Superelastic cycling and room temperature recovery of Ti74Nb26 shape memory alloy

The superelastic cyclic response of Ti74Nb26 shape memory alloy (SMA), and the nature of cyclic evolution of its superelastic properties and their unexpected static recovery process after cycling, were investigated at room temperature. The critical stress for stress-induced martensitic transformation (σSIM) and stress hysteresis (Δσ) continuously decrease with increasing number of superelastic cycles. However, cumulative irrecoverable strain during cycling in samples of particular processing conditions increases only up to a certain number of cycles before decreasing with further cycling. Stress-free aging at room temperature after cycling was shown to increase σSIM and Δσ. The unexpected room temperature recovery is attributed to the recovery of retained martensite and point defects. Similar experiments conducted on conventional Ni-rich Ni–Ti SMAs also show static recovery at room temperature, indicating that the recovery process is not unique to Ti–Nb SMAs.