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 Fe43.5−xMn34Al15Ni7.5Tix (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 Fe42Mn34Al15Ni7.5Ti1.5 SMA reaches about 9.0 mm, which is about twice of that for Fe42.5Mn34Al15Ni7.5Ti1 SMA. This is attributed to the small subgrains can provide a higher subgrain boundary energy (ΔGs) and grain boundary (GB) migration rate. The subgrain size of Fe42Mn34Al15Ni7.5Ti1.5 SMA (9.7 μm) is significantly smaller than that of Fe42.5Mn34Al15Ni7.5Ti1 SMA (21.3 μm). Thereby, the ΔGs (15.3 × 10−2 J mol−1) and GB migration rate (11.3 × 10−6 m s−1) of Fe42Mn34Al15Ni7.5Ti1.5 SMA are significantly higher than those of Fe42.5Mn34Al15Ni7.5Ti1 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 Fe42Mn34Al15Ni7.5Ti1.5 and Fe42.5Mn34Al15Ni7.5Ti1 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.