Mechanisms of Shock Strength Exhibited by a Nickel-Rich Nickel-Titanium-Hafnium Alloy

Nickel-rich NiTiHf alloys that are heat treated to strengthen the microstructures with a dense distribution of Ni4Ti3 nanoprecipitates exhibit very high strengths and good quasi-static indentation resistance and rolling contact fatigue performances. To determine whether these properties are maintained at high rates of loading, in situ and recovery flyer plate impact shock experiments are performed on a Ni54Ti45Hf1 alloy at impact velocities ranging from approximately 150 m s-1 (2.5 GPa) to 700 m s-1 (12.40 GPa). Analysis of shocked samples indicated less cracking is observed to emanate from spall failures resulting from impact velocities greater than 250 m s-1 (4.23 GPa), concurrent with observations of intragranular microbands within the microstructures. Analyses show clear evidence that, like responses to quasi-static loading, martensitic phase transformation occurs upon shock compression in all cases. However, dissimilarly, for the higher impact velocities it reverses upon stress release, leaving behind microbands that show no evidence for retained martensite and within which the Ni4Ti3 nanoprecipitates dissolved. These results indicate that strain-rate dependence of these SMAs under shock loading is not only governed by the expected physics of rate-dependence of the martensitic transformations themselves but may also be enhanced by inelastic deformation mechanisms that result in precipitate dissolution. Heat-treated NiTiHf alloys exhibit less cracking due to impact from projectiles traveling at velocities greater than 250 m s-1 versus slower projectiles. Microstructure analyses indicate that shock loading responses of these materials are governed by rate-dependence of martensitic transformations and further enhanced upon stronger impacts by inelastic deformation mechanisms that result in precipitate dissolution.image (c) 2023 WILEY-VCH GmbH