Impact response of nitinol over 300-473 K temperature range

The response of plane-parallel 2 mm thick samples of 47.3Ni-52.7Ti alloy was studied in two series of planar impact tests at temperatures between 300 and 473 K and between 473 and 318 K (heating to 473 K followed by cooling). In two additional series, the samples of 0.4- 4 mm thickness were tested at 300 and 338 K (after preheating up to 473 K). In all the tests, the samples were loaded by 1 mm thick copper impactors having velocities equal to 314 +/- 2 m/s. The velocity of the rear sample surface was continuously monitored by a laser Doppler velocimeter. It was shown that substantial, by an order of magnitude, variation of Hugoniot elastic limit sigma(HEL )and compressive strength Y of the nitinol with temperature are caused by the martensite-austenite transformation and its reversal. The variation of the dynamic tensile (spall) strength asp of the nitinol along the heating-cooling path was found similar to that of sigma(HEL) although the difference between sigma(sp) values of austenite and martensite, similar to 20%, is much more modest than in the case of sigma(HEL). The test series performed at constant temperatures with samples of different thicknesses allows one to conclude that the plastic deformation in shocked austenite is presumably realized by dislocation motion and multiplication controlled by phonon viscosity. In the shocked martensite, the plastic deformation mechanism at a stress lower than -0.3 GPa is likely a thermally activated combination of deformation twinning and slip of kinking dislocations.