Phase transformations in sputtered thin films of Ti_x(Ni, Cu)_1−x II: Displacive transformations

Abstract

Part I of this series described diffusional transformations in sputtered films of Ti _x (NiCu)_1−x containing about 5 at.% Cu. The as-deposited amorphous films supported substantial supersaturation either of Ti or of Ni + Cu at 300 K. Subsequent isothermal annealing produced fine oriented precipitates which altered the matrix chemistry and often exerted long-range stress fields arising from coherency strains. These phenomena will be shown in part II to influence strongly the subsequent displacive transformation behaviour. In Ti_47.4Ni_46.5Cu_6.1 films, precipitates of (NiCu)₂ Ti₂ resulted in B2←R and R←B19′ transformations. Extensive precipitation of Ti₂(Ni, Cu) in Ti_51.0Ni_44.4Cu_4.6 films led to formation of an orthorhombic martensite, with preferred [010]_B19‖[011]_B2 and (202)_B19‖(211)_B2 orientation relationships, prior to transformation to a monoclinic phase that formed along alternating {322}_B2 and {112}_B2 steps. Even in films with compositions near stoichiometry. small quantities of transgranular Ti₄₀Ni_56.5Cu_3.5 and Ti₂(NiCu) precipitates significantly influenced transformation sequences, temperatures and details of martensite crystallography.