Temperature effect on austenite–martensite transformation in shock-loaded pseudoelastic nitinol

The shock response of the samples of “pseudoelastic” nitinol [Ni/Ti = (51.1 ± 0.1)/(48.9 ± 0.1)] of different, 0.5–6 mm, thicknesses were studied in a series of planar impact tests (1 mm thick aluminum impactor, impact velocities 580–744 m/s) characterized by initial test temperatures T0 = 300, 473, 573, and 673 K. In all these tests, the velocity of the rear sample surface was continuously monitored by a laser Doppler velocimeter. By combining the recorded velocity histories with analysis of the available sets of elastic constants of cubic B2 and monoclinic B19′ lattices, we found that the thermodynamic pT-plane is divided by the temperature Tcr = 580–585 K on two domains. Below Tcr, shock loading of the studied B2 nitinol transforms it, at stress equal to σHEL, from elastically compressed to plastically compressed B2, and then, at σ=σTR>σHEL, transforms plastically deformed B2 into B2+B19′ mixture. Above Tcr, the shock loaded B2 transforms, at σ=σHEL, into its plastically deformed modification and does not experience any additional transformation.