Hierarchically structured powder metallurgy austenitic stainless steel with exceptional strength and ductility

Abstract

Powder metallurgy stainless steels have attracted much attention due to their wide applications. However, they often exhibit a low relative density (<96%) and a strength-ductility trade-off dilemma. We herein study the microstructure and mechanical property of a dense hierarchically structured stainless steel, which is synthesized by a high-pressure (4 GPa)/high-temperature (1200 °C) sintering technique and exhibits a relative density of approx. 99%. The hierarchically structured stainless steel is composed of 27.5% microcrystalline phase with an average grain size of 9 μm and 72.5% ultrafine-grained phase with an average grain size of 259 nm. These unique microstructures together with the high relative density enable us to achieve an exceptional combination of high yield strength of 1188 ± 59 MPa and a relatively high tensile elongation of 15 ± 2.6%, evading the strength-ductility dilemma observed in conventional powder metallurgy stainless steels synthesized by pressureless or low-pressure sintering. The high strength is mainly caused by the ultrafine-grained phase, which can be obtained even after sintering nanocrystalline stainless steel powders at a high temperature of 1200 °C (∼87% of melting temperature). The ultrafine-grained phase and the grain boundary strengthening factor in the ultrafine-grained phase contributes 91% and 48% to the total strength, respectively. Both experiments and calculations indicate that increasing the sintering pressure from 40 MPa to 4 GPa can significantly lower the grain growth rate by approx. 64%. The high ductility is mainly caused by the microcrystalline phase, which causes the formation of a large number of dislocations and the generation of nano-twins.