| 1. |
- Akurati, Sashanka, et al.
(author)
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Deformation mechanisms in ice-templated alumina–epoxy composites for the different directions of uniaxial compressive loading
- 2021
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In: Materialia. - : Elsevier. - 2589-1529. ; 16
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Journal article (peer-reviewed)abstract
- The ice-templating technique enables the fabrication of multilayered ceramic-based composite materials. Very little is known on the inelastic deformation mechanisms that evolve in this class of composite materials under compressive loading conditions and cause macroscopic failure. The current investigation is motivated by a recent study by the authors, which revealed that the uniaxial compressive response of ice-templated ceramic–polymer composites is strongly dependent on the loading direction relative to the layer orientation. The current investigation reveals that the inelastic deformation mechanisms in ice-templated alumina–epoxy composites are strongly influenced by the compressive loading orientation relative to the growth direction of ice crystals. The deformation mechanisms were investigated for the loading directions of 0° (parallel to the growth direction), 45° (to the growth direction), and 90° (to the growth direction). For 0°, kink band formation and longitudinal splitting were observed to be the primary strength limiting mechanisms. Kink band formation could be the primary strength limiting factor and responsible for the catastrophic-type compressive failure response. For the loading directions of 45° and 90°, interface delamination and fracture within the lamella walls and across the alumina–epoxy interfaces were the main deformation mechanisms. These mechanisms significantly reduced the compressive strength but attributed progressive-type failure behavior in ice-templated composites. The knowledge of the inelastic deformation mechanisms in ice-templated ceramic–polymer composites under compressive loading is vital for an improved understanding of structure–mechanical property relationships and hierarchical materials design.
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| 2. |
- Baghdadchi, Amir, et al.
(author)
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Effect of Ni content on 475°C embrittlement of directed energy deposited duplex stainless steel using a laser beam and wire feedstock
- 2024
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In: Materialia. - 2589-1529. ; 36
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Journal article (peer-reviewed)abstract
- Duplex stainless steel (DSS), specifically the 2209 grade, is increasingly employed in additive manufacturing, particularly in processes like directed energy deposition using a laser beam with wire (DED-LB/w). However, a significant challenge arises when DSS faces brittleness within the temperature range of 250–500 °C. This study employs advanced characterization techniques, including atom probe tomography (APT) and transmission electron microscopy (TEM), to investigate DSS embrittlement after aging at 400 °C for up to 1000 h. The hardness analysis revealed that the higher Ni content in DED-LB/w-fabricated DSS cylinder promotes the age hardening compared to 2205 wrought DSS plate. Furthermore, APT and TEM demonstrated that, alongside the decomposition of ferrite into Fe-rich (α) and Cr-rich (αʹ) phases, clustering of Ni, Mn, and Si atoms contributes to the embrittlement. Although the Ni-Mn-Si-rich clusters could suggest nucleation of G-phase, the G-phase crystal structure was not observed by TEM. This might be attributed to the short aging time or limitations in the characterization technique. This work underscores the impact of characterization techniques on the measurement of spinodal decomposition, with APT providing capability of detecting nanometer sized clusters. By elucidating the complexities of 475 °C-embrittlement in DED-LB/w DSS, this study offers valuable insights for industrial applications and a deeper understanding of age hardening in duplex DSSs under specific manufacturing conditions.
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| 3. |
- Bogdanoff, Toni, et al.
(author)
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The influence of copper addition on crack initiation and propagation in an Al–Si–Mg alloy during cyclic testing
- 2020
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In: Materialia. - : Elsevier. - 2589-1529. ; 12
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Journal article (peer-reviewed)abstract
- The effect of copper (Cu) addition up to 3.2 wt% on crack initiation and propagation in an Al–Si–Mg cast alloy was investigated using in-situ cyclic testing in the as-cast condition. A combination of digital image correlation, electron backscatter diffraction, and scanning electron microscopy was used to investigate crack initiation and propagation behaviour during in-situ cyclic testing. The results showed that Cu-rich intermetallic compounds with the addition of Cu up to 1.5 wt% do not affect the fatigue behaviour of these alloys, and that crack propagation in these cases is trans-granular and trans-dendritic. However, increasing the concentration of the Cu retained in the primary α-Al matrix in solid solution and Cu-containing precipitates delayed crack propagation during cyclic testing. The results showed that strain accumulation was highest at the grain boundaries; however, the crack preferred to propagate along or across primary α-Al dendrites due to the relatively lower mechanical strength of the matrix compared to the eutectic and intermetallic phases. Moreover, the addition of Cu of more than 3.0 wt% to Al-Si-Mg alloys changes the fatigue behaviour that a rapid failure occurs.
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| 4. |
- Bäcke, Olof, 1984, et al.
(author)
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Decomposition pathways in nano-lamellar CVD Ti 0.2 Al 0.8 N
- 2023
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In: Materialia. - 2589-1529. ; 30
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Journal article (peer-reviewed)abstract
- Recent progress in chemical vapour deposition (CVD) technology has enabled synthesis of metastable cubic Ti1−xAlxN coatings with x as high as 0.8–0.9. These coatings have unique micro- and nano-structures consisting of grains with epitaxially grown nanolamellae with different Al/Ti ratios, and exhibit exceptional hardness and resistance to wear and oxidation. Here, the thermal stability and decomposition of nano-lamellar CVD Ti0.2Al0.8N at temperatures between 800 and 1000 °C have been investigated using a combination of cross-sectional transmission X-ray nano-diffraction and scanning transmission electron microscopy. The decomposition started by formation of hexagonal AlN (h-AlN) in the grain boundaries throughout the coating. Below 900 °C, only limited further decomposition of the grain interiors occurred. At higher temperatures the formation of grain boundary h-AlN was followed by a bulk transformation of the nano-lamellar structure, starting at the top of the coating and subsequently sweeping inwards. The bulk transformation occurred initially through spinodal decomposition, followed by transformation of the Al-rich cubic phase to h-AlN, leading to a coarsened structure with Ti-rich domains in a h-AlN matrix. The behaviour is explained by the higher capability of grain boundaries and free surfaces to accommodate the volumetric expansion from the h-AlN formation. The results increase our understanding of the complicated decomposition processes in these metastable cubic coatings, which are of utmost importance from both technological and scientific perspectives.
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| 5. |
- Bäcke, Olof, et al.
(author)
-
Decomposition pathways in nano-lamellar CVD Ti0.2Al0.8N
- 2023
-
In: Materialia. - 2589-1529. ; 30
-
Journal article (peer-reviewed)abstract
- Recent progress in chemical vapour deposition (CVD) technology has enabled synthesis of metastable cubic Ti1−xAlxN coatings with x as high as 0.8–0.9. These coatings have unique micro- and nano-structures consisting of grains with epitaxially grown nanolamellae with different Al/Ti ratios, and exhibit exceptional hardness and resistance to wear and oxidation. Here, the thermal stability and decomposition of nano-lamellar CVD Ti0.2Al0.8N at temperatures between 800 and 1000 °C have been investigated using a combination of cross-sectional transmission X-ray nano-diffraction and scanning transmission electron microscopy. The decomposition started by formation of hexagonal AlN (h-AlN) in the grain boundaries throughout the coating. Below 900 °C, only limited further decomposition of the grain interiors occurred. At higher temperatures the formation of grain boundary h-AlN was followed by a bulk transformation of the nano-lamellar structure, starting at the top of the coating and subsequently sweeping inwards. The bulk transformation occurred initially through spinodal decomposition, followed by transformation of the Al-rich cubic phase to h-AlN, leading to a coarsened structure with Ti-rich domains in a h-AlN matrix. The behaviour is explained by the higher capability of grain boundaries and free surfaces to accommodate the volumetric expansion from the h-AlN formation. The results increase our understanding of the complicated decomposition processes in these metastable cubic coatings, which are of utmost importance from both technological and scientific perspectives.
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| 6. |
- Chai, Guocai, 1956-, et al.
(author)
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On fatigue crack origin with a fine granular area in matrix without defect
- 2024
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In: Materialia. - : ELSEVIER SCI LTD. - 2589-1529. ; 33
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Journal article (peer-reviewed)abstract
- Fatigue crack initiation in metallic materials during very high cycle fatigue has been characterized by a subsurface crack origin with defect such as an inclusion. In this paper, fatigue damage behavior in an austenitic stainless steel has been studied using a novel progressive stepwise load increasing test method with each cycle step higher than 108 cycles. Subsurface crack origin with a fine granular area has formed in the matrix without defect. This is a new phenomenon. The mechanism has been investigated using Focused Ion Beam crosssectioning and electron channeling contrast imaging techniques. Strain localization, grain fragmentation and local plasticity exhaustion are the main factors that cause fatigue damage and crack initiation in the matrix. This study provides a fundamental understanding how material damage and crack initiation occur in material matrix during very high cycle fatigue.
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| 7. |
- Dahlqvist, Martin, et al.
(author)
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Impact of vacancies on structure, stability and properties of hexagonal transition metal diborides, MB2 (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, and Fe)
- 2022
-
In: Materialia. - : Elsevier Science Ltd. - 2589-1529. ; 26
-
Journal article (peer-reviewed)abstract
- In this study, we have used density functional theory (DFT) calculations to characterize if and how defects influence the stability and electronic/mechanical properties of MB2 (AlB2-type) for different transition metal M. From a point defect analysis including vacancies, interstitials, and anti-sites, we identify vacancies to be most favored, or least unfavored. To provide insight into possible vacancy ordering, we focus on vacancies on M- and B-sublattices for nine metals (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), modelled both as disordered and ordered. We demonstrate and explain why vacancies have a significant impact for M from Group 4 (Ti, Zr, Hf), Group 5 (Nb, Ta) and 6 (Mo, W) with improved thermodynamical and dynamical stability as well as mechanical properties. This by diverging from the ideal composition through controlled off-stoichiometry in terms of vacancies in M- or B-deficient structures. Line compounds TiB2, ZrB2 and HfB2 account for B-poor or M-rich conditions by forming planar defects comprised of vacant B. This in contrast to the ordered M- and B vacancies identified for MoB2 and WB2, with an optimal result at 33.33% M- and 25% B-vacancies, respectively, which significantly improves the stability and concurrent properties through elimination of antibonding states and minimization of non-bonding states. Similar behavior with enhanced stability and properties is demonstrated for NbB2 and TaB2 with an optimum around 10% M- and 17% B-vacancies, respectively.
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| 8. |
- Das, Yadunandan, et al.
(author)
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Small-angle neutron scattering quantification of phase separation and the corresponding embrittlement of a super duplex stainless steel after long-term aging at 300 degrees C
- 2020
-
In: Materialia. - : ELSEVIER SCI LTD. - 2589-1529. ; 12
-
Journal article (peer-reviewed)abstract
- Small-angle neutron scattering (SANS) was applied to quantify the nanostructural evolution during spinodal decomposition in a 25Cr-7Ni (wt.%) super duplex stainless steel isothermally aged at 300 degrees C, for up-to 48,000 h. Prior to the application on the 25Cr-7Ni alloy, the SANS methodology was validated by comparing results from SANS measurements on binary Fe-Cr alloys with atom probe tomography results. SANS results on the 25Cr-7Ni alloy indicated that decomposition wavelength decreased from 5.1 nm to 4.5 nm, whereas the amplitude increased from 15.0 to 33.4 at.%. This quantitative nanostructural evolution correlated to a hardening of the ferrite phase by 190 HV and a reduction of the sub-size Charpy-V impact toughness from 60 J to 25 J.
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| 9. |
- Dong, Xixi, et al.
(author)
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A quantitative strategy for achieving the high thermal conductivity of die-cast Mg-Al-based alloys
- 2022
-
In: Materialia. - : Elsevier. - 2589-1529.
-
Journal article (peer-reviewed)abstract
- A quantitative strategy was reported to design and develop Mg-Al-based alloys to achieve high thermal conductivity, in which the specific RE elements can be introduced to reduce the Al concentration in Mg matrix and to suppress the formation of Mg17Al12 phase through the formation of new intermetallic phases. Based on quantitative calculations, the strategy was demonstrated by a novel die-cast Mg3.2Al4.4La0.4Nd (in wt.%) alloy, which provided the thermal conductivity of 114.3 W/(m∙K) at ambient temperature and 137.5 W/(m∙K) at 300 °C, ∼25% higher than the commercial Mg4Al4RE (AE44) alloy. Meanwhile, the alloy also offered excellent ambient yield strength of 143.2 MPa and elongation of 8.2%, and superior strength and ductility than the AE44 alloy at elevated temperatures.
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| 10. |
- Edén, Mattias
(author)
-
Structure and formation of amorphous calcium phosphate and its role as surface layer of nanocrystalline apatite : Implications for bone mineralization
- 2021
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 17
-
Journal article (peer-reviewed)abstract
- We provide a critical review of the chemical composition and structure of synthetic and biogenic (bone/dentin mineral) nanocrystalline hydroxy-carbonate apatite (HCA). Such particles exhibit a core-shell organization, where an ordered HCA core is coated by a surface layer, whose nature is best captured by amorphous calcium phosphate (ACP), which is known to be a precursor phase of synthetic HCA, but whose role of bone/dentin mineralization has remained a most controversial subject. After reviewing the structure of each HCA and ACP component, as well as the most recent findings on their in vitro formation mechanisms, we examine the core-shell HCA organization further, with a focus on the disordered surface (shell ) domain. In most of recent literature, the surface portion is often referred to as the hydrated surface layer , but without identifying its shared chemical and structural features of (synthetic) ACP. Unfortunately, that missing surface-layer/ACP equivalence obscures that the surface layer at the synthetic/biogenic nanocrystallites may simply constitute a remnant of the ACP phase from which the ordered HCA core nucleated. Although many topics reviewed herein have been investigated for more than six decades, several remain unsettled and heavily debated. Notably, decades-old articles offer suggestions that have passed unnoticed by the younger generations of researchers; we contrast and discuss both the latest and early contributions of this field, as well as highlighting several unsettled topics that should be revisited to improve our understanding of the ACP and HCA structures and in vitro / in vivo formation mechanisms.
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| 11. |
- Ghafoor, Naureen, et al.
(author)
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Phase separation paths in metastable Zr1-xAlxN monolithic layers compared to multilayers with TiN: Growth versus annealing temperatures
- 2023
-
In: Materialia. - : ELSEVIER SCI LTD. - 2589-1529. ; 28
-
Journal article (peer-reviewed)abstract
- Metastable super-saturated Zr1_xAlxN alloys tend to phase separate into the equilibrium cubic (c) ZrN and wurtzite (w) AlN due to a deep miscibility gap. Transformation is shown here to follow distinctly different paths depending on if Zr1_xAlxN (x = 0.3 and 0.6) is sputter deposited as a single layer or multi-layered with TiN, and further varied by post-deposition annealing. Using in situ high-energy synchrotron wide-angle X-ray scattering and analytical transmission electron microscopy, surface segregation effects are compared to secondary phase transformations occurring in as-deposited layers during thermal annealing up to 1000 degrees C. For the primary phase transformation from the vapor phase, w-AlN nucleates and an AlN-ZrN labyrinthine structure evolves at elevated deposition temperature with semi-coherent interfaces over several nanometers, where the higher Al content narrows the structure in both single and multilayers. Transformation in thinner alloy layers is governed by epitaxial forces which stabilize single-phase c-Zr0.4Al0.6N, which enables c-Zr0.4Al0.6N/TiN superlattice growth at temperatures as low as 350 degrees C. Regardless of the decomposition route, the formation of c-AlN is impeded and w-AlN instantaneously forms during transformation. In contrast, isostructural decomposition into w-AlN and w-Zr (Al)N occurs in w-Zr0.4Al0.6N alloys during annealing.
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| 12. |
- Ghatei-Kalashami, A., et al.
(author)
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Occurrence of liquid-metal-embrittlement in a fully ferritic microstructure
- 2021
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In: Materialia. - : Elsevier. - 2589-1529. ; 15
-
Journal article (peer-reviewed)abstract
- Despite numerous studies making an effort to attain a thorough understanding of the liquid-metal-embrittlement (LME) phenomenon, the metallurgical facet of this catastrophic event remains unclear in iron/zinc (Fe/Zn) systems. While it has been frequently reported that the presence of austenite is an essential prerequisite for LME formation, the present study showed that fully ferritic structure is prone to LME phenomenon and has a high susceptibility to LME-cracking which makes it a novel observation adding to a pool of knowledge regarding LME occurrence. The elemental distribution analysis near the LME crack-tip indicated that liquid Zn was not present which confirmed solid-state grain boundary diffusion was a plausible description of LME-cracking. The occurrence of grain dropout as well as a Zn-containing crack in grain boundary without any branches with other cracks showed that grain boundary sensitization has assisted LME-cracking.
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| 13. |
- Gren, Martin, 1989, et al.
(author)
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Wetting of surfaces and grain boundaries in cemented carbides and the effect from local chemistry
- 2019
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 8
-
Journal article (peer-reviewed)abstract
- Wettability is an important factor in the sintering of cemented carbides. We study wetting of surfaces and grain boundaries in WC–Co och WC–Ni cemented carbides using density functional theory (DFT). Based on experimental observations of WC grain orientations in cemented carbides, relevant model interfaces are created. The local chemical composition at the interfaces is taken into account by substitution of interface atoms, and the effect of temperature is evaluated based on a companion study of temperature dependent interface energies in cemented carbides. The results indicate that the wettability of Ni and Co on WC surfaces are similar. Furthermore, the wettability of Co on the commonly occurring basal WC surface is better in W-rich materials compared to C-rich materials. At liquid phase sintering temperatures we get perfect wetting in W-rich materials, while only partial wetting in C-rich materials, which is in agreement with recent experiments on wetting in WC–Co cemented carbides. The segregation of binder phase atoms to WC/WC grain boundaries stabilize grain boundaries and make them more resistance against infiltration (wetting) by binder phase. We find that the amount of dissolved binder atoms in essentially all studied WC/WC grain boundaries are of half a monolayer proportion, which is in agreement with experimental studies. Further, in WC–Co there is a stronger resistance against grain boundary infiltration compared to WC–Ni. We find that the continuous skeleton of WC grains seen after sintering and which is crucial for the superior mechanical strength of the material exists already during liquid phase sintering.
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| 14. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(author)
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Fracture of Cr2O3 single crystals on the microscale
- 2021
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 15
-
Journal article (peer-reviewed)abstract
- Studying cleavage properties of protective oxide scales is imperative to understand their fracture behaviour, since transgranular fracture is observed in many cases. The small thickness and polycrystalline structure of such scales makes it difficult to identify active cleavage planes directly from mechanical testing. To resolve this issue for Cr2O3, we present an approach to experimentally identify cleavage planes through micro-cantilever bending. Single crystal wafers are used to prepare micro-cantilevers of pentagonal cross-section in different orientations, targeting possible cleavage planes. Fracture surface imaging showed rhombohedral and pyramidal fracture, though surface energy studies predict rhombohedral as the dominant plane. There does exist a preference for rhombohedral fracture over pyramidal, which is also revealed from the experiments.
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| 15. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(author)
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Microscale fracture of chromia scales
- 2019
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 8
-
Journal article (peer-reviewed)abstract
- Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the mm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were obtained as well. The observed mixed trans- and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales.
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| 16. |
- Hearn, William, 1992, et al.
(author)
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In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel
- 2022
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 23
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Journal article (peer-reviewed)abstract
- During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters.
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| 17. |
- Hernando, Juan Carlos, et al.
(author)
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Evolution of primary austenite during coarsening and impact on eutectic microstructure in Fe–C–Si alloys
- 2019
-
In: Materialia. - : Elsevier. - 2589-1529. ; 7
-
Journal article (peer-reviewed)abstract
- The evolution of primary austenite morphology during isothermal coarsening has been studied in the three main Fe–C–Si alloys used in industry, LGI, CGI, and SGI. The dendritic microstructure increases length scale during coarsening accompanied by fragmentation and coalescence of austenite crystals. The morphological parameters, SDAS, Mγ, DhydID, and Dγ show a linear relation with the cube root of coarsening time, t1/3, with similar rates for the three different Fe–C–Si alloys. The eutectic microstructures after coarsening of primary austenite in CGI and SGI alloys are not significantly affected by the surface area of primary austenite and the size of the interdendritic regions. Fraction, nodularity, shape distribution of graphite particles and the number of nodules and eutectic cells are similar when studied as a function of coarsening time. These results suggest that the nucleation frequency in CGI and SGI, and the growth of eutectic microstructures in CGI, are not significantly influenced by the morphology of primary austenite.
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| 18. |
- Hosseini, Vahid, 1987-, et al.
(author)
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Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels
- 2019
-
In: Materialia. - : Elsevier. - 2589-1529. ; 6
-
Journal article (peer-reviewed)abstract
- High temperature heat affected zone (HTHAZ) of duplex stainless steels is prone to local corrosion attack due to a high ferrite fraction and nitride formation. Literature commonly attributes formation of this undesirable microstructure to rapid cooling from high peak temperatures. However, this study investigated the possible role of nitrogen loss in HTHAZ using a combination of physical and kinetics simulation. Applying a stationary gas-tungsten arc (GTA) on a water-cooled plate, a technique known as arc heat treatment, showed that considerable nitrogen loss occurred already after 0.5 min up to 150 µm from the fusion boundary. This zone was extended to 1300 µm after 600 min arc heat treatment. The results of bead-on-plate GTA welding and Gleeble testing replicating the thermal cycle in HTHAZ showed that the ferrite fraction of the real HTHAZ was 7% higher than that for Gleeble samples. This agrees with results from arc heat treatment, where ferrite fraction was found to increase due to nitrogen loss. Numerical and Dictra approaches were developed to simulate the kinetics of nitrogen loss in HTHAZ considering ferrite as the nitrogen rapid diffusion path towards the weld pool. Simulation showed good agreement with both welding and physical simulation. A combination of thermodynamic and kinetics simulations properly predicted the ferrite fraction at 1100 °C for different arc heat treatment times. In conclusion, the experiments (physical simulations and GTA welding) and kinetics simulation showed that nitrogen was lost from HTHAZ to the weld pool. © 2019 Acta Materialia Inc.
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| 19. |
- Jabir Hussain, Ahmed Fardan, 1996, et al.
(author)
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Fine-Tuning Melt Pools and Microstructures: Taming Cracks in Powder Bed Fusion—Laser Beam of a non-weldable Ni-base Superalloy
- 2024
-
In: Materialia. - : ELSEVIER SCI LTD. - 2589-1529. ; 34
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Journal article (peer-reviewed)abstract
- Powder Bed Fusion – Laser Beam (PBF-LB) of high γ’ strengthened Ni-base superalloys, such as CM247LC, is of great interest for high temperature applications in gas turbines. However, PBF-LB of CM247LC is challenging due to the high cracking susceptibility during PBF-LB processing (solidification cracking) and heat treatment (strain age cracking, mostly caused by residual stresses). This study focuses on understanding the impact of process parameters on microstructure, residual stresses and solidification cracking. Laser power (P), speed (v) and hatch spacing (h) were varied while the layer thickness (t) was fixed. The melt pool size and shape were found to be key factors in minimizing solidification cracking. Narrower and shallower melt pools, achieved using a low line energy density (LED = P/v ≤ 0.1 J/mm), gave low crack densities (0.7 mm/mm2). A tight hatch spacing (h = 0.03 mm) resulted in reduced lack of fusion porosity. Electron backscatter diffraction investigations revealed that parameters giving finer microstructure with 〈100〉 crystallographic texture had low crack densities provided they were processed with a low LED. Atom probe tomography elucidated early stages of spinodal decomposition in the as-built condition, where Cr and Al cluster separately. The extent of spinodal decomposition was found to be affected by the LED and the hatch spacing. Samples with low LED and small hatch spacing showed higher degrees of spinodal decomposition. X-ray diffraction residual stress investigations revealed that the residual stress is proportional to the volumetric energy density (VED = P/(v. h. t)). Although low residual stresses can be achieved by using low VED, there is a high risk of lack of fusion. Hence, other parameters such as modified scan strategy, build plate pre-heating and pulsed laser mode, must be further explored to minimize the residual stresses to reduce the strain age cracking susceptibility.
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| 20. |
- Jaladurgam, Nitesh Raj, 1993, et al.
(author)
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Load redistribution in eutectic high entropy alloy AlCoCrFeNi 2.1 during high temperature deformation
- 2022
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 22
-
Journal article (peer-reviewed)abstract
- The load redistribution between and within phases in eutectic high entropy alloy AlCoCrFeNi2.1 was measured using in-situ neutron diffraction during tensile deformation at 973 K. The load partitioning between phases is reversed compared to lower temperatures, with L12 becoming the stronger phase. The evolution of the orientation-specific stresses and strains in the L12 phase suggests that cube slip dominates the response. The low strength, internal load transfer and ideally plastic response of the B2 phase indicate a change in deformation mechanism compared to lower temperatures.
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| 21. |
- Jaladurgam, Nitesh Raj, 1993, et al.
(author)
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Temperature dependent load partitioning and slip mode transition in a eutectic AlCoCrFeNi 2.1 high entropy alloy
- 2021
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 17
-
Journal article (peer-reviewed)abstract
- Eutectic high entropy alloys are gaining increasing attention due to their excellent castability and combination of strength and ductility in the as-cast state. However, the detailed behavior of the nano-scale lamellar microstructure during deformation, and in particular the interaction between the phases, is not well understood. Here we use in-situ neutron diffraction during tensile testing over a wide temperature range (77–673 K) to obtain new insights into the temperature dependent mechanical interactions between and within phases during initial plastic deformation of an AlCoCrFeNi eutectic high entropy alloy. The load was transferred from the L1 to the B2 phase during the yielding process, and the changing load distribution within the L1 phase with increasing temperature strongly suggests that <110>{001} cube slip is activated at room temperature and above. This points towards alloying design for delayed octahedral-to-cube slip transition as a possible strategy for increasing the high temperature strength of material. 2.1 2 2
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| 22. |
- Kaplan, Maciej, 1991-, et al.
(author)
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Hydrogen-induced enhancement of thermal stability in VZr(H) metallic glasses
- 2022
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 24
-
Journal article (peer-reviewed)abstract
- Prediction of crystallization temperatures in metallic glasses is still an open question. Investigations of multi component alloys are common in the literature, however, binary and ternary alloys are more suitable for funda-mental studies due to their simplicity. Here, we show that a low thermodynamic driving force for crystallization can be associated with a high crystallization temperature. The driving force is determined by calculating - for the first time in metallic glasses - the temperature dependent Gibbs free energies of the alloys using ab initio density functional theory, in combination with the stochastic quenching method. The crystallization tempera-tures of VxZr100-x and VxZr67-xH33 have been determined using simultaneous in-situ x-ray scattering techniques and resistivity measurements. The onset of crystallization is found to exhibit a parabolic dependence throughout the composition range, whereas alloying with hydrogen increases the thermal stability up to 150 K close to the amorphous-crystalline boundaries. These findings suggest that hydrogen acts as an alloying element with the ability to dynamically tune the intrinsic properties of the material. Lastly, temperature-dependent descriptions of the Gibbs free energy and kinetic considerations of a metallic glass are necessary for a complete characterization of the crystallization process.
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| 23. |
- Kjellin, P., et al.
(author)
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A nanosized zirconium phosphate coating for PEEK implants and its effect in vivo
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 10
-
Journal article (peer-reviewed)abstract
- Surface treatments and coatings can be applied to polyether ether ketone (PEEK) implants to improve their ability to osseointegrate. A new coating, consisting of amorphous nanosized zirconium phosphate (ZrP) was applied to PEEK and titanium substrates. The coating was applied by using a microemulsion as a carrier for the nanoparticles. It was found that the coating formed a thin continuous porous layer on top of the substrate, with pore diameters of 10–50 nm. The thickness of the coating was estimated to <100 nm. The resistance to acidic (pH = 4) conditions and exposure to ultrasonication was investigated with XPS, which showed no loss of coating. The adherence of the coating was investigated by insertion of implants in simulated bone material, which showed a minor loss in coating. In vitro (SBF) testing showed that the coating promoted crystallization of calcium phosphates, for uncoated PEEK, no crystallization was detected. The in vivo performance of the ZrP coating was examined by coating screw shaped PEEK implants which were implanted in rabbit tibia for 6 weeks. The anchoring strength of the implants was evaluated with removal torque (RTQ) measurements. The average RTQ for the ZrP coated implants was significantly higher compared to the non-coated implants. The results show that a nanosized ZrP coating on PEEK implants can transform the surface from having a low ability to osseointegrate to a surface which stimulates bone tissue growth. This makes the ZrP coating an interesting alternative for coating PEEK implants, such as spinal fusion cages and tendon fixation screws. © 2020
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| 24. |
- Kumara, Chamara, et al.
(author)
-
Toward a better understanding of phase transformations in additive manufacturing of Alloy 718
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 13
-
Journal article (peer-reviewed)abstract
- This paper presents a discussion on the phase-transformation aspects of additively manufactured Alloy 718 during the additive manufacturing (AM) process and subsequent commonly used post-heat treatments. To this end, fundamental theoretical principles, thermodynamic and kinetics modeling, and existing literature data are employed. Two different AM processes, namely, laser-directed energy deposition and electron-beam powder-bed fusion are considered. The general aspects of phase formation during solidification and solid state in Alloy 718 are first examined, followed by a detailed discussion on phase transformations during the two processes and subsequent standard post heat-treatments. The effect of cooling rates, thermal gradients, and thermal cycling on the phase transformation in Alloy 718 during the AM processes are considered. Special attention is given to illustrate how the segregated composition during the solidification could affect the phase transformations in the Alloy 718. The information provided in this study will contribute to a better understanding of the overall process–structure–property relationship in the AM of Alloy 718 718. © 2020
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| 25. |
- Kuzminova, Yulia O., et al.
(author)
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Structural and mechanical properties of the additive manufactured CrFeCoNi(Al,Ti) high-entropy alloys produced using powder blends
- 2023
-
In: Materialia. - : Elsevier. - 2589-1529. ; 32
-
Journal article (peer-reviewed)abstract
- High-entropy Alloys (HEAs) are considered prospective materials demonstrating the new approach of alloy design creating new compositions for harsh conditions. However, searching for alloy chemical composition providing the best material properties is a costly process. Additive manufacturing (AM) can be an effective technique for adjusting the alloy composition by using several initial materials. The powder bed fusion (PBF) AM process allows the printing of solid parts using powder blends. In the present study, the CrFeCoNi(Al,Ti) HEAs were printed by the PBF technique using the blends of three powders. The structural and phase investigations revealed the chemical inhomogeneity in the materials that led to the new phase formations affecting the mechanical characteristics. The high-temperature annealing at 1200 °C can be considered a post-treatment process for the printed alloys as a homogenization process while the annealing at a lower temperature of 800 °C initiates the decomposition of the initially formed f.c.c. phase.
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| 26. |
- Liang, Zhi, et al.
(author)
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Development of computational framework for titanium alloy phase transformation prediction in laser powder-bed fusion additive manufacturing
- 2020
-
In: Materialia. - : Elsevier. - 2589-1529. ; 14
-
Journal article (peer-reviewed)abstract
- In conjunction with bare metal single laser track validation experiments, a computational framework is proposed to accelerate the design and development of new additive manufacturing (AM) specific alloys. Specifically, Additive Manufacturing-Computational Fluid Dynamics (AM-CFD) and Calculation of Phase Diagram (CALPHAD), were combined to predict location-specific beta ->alpha phase transformation for a new Ti-Al-Fe-alloy. This modeling work was validated by rigorous spatially resolved synchrotron-based X-ray diffraction measurements. This framework reasonably predicts the melt pool and heat affected zone features in the experiment and reveals their significance in actual AM conditions. This framework can be applied for rapid and comprehensive evaluation of location-specific thermal history, phase, microstructure, and properties for new AM titanium alloy development.
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| 27. |
- Lindgren, Kristina, 1989, et al.
(author)
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On the role of Zr and B addition on solidification cracking of IN738LC produced by laser powder bed fusion
- 2022
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 26
-
Journal article (peer-reviewed)abstract
- The demand for manufacturing increasingly complex geometries for high temperature applications drives the increasing interest into additive manufacturing of nickel-based superalloys. Of particular interest are superalloys with high contents of the strengthening phase γ' such as IN738LC. Previous research suggests that especially B and Zr have a detrimental influence on crack formation during the laser powder bed fusion (LPBF) process. The present study investigates solidification cracks in an IN738LC derivative with increased B (0.03 wt.%) and Zr (0.07 wt.%) in more detail using high resolution techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT). Analysis of the bulk material shows a high number of MC carbides containing Ti and Cr. The concentration profiles indicate non-equilibrium carbide compositions by suggesting that Cr is pushed out of these particles. The carbides are surrounded by a thin B-rich layer at the metal/carbide interface. Analysis of the fracture surface shows both Zr and small amounts of B in the formed oxide layer. The presence of these elements together with thermodynamic calculations and previously reported findings of the same material variant support the hypothesis that low-melting phases are likely reasons for cracking of IN738LC.
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| 28. |
- Lindgren, Lars-Erik, et al.
(author)
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Elastic properties of ferrite and austenite in low alloy steels versus temperature and alloying
- 2019
-
In: Materialia. - : Elsevier. - 2589-1529. ; 5
-
Journal article (peer-reviewed)abstract
- Models for elastic properties, as a function of temperature, are required when simulating various thermo-mechanical processes. A model for hypoeutectoid steels is proposed that accounts for this temperature dependency as well as the influence of alloying. The model consists of separate parts for the ferrite and austenite phases. The latter also includes a specific contribution due to ferromagnetism. The model is calibrated versus iron and evaluated against various steels.
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| 29. |
- Malladi, Bala, 1993, et al.
(author)
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Single track versus bulk samples : Understanding the grain refinement in inoculated ferritic stainless steels manufactured by powder bed fusion-laser beam
- 2023
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 32
-
Journal article (peer-reviewed)abstract
- One generic challenge in powder bed fusion - laser beam (PBF-LB) is the formation of epitaxially grown columnar grains, which lead to the undesirable anisotropy of mechanical properties. This anisotropy could be rectified by ex-situ or in-situ inoculation in some particular alloy systems. Understanding the grain refinement mechanism caused by in-situ inoculation is, however, complicated by remelting caused by the overlapping between neighboring scan tracks, when printing bulk samples using multiple tracks. Here in this work, a series of single tracks using ferritic stainless steels feedstock powder with and without pre-alloyed inoculant-forming elements, were printed at different scanning speeds to gain refreshed understanding on the mechanism of the observed grain refinement. Interestingly, the grain refinement in single tracks and bulk samples printed from the powder with and without inoculant-forming elements showed an opposite tendency. When using the powder without inoculant-forming elements, the single tracks showed large columnar grains, while the bulk samples showed even larger grain sizes; when using the powder with pre-alloyed inoculant-forming elements, fine equiaxed grains are found at the centers of the melt pools, surrounded by slightly coarser columnar grains at melt pool boundaries, in both single tracks and bulk samples. Noticeably, the mean grain sizes in the bulk samples are however smaller compared to those for single tracks because of remelting. Our work provides new insights on the grain refinement via in-situ inoculation during the PBF-LB process and highlights the importance of studying single tracks to better understand the melting and solidification behavior.
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| 30. |
- Masese, Titus, et al.
(author)
-
Unveiling structural disorders in honeycomb layered oxide: Na2Ni2TeO6
- 2021
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 15
-
Journal article (peer-reviewed)abstract
- Honeycomb layered oxides have garnered tremendous research interest in a wide swath of disciplines owing not only to the myriad physicochemical properties they exhibit, but also their rich crystal structural versatility. Herein, a comprehensive crystallographic study of a sodium-based Na2Ni2TeO6 honeycomb layered oxide has been performed using atomic-resolution transmission electron microscopy, elucidating a plethora of atomic arrangement (stacking) disorders in the pristine material. Stacking disorders in the arrangement of honeycomb metal slab layers (stacking faults) occur predominantly perpendicular to the slabs with long-range coherence length and enlisting edge dislocations in some domains. Moreover, the periodic arrangement of the distribution of alkali atoms is altered by the occurrence of stacking faults. The multitude of disorders innate in Na2Ni2TeO6 envisage broad implications in the functionalities of related honeycomb layered oxide materials and hold promise in bolstering renewed interest in their material science. Correction published, see: https://doi.org/10.1016/j.mtla.2021.101104
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| 31. |
- Moldarev, Dmitrii, et al.
(author)
-
Photochromic properties of yttrium oxyhydride thin films : Surface versus bulk effect
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 11
-
Journal article (peer-reviewed)abstract
- Photochromic yttrium oxyhydride (YHO) films of different thicknesses but similar chemical composition were grown by reactive magnetron sputtering. Photochromie response of the films defined as the relative change in transmittance upon illumination increases almost linearly with thickness for films below 600 nm and saturates (approximate to 50%) for thicker ones. These results suggest that the photochromic effect on YHO films has a bulk nature and might be limited by material transport on the microscale.
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| 32. |
- Pacheco, Victor, et al.
(author)
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On the relationship between laser scan strategy, texture variations and hidden nucleation sites for failure in laser powder-bed fusion
- 2022
-
In: Materialia. - : Elsevier. - 2589-1529. ; 26
-
Journal article (peer-reviewed)abstract
- While laser powder-bed fusion has overcome some of the design constraints of conventional manufacturing meth-ods, it requires careful selection of process parameters and scan strategies to obtain favorable properties. Here we show that even simple scan strategies, complex ones being inevitable when printing intricate designs, can inadvertently produce local alterations of the microstructure and preferential grain orientation over small areas - which easily remain unnoticed across the macroscale. We describe how a combined usage of neutron imaging and electron backscatter diffraction can reveal these localized variations and explain their origin within cm-sized parts. We explain the observed contrast variations by linking the neutron images to simulated data, pole figures and EBSD, providing an invaluable reference for future studies and showing that presumably minor changes of the scan strategy can have detrimental effects on the mechanical properties. In-situ tensile tests reveal that fracture occurs in a region that was re-melted during the building process.
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| 33. |
- Palisaitis, Justinas, et al.
(author)
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On the nature of planar defects in transition metal diboride line compounds
- 2022
-
In: Materialia. - : Elsevier Science Ltd. - 2589-1529. ; 24
-
Journal article (peer-reviewed)abstract
- Planar defect structures appearing in transition metal diboride (TMB2) thin films, grown by different magnetron sputtering-deposition approaches over a wide compositional and elemental range, were systematically investi-gated. Atomically resolved scanning transmission electron microscopy (STEM) imaging, electron energy loss spec-troscopy (EELS) elemental mapping, and first principles calculations have been applied to elucidate the atomic structures of the observed defects. Two distinct types of antiphase boundary (APB) defects reside on the {1(1) over bar 00} planes. These defects are without (named APB-1) or with (APB-2) local deviation from stoichiometry. APB-2 de-fects, in turn, appear in different variants. It is found that APB-2 defects are governed by the films composition, while APB-1 defects are endemic. The characteristic structures, interconnections, and circumstances leading to the formation of these APB-defects, together with their formation energies, are presented.
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| 34. |
- Pitthan, Eduardo, et al.
(author)
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In-situ, real-time investigation of the formation of oxygen-containing rare-earth hydrides by combining a quartz crystal microbalance and ion beam analysis
- 2023
-
In: Materialia. - : Elsevier. - 2589-1529. ; 27
-
Journal article (peer-reviewed)abstract
- We present an in-situ and real-time investigation of the formation of YHO and GdHO thin films grown by reactive e(-)-beam evaporation. Mass changes were continuously monitored during deposition, oxidation, and illumination using a highly sensitive quartz crystal microbalance, while changes in chemical composition and depth profiles were investigated simultaneously by ion beam analysis. Results highlight the strong reactivity of freshly deposited YHx and GdHx films, even under ultra-high vacuum conditions. Oxidation starts at the surfaces of the films and the oxidation rate is strongly dependent on the O-2 pressure. The response of the system under ion beam irradiation and in-situ illumination is also presented and discussed. For the measured mass changes, a quantitative agreement better than 2% was observed between both techniques and demonstrates the consistency and sensitivity of this approach.
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| 35. |
- Qiu, Ren, 1993, et al.
(author)
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Effects of gas flow on detailed microstructure inhomogeneities in LPCVD TiAlN nanolamella coatings
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 9
-
Journal article (peer-reviewed)abstract
- Depositing homogeneous TiAlN coatings with a high Al content on cutting tool inserts is a challenging task. In this work, high-Al cubic Ti1-xAlxN coatings (average x = 0.8) with periodic Ti(Al)N (x = 0.5) and Al(Ti)N (x = 0.9) nanolamellae structure were synthesized by low pressure chemical vapour deposition (LPCVD) with different gas flow velocities, and the detailed microstructure was investigated by electron microscopy and simulations. Using a high gas flow rate, the columnar TiAlN grains with regular periodic nanolamella structures disappeared, the coating became enriched in Ti and hexagonal AlN (h-AlN) formed in the coating. The high Ti content is suggested to be caused by the high gas flow rate that increases the mass transport of the reactants. However, this does not influence the Al-deposition much as it is mainly limited by the surface kinetics due to the relatively low deposition temperature. Density functional theory (DFT) modelling and electron microscopy showed that h-AlN tends to form on the Ti(Al)N phase with a specific crystallographic orientation relationship. The Ti enrichment due to high gas flow rate promotes the formation of h-AlN, which therefore deteriorates the nanolamella structure and causes the disappearance of the columnar TiAlN grains. Thus, by designing the CVD process conditions to avoid too high gas flow rates, homogenous TiAlN coatings with high Al content and nanolamella structures can be deposited, which should yield superior cutting performance.
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| 36. |
- Rashidi, Masoud, 1987, et al.
(author)
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Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels
- 2018
-
In: Materialia. - : Elsevier B.V.. - 2589-1529. ; 4, s. 247-254
-
Journal article (peer-reviewed)abstract
- Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy investigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Besides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.
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| 37. |
- Spartacus, Gabriel, et al.
(author)
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Characterization of the nature and morphology of coarse precipitation in various oxide dispersion strengthened steels
- 2021
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 17
-
Journal article (peer-reviewed)abstract
- Oxide Dispersion Strengthened (ODS) steels are candidate materials for both fission and fusion nuclear reactors. In this study, the microstructure of ferritic (Fe-14Cr-1W) and ferritic / martensitic (Fe-9Cr-1W) ODS steels has been characterized after two different processing routes: Hot Extrusion (HE) and Hot Isostatic Pressing (HIP). Transmission Kikuchi Diffraction (TKD) revealed the presence of Ti-rich precipitates on every specimen. They are identified by X-Ray Diffraction (XRD) as Ti(C,N) and Ti-O (Ti2O3 or TiO2). Intergranular M7C3 and M23C6 have been observed on most Fe-14Cr ODS steels except one, where no Cr-carbides have been found. In Fe-9Cr ODS, intergranular Cr-rich M23C6 carbides have been found. The lack of M7C3 on Fe-9Cr ODS is possibly linked to the matrix phase transformation, not occurring in the Fe-14Cr ODS. Cr-carbides display highly elongated shapes for the Fe-14Cr HE specimens that could be detrimental to the mechanical behavior of the material.
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| 38. |
- Tsakadze, Z., et al.
(author)
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Accelerated multi-property discovery of promising Fe-Si-Al magnetic alloys
- 2024
-
In: Materialia. - 2589-1529. ; 36
-
Journal article (peer-reviewed)abstract
- Fe-Si binary alloys are one of the most widely used magnetic materials, while some ternary Fe-Si-Al alloys also have excellent magnetic properties and high electrical resistivity. However, none of these alloy compositions meet the multiple superior properties needed in next generation electrical motors. Hence, there is an urgent need to identify novel Fe-Si-Al alloy compositions with a good property set. Compositionally graded materials libraries of Fe-xSi-yAl (x = 3, 7, 9; 0 ≤ y ≤ 8, in wt%) were prepared by mechanical alloying and high throughput spark plasma sintering. The structural, magnetic, mechanical, and electrical properties were rapidly assessed and found to vary significantly with composition. Four promising compositions were down selected based on rapid characterization and property evaluation of the materials libraries. Validation studies of these compositions revealed that the Fe-7Si-8Al composition exhibited an attractive combination of properties. Thus, an accelerated methodology was used to discover new attractive alloy compositions for rotating electrical machines.
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| 39. |
- Vaché, N., et al.
(author)
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Microstructural study of the NbC to G-phase transformation in HP-Nb alloys
- 2020
-
In: Materialia. - : Elsevier B.V.. - 2589-1529. ; 9
-
Journal article (peer-reviewed)abstract
- The microstructure of a centrifugally cast HP alloy was studied in its as-received state and after ageing at 900 °C. A multi-scale approach combining X Ray Diffraction (XRD), advanced electron microscopy modes (scanning and transmission electron microscopies (SEM, TEM), together with focused ion beam/SEM nanotomography (FIB-nt)) has been carried out to characterize the evolution of niobium carbides during ageing. After thermal treatment, the carbides exhibit a complex microstructure, consisting of a core of untransformed NbC, an intermediate layer of G-phase (Ni16Nb6Si7) with embedded nanometric titanium carbide precipitates, and an outer shell of alternating chromium carbides Cr23C6 and G-phase. A simple diffusion model was used to explain the faster external growth of G-phase compared to the internal NbC dissolution, and to determine a diffusion coefficient of niobium in the G-phase at 900 °C.
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| 40. |
- Varahabhatla, S. M., et al.
(author)
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Influence of energy density on the microstructure, growth orientation, and anisotropy of magnetic properties in additively manufactured Fe-3.8wt%Si transformer steels
- 2023
-
In: Materialia. - 2589-1529. ; 30
-
Journal article (peer-reviewed)abstract
- Fe-3.8wt%Si transformer steels were processed using two different additive manufacturing (AM) techniques, laser powder bed fusion (LPBF) and directed energy deposition (DED). While the LPBF processed samples exhibited a strong <001> orientation of the BCC grains along the build axis, the DED processed samples exhibited a randomized texture along the build axis. DED processed samples showed substantially coarser columnar grains as compared to their LPBF counterparts. The columnar grains exhibited a substantial number of low-angle sub-grain boundaries. All samples exhibited very good soft magnetic properties, with saturation magnetization (Ms) values ranging from 205 - 232 emu/gm, and coercivity (Hc) values ranging from 1.2 – 4.2 Oe. The Coercivity (Hc) values were significantly lower when the magnetic field was applied parallel to the build axis, as compared to being perpendicular, which can be rationalized based on the columnar nature of the grains, resulting in a higher number density of grain boundaries in case of the field applied perpendicular to the build axis.
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| 41. |
- Wang, Wei, et al.
(author)
-
In-situ real time observation of martensite transformation in duplex fcc+hcp cobalt based entropic alloys
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 14
-
Journal article (peer-reviewed)abstract
- Athermal martensite transformation in duplex fcc+hcp Co-based entropic alloys during continuous cooling was investigated in-situ. The real time observation was carried out using high temperature confocal laser scanning microscopy (HT-CLSM). This technique enables the detection of the athermal fcc to hcp transformation in entropic alloys, which is not sensitively detected by conventional thermomechanical methods e.g. dilatometer. The martensite fraction increases with increasing martensite starting temperature, and vice versa. Meanwhile, the martensite starting temperature decreases with the increasing grain size. In addition, the morphology and nucleation sites for martensite formation is discussed. This is the first time the that HT-CLSM technique is utilized in the field of entropic alloys. This in-situ observation technique coupled with thermodynamic calculations may help in the design of entropic alloys through the tailoring of the desired microstructure.
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| 42. |
- Wang, Xuying, et al.
(author)
-
Bioaccessibility and reactivity of alloy powders used in powder bed fusion additive manufacturing
- 2021
-
In: Materialia. - : Elsevier. - 2589-1529. ; 19, s. 1-10
-
Journal article (peer-reviewed)abstract
- Exposure to metal particles via the inhalation route unavoidably takes place at occupational settings during additive manufacturing of metals and alloys. This calls for investigations on possible adverse health effects. This study focuses on virgin and reused powders of three iron- and nickel-based alloy powders (316L, IN718, 18Ni300) widely used in additive manufacturing, and dust powder of 18Ni300 generated during laser melting. Investigations were performed from a physico-chemical and toxicological perspective assessing their bioaccessibility in artificial lysosomal fluid (ALF, simulating lung exposure to respirable particles), corrosion behavior, surface morphology and composition, microstructure, hydrodynamic size distribution in ALF, and in-vitro toxicity towards cultured human lung cells. Less than 1 % of the powder mass was dissolved from the passive alloys (316L, IN718) under simulated physiological conditions (pH 4.5, 37°C, 24 h), whereas the 18Ni300 iron-nickel alloy showed an active behavior and dissolved completely. Reused powders of 18Ni300 and IN718 showed no, or only minor, differences in surface oxide composition, metal release pattern, and corrosion behavior compared with virgin powders. After reuse, the 316L powder showed an enrichment of manganese within the outermost surface, an increased corrosion current, increased amounts of released iron and an increased fraction of particles with ferritic microstructure, which increased the extent of particle aggregation. All powders showed low, or negligible, cytotoxic potency and reactive oxygen species formation. Powder bed fusion using laser melting can hence affect the chemical, physical, and surface properties of non-fused powders, which, if reused, could influence the properties of the printed part.
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| 43. |
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| 44. |
- Werner, Konstantin V., et al.
(author)
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Reconciling experimental and theoretical stacking fault energies in face-centered cubic materials with the experimental twinning stress
- 2023
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 27
-
Journal article (peer-reviewed)abstract
- Stacking fault energy and twinning stress are thought to be closely correlated. All currently available models predict a monotonous decrease in twinning stress with decreasing stacking fault energy and depart from the assumption that the intrinsic stacking fault energy has a positive value. Opposite to this prediction, for mediumand high-entropy alloys the twinning stress was shown to increase with decreasing SFE. Additionally, for metastable materials, first principles methods predict negative intrinsic stacking fault energy values, whilst experimentally determined values are always positive. In the present communication, it is postulated that the twinning stress scaled by the Burgers vector bridges the difference between intrinsic and experimentally measured stacking fault energy. The assumption is tested for Cu-Al alloys, for pure metals and for medium- and high-entropy alloys and, for the first time, provides a consistent quantitative interpretation of data for both alloys with positive and negative stacking fault energy.
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| 45. |
- Xu, Jinghao, et al.
(author)
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On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy
- 2020
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 10
-
Journal article (peer-reviewed)abstract
- The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.
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| 46. |
- Xu, X., et al.
(author)
-
Effects of mixing enthalpy and cooling rate on phase formation of Al x CoCrCuFeNi high-entropy alloys
- 2019
-
In: Materialia. - : Elsevier BV. - 2589-1529. ; 6
-
Journal article (peer-reviewed)abstract
- We investigated the influence of mixing enthalpy and cooling rate on phase formation and selected the melt-spun Al x CoCrCuFeNi (x = 0, 0.1, 0.5, 0.8 and 1.0 in molar ratios) high-entropy alloys (HEAs) as a model system. The mean mixing enthalpy (ΔH mix ) of the alloy system is tuned from positive to negative by increasing Al molar ratios while the mixing entropy (ΔS mix ) only has an insignificant variation. Microstructure analyses revealed that the dominant phase in the Al x CoCrCuFeNi HEAs changes from a face-centered cubic (FCC) to body-centered cubic (BCC) structure with the variation of the ΔH mix . Accompanying with phase changing, the lattice constant of the FCC phase increases linearly with Al molar ratio regardless of cooling rate, indicating that the lattice expansion caused by the substitutional alloying of Al plays an important role in the phase evolution, in addition to the effect of the ΔH mix . The increasingly negative enthalpy ΔH mix with Al addition also leads to more pronounced phase separation with the formation of ordered intermetallic phases in the BCC-dominant HEAs than the FCC-dominant ones. Interestingly, when the magnitude of the mean ΔH mix is small, both the coarsening of Cu-rich nanophase and decomposition of the solid-solution phase in the Al 0.5 CoCrCuFeNi HEA are suppressed. This observation is in line with thermodynamic predictions that a weak ΔH mix benefits the stabilization of the solid-solution phase.
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| 47. |
- Zhou, Tao, et al.
(author)
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Precipitation of multiple carbides in martensitic CrMoV steels - experimental analysis and exploration of alloying strategy through thermodynamic calculations
- 2020
-
In: Materialia. - : ELSEVIER SCI LTD. - 2589-1529. ; 9
-
Journal article (peer-reviewed)abstract
- Martensitic microstructures with engineered precipitation of nano-scale carbides formed during tempering are key in the development of, for example, wear-resistant steels. These steels often experience multiple carbide precipitation with evolving compositions and where the metastable phases transition to more stable carbide phases. In the case of low alloy CrMoV steels, the incomplete understanding of the complex precipitation evolution during tempering is preventing their further optimization. Therefore, in the present work we perform an in-depth experimental investigation of the precipitation of carbides in an Fe-0.32 C-1.4 Cr-0.8 Mo-0.14 V-1.1 Si-0.8 Mn-0.7 Ni (wt.%) martensitic steel tempered at 550 degrees C by transmission electron microscopy and atom probe tomography. The experimental data is compared to thermodynamic calculations and these are subsequently used to expose further potential improvements to the alloying strategy.
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