Effects of Ti and Nb Additions on Microstructure and Shape Memory Effect of Fe-10Mn-6Si-4Ni-7Cr-0.3C Shape Memory Alloy

The iron-based shape memory alloys have found applications in the pharmaceutical and aerospace industries for their light weight and excellent shape memory effects. The shape memory effects and corrosion properties are significantly influenced by additives and heat treatments. Vacuum arc remelting was employed to prepare Fe-10Mn-6Si-4Ni-7Cr-0.3C-mTi/nNb (m = 0.1, 0.3, 0.5, 0.7, n = 0.05, 0.1, 0.3, 0.5) shape memory alloys. The samples were hot-rolled at 1100°C and solution heat-treated at 1150°C for 1 h. The shape memory effect, microstructure, and corrosion performance of Fe-10Mn-6Si-4Ni-7Cr-0.3C-mTi/nNb were analyzed at higher aging temperatures. The microstructural investigations indicate that large amounts of TiC, NbC, Cr₂₃C₆, and Cr₇C₃ phases are precipitated when aged at 800°C, leading to improvement of the shape memory effect of the alloys. The shape recovery ratio reaches 89.9%, 89.5%, 91.1%, and 74.4% for the alloys with 0.1 Ti, 0.3 Ti, 0.5 Ti, and 0.7Ti additions, respectively, at 800°C aging temperature under 3% bending strain. The aging at 500°C increased the shape memory effect by 20–30%, while further aging at 700°C only improved it by a further 10–20%. The shape recovery ratio of alloys with 0.05 Nb and 0.1 Nb reached a maximum of 88.6% and 88.4%, respectively, when aged at 900°C, while, when aged at 800°C, 0.3 Nb and 0.5 Nb reached 83.7%, and 82.9%, respectively. The shape recovery ratio of the alloys with Nb was increased by about 5–30% compared to that with no Nb. The addition of Ti/nNb and aging at higher temperatures improved the corrosion resistance in 3.5-wt.% NaCl solution owing to the formation of Ti/Nb carbide more easily than Cr carbide, leaving a higher content of Cr in the matrix of the alloys.