Superelastic behavior of novel Ni–Ti–Co shape memory alloys for seismic applications

Ni–Ti–Co is a new shape memory alloy (SMA) composition that has a higher strength and a lower superelastic temperature range than the traditional binary Ni–Ti SMA composition. In this paper, the superelastic properties of Ni–Ti–Co bars that are important for seismic applications were studied and compared with those of Ni–Ti bars. The superelastic behavior, strength, strain recovery, low-cycle fatigue characteristics, and fracture strains of Ni–Ti–Co and Ni–Ti SMA with different heat treatment strategies and testing temperatures from −40 °C to 50 °C were investigated with seismic applications in mind. The effect of low-cycle fatigue loading and temperature variations on the superelasticity degradation of Ni–Ti–Co SMA were evaluated. The results showed that the yield stress of Ni–Ti–Co SMA at room temperature was 1.41–1.74 times that of the Ni–Ti SMA. Ni–Ti–Co SMA exhibited 100% strain recovery when unloaded from 5% strain in a temperature range from −40 °C to room temperature; while at 50 °C, a residual strain of 0.6% was observed when unloaded from 5% strain. For comparison, the Ni–Ti SMA lost superelasticity when the temperature was reduced to 0 °C. When subjected to low-cycle fatigue loading, the stability of the superelastic behavior (maintaining yield stress, energy dissipation and strain recovery) of Ni–Ti–Co SMA at −40 °C was better than that at room temperature and 50 °C. In particular, the superelasticity of Ni–Ti–Co SMA at −40 °C was superior to that of the Ni–Ti SMA at 0 °C. At −40 °C, the Ni–Ti–Co SMA showed almost no loss in yield stress, damping ratio and recovery strain during the first 100 cycles of fatigue loading. Moreover, from 100^th to 471^st cycle, at which the fracture occurred, the strain recovery of Ni–Ti–Co SMA showed almost no degradation.