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

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 1073 K for 30 min than for the cast 18Mn alloy annealed at 973 K for 30 min, 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.