Low temperature macro- and micro-mechanical behavior of an ultrafine-grained metastable 304 austenitic stainless steel investigated by in situ high-energy X-ray diffraction

Abstract

Tensile tests of an ultrafine-grained metastable austenitic stainless steel (ASS) were carried out at 298, 253, and 173 K along with in situ high-energy X-ray diffraction (HE-XRD). We found that the interplane elastic compatibility of austenite and strain-induced α′-martensite (SIMα′) at 253 K was better than that at other temperatures. It was further detected that the martensitic transformation of all the studied austenite planes caused the SIMα′ content at 253 K to be higher than that at 298 K during uniform plastic deformation (UPD). Furthermore, the SIMα′ transformation at 173 K was slow initially, but then occurred explosively during the early-middle UPD stage. The dislocation density and the coherent domain size of austenite were observed to increase and decrease at a moderate rate during UPD at 253 K, respectively; however, abrupt dislocations accompanied with the rapid increase in SIMα′ content were evidenced in austenite at 173 K during the middle UPD stage. The work-hardening behavior was initially induced by austenite, and then by the strain-induced ε-martensite (SIMε); finally, it was SIMα′ that pronouncedly affected the work-hardening behavior at 298 and 253 K. However, the influence of SIMε on the work -hardening behavior at 173 K was equivalent to that of SIMα′ during the middle-late UPD stage, which resulted in the formation of two peaks in the work-hardening curve. This paper practically demonstrates that the macro- and micro-mechanical behaviors of an ultrafine-grained metastable ASS can be significantly tailored by reducing temperature, which is beneficial for an in-depth understanding of the low temperature mechanical behavior exhibited by other metastable alloys.