Unique deformation behavior of ultrafine-grained 304 stainless steel at 20 K

An ultrafine-grained 304 austenitic stainless steel showed significant serrated Lüders deformation at 20 K, with stress and temperature oscillations of up to 200 MPa and 20 K, respectively. In - situ neutron diffraction and digital image correlation were used to investigate microstructure evolution and phase stress partitioning. The results indicated that Lüders bands propagated discontinuously, accompanied by burst martensite formation . During Lüders deformation, the phase stress in austenite remained lower than that at macroscopic upper yielding, suggesting that the austenite deformed elastically. More interestingly, a unique stress partitioning behavior where the phase stress of martensite remained smaller than that of austenite until fracture was observed during the deformation at 20 K. This might be attributed to the dramatic stress relaxation in martensite caused by the burst martensitic transformation at 20 K. The relatively low phase stress in martensite delayed brittle fracture until the austenite plastically yielded during uniform deformation.