The Influence of α′ (bcc) Martensite on the Dynamic and Magnetic Response of Powder Metallurgy FeMnSiCrNi Shape Memory Alloys

Abstract

In FeMnSi-based SMAs the “executive” phase is e (hexagonal close packed, hcp) stress induced martensite which retransforms to γ (face center cubic, fcc) austenite during heating, causing free-recovery shape memory effect (SME). At low Mn content or high deformation degrees, α′ (body center cubic, bcc) martensite can be additionally induced by cooling or deformation, being considered as detrimental for the magnitude of SME. In the case of powder metallurgy (PM) Fe-14Mn-6Si-9Cr-5Ni (wt%) SMAs containing 5 fractions of mechanically alloyed (MA’ed) powders (0–40%vol.) solution treated to 5 temperatures (700–1100 °C), large amounts, (20–90%) of α′-bcc martensite were detected by XRD and observed by SEM. Nevertheless, free-recovery SME was obtained and enhanced by training, up to bending strokes of 24 mm, developed with a rate of 1.71 mm/°C. The paper corroborates the qualitative and quantitative evolutions of α′, e and γ phases with DMA and thermomagnetic measurements performed on the 25 sets of specimens, during heating up to 500 °C.