Superelasticity and functional fatigue of single crystalline FeNiCoAlTi iron-based shape memory alloy

Highlights: • FeNiCoAlTi iron-based shape memory alloy was investigated following various heat treatments and loading conditions • Local accumulation of irrecoverable strains was measured despite high recovery rates based on global strain measurements • Under cyclic loading conditions, loss of superelasticity was limited initially but became significant after 10 cycles • Functionality loss was attributed to the buildup of local irrecoverable strains and plasticity at the martensite boundary The development of Iron-based shape memory alloys is primarily motivated by the need for a cost-effective alternative to NiTi. This work explores the superelastic and functional fatigue properties of Fe_42.5Ni₃₀Co₁₅Al₁₀Ti_2.5 SMA. Single crystalline samples were subjected to various heat treatments to optimize the precipitation content and achieve superelastic response. Precipitation heat treatments between 180 and 200 min at 600 °C were conducive to superelasticity with large recoverable strains (~7%) and high levels of recovery (>95% recovery). Heat treatments at lower temperatures altered the strength but without achieving superelasticity. A phenomenon which was attributed to plastic slip resulting from the critical transformation stress being higher than the slip resistance. Treatment times beyond 200 min induced a brittle response and premature fracture prior to transformation. Cyclic experiments were also conducted following different heat treatments and loading conditions to study the functional fatigue properties. In all cases, limited degradation of superelastic properties took place in the first 10 cycles. However, with continued loading, reduction of superelastic strains and loss of functionality was observed. This was attributed to the gradual buildup of local irrecoverable strains due to plasticity at the martensite boundary which acts to pin the interface and prevent reverse transformation.