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The present study focuses on the corrosion behavior of a single-phase FCC high entropy alloy (VCrNiCoFeCu) casted by two different methods: induction melting and spark plasma sintering. The corrosion resistance has been evaluated using immersion tests in 3.5% NaCl solution, the potentiodynamic polarization measurements and the results are compared how is dependent the corrosion rate as a function of the production methods. Our results show that induction melted sample is stable in salty environment. On the other hand, based on the changes of polarization curves, there must be an evolution of oxide films on the SPSed sample until reaching the stable oxide layer. 

First-principle alloy theory and key experimental techniques are applied to determine the thermal expansion of FeCrCoNiGa high-entropy alloy. The magnetic transition, observed at 649 K, is accompanied by a significant increase in the thermal expansion coefficient. The phase stability is analyzed as a function of temperature via the calculated free energies accounting for the structural, magnetic, electronic, vibrational and configurational contributions. The single-and polycrystal elastic modulus for the ferro-and paramagnetic states of the face-centered and body-centered cubic phases are presented. By combining the measured and theoretically predicted temperature-dependent lattice parameters, we reveal the structural and magnetic origin of the observed anomalous thermal expansion behavior. Published by AIP Publishing.

Austenitic stainless steels are primarily known for their exceptional corrosion resistance. They have the face centred cubic (FCC) structure which is stabilised by adding nickel to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mn, Mo, N, C, etc. in order to improve the properties. Since austenitic stainless steels are often used as structural materials, it is important to be able to predict their mechanical behaviour based on their composition, microstructure, magnetic state, etc.

In this work, we investigate the plastic deformation behaviour of austenitic stainless steels by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, dissociated or undissociated dislocation glide. All these features influence the behaviour differently, therefore it is desired to be able to predict their occurrence. Alloying and temperature have strong effect on the SFE and thus on the mechanical properties of the alloys. Several models based on the SFE and more recently on the so called generalised stacking fault energy (GSFE or γ-surface) are available to predict the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the twinning deformation mechanism. One can employ well established experimental techniques to measure the SFE. On the other hand, one needs to resort to ab initio calculations based on density functional theory (DFT) to compute the GSFE of austenitic steels and derive parameters like the twinnability and the critical twinning stress. 

We discuss the effect of the stacking fault energy on the deformation behaviour for two different austenitic stainless steels. We calculate the GSFE of the selected alloys and based on different models, we predict their tendency for twinning and the critical twinning stress. The theoretical predictions are contrasted with tensile tests and electron backscatter diffraction (EBSD) measurements. Several conventional and in situ tensile test are performed to verify the theoretical results. We carry out EBSD measurements on interrupted and fractured specimens and during tensile tests to closely follow the development of the microstructure. We take into account the role of the intrinsic energy barriers in our predictions and introduce a new and so far unique way to experimentally obtain the GSFE of austenitic stainless steels. Previously, only the SFE could be measured precisely by well-designed experiments. In the present thesis we go further and propose a technique that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning. 

Austenitic stainless steels are primarily known for their exceptional corrosion resistance. They have the face centred cubic (FCC) structure which is stabilised by adding nickel, manganese or nitrogen to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mo, Cu, Ti, C, etc. to improve the properties. Since austenitic stainless steels are often used as structural materials, it is important to be able to predict their mechanical behaviour based on their composition, microstructure, magnetic state, etc.

In this work, the plastic deformation behaviour of austenitic stainless steels is investigated by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the description and prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, or dislocation glide. All these deformation modes influence the material behaviour, therefore it is desired to predict and control their occurrence. Alloying elements and temperature have a strong effect on the SFE and thus on the mechanical properties of the alloys. Several models based on the SFE and more recently on the so-called generalised stacking fault energy (GSFE or γ-surface) are available to describe the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the deformation twinning mechanism. One can employ well established experimental techniques to measure the SFE. On the other hand, one needs to resort to ab initio calculations based on density functional theory (DFT) to compute the GSFE of austenitic steels and derive parameters like the twinnability and the critical twinning stress.

The correlation between the stacking fault energy and the deformation behaviour for four different austenitic stainless steels is discussed in this work. The SFE of the selected alloys is obtained by ab initio calculations and based on different models, their tendency for twinning and their critical twinning stress is predicted. The mechanical behaviour and the affinity for twinning and martensitic transformation is mapped across a broad range of temperature (-70°C to +500°C) for the four alloys. The theoretical predictions are contrasted with tensile tests and electron backscatter diffraction (EBSD) measurements. Several conventional and in situ tensile test are performed to verify the theoretical results. EBSD measurements on interrupted and fractured specimens, and during in situ tensile tests were carried out to closely follow the development of the microstructure. In the present thesis, a technique is proposed that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning at low stress values. The importance of temperature and interstitial alloying on mechanical behaviour is also investigated.

We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.

In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.

In this study, three austenitic stainless steels with different compositions are compared in terms of their deformation behaviour. Two of the investigated steels have considerable high nitrogen content which affects their deformation behaviour. The deformation properties and microstructure of the materials were studied by tensile testing and electron backscatter diffraction. We observe that the strain hardening rate curve of the alloy with low nitrogen content shows a continuously decreasing slope, whereas those of the high‑nitrogen steels exhibit a clear plateau. Since no twinning or ε-phase formation is observed at the corresponding strain levels, we suggest that the addition of a large amount of nitrogen suppresses cross-slip and promotes dislocation planarisation. The microstructural evolution of the materials during deformation supports the above scenario. Based on the results of the ab initio calculations, the deformation behaviour of high‑nitrogen alloys cannot be explained in terms of the stacking fault energy.

The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.

The effect of cooling rate on the microstructure and mechanical properties of equimolar NiCoFeCrGa high-entropy alloy (HEA) was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD), as well as by microhardness tests. Experimental results show that the cooling rate has a crucial impact on the developing microstructure which has a mixture of two—FCC and BCC—phases, leading to a self-similarity of the solidified structure formed in the sample. Furthermore, the cooling rate influences both the composition of the two phase-components and the ratio of their volume fractions, determining the mechanical properties of the sample. The present results confirm the grouping of Co, Fe and Cr in the FCC phase and that of Ni and Ga in BCC phase in the NiCoFeCrGa high-entropy alloy system. An empirical rule is suggested to predict how the phase-components can be expected in this complex high-entropy alloy.

The present investigation is directed to phase transitions in the equimolar NiCoFeCrGa high entropy alloy, which is a mixture of face-centered cubic (FCC) and body-centered cubic (BCC) crystalline phases. The microstructure of the samples was investigated by using scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), transmission electron microscopy-based energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), as well as X-ray diffraction (XRD) measurements. Based on the phases observed in different temperature ranges, a sequence of the phase transitions can be established, showing that in a realistic process, when freely cooling the sample with the furnace from high to room temperature, a microstructure having spinodal-like decomposition can also be expected. The elemental mapping and magnetic behaviors of this decomposed structure are also studied.