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

摘要

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 Ni_(50)Ti_(30)Hf_(20)alloy during isothermal aging at temperatures between 450℃and 650℃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 nondilute 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 Ni_(50)Ti_(30)Hf_(20)alloy.