Effect of titanium addition on shape memory effect and recovery stress of training-free cast Fe–Mn–Si–Cr–Ni shape memory alloys

Abstract

The shape memory effect and recovery stress of cast Fe–17.2Mn–5.28Si–9.8Cr–4.57Ni (18Mn) and Fe–17.5Mn–5.29Si–9.68Cr–4.2Ni–0.09Ti (18Mn–Ti) alloys have been investigated by optical microscopy, scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and resistivity–temperature curves. The cast 18Mn and 18Mn–Ti alloys solidified as the ferritic mode for which liquid phase fully transforms into primary δ ferrite. The role of titanium is to indirectly refine the austenite through refining the primary δ ferrite. In this case, the austenitic grains of the cast 18Mn alloy were much bigger than that of the cast 18Mn–Ti alloy, although the two alloys underwent δ→γ phase transformation. Grain refinement suppresses the stress-induced ε martensitic transformation, and thus the shape memory effect of the cast 18Mn–Ti alloy is worse than that of the cast 18Mn alloy. On the contrary, the maximum recovery stress and the recovery stress at room temperature are higher for the cast 18Mn–Ti alloy annealed at 1073K for 30min than for the cast 18Mn alloy annealed at 973K for 30min, because grain refinement suppresses the relaxation of recovery stress caused by the plastic deformation and the stress-induced ε martensitic transformation during cooling process. It is difficult to obtain the training-free cast Fe–Mn–Si based shape memory alloys with excellent shape memory effect and high recovery stress only by grain refinement.