TENSILE BEHAVIOR OF ULTRAFINE-GRAINED METASTABLE AUSTENITE IN AN Fe-24NI-0.3C ALLOY

摘要

Tensile behaviors of ultrafine-grained (1.1 μm) and coarse-grained (35 μm) austenite in an Fe-24Ni-0.3C (wt.%) alloy were investigated in this study. The ultrafine-grained austenite was prepared through high-pressure torsion and subsequent heat treatment. With the grain refinement, the martensitic transformation starting (M_S) temperature of the austenite decreased from -26 °C to -60 °C, and the yielding behavior changed from continuous into discontinuous yielding. At high temperature, the ultimate tensile strength and yield strength of the ultrafine-grained austenite increased but the work-hardening capability was deteriorated compared with the coarse-grained austenite. Below 40 °C for the ultrafine-grained austenite and below 60 °C for the coarse-grained austenite, deformation-induced martensitic transformation occurred and the ultimate tensile strength increased with decreasing temperature. Below the M_S temperature, the yield strength of the ultrafine-grained austenite decreased with decreasing temperature due to stress-assisted martensitic transformation. At higher temperatures, the ultrafine-grained austenite had a smaller ductility than the coarse-grained austenite, and the peak deformation temperature shifted towards a lower temperature since the martensitic transformation was suppressed by the grain refinement.