Very high strain-rate response of a NiTi shape-memory alloy

The compressive response of a NiTi shape-memory alloy is investigated at high strain rates, using UCSD's modified split Hopkinson pressure bar and a mini-Hopkinson bar with specially designed striker bars. To obtain a constant strain rate during the formation of the stress-induced martensite phase in a Hopkinson test, a copper-tube pulse shaper of suitable dimensions or a stepped striker bar is employed, since without a pulse shaper or with a uniform striker bar, the strain rate of the sample will vary significantly as the material's microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. At a very high strain rate, the yield stress and the stress-induced martensite formation process are significantly different from those at moderately high strain rates, suggesting that, correspondingly, different microstructural changes may be involved in the phase transition regime. The material's yield stress appears lower when measured in a mini-Hopkinson bar (with very small samples) as compared with that measured by a 1/2-in. Hopkinson bar (with relatively large samples), possibly due to the sample size that may produce different deformation mechanisms within the superelastic strain range. The transition stress from the austenite to the martensite phase shows strain-rate sensitivity. This may be explained by considering the interfacial motion of the formed martensite phase, based on the thermally activated and dislocation-drag models. There exists a certain critical strain-rate level, at which the transition stress for the stress-induced martensite formation equals the yield stress of the austenite phase. Therefore, the shape-memory alloy deforms by the formation of stress-induced martensites, accompanied by the yielding of the martensite phase at this critical strain rate, while the material deforms plastically by the dislocation-induced plastic slip at strain rates above this critical level.