Simulation and experimental characterization of VC precipitation and recovery stress formation in an FeMnSi-based shape memory alloy

Abstract

In this study, TC-PRISMA was employed to simulate the precipitation (number density and radius) of an FeMnSi-based shape memory alloy (FeMnSi-SMA), with a chemical composition of Fe–17Mn–5Si–10Cr–4Ni-1(V, C) (wt%), during one-step and two-step aging processes. In addition, transmission electron microscopy (TEM) was used to study the microstructures of the specimens, particularly the VC precipitates, under several selected two-step aging conditions to validate the simulation results. Furthermore, thermomechanical characterization was conducted to investigate the recovery stress of the FeMnSi-SMA under the selected two-step aging conditions. It was found that, compared to only one-step aging, the first aging at a lower temperature (600 °C) followed by a second aging at a higher temperature (670 °C) can take advantage of both high nucleation rate and high growth rate. This achieves a similar precipitation state in a significantly shorter time (approximately 1/6 of the total duration of the one with one-step aging). The recovery stress value (509 MPa) corresponding to the two-step aging condition (first step: 600 °C for 20 h; second step: 670 °C for 6 h) shows a similar stress value range (514 MPa) corresponding to the one-step aging condition (600 °C for 144 h); however, the fracture strain is 40% higher. The number density and VC radii simulated by TC-PRISMA are generally consistent with the TEM observations.