In situ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

The yield stress of Fe-24Ni-0.3C (wt.%) metastable austenitic steel increased 3.5 times (158 → 551 MPa) when the average grain size decreased from 35 μm (coarse-grained [CG]) to 0.5 μm (ultrafine-grained [UFG]), whereas the tensile elongation was kept large (0.87 → 0.82). In situ neutron diffraction measurements of the CG and UFG Fe-24Ni-0.3C steels were performed during tensile deformation at room temperature to quantitatively elucidate the influence of grain size on the mechanical properties and deformation mechanisms. The initial stages of plastic deformation in the CG and UFG specimens were dominated by dislocation slip, with deformation-induced martensitic transformation (DIMT) also occurring in the later stage of deformation. Results show that grain refinement increases the initiation stress of DIMT largely and suppresses the rate of DIMT concerning the strain, which is attributed to the following effects. (i) Grain refinement increased the stabilization of austenite and considerably delayed the initiation of DIMT in the <111>//LD (LD: loading direction) austenite grains, which were the most stable grains for DIMT. As a result, most of the <111>//LD austenite grains in the UFG specimen failed to transform into martensite. (ii) Grain refinement also suppressed the autocatalytic effect of the martensitic transformation. Nevertheless, the DIMT with the low transformation rate in the UFG specimen was more efficient in increasing the flow stress and more appropriate to maintain uniform deformation than that in the CG specimen during deformation. The above phenomena mutually contributed to the excellent combination of strength and ductility of the UFG metastable austenitic steel.