Tensile deformation of 316L austenitic stainless steel using in-situ electron backscatter diffraction and crystal plasticity simulations

Abstract

In-situ electron backscatter diffraction of low stacking fault energy 316L austenitic stainless steel was carried out in tension to study the evolution of microstructure and micro-texture as a function of strain till fracture. The microstructure was characterized by extensive twinning throughout the deformation process. At low and intermediate strain, scattered areas of twinned regions are observed in the microstructure with <101> grains with higher Schmid factor showing extensive twinning. However, not all the grains with <101> orientation show twinning despite the higher Schmid factor during initial stages of deformation. However, the entire microstructure appeared uniformly twinned irrespective of the orientation of the parent grains near the fractured region. Twinning was also accompanied with evolution of intragranular misorientation and concomitant roughness evolution in the deformed state. It was observed that the grains with <100> orientation show higher roughness evolution and contribute to failure. Crystal plasticity simulations indicate that saturation in twinning leads to lower work hardening rate, ultimately leading to failure.

Introduction

Austenitic stainless steels are important for their high temperature tensile and creep strength as well as excellent corrosion resistance. 316L stainless steel (SS) shows excellent strength and superior corrosion resistance [1], [2], [3], [4], [5], [6] due to which it is an important nuclear material [7], [8]. At room temperature, this grade has good combination of strength and ductility with 332 MPa yield strength, 673 MPa ultimate tensile strength and 35.5% elongation at break [9]. The room temperature deformation and fracture behaviour of this material is an interesting subject due to its characteristic strain hardening response attributed to deformation twinning along with conventional octahedral slip as a result of low stacking fault energy. Twinning unlike slip is polar in nature and has a strong grain size and strain rate dependence. Hence, the strain hardening behaviour of SS 316L is also a strong function of grain size [10]. However, there have been few studies trying to correlate microstructural evolution with strain hardening response for this alloy. In order to understand the plastic deformation mechanisms during tensile loading, the mechanical behaviour must be correlated with microstructural evolution. This is achieved by in-situ characterization of deformation and fracture behaviour. In-situ testing has gained importance over the years because it enables dynamic observation of deformation processes correlated with microstructure [11], [12], [13], [14], [15], [16], [17], [18]. Also, measurement and analysis can be carried out at the local level which offers significant insight into small-scale deformation processes.

The various stages of strain hardening depend on the underlying deformation mechanisms and phenomena like dynamic strain aging and dynamic recovery play important role during testing at various temperature or strain rate [19], [20], [21], [22]. However, even at room temperature, the nature of strain hardening is determined by the interplay between mechanisms like crystallographic slip and deformation twinning. Deformation twinning is common in hexagonal close packed (hcp) materials which do not have enough independent slip systems to accommodate the strain according to von Mises criterion during plastic deformation. Among materials with face-centred cubic (fcc) crystal structure, low stacking fault energy (SFE) materials like silver, brass and austenitic stainless steels exhibit deformation twinning. Contrary to slip which occurs by sliding of atomic planes, twinning involves sudden reorientation by shear displacement. During twinning, the deformed part of the crystal differs in orientation from the parent while retaining the original crystal structure. The two parts of the crystal are a mirror reflection of one another, hence they are said to be twinned with respect to each other. Twins could be of various types like annealing, deformation or growth twin and the mechanisms due to which the different kinds of twins form are reviewed by Mahajan [23]. Primarily, annealing twins are formed by growth accidents during recrystallization in deformed material while deformation twins are formed during plastic deformation. The crystallography, nucleation and growth mechanisms of deformation twins are discussed in great detail by Christian and Mahajan [24]. The importance of deformation twinning in the present context is that it leads to high strain hardening rate.

While the standard stress–strain curve of a fcc single crystal shows three stages of strain hardening corresponding to easy glide, multiple slip and cross-slip [25], strain hardening rate curve of low SFE fcc polycrystals has 4 regimes (stages A–D) as discussed by Asgari et al. [26]. In twinning induced plasticity (TWIP) steels where manganese levels are varied to obtain desired SFE, there may be five stages [27], [28]. In summary, greater work hardening is observed in systems that deform by both slip and twinning as in fcc polycrystal where deformation occurs mainly by movement of $(1/2)\left\{111\right\}1\overline{1}0$ dislocations that contribute to slip and $(1/6)\left\{111\right\}11\overline{2}$ partial dislocations that contribute to twinning [29].Further, twinning saturates at higher strain leading to decrease in strain hardening rate.

Many authors have reported the influence of twinning on the strain hardening rate and the related laws governing the stress–strain response. Previous authors discussed high strain and strain rate deformation of 316L SS, underlying micro-mechanisms of deformation as well as texture evolution from experiments and simulation [30], [31], [32], [33]. However, earlier investigations are based on post-processing characterization whereas in-situ observation of microstructure and texture evolution during deformation of 316L SS is yet to be explored. In the present work on tensile deformation of 316L SS up to failure, an attempt has been made to understand the underlying micro-mechanisms for the occurrence of twinning from misorientation measures using in-situ EBSD characterization. This investigation also provides evidence of the contribution of twinning to failure along with the effect of microstructure and micro-texture, illustrating the need for texture control to extenuate failure. Orientation mapping is used to supplement SEM observation of fracture surface evolution. Visco-plastic self-consistent (VPSC) simulations were carried out to compare the mechanical response with the experimental results and analyse the nature of hardening caused by twinning to support experimental observations. The simulations also made it possible to correlate micro-texture with twin activity. Thus, a combinatorial approach of in-situ testing and crystal plasticity simulations was employed to understand the effect of microstructure and micro-texture on twinning that contributes to strain hardening and ultimate failure in 316L stainless steel.