Precipitation and coarsening kinetics of H-phase in NiTiHf high temperature shape memory alloy

Abstract

Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys. This paper discusses the precipitation, coarsening and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy during isothermal aging at temperatures between 450 °C and 650 °C for time to 75 h. The H-phase mean size and volume fraction were determined using transmission electron microscopy. Precipitation kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a Time-Temperature-Transformation diagram was constructed. The evolution of H-phase size suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening for all considered temperatures. The coarsening rate constants of H-phase precipitation have been determined using a modified coarsening rate equation for non-dilute solutions. Critical size of H-phase precipitates for breaking down the precipitate/matrix interface coherency was estimated through a combination of age hardening and precipitate size evolution data. Moreover, time-temperature-hardness diagram was constructed from the precipitation and coarsening kinetics and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy.