Effects of Cr on the shape memory effect and tensile properties of Fe-Mn-Si shape memery alloy

Abstract

The effects of Cr content (1-5 wt\%) on the shape memory effect and tensile properties of Fe-30Mn-5Si-Cr shape memory alloys have been investigated. In contrast to the usual shape memory alloys such as Ni-Ti and Cu-based alloys, the shape memory effect of the Fe-Mn-Si alloy was attributed to the stress induced $\gamma$ (fcc) $\to\varepsilon$ (hcp) martensitic transformation which was caused by the motion of Shockley partial dislocations. It was found that the addition of Cr improved the tensile strength of the Fe-Mn-Si-Cr alloy. Teh yield strength increased from 270 MPa for 1Cr alloy to 364 MPa for 5Cr alloy at RT. The deformation mode changed from the usual slip and twinning to the stress induced $\gamma\to\varepsilon$ martensitic transformation with lowering temperature. Below M$_s^\sigma$ temperature, the stress induced $\gamma\to\varepsilon$ martensitic transformation was found, and the failure occurred without the formation of necking in a brittle manner due to the high work hardening rate. The Neel temperature (T$_N$), at which paramagnetic to antiferromagnetic transformation takes place, related to Ms $\sigma$ temperature, plays an important role in obtaining a good shape memory effect. Below this temperature, the $\gamma\to\varepsilon$ martensitic transformation is suppressed by magnetic ordering. The addition of Cr decreased the Neel temperatur (T$_N$) of the Fe-30Mn-5Si-Cr alloy by $8\,^\circ\!C$/wt\%. It was found that the additions of Cr to the Fe-30Mn-5Si alloy increased the yield strength and reduced the Neel temperature (T$_N$) of the $\gamma$ phase, which promoted the stress induced $\gamma\to\varepsilon$ martensitic transformation and inhibited the intrusion of permanent slip. As a consequence, the shape memory effect of he Fe-Mn-Si-Cr alloy was enhanced by the addition of Cr. Nearly complete shape memory effect was observed below the M$_s^\sigma$ temperature. However, at above M$_s^\sigma$ temperature, the degree of the shape memory effect wea...