Effect of microstructure on the thermodynamics of Ni 48.4 Ti 31.3 Hf 20.3 (at. %) shape memory thin films

Abstract

The martensitic transformation of sputteredNi48.4Ti31.3Hf20.3 (at. %) thin films was investigated. Amorphous samples werecrystallized at 773 K, 873 K, 973 K and 1073 K to establish a correlation betweenmicrostructure and thermodynamic properties. Regardless of the crystallizationtemperature, all the samples exhibited similar grain size dispersions withaverage values around 100 nm. Grains with similar sizes revealed a widening ofmartensite plates with increasing crystallization temperature. Calorimetrymeasurements showed that both transformation temperatures and transformationheats increase as the crystallization temperature increases. Experimental data allowedfor frictional work and stored elastic energy calculations. It was found thatthe reduction in stored elastic energy and the increase in transformation heatwith rising crystallization temperature are interrelated phenomena. The storedelastic energy reduction with rising crystallization temperature is caused bythe larger size of precipitates and their higher concentration at grain boundaries.The frictional work showed approximately the same value regardless of thecrystallization temperature. Both parameters are higher in NiTiHf thin filmscompared to those calculated for a NiTiHf bulk sample with the same Hf content,mainly attributed to the reduced grain size in the thin films.