Abstract
We describe here the relationship between grain structure, deformation mechanism and fracture characteristics in an austenitic stainless steel. This was accomplished using the novel concept of phase reversion that enabled a wide range of grain size from nanograined/ultrafine grained (NG/UFG) to coarse-grained (CG) regime to be obtained in a single material through change in temperature-time annealing sequence. In the NG/UFG structure, a marked increase in abundance of stacking faults (SFs) and twin density with strain was observed that led to a decrease in the average spacing between adjacent SFs, thus converting stacking faults into twins. Twinning in NG/UFG structure involved partial dislocations and their interaction with the grain boundaries, including SF overlapping and the coordinated nucleation of partial dislocations from the grain boundaries. The plastic zone in the NG/UFG structure resembled a network knitted by the intersecting twins and SFs. With SFE ~30 mJ/m2, the minimum stress for twin nucleation was ~250 MPa for the experiment steel and the corresponding optimal grain size (dop) wa ~120 nm. In contrast, in the CG structure, strain induced martensite formation was the deformation mechanism. The difference in the deformation mechanism led to a clear distinction in the fracture behavior from striated fracture in high strength-high ductility NG/UFG alloy to microvoid coalescence in the low strength-high ductility CG counterpart. The underlying reason for the change in fracture behavior was consistent with change in deformation mechanism from nanoscale twinning in NG/UFG alloy to strain-induced martensite in the CG alloy, which is related to change in the stability of austenite with grain size. An analysis of critical shear stress required to initiate twinning partial dislocations in comparison to that required to nucleate shear bands is presented. The appearance of striated fracture in the NG/UFG alloy suggests a quasi-static step wise crack growth process.
Highlights
There is a strong interest to understand and develop advanced high strength steels, including austenitic stainless steels that are characterized by nano/ultrafine grains with high strength-high ductility combination as light-weight structural materials
We have recently developed an innovative concept of phase reversion (Fig. 1) to obtain nanograined/ultra-fine grained (NG/UFG) structure in austenitic stainless steels[9,10,11,12]
It is widely recognized that the deformation mechanisms in nanograined/ultrafine grained (NG/UFG) structure can be significantly different from those occurring in the CG structure
Summary
There is a strong interest to understand and develop advanced high strength steels, including austenitic stainless steels that are characterized by nano/ultrafine grains with high strength-high ductility combination as light-weight structural materials. Traditional austenitic stainless steels have low yield strength of 350–450 MPa. Grain refinement is a practical approach to enhance the yield strength of metallic materials[1,2]. The unique aspect of the phase reversion concept is that a wide range of yield strength can be obtained in a single material depending on the grain size varying in a wide regime from nano-grained (NG) to coarse-grained (CG), by altering the degree of cold deformation and annealing temperature-time sequence. The present study focuses on the dependence of grain structure on the deformation behavior and consequent fracture mechanism. In this context, we have used the innovative concept of phase reversion to obtain NG/UFG to CG structure in a single material through change in the reversion annealing temperature-time sequence. The experimental details are described in detail elsewhere[10,12,15,16,17]
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