Generalised stacking fault energy and plastic deformation of austenitic stainless steels

Abstract: 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.