for flexibility, higher temperature and higher pressure of future biomass power plants, the demands of improved mechanical properties of the materials used for the components are also increased. Properties like creep strength, maintained structural integrity, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Highly alloyed austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already used in high temperature cyclic conditions for other applications. The behaviour of austenitic stainless steels subjected to future biomass power plants operating conditions are not yet fully investigated.This thesis presents research that includes investigations of the mechanical and microstructural behaviour during high temperature cyclic conditions of austenitic stainless steels. This is done using thermomechanical fatigue testing, dwell-fatigue testing and impact toughness testing at elevated temperatures. Material service degradation as an effect of microstructural evolution is investigated by ageing of some test specimens before testing. Microscopy is used to investigate the connection between the mechanical behaviour and the microstructural deformation- and damage mechanisms of the austenitic stainless steels after testing.The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present during high temperature cyclic conditions. In addition, ageing results in a less favourable microstructural configuration which negatively affects the resistance to high temperature damage mechanisms. An example of this is the lowering of impact toughness due to precipitation and coarsening of detrimental phases of some aged austenitic stainless steels. Moreover, TMF testing of aged austenitic stainless steels induce oxidation assisted cracking and an embrittling effect that cause significant cyclic life decrease. The creep-fatigue interaction behaviour during dwell-fatigue testing of two austenitic stainless steels generates various crack propagation characteristics. The higher alloyed material shows interchanging intra- and intergranular propagation with dynamic recrystallization, while the lower alloyed material shows propagation exclusively along the grain boundaries by the assistance of fatigue induced slip bands interaction with grain boundary precipitates.The research of this thesis provides a deeper understanding of the structural integrity, deformation mechanisms, damage mechanisms and fracture mechanisms during high temperature cyclic conditions of austenitic stainless steels. Long term, this is believed to contribute to development of suitable materials used as components of future biomass-fired power plants to achieve sustainable power generation.
10.	Xu, Jinghao, 1992- (författare) Alloy Design and Characterization of γ′ Strengthened Nickel-based Superalloys for Additive Manufacturing 2021 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Nickel-based superalloys, an alloy system bases on nickel as the matrix element with the addition of up to 10 more alloying elements including chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. Through the development and improvement of nickel-based superalloys in the past century, they are well proved to show excellent performance at the elevated service temperature. Owing to the combination of extraordinary high-temperature mechanical properties, such as monotonic and cyclic deformation resistance, fatigue crack propagation resistance; and high-temperature chemical properties, such as corrosion and oxidation resistance, phase stability, nickel-based superalloys are widely used in the critical hot-section components in aerospace and energy generation industries.The success of nickel-based superalloy systems attributes to both the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed manufacturing processes. The microstructure of the modern nickel-based superalloys consists of a two-phase configuration: the intermetallic precipitates (Ni,Co)3(Al,Ti,Ta) known as γ′ phase dispersed into the austenite γ matrix, which is firstly introduced in the 1940s. The recently developed additive manufacturing (AM) techniques, acting as the disruptive manufacturing process, offers a new avenue for producing the nickel-based superalloy components with complicated geometries. However, γ′ strengthened nickel-based superalloys always suffer from the micro-cracking during the AM process, which is barely eliminated by the process optimization.On this basis, the new compositions of γ′ strengthened nickel-based superalloy adapted to the AM process are of great interest and significance. This study sought to design novel γ′ strengthened nickel-based superalloys readily for AM process with limited cracking susceptibility, based on the understanding of the cracking mechanisms. A two-parameter model is developed to predict the additive manufacturability for any given composition of a nickel-based superalloy. One materials index is derived from the comparison of the deformation-resistant capacity between dendritic and interdendritic regions, while another index is derived from the difference of heat resistant capacity of these two spaces. By plotting the additive manufacturability diagram, the superalloys family can be categorized into the easy-to-weld, fairly-weldable, and non-weldable regime with the good agreement of the existed knowledge. To design a novel superalloy, a Cr-Co-Mo-W-Al-Ti-Ta-Nb-Fe-Ni alloy family is proposed containing 921,600 composition recipes in total. Through the examination of additive manufacturability, undesired phase formation propensity, and the precipitation fraction, one composition of superalloy, MAD542, out of the 921,600 candidates is selected.Validation of additive manufacturability of MAD542 is carried out by laser powder bed fusion (LPBF). By optimizing the LPBF process parameters, the crack-free MAD542 part is achieved. In addition, the MAD542 superalloy shows great resistance to the post-processing treatment-induced cracking. During the post-processing treatment, extensive annealing twins are promoted to achieve the recrystallization microstructure, ensuring the rapid reduction of stored energy. After ageing treatment, up to 60-65% volume fraction of γ′ precipitates are developed, indicating the huge potential of γ′ formation. Examined by the high-temperature slow strain rate tensile and constant loading creep testing, the MAD542 superalloy shows superior strength than the LPBF processed and hot isostatic pressed plus heat-treated IN738LC superalloy. While the low ductility of MAD542 is existed, which is expected to be improved by modifying the post-processing treatment scenarios and by the adjusting building direction in the following stages of the Ph.D. research.MAD542 superalloy so far shows both good additive manufacturability and mechanical potentials. Additionally, the results in this study will contribute to a novel paradigm for alloy design and encourage more γ′-strengthened nickel-based superalloys tailored for AM processes in the future.
11.	Xu, Jinghao, 1992- (författare) High-performance Nickel-based Superalloys for Additive Manufacturing 2022 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM), e.g., laser powder bed fusion (LPBF) technique, has become a powerful manufacturing process for producing metallic components with the advantages of design freedom, net-shape-forming flexibility, product customization, and reduced lead time to market. Nickel-based superalloys is one of the most significant alloy families used at elevated temperatures. Nickel-based superalloys commonly contain up to 10 more alloying elements like chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. The great capacity of the nickel-based superalloys for high-temperature operation is ensured by the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed corresponding manufacturing processes. However, high-performance nickel-based superalloys generally suffer from structural integrity issues during AM process, i.e., this class of superalloys is highly susceptible to crack. Therefore, new nickel-based superalloys adapted to AM process with tailored chemical composition are under the urgent call. Meanwhile, high-temperature performance is another prioritized target for the new superalloys.The first topic is the chemical composition-dependent cracking mechanisms. The interdendritic region formed at the last-stage solidification has been found as the cracked spaces. Owing to the suppression of precipitate formation, the cracking mechanism is generalized as (1) the large mismatch of the solidification steps accounting for the crack initiation, and (2) the large mismatch of load-bearing capacity accounting for the crack propagation, between the dendritic and interdendritic regions. To quantitatively formulate the additive manufacturability of nickel-based superalloys, herein a two-parameter-based, heat resistance, and deformation resistance (HR-DR) model, has been successfully proposed to predict the printability on accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility. The concept of this model is formulated as that if the interdendritic region obtains both higher heat and deformation resistances than the rest dendritic region, this alloy is expected to be crack resistant. Validated by the experimental results and hitherto reported literature data, the HR-DR model provides an excellent sound prediction on the crack susceptibility of nickel-based superalloy during AM process. By considering the combination of additive manufacturability and high-temperature performance, a novel high-strength nickel-based superalloy, MAD542 has been developed based on the materials selection procedure from 921,600 candidate compositions. In addition, another precipitation-strengthened nickel-based superalloy, Alloy738+ has been developed based on the modification of the composition of heritage superalloy IN738LC, aiming for improving the additive manufacturability, creep, and oxidation resistance.The second topic is the post-processing treatments related to microstructural evolution and mechanical properties. Owing to the thermal history during the LPBF process, the as-built microstructure commonly consists of columnar grains nearly parallel to the building direction with strong crystallographic texture. Subjected to the post-processing treatments, the solution treatment is the key to controlling the grain evolution. It has been shown for both LPBF MAD542 and heritage LPBF CM247 superalloys, the high crystallographic texture is maintained at the sub-γ′-solvus temperatures because of the grain boundary pinning effect from grain boundary precipitates. Whilst the crystal anisotropy is highly reduced by the treatment at super-γ′-solvus temperatures driven by the means of recrystallization. However, fully recrystallized microstructure with low texture largely reduced the mechanical properties by the embrittlement manner at elevated temperatures accordingly.The third topic is the examination of creep and oxidation performance of various LPBF superalloys. A strong building direction-dependent creep performance is found for an LPBF IN738LC superalloy fabricated by the vertical and horizontal build. Vertically built samples show 7-40 times longer rupture life and approximately 2 times longer elongation at fracture than the horizontally produced samples, for the creep at 150-300 MPa at 850 °C. To evaluate the short-term creep performance, constant displacement rate-controlled slow strain rate tensile (SSRT) testing was carried out. The constant load-controlled creep and SSRT are correlated by deformation rate-based power-law type analysis. The new superalloy LPBF MAD542 generally displays a 5 times slower deformation rate than the LPBF IN738LC superalloy at 850 °C. The new superalloy Alloy738+ shows a comparable creep performance to LPBF IN738LC. Oxidation tests were conducted at 850/950/1050 °C. The new superalloy Alloy738+ presents an excellent oxidation resistance at 850 and 950 °C. By comparison, for example, Alloy738+ has 3 times slower oxidation kinetics than IN738LC at 950 °C.The several investigations associated with the composition/processing/property in multiple precipitation-strengthened nickel-based superalloys fabricated by AM in this thesis have proven that the materials development requires comprehensive in-depth considerations. The presented results can contribute to the fundamental understanding and/or serve as the reference data for other superalloys by AM from the properties perspective.
12.	Busse, Christian, 1989- (författare) Aspects of Crack Growth in Single-Crystal Nickel-Base Superalloys 2017 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract This Licentiate of Engineering thesis is a product of the results generated in the research project KME-702, which comprises modelling, microstructure investigations and material testing of cast nickel-base superalloys.The main objective of this work is to model the fatigue crack propagation behaviour in single-crystal nickel-base superalloys. To achieve this, the influence of the crystal orientations on the cracking behaviour is assessed. The results show that the crystal orientation is strongly affecting the material response and must be accounted for. Furthermore, a linear elastic crack driving force parameter suitable for describing crystallographic cracking has been developed. This parameter is based on resolved anisotropic stress intensity factors and is able to predict the correct crystallographic cracking plane after a transition from a Mode I crack. Finally, a method to account for inelastic deformations in a linear elastic fracture mechanics context was investigated. A residual stress field is extracted from an uncracked finite-element model with a perfectly plastic material model and superimposed on the stress field from the cracked model with a linear elastic material model to account for the inelastic deformations during the determination of the crack driving force. The modelling work is validated by material testing on two different specimen geometries at different temperatures.This Licentiate of Engineering thesis consists of two parts, where Part I gives an introduction and background to the research area, while Part II consists of three papers.
13.	Nordström, Joakim, 1971-, et al. (författare) Temperature study of deformation twinning behaviour in nickel-base Superalloy 625 2024 Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 907 Tidskriftsartikel (refereegranskat)abstract Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and therefore the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that critical strain and strain hardening rate influences the deformation twinning onset and twinning rate.
14.	Palmert, Frans, 1986- (författare) Crack growth in single crystal nickel base superalloys under isothermal and thermomechanical fatigue 2019 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and also to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close-packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Fatigue crack growth testing has been performed, both using conventional isothermal testing methods and also using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are however only presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, in order to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance in the understanding of the crack driving force under different testing conditions.
15.	Pant, Prabhat, 1990- (författare) Residual Stress Distributions in Additively Manufactured Parts : Effect of Build Orientation 2020 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM) of parts using a layer by layer approach has seen a rapid increase in application for production of net shape or near-net shape complex parts, especially in the field of aerospace, automotive, etc. Due to the superiority of manufacturing complex shapes with ease in comparison to the conventional methods, interest in these kinds of processes has increased. Among various methods in AM, laser powder bed fusion (LPBF) is one of the most widely used techniques to produce metallic components.As in all manufacturing processes, residual stress (RS) generation during manufacturing is a relevant issue for the AM process. RS in AM are generated due to a high thermal gradient between subsequent layers. The impact of residual stresses can be significant for the mechanical integrity of the built parts and understanding the generation of RS and the effect of AM process parameters is therefore important for a broader implementation of AM techniques. The work presented in this licentiate thesis aims to investigate the influence of build orientation on the RS distribution in AM parts. For this purpose, L-shaped Inconel 718 parts were printed by LPBF in three different orientations, 0°, 45°, and 90°, respectively. Inconel 718 was selected because it is a superalloy widely used for making gas turbine components. In addition, IN718 has in general good weldability which renders it a good material for additive manufacturing.Residual stress distributions in the parts removed from the build plate were measured using neutron diffraction technique. A simple finite element model was developed to predict the residual stresses and the effect of RS relaxation due to the separation of the parts and build plate. The trend of residual stress distribution predicted was in good agreement with experimental results. In general, compressive RS at the part center and tensile RS near the surface were found. However, while the part printed in 0° orientation had the least amount of RS in all three principal directions of part, the part built in 90° orientation possessed the highest amount of RS in both compression and tension. The study has shown that residual stress distributions in the parts are strongly dependent on the building process. Further, it has shown that the relaxation of RS associated with the removal of the parts from the build plate after printing has a great impact on the final distribution of residual stress in the parts. These results can be used as guidelines for choosing the orientations of the part during printing.
16.	Saarimäki, Jonas, 1985- (författare) Cracks in superalloys 2018 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Gas turbines are widely used in industry for power generation and as a power source at hard to reach locations where other possibilities for electrical power supplies are insufficient. New ways of producing greener energy is needed to reduce emission levels. This can be achieved by increasing the combustion temperature of gas turbines. High combustion temperatures can be detrimental and degrade critical components. This raises the demands on the high temperature performance of the superalloys used in gas turbine components. These components are frequently subjected to different cyclic loads combined with for example dwell-times and overloads at elevated temperatures, which can influence the crack growth. Dwell-times have been shown to accelerate crack growth and change cracking behaviour in both Inconel 718, Haynes 282 and Hastelloy X. On the other hand, overloads at the beginning of a dwell-time cycle have been shown to retard the dwell-time effect on crack growth in Inconel 718. More experiments and microstructural investigations are needed to better understand these effects.The work presented in this thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", where I have mainly looked at fatigue crack growth mechanisms in superalloys subjected to dwell-fatigue, which can have a devastating effect on crack propagation behaviour. Mechanical testing was performed under operation-like cycles in order to achieve representative microstructures and material data for the subsequent microstructural work. Microstructures were investigated using light optical microscopy and scanning electron microscopy (SEM) techniques such as electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at maximum load (0 % overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. The crack growth rate decreases with increasing overload levels in Inconel 718 when an overload is applied prior to the dwell-time. At high temperature, intergranular crack growth was observed in Inconel 718 as a result of oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along δ-phase boundaries with subsequent oxidation of the δ-phase. This thesis comprises two parts. Part I gives an introduction to the field of superalloys and the acting microstructural mechanisms related to fatigue and crack propagation. Part II consists of five appended papers, which report the work completed as part of the project.
17.	Sjöström, William, 1994- (författare) Material Development for Electron Beam-based Powder Bed Fusion 2024 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Electron beam powder bed fusion (PBF-EB) is an additivemanufacturing (AM) method based on layer-by-layer melting of apowder bed. The technology is industrialized in certain applicationsbut still considered as immature and is not as widely used as laserbeam-based systems (PBF-LB). PBF-EB can offer several benefits overPBF-LB such as process cleanliness, thermal efficiency, fast beam speed,higher power and energy transfer, low residual stresses in built partsand a good signal environment for process monitoring. This can beadded on top of the general benefits of AM such as geometricalfreedom, manufacturing efficiency, easy design revisions, short leadtimes and so on. This suggests that PBF-EB holds potential as atechnology for the sustainable production of materials andcomponents. This thesis investigates how PBF-EB can be furtherdeveloped to create new and unique materials features. This isachieved by introducing innovative methods for material processingand by further developing the PBF-EB process itself. The thesisintroduces a charge-free heating method for PBF-EB and the resultssuggest an enhanced processability of difficult-to-process materialsand powders. A method for building multi-materials in PBF-EB isintroduced and demonstrated by the manufacturing of direct andlamellar transitions between different alloys. Methods for processmonitoring and powder bed resistivity evaluation are proposed andxiidemonstrated. It is concluded that the results presented in this thesisenabled new PBF-EB processing modes, increased the knowledge ofthe process, and introduced a new material group by demonstratingthat ceramics can be processed at high temperatures (~1600C).
18.	Sun, Xiaoyu, 1994- (författare) Thermodynamic investigation of high-temperature coatings : microstructure, oxidation and interdiffusion 2024 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract High-temperature coatings, such as metallic MCrAlY (M: Ni and/or Co) coatings, are widely employed to improve the oxidation resistance of superalloys. However, interdiffusion between the coatings and the superalloys under elevated service temperatures leads to microstructural degradation in both. Some of the underlying degradation mechanisms are still elusive, such as the γ′ (Ni3Al) phase depletion in the superalloys, where a large amount of γ′ precipitates dissolve in the γ matrix, even though the incoming Al from the coatings locally increases the Al content. In this research, we investigated the interdiffusion and oxidation behaviors in coating/superalloy systems of Amdry365 (MCrAlY)/IN792, Ta-containing (MCrAlYTa) coatings/IN792 and high-entropy-alloying (AlCrFeCoNi) coatings/IN792 at 1100 °C. Multiple microscopic techniques were employed to study microstructural evolution and chemical compositional changes. Thermodynamic calculations were used to reveal the underlying mechanisms of these changes.The experimental and simulation results reveal a dominant effect of fast-diffusion Al from the coating towards the superalloy in the early stage of phase transitions. This leads to β phase depletion on the coating side and the formation of β as well as and increment of γ′ phase on the superalloy side. We propose an Al-Cr interference (ACI) effect to account for the pile-up behavior of Cr and the reduced Al content observed near the coating/superalloy interface. The incoming Al from the coating enhances the Cr potential in the superalloy, causing Cr diffusion from the interior of the superalloy to the coating/superalloy interface., Meanwhile, the pile-up of Cr increases the Al potential, which accelerates Al diffusion from this region into the interior of the superalloy. Furthermore, local-phase- equilibrium calculations reveal that γ′ phase depletion in the substrate is ascribed to the loss of γ′ forming elements, Ti and Ta. The incoming Al accelerates γ′ phase depletion, since it enhances the potential of Ti and Ta promoting their diffusion from the superalloy side to the coating side. Adding Ta to coatings impedes γ′ depletion in the substrate superalloys and also promotes the phase transition from β (NiAl) to γ′ in the coatings. As for the oxidation behavior, a high Ta content in coatings, such as the coating with 7.2 wt.% Ta reduces the oxidation resistance due to the formation of excessive Ta-rich oxides. It is observed in the high entropy alloy coatings that a lower Al/Cr ratio benefits oxidation resistance as its protective scale consists of larger Al2O3 grains, reducing the number of grain boundaries that function as fast diffusion paths for oxygen.This research clarifies a confusing point: the reason for γ′ phase depletion is the increase of Cr and Co related to interdiffusion. These results answer a long-standing but critical question in the coating/superalloy interdiffusion research, i.e., why the γ′ precipitates dissolve in the γ matrix even though the incoming Al from coatings increases the Al content. The γ′ phase depletion is ascribed to the loss of γ′ forming elements, Ti and Ta, whereas the incoming Al cannot offset their effect. The diffusion simulation performed in this work still requires a long calculation time, as a large number of elements were considered in the diffusion model. According to our findings, γ′ depletion is mainly dependent on the loss of Ta, Ti. Hence, a simplified prediction model for γ′ depletion for fast computation can be established by considering only the diffusion of Ta, Ti and Al.
19.	Wärner, Hugo, 1988- (författare) High-Temperature Fatigue Behaviour of Austenitic Stainless Steel : Influence of Ageing on Thermomechanical Fatigue and Creep-Fatigue Interaction 2018 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract The global energy consumption is increasing and together with global warming from greenhouse gas emission, create the need for more environmental friendly energy production processes. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section and this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The power generation must also be flexible to be able to follow the demands of the energy market, this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs.Because of the demands of flexibility, higher temperature and higher pressure in the boiler section of future biomass power plants, the demands on improved mechanical properties of the materials of these components are also increased. Properties like creep strength, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already operating at high temperature cyclic conditions in other applications. The behaviour of austenitic stainless steels during these widened operating conditions are not yet fully understood.The aim of this licentiate thesis is to increase the knowledge of the mechanical behaviour at high temperature cyclic conditions for austenitic stainless steels. This is done by the use of thermomechanical fatigue- and creepfatigue testing at elevated temperatures. For safety reasons, the effect of prolonged service degradation is investigated by pre-ageing before mechanical testing. Microscopy is used to investigate the microstructural development and resulting damage behaviour of the austenitic stainless steels after testing. The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present at high temperature cyclic conditions. In addition, simulated service degradation resulted in a detrimental embrittling effect due to the deterioration by the microstructural evolution.
20.	Yu, Cheng-Han, 1992- (författare) Anisotropic mechanical behaviors and microstructural evolution of thin-walled additively manufactured metals 2020 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM), also known as 3D printing, is a concept and method of a manufacturing process that builds a three-dimensional object layer-by-layer. Opposite to the conventional subtractive manufacturing, it conquers various limitations on component design freedom and raises interest in various fields, including aerospace, automotive and medical applications. This thesis studies the mechanical behavior of thin-walled component manufactured by a common AM technique, laser powder bed fusion (LPBF). The studied material is Hastelloy X, which is a Ni-based superalloy, and it is in connection to a component repair application in gas turbines. The influence of microstructure on the deformation mechanisms at elevated temperatures is systematically investigated. This study aims for a fundamental and universal study that can apply to different material grades with FCC crystallographic structure.It is common to find elongated grain and subgrain structure caused by the directional laser energy input in the LPBF process, which is related to the different printing parameters and brands of equipment. This thesis will start with the study of scan rotation effect on stainless steel 316L in an EOS M290 equipment. The statistic texture analysis by using neutron diffraction reveals a clear transition when different level of scan rotation is applied. Scan rotation of 67° is a standard printing parameter with intention to lower anisotropy, yet, the elongated grain and cell structure is still found in the as-built microstructure. Therefore, the anisotropic mechanical behavior study is carried out on the sample printed with scan rotation of 67° in this thesis.Thin-walled effects in LPBF are investigated by studying a group of plate-like HX specimens, with different nominal thicknesses from 4mm down to 1mm, and a reference group of rod-like sample with a diameter of 18mm. A texture similar to Goss texture is found in rod-like sample, and it becomes <011>//BD fiber texture in the 4mm specimen, then it turns to be <001> fiber texture along the transverse direction (TD) in the 1mm specimen. Tensile tests with the strain rate of 10−3 s−1 have been applied to the plate-like specimens from room temperature up to 700 ℃. A degradation of strength is shown when the sample becomes thinner, which is assumed to be due to the overestimated load bearing cross-section since the as-built surface is rough. A cross-section calibration method is proposed by reducing the surface roughness, and a selection of proper roughness parameters is demonstrated with the consideration of the calculated Taylor’s factor and the residual stress. The large thermal gradient during the LPBF process induces high dislocation density and strengthens the material, hence, the LPBF HX exhibits better yield strength than conventionally manufactured, wrought HX, but the work hardening capacity and ductility are sacrificed at the same time.Two types of loading condition reveal the anisotropic mechanical behavior, where the vertical and horizontal tests refer to the loading direction being on the BD and TD respectively. The vertical tests exhibit lower strength but better ductility that is related to the larger lattice rotation observed from the samples with different deformation level. Meanwhile, the elongated grain structure and grain boundary embrittlement are responsible for the low horizontal ductility. A ductile to brittle transition is traced at 700 ℃, so a further study with two different slow strain rates, 10−5 s−1 and 10−6 s−1, are carried out at 700 ℃. Creep damage is shown in the slow strain rates testing. Deformation twinning is found only in the vertical tests where it forms mostly in the twin favorable <111> oriented grain along the LD. The large lattice rotation and the deformation twinning make the vertical ductility remain high level under the slow strain rates. The slow strain rate tensile testing lightens the understanding of creep behavior in LPBF Ni-based superalloys.In summary, this thesis uncovers the tensile behavior of LPBF HX with different variations, including geometry-dependence, temperature-dependence, crystallographic texture-dependence and strain rate-dependence. The generated knowledge will be beneficial to the future study of different mechanical behavior such as fatigue and creep, and it will also enable a more robust design for LPBF applications.
21.	Yu, Cheng-Han, 1992- (författare) Anisotropic Mechanical Behaviours and Thin-wall Effects of Additively Manufactured Austenitic Alloys 2022 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM), also known as 3D printing, is a general concept of building a three-dimensional object layer-by-layer. AM breaks through the manufacturing limitations in conventionally subtractive manufacturing, leading to a great design freedom of components with complex geometries. The potential of integrating AM into existing manufacturing process with additional functionality raises interest in various ﬁelds, such as aerospace, automotive and medical applications. To ensure robust AM applications, this PhD project has carried out investigations on the mechanical behaviours of AM components with respect to the characteristic microstructure and the geometrical eﬀects. The investigated materials include Hastelloy X (HX, a solid-solution strengthened Ni-based superalloy) and stainless steel 316L (SS 316L, a solid-solution strengthened austenitic stainless steel) manufactured by laser powder bed fusion (LPBF). The high temperature tensile behaviours, short-term creep resistance and low cycle fatigue performance have been examined. The aim of this thesis is to conduct a fundamental studies that can be applied to diﬀerent material grades with single phase face-centred cubic (FCC) crystallographic structure. LPBF HX shows a great potential for the burner tip repair application in gas turbines. Due to the complex geometry of the burner and the requirement of high temperature mechanical performance, the tensile properties have been systematically examined. Multiple testing variables have been applied, including the specimen geometry, the elevated temperature, the strain rate and the loading direction (LD). Combined with the prior and post microstructural analysis, the deformation and fracture mechanisms have been investigated. For the thin-walled specimens, a clear texture transition is found when it comes to the thinner specimen, and it leads to the lower yield strength as a result. In addition, as the high surface roughness of the LPBF as-built specimen can cause inaccuracy of the yield strength determination due to the overestimated loading cross section, especially for the thin-walled specimen, a calibration method based on the crystallographic texture results has been proposed. Meanwhile, anisotropic tensile behaviours are observed at all the testing conditions due to the elongated grain structure and the characteristic texture along the building direction (BD). At elevated temperatures, the grain boundary embrittlement takes place at 700 °C that leads to the ductility loss in the horizontal loading (LD ⊥ BD). Slow strain rate tensile testing (SSRT) has been performed to probe the short-term creep resistance at 700 °C, since it is a useful tool to address the strain rate dependent in elastic strain accumulation. Surprisingly, the ductility of the vertical loading (LD // BD) remains at a high level not only at 700 °C but also at SSRT condition, and the high ductility results from the evident texture evolution and crystallographic orientation dependent deformation twinning. The good ductility of the vertical loading indicates a better creep performance compared to the horizontal loading. In-situ and ex-situ neutron diﬀraction measurements upon loading have also been applied for a full-length investigation on the anisotropic tensile behaviours. Thin-wall eﬀects on strain-controlled low cycle fatigue (LCF) behaviours of LPBF SS 316L have been investigated by using the tubular fatigue specimens with diﬀerent wall thicknesses. The comparison between the machined and as-built surface conditions have been drawn. The fully reversed LCF tests were successfully performed without the buckling problem in thin-walled structures owing to the tubular geometry. The surface roughness and the distinct microstructure at the surface region lead to the inferior fatigue strain-life, especially with the low applied strain range. The combined eﬀects have been quantiﬁed by estimating the fatigue notch factor, Kf . The LCF tests have also been performed on the regular cylindrical specimens and compared to the wrought SS 316L. A comparable fatigue strain-life is found between the LPBF and the wrought SS 316L. Yet, the secondary hardening caused by strain-induced martensitic phase transformation is only observed in the wrought SS 316L, while continuous cyclic softening is shown in the LPBF SS 316L. In addition, as high level of residual stress (RS) is commonly found in the as-built specimen, the eﬀect of stress relief heat treatment (600 °C /4 hours) on the LCF behaviours has been examined. A great reduction of RS is found after the heat treatment, and higher responding stresses are shown in the stress relieved specimen, which indicates a better fatigue stress-life. In summary, the deformation and fracture mechanisms of LPBF HX and SS 316L under diﬀerent loading conditions have been systematically investigated. Via increasing deeper knowledge of LPBF material behaviours, the LPBF applications can be expanded to a greater extent.
22.	Azeez, Ahmed, 1991-, et al. (författare) The effect of dwell times and minimum temperature on out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel - Crack closure prediction 2022 Ingår i: International Journal of Fatigue. - : Elsevier Science Ltd. - 0142-1123 .- 1879-3452. ; 162 Tidskriftsartikel (refereegranskat)abstract Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.
23.	Deng, Dunyong, 1989- (författare) Additively Manufactured Inconel 718 : Microstructures and Mechanical Properties 2018 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Additive manufacturing (AM), also known as 3D printing, has gained significant interest in aerospace, energy, automotive and medical industries due to its capabilities of manufacturing components that are either prohibitively costly or impossible to manufacture by conventional processes. Among the various additive manufacturing processes for metallic components, electron beam melting (EBM) and selective laser melting (SLM) are two of the most widely used powder bed based processes, and have shown great potential for manufacturing high-end critical components, such as turbine blades and customized medical implants. The futures of the EBM and SLM are doubtlessly promising, but to fully realize their potentials there are still many challenges to overcome.Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties and low cost. Though IN718 is being mostly used as a turbine disk material now, the initial introduction of IN718 was to overcome the poor weldability of superalloys in 1960s, since sluggish precipitation of strengthening phases λ’/λ’’ enables good resistance to strain-age cracking during welding or post weld heat treatment. Given the similarity between AM and welding processes, IN718 has been widely applied to the metallic AM field to facilitate the understandings of process-microstructure-property relationships.The work presented in this licentiate thesis aims to better understand microstructures and mechanical properties EBM and SLM IN718, which have not been systematically investigated. Microstructures of EBM and SLM IN718 have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the process conditions. Monotonic mechanical properties (e.g., Vickers microhardness and tensile properties) have also been measured and rationalized with regards to the microstructure evolutions before and after heat treatments.For EBM IN718, the results show the microstructure is not homogeneous but dependant on the location in the components, and the anisotropic mechanical properties are probably attributed to alignment of porosities rather than texture. Post heat treatment can slightly increase the mechanical strength compared to the as-manufactured condition but does not alter the anisotropy. SLM IN718 shows significantly different microstructure and mechanical properties to EBM IN718. The as-manufactured SLM IN718 has very fine dendritic microstructure and Laves phases in the interdendrites, and is “work-hardened” by the residual strains and dislocations present in the material. Mechanical properties are different between horizontally and vertically built samples, and heat treatment can minimize this difference. Results from this licentiate thesis provide the basis for the further research on the cyclic mechanical properties of EBM and SLM IN718, which would be the focus of following phase of the Ph.D. research.
24.	Jiang, Shuang, et al. (författare) Shear banding-induced ⟨c + a⟩ slip enables unprecedented strength-ductility combination of laminated metallic composites 2022 Ingår i: Journal of Materials Science & Technology. - Shenyang, China : Elsevier. - 1005-0302. ; 110, s. 260-268 Tidskriftsartikel (refereegranskat)abstract Shear bands in metallic materials have been reported to be catastrophic because they normally lead to non-uniform plastic deformation. Ductility of laminated metallic composites deteriorates with increasing processing strain, particularly for those having hexagonal-close-packed (hcp) constituents due to inadequate slip systems and consequently prominent shear banding. Here, we propose a design strategy that counterintuitively tolerates the bands with localized strains, i.e. the shear banded laminar (SBL) structure, which promotes ⟨c + a⟩ dislocation activation in hcp metals and renders unprecedented strengthductility combination in hcp-metal-based composites fabricated by accumulative roll bonding (ARB). The SBL structure is characterized with one soft hcp metal constrained by adjacent hard metal in which dislocations have been accumulated near the bimetal interfaces. High-energy X-ray diffraction astonishingly reveals that more than 90% of dislocations are non-basal in Ti layers of the SBL Ti/Nb composite processed by eight ARB cycles. Moreover, ⟨c + a⟩ dislocations occupy a high fraction of ∼30%, promoting further ⟨c + a⟩ cross slip. The unique stress field tailored by both shear banding and heterophase interface-mediated deformation accommodation triggers important ⟨c + a⟩ slip. This SBL design is of significance for developing hcp-based laminates and other heterostructured materials with high performances.
25.	Kjellsson, Henrik, et al. (författare) High Temperature Fatigue Performance of Electron Beam Powder Bed Fusion Manufactured Alloy 718 2022 Ingår i: Metallurgical and Materials Transactions. A. - : Springer Nature. - 1073-5623 .- 1543-1940. ; 53, s. 2496-2514 Tidskriftsartikel (refereegranskat)abstract The microstructure and mechanical properties of additively manufactured (AM) parts have been shown to be different from that of cast and wrought counterparts. In this study, electron beam powder bed fusion (EB-PBF) fabricated Alloy 718 was exposed to three different heat treatment routes followed by strain-controlled fatigue testing at 550 degrees C. The fatigue tests were performed with specimens built with their center axis parallel and transverse relative to the build direction. The microstructure showed saturated precipitation of delta-Ni3Nb after repeated solution treatment at 954 degrees C. In contrast, no delta-Ni3Nb precipitates could be observed after a single-step solution treatment at 1025 degrees C. However, the disparity of secondary phases showed no noticeable influence on the fatigue life. A significant difference in fatigue behavior was noted between the parallel and transverse directions. The specimens loaded parallel to the elongated grains showed on average similar to 5x greater life in comparison to the perpendicularly loaded specimens. Compared to corresponding heat-treated material conditions tested at ambient temperature, the specimens showed lower life at high strain amplitude and superior life at low strain amplitude. Moreover, competitive internal and surface failure modes were observed at the lower strain amplitudes while for the higher strain ranges, surface failure modes dominated. (C) The Author(s) 2022
26.	Leijon, Freddy, 1985- (författare) Aluminium alloy development for Additive Manufacturing 2023 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Powder Bed Fusion (PBF Additive Manufacturing AM have emerged as a promising manufacturing process possessing a powerful combination of characteristics. Most noticeable are the near-net-shape, short lead time and flexibility, both with regard in design freedom and in part-to-part variation. Aluminium alloys are used in everything from food packaging, furniture's, to cars and airplanes. To accommodate for this wide range in material requirements, different alloys have been developed over the past century. To reach the full potential of AM, a thoroughly work lies ahead of the research community to find, tailor and refine alloys.This work has focused on experimentally screening of AM alloys, for their printability and potential properties. To accelerate this, a novel high through put method was first developed to efficiently produce a broad range of alloys both with respect to compositions and alloying elements. This method consists of two steps; in the first step a compositional alloy gradient film is deposited on an aluminium substrate, and in a second step a microstructure mimicking PBF is formed by either laser or electron beam melting of the film. Gradients up to 500mm in length ranging from 0-85wt% in alloying content were achieved. This enabled high resolution studies of the influence of alloying elements over wide compositional intervals. Various aspects of the material were possible to investigate such as: Grain size, hardness, printability, evaporation losses, solid solution, electrical conductivity and microstructure. The results were verified against the available literature, and a strong correlation between properties of the PBF mimicked materials and actual PBF materials were confirmed.With the developed screening method, printability i.e. the material's capacity to be processed in PBF without formation of cracks , could be studied and mapped out for a large set of alloys. The AlMgSi system were found to be printable without grain refinement if Si+Mg<0.7wt% or Si+2/3Mg>4wt% for Mg < 3wt% and Si > 3wt%. Investigations of Til-xMxB2 and Al3Til-xMx grain refiners in 2wt% Cu alloys reveled that grain refinement and printability strongly correlated to both x and the element M(Zr,Ta,V,W). However, no clear relationship between the grain size and the lattice parameters of Til-xMxB2 and Al3Til-xMx were found.In addition to mapping out printability, hardness as a function of composition was also mapped out for the binary alloys Al -Ti, -Zr, -Nb, -Sr and the AlMgSi system. Other important findings are that the Mg loss due to evaporation and the solid solution of Mg was found to depend linearly on the amount of Mg, and a transition from equiaxed to fine lamellar Al4Sr intermetallic going above 5wt%. Altogether, the screening method developed in this work offer a unique way to efficiently study composition dependent transitions in printability, microstructure and other material properties which are otherwise difficult to foresee or experimentally laborious to study.
27.	Moverare, Johan, Professor, 1973- (författare) Mechanical properties of high-performance AM materials 2023. - 1. Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 429-458 Bokkapitel (refereegranskat)abstract Due to the inherent nature of the processes used for additive manufacturing (AM), the AM materials often display uniquemicrostructures that differ from their conventionallymanufactured counterparts. This will also influence the mechanical properties of these alloys. This chapter gives anoverview of the impact of the AM-specific features typicallyfound in the microstructure of high-performance materials. This includes the strengthening effects from the cell and grainstructures in powder bed fused materials. The origin of theanisotropic behaviors and fracture mechanisms present in thematerials, especially at elevated temperatures are reviewed. Finally, the influence of typical surface conditions, internal defects, and microstructure on fatigue behavior will be discussed.
28.	Moverare, Johan, 1973- (författare) Microstresses and anisotropic mechanical behaviour of duplex stainless steels 2001 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The evolution of deformation during monotonic and cyclic loading of a two-phase material like duplex stainless steel is more complex than in a single-phase material. One reason for this is the microstresses formed due to differences in thermal and mechanical properties between the two phases. Another factor contributing to a complex load partitioning between the two phases is that hot and cold rolled duplex stainless steel exhibits anisotropic material properties. The aim of this thesis has therefore been to investigate the influence of microstresses and an isotropy on the mechanical properties of duplex stainless steels.The effect of microstresses was clearly revealed when X-ray diffraction was used to study the evolution of microstresses during cyclic loading. Even if the hardness and yield strength were found to be higher in the austenitic phase compared to the ferritic phase more plastic deformation occurs in austenite during cyclic tensile loading. This was also confirmed by transmission electron microscopy investigations of the dislocation structure in both phases. The main reason for the higher degree of plastic deformation in the austenitic phase is that the microstresses are tensile in this phase and compressive in the ferritic phase.Measurements of the crystallographic texture were used as input to theoretical predictions of both elastic and plastic anisotropy. The predicted anisotropic material properties were then used in finite element simulations to study the flow behavior and the load partitioning between phases during deformation in different loading directions. The experiments and the simulations show that the microstresses and the anisotropy make the load partitioning between the two phases dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45°-direction more plastic deformation occurs in the ferritic phase.The anisotropic flow behaviour of the as-received material can be predicted from the crystallographic texture. However, it was found that prestraining introduces a transient work hardening behaviour during the second stage deformation, whjch causes an anisotropic flow behaviour immediately after yielding that cannot be described by the crystallographic texture. Instead the an isotropy can be associated with the rearrangement of the dislocation structure that occurs during changes in the loading path. Prestraining also alters the microstresses from being higher in the transverse direction to being higher in the rolling direction. At the same time the fatigue limit is changed from being higher in the rolling direction to being higher in the transverse direction. This study shows that microstresses have a significant influence on fatigue crack initiation and the fatigue limit of duplex stainless steels.
29.	Pauzon, Camille Nicole Géraldine, 1994, et al. (författare) Effect of layer thickness on spatter properties during laser powder bed fusion of Ti–6Al–4V 2023 Ingår i: Powder Metallurgy. - : Informa UK Limited. - 0032-5899 .- 1743-2901. ; 66:4, s. 333-342 Tidskriftsartikel (refereegranskat)abstract High layer thicknesses for laser powder bed fusion are promising for productivity increase. However, these are associated with increased process instability, spatter generation and powder degradation, crucial for alloys sensitive to oxygen. The effect of increasing layer thickness from 30 to 60 µm is studied focusing on Ti-6Al-4V spatter formation during LPBF and its characterisation, with scanning and transmission electron microscopy, combustion analysis and X-ray photoelectron spectroscopy. Results indicate that spatters are covered with a uniform Ti-Al-based oxide layer and Al-rich oxide particulates, the thickness of which is about twice that present on virgin powder. The oxygen content was about 60% higher in spatters compared to the virgin powder. The study highlights that increasing the layer thickness to 60 µm permits to reduce the total generation of spatters by ∼40%, while maintaining similar spatter characteristics and static tensile properties. Hence, this allows to increase build rate without compromising process robustness.
30.	Sun, Xiaoyu, et al. (författare) Impeding the γ' depletion during the interdiffusion between bond coatings and superalloys via introduction of tantalum in bond coatings 2023 Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 227 Tidskriftsartikel (refereegranskat)abstract The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to microstructural degradation of superalloys and thus loss of mechanical properties. In this study, we mod- ified MCrAlY coatings by adding Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt% Ta MCrAlY exhibits an outstanding resistance to c0 depletion in the substrate and compa- rable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase- transformation mechanism induced by initial interdiffusion, uncovering the cause of c0 depletion in the substrate and the mechanism behind improving resistance to c0 depletion by Ta addition.
31.	Sun, Xiaoyu (författare) Performance of High-temperature Coatings : Oxidation and Interdiffusion 2023 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract The use of aluminiferous coatings profoundly improves the service life of superalloys but leads to the microstructural degradation of superalloys and thus the loss of mechanical properties. To solve this trade-off, two strategies were employed in this research.At first, we modified MCrAlY coatings by inducing Ta to reduce the interdiffusion effect on substrate alloys. This strategy was verified by 2000 h/1100 °C oxidation tests in two Ta-containing MCrAlY-IN792 systems. The system with 3.3 wt.% Ta MCrAlY displays an outstanding resistance to γ′ depletion in the substrate and comparable oxidation property in comparison with a reference system of Ta-free MCrAlY-IN792. Increasing Ta to 7.4 wt.% results in reduced oxidation resistance. Thermodynamic simulations revealed the phase-transformation mechanism induced by initial interdiffusion, uncovering the cause of γ′ depletion in the substrate and the mechanism behind improving resistance to γ′ depletion by Ta addition.In addition, we developed novel Y-doped AlCoCrFeNi high-entropy alloys by tuning Al/Cr ratio ACR. After a long-term isothermal test in air at 1100 °C up to 1000 h, the higher ACR alloy displayed a stronger oxidation resistance at the early oxidation stage, whereas a contrary result could be detected in the later stage. The microstructural analysis confirmed that the fast growth of spinel dominated the early oxidation process, leading to higher oxidation rate of the lower ACR alloys. The later stage was governed by the growth of Al2O3. Lager size Al2O3 gain formed on the lower ACR alloy impeded the inward diffusion of oxygen and thus reduced the oxidation rate, which was further verified by our thermodynamic calculations.
32.	Wennersten, Karin, et al. (författare) Feasibility of Melting NbC Using Electron Beam Powder Bed Fusion 2024 Ingår i: Advanced Engineering Materials. - : WILEY-V C H VERLAG GMBH. - 1438-1656 .- 1527-2648. Tidskriftsartikel (refereegranskat)abstract High melting point materials such as ceramics and metal carbides are in general difficult to manufacture due to their physical properties, which imposes the need for new manufacturing methods where electron beam powder bed fusion (EB-PBF) seems promising. Most materials that have been successfully printed with EB-PBF are metals and metal alloys with good electrical conductivity, whereas dielectric materials such as ceramics are generally difficult to print. Catastrophic problems such as smoking and spattering can occur during the EB-PBF processing owing to inappropriate physical properties such as lack of electrical, and thermal conductivity and high melting point, which are challenging to overcome by process optimization. Due to these difficulties, a limited level of understanding has been achieved regarding melting ceramics and refractory alloys. Herein, three different substrates of niobium carbide (NbC) are melted using EB-PBF. The established process parameter window shows a good correlation between EB-PBF process parameters, surface, and melt characteristics, which can be used as a baseline for a printing process. Melting NbC is proven feasible using EB-PBF; the work also points out challenges related to arc trips and spattering, as well as future investigations necessary to create a stable printing process. Additive manufacturing offer new ways of manufacturing ceramics and metal carbides otherwise hard to produce. This study presents one of the first attempts at melting niobium carbide using electron beam powder bed fusion by identifying process window and investigating how the different process parameters affect the melt characteristics, as well as identifying potential issues regarding printing metal carbides.image (c) 2024 WILEY-VCH GmbH
33.	Yu, Cheng-Han, 1992-, et al. (författare) Anisotropic behaviours of LPBF Hastelloy X under slow strain rate tensile testing at elevated temperature 2022 Ingår i: Materials Science & Engineering. - : ELSEVIER SCIENCE SA. - 0921-5093 .- 1873-4936. ; 844 Tidskriftsartikel (refereegranskat)abstract To improve the understanding of high temperature mechanical behaviours of LPBF Ni-based superalloys, this work investigates the influence of an elongated grain structure and characteristic crystallographic texture on the anisotropic tensile behaviours in LPBF Hastelloy X (HX) at 700 °C. Two types of loading conditions have been examined to analyse the anisotropy related to the building direction (BD), including the vertical loading (loading direction//BD) and the horizontal loading (loading direction ⊥ BD). To probe the short-term creep behaviours, slow strain rate tensile testing (SSRT) has been applied to address the strain rate dependent inelastic strain accumulation. In-situ time-of-flight neutron diffraction upon loading was performed to track the anisotropic lattice strain evolution in the elastic region and the texture evolution in the plastic region. Combined with the post microstructure and fracture analysis, the anisotropic mechanical behaviours are well correlated with the different microstructural responses between vertical and horizontal loading and the different strain rates. A better creep performance is expected in the vertical direction with the consideration of the better ductility and the higher level of texture evolution.

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