Deformation Mechanisms and Shock Loading Responses of a Tribology-Grade NiTiHf Alloy

Small Hf additions to tribology-grade nickel-rich NiTi alloys improve processability of the alloys by reducing sensitivity to cooling rate after solid solution annealing and precipitate hardening heat treatments. Such alloys have shown improved denting resistance and rolling contact fatigue performances. Here, we report upon a first look at their dynamic compressive performances when subjected to impacts from projectiles moving at 150-600 m/s. Characterization of the microstructure before and after shock loading using a combination of scanning and transmission electron microscopy techniques reveals that samples subjected to the higher impact velocities had lower levels of cracking that correlated with dissolution of strengthening nanoprecipitates within plastic deformation bands, whereas samples subjected to lower impact velocities showed plastic deformation bands that contained the strengthening nanoprecipitates. This outcome indicates that precipitates will dissolve under sufficiently high loading, increasing Ni content and causing a superelastic response with a higher spall strength.