| 1. |
- Nordström, Joakim, 1971-
(författare)
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Deformation twinning in corrosion-resistant nickel alloys : with a rising nickel content
- 2024
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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.
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| 2. |
- Calmunger, Mattias
(författare)
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High-Temperature Behaviour of Austenitic Alloys : Influence of Temperature and Strain Rate on Mechanical Properties and Microstructural Development
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global increase in energy consumption and the global warming from greenhouse gas emission creates the need for more environmental friendly energy production processes. Biomass power plants with higher efficiency could generate more energy but also reduce the emission of greenhouse gases, e.g. CO2. Biomass is the largest global contributor to renewable energy and offers no net contribution of CO2 to the atmosphere. One way to increase the efficiency of the power plants is to increase temperature and pressure in the boiler parts of the power plant.The materials used for the future biomass power plants, with higher temperature and pressure, require improved properties, such as higher yield strength, creep strength and high-temperature corrosion resistance. Austenitic stainless steels and nickel-base alloys have shown good mechanical and chemical properties at the operation temperatures of today’s biomass power plants. However, the performance of austenitic stainless steels at the future elevated temperatures is not fully understood.The aim of this licentiate thesis is to increase our knowledge about the mechanical performance of austenitic stainless steels at the demanding conditions of the new generation power plants. This is done by using slow strain rate tensile deformation at elevated temperature and long term hightemperature ageing together with impact toughness testing. Microscopy is used to investigate deformation, damage and fracture behaviours during slow deformation and the long term influence of temperature on toughness in the microstructure of these austenitic alloys. Results show that the main deformation mechanisms are planar dislocation deformations, such as planar slip and slip bands. Intergranular fracture may occur due to precipitation in grain boundaries both in tensile deformed and impact toughness tested alloys. The shape and amount of σ-phase precipitates have been found to strongly influence the fracture behaviour of some of the austenitic stainless steels. In addition, ductility is affected differently by temperature depending on alloy tested and dynamic strain ageing may not always lead to a lower ductility.
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| 3. |
- Calmunger, Mattias, 1986-
(författare)
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On High-Temperature Behaviours of Heat Resistant Austenitic Alloys
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Advanced heat resistant materials are important to achieve the transition to long term sustainable power generation. The global increase in energy consumption and the global warming from greenhouse gas emissions create the need for more sustainable power generation processes. Biomass-fired power plants with higher efficiency could generate more power but also reduce the emission of greenhouse gases, e.g. CO2. Biomass offers no net contribution of CO2 to the atmosphere. To obtain greater efficiency of power plants, one option is to increase the temperature and the pressure in the boiler section of the power plant. This requires improved material properties, such as higher yield strength, creep strength and high-temperature corrosion resistance, as well as structural integrity and safety.Today, some austenitic stainless steels are design to withstand temperatures up to 650 °C in tough environments. Nickel-based alloys are designed to withstand even higher temperatures. Austenitic stainless steels are more cost effective than nickel-based alloys due to a lower amount of expensive alloying elements. However, the performance of austenitic stainless steels at the elevated temperatures of future operation conditions in biomass-red power plants is not yet fully understood.This thesis presents research on the influence of long term high-temperature ageing on mechanical properties, the influence of very slow deformation rates at high-temperature on deformation, damage and fracture, and the influence of high-temperature environment and cyclic operation conditions on the material behaviour. Mechanical and thermal testing have been performed followed by subsequent studies of the microstructure, using scanning electron microscopy, to investigate the material behaviours.Results shows that long term ageing at high temperatures leads to the precipitation of intermetallic phases. These intermetallic phases are brittle at room temperature and become detrimental for the impact toughness of some of the austenitic stainless steels. During slow strain rate tensile deformation at elevated temperature time dependent deformation and recovery mechanisms are pronounced. The creep-fatigue interaction behaviour of an austenitic stainless steel show that dwell time gives shorter life at a lower strain range, but has none or small effect on the life at a higher strain range.Finally, this research results in an increased knowledge of the structural, mechanical and chemical behaviour as well as a deeper understanding of the deformation, damage and fracture mechanisms that occur in heat resistant austenitic alloys at high-temperature environments. It is believed that in the long term, this can contribute to material development achieving the transition to more sustainable power generation in biomass-red power plants.
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| 4. |
- Wärner, Hugo, 1988-
(författare)
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High Temperature Fatigue Behaviour of Austenitic Stainless Steel : Microstructural Evolution during Dwell-Fatigue and Thermomechanical Fatigue
- 2021
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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.
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| 5. |
- Nordström, Joakim, 1971-, et al.
(författare)
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Temperature study of deformation twinning behaviour in nickel-base Superalloy 625
- 2024
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 907
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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.
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| 6. |
- Calmunger, Mattias, 1986-, et al.
(författare)
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Thermomechanical Fatigue of Heat Resistant Austenitic Alloys
- 2023
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Ingår i: Procedia Structural Integrity. - : Elsevier. - 2452-3216. ; 43, s. 130-135
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Tidskriftsartikel (refereegranskat)abstract
- Rising global energy consumption and the increase in emissions of greenhouse gases (e.g. CO2) causing global warming, make need for more sustainable power generation. This could be accomplished by increasing the efficiency of the biomass-fired power plants, which is achieved by increasing the temperature and pressure. In addition, flexible generation of power is critical if only renewable power generation is to be achieved and this will increase the number of start-and stop cycles. Cyclic condition in a long-term high temperature environment is an operation process that such materials must withstand, in order to satisfy the needs for future power generation.Commonly austenitic stainless steel are used for critical components of power plants. Because of future change in operating conditions, further investigations are needed to verify that the demands on safety for cyclic long-term usage is fulfilled. This work includes investigation of two commercial austenitic steels: Esshete 1250 and Sanicro 25. The materials were exposed to thermomechanical fatigue (TMF) in strain control under In-Phase and Out-of-Phase conditions and main testing temperature ranges of 100-650°C and 100-800°C respectively. Some of the specimens were pre-aged to simulate prolonged service condition. Mechanical test data were obtained and analysed in order to define the TMF performance of the investigated alloys. The differences in performance were discussed in relation to mechanical and microstructural characterization.
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| 7. |
- Eriksson, Mirva
(författare)
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Spark plasma sintering and deformation behaviour of Titanium and Titanium/TiB2Spark plasma sintering and deformation behaviour of Titanium and Titanium/TiB2 composites
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Titanium has been used as a model substance to study how it behaves in a SPS apparatus when heating rate and/or pressure were varied during the sintering and deformation process. The sintering and deformation of Ti in SPS were compared with that occurring in the conventional hot pressing (HP) in order to reveal if there are any positive effects added by the use of SPS. The ductility of Ti was explored in order to understand the sintering and deformation of (Ti) x (TiB2)1-x composites with x = 0.05, 0.1, and 0.2, respectively, expressed in mol ratio. The temperature difference (DT) between the monitored and the temperature that the samples are exposed to was evaluated. It was noticed that Ti can be sintered at relatively low temperatures. High heating rate implied that the onset temperatures of the sintering and deformation processes decreased. Increasing pressure did not affect the onset temperature but revealed that the deformation of Ti is different if the experiments are conducted within the stability region of the a -phase region of Ti or if the deformation takes place in a temperature region that covers both a-and b-phase areas, i. e. the use of high pressures implied a one step deformation process while the use of low implied that the main part of the deformation took place in the b-phase region. (Ti) x (TiB2)1-x composites were prepared to full densities at 1500 °C using a holding time of 3 min and pressure of 50 MPa. During the SPS sintering the composite with x= 0.2 revealed the presence of TiB due to the reaction Ti + TiB 2 -> 2TiB while the composites with low x values did not show any formation of TiB. The formation of TiB impaired the mechanical properties. The deformation of composites was very difficult. Their deformability increased with increasing x and temperature as well as pressure. During the deformations of pre-sintered samples TiB was formed in all of the composites.
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| 8. |
- Wärner, Hugo, 1988-
(författare)
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High-Temperature Fatigue Behaviour of Austenitic Stainless Steel : Influence of Ageing on Thermomechanical Fatigue and Creep-Fatigue Interaction
- 2018
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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.
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