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 B 2 and monoclinic B 19 ′ lattices, we found that the thermodynamic pT-plane is divided by the temperature T_cr = 580-585 K on two domains. Below T_cr, shock loading of the studied B 2 nitinol transforms it, at stress equal to σ H E L , from elastically compressed to plastically compressed B 2 , and then, at σ = σ T R > σ H E L , transforms plastically deformed B 2 into B 2 + B 1 9 ′ mixture. Above T_cr, the shock loaded B 2 transforms, at σ = σ H E L , into its plastically deformed modification and does not experience any additional transformation.