Effect of Ti content on abnormal grain growth of Fe–Mn–Al–Ni–Ti shape memory alloy

In this paper, the effects of Ti content on the solvus temperature of γ-phase and abnormal grain growth (AGG) in Fe_43.5−x Mn₃₄Al₁₅Ni_7.5Ti _x (x = 0, 0.5, 1 and 1.5) shape memory alloys (SMAs) were investigated. It is found that, the increase of Ti content leads to a significant reduction of the solvus temperature of γ-phase, a significant refinement of γ-phase, and a decrease of subgrain size. After 3 times cyclic heat treatments, the average grain size of Fe₄₂Mn₃₄Al₁₅Ni_7.5Ti_1.5 SMA reaches about 9.0 mm, which is about twice of that for Fe_42.5Mn₃₄Al₁₅Ni_7.5Ti₁ SMA. This is attributed to the small subgrains can provide a higher subgrain boundary energy (ΔG _s) and grain boundary (GB) migration rate. The subgrain size of Fe₄₂Mn₃₄Al₁₅Ni_7.5Ti_1.5 SMA (9.7 μm) is significantly smaller than that of Fe_42.5Mn₃₄Al₁₅Ni_7.5Ti₁ SMA (21.3 μm). Thereby, the ΔG _s (15.3 × 10⁻² J mol⁻¹) and GB migration rate (11.3 × 10⁻⁶ m s⁻¹) of Fe₄₂Mn₃₄Al₁₅Ni_7.5Ti_1.5 SMA are significantly higher than those of Fe_42.5Mn₃₄Al₁₅Ni_7.5Ti₁ SMA (7.1 × 10⁻² J mol⁻¹, 6.3 × 10⁻⁶ m s⁻¹). In addition, when the applied strain was up to 10%, the maximum superelastic strain of Fe₄₂Mn₃₄Al₁₅Ni_7.5Ti_1.5 and Fe_42.5Mn₃₄Al₁₅Ni_7.5Ti₁ were 5.5% and 5.1%, respectively. In summary, the addition of 1.5 at.% Ti in Fe–Mn–Al–Ni–Ti SMA can promote the AGG with relatively small loss in superelasticity.