Strain-Rate-Dependent Thermo-Microstructural Evolution in Fe-Mn-Si Shape Memory Alloys Under Cyclic Tensile Training Process

Iron-based shape memory alloys (Fe-SMAs) are promising for structural retrofitting because of their low cost, corrosion resistance, and manufacturability. However, the effect of strain rate on the coupled thermo-microstructural evolution during cyclic training remains underexplored. In this study, samples underwent cyclic tensile training at quasi-static and impact strain rates. After each cycle, DSC was adopted to obtain transformation temperatures and enthalpies, and selected cycles were characterized by EBSD (KAM and IPF) to quantify phase fractions and variant statistics. Results show tensile loading shifts transformation temperatures, with the principal difference between regimes appearing in the evolution of martensite finish temperature. Under impact loading, the transformation enthalpy increases more rapidly (0.18 to 0.8 J/g in absolute value), and the driving force decreases more markedly by the fourth cycle (−0.0578 to −0.1117 J/g), indicating faster thermodynamic changes at high strain rates. Internal stress and dislocation storage accumulate faster under impact, lowering the effective stress (−17.01 MPa) for transformation and promoting martensite nucleation/growth. EBSD reveals increasing lattice distortion; in impact-trained samples, single-variant martensite and higher stored energy reduce interface resistance and enable elastic energy release, accelerating transformation and improving shape recovery.