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

Abstract

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 were performed on a Ni54Ti45Hf1 alloy at impact velocities ranging from approximately 150 m/s (2.5 GPa) to 700 m/s (12.40 GPa). Analysis of shocked samples indicated less cracking was observed to emanate from spall failures resulting from impact velocities greater than 250 m/s (4.23 GPa), concurrent with observations of intragranular microbands within the microstructures. Analyses showed clear evidence that, like responses to quasi‐static loading, martensitic phase transformation occurred upon shock compression in all cases. However, dissimilarly, for the higher impact velocities it reversed upon stress release, leaving behind microbands that showed 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 are not only governed by expected physics of rate‐dependence of the martensitic transformations themselves but may also be enhanced by inelastic deformation mechanisms that result in precipitate dissolution.This article is protected by copyright. All rights reserved.