Strain rate sensitivity of the ultrastrong gradient nanocrystalline 316L stainless steel and its rate-dependent modeling at nanoscale

We fabricate ultrastrong gradient nanocrystalline (NC) 316L stainless steel with an extremely fined grain size of 5 nm using the Ultrasonic Strain Engineering Technology (USET) at ambient temperature. We evaluate the strain rate sensitivity (SRS), an indicator of the rate controlling mechanism in the plastic deformation of metals and alloys, for the ultrastrong gradient NC 316L stainless steel by strain rate jump nanoindentation and micropillar compression test. The significant decrease of the SRS for the NC 316L stainless steel with a grain size of 5 nm is attributed to the phase transformation from the austenite face center cubic (FCC) to the martensite body center cubic (BCC) structures during the grain refinement process. In addition, the grain boundary activities in NC 316L stainless steel is largely suppressed by the embodied nanosized intermetallic phases and metal precipitates. We propose an in-situ grain structure stability mechanism to achieve such extremely fined NC stainless steel via USET at ambient temperature. We further adopt and validate a rate-dependent constitutive model for the NC 316L stainless steel. The work provides an efficient way to produce ultrastrong gradient NC 316L stainless steel that can be widely used in the light-weighting and mechanical strengthening of aerospace and transportation engineering.