Gradation of Nanostructures in Cold-Rolled and Annealed Ti–Ni Shape Memory Alloys

Abstract

Nanostructures formed in Ti–50.26 at.%Ni shape memory alloy as a result of post-deformation annealing (PDA) at 400 °C (1 h) after cold rolling (CR) in the e = 0.3–1.9 true strain range are classified and quantitatively studied. The statistical quantitative transmission electron microscopy analysis of bright and dark field images and selected area diffraction patterns reveal the following regularities. Two types of nanostructure form in B2-austenite as a result of PDA after CR: (a) a nanosubgrained structure, which consists of subgrains formed as a result of polygonization of the initially highly dislocated substructure; (b) a nanocrystalline structure, which represents a combination of the deformation-induced nano-grains grown during PDA and new nano-grains formed during crystallization of the amorphous phase. After moderate CR (e = 0.3), mainly nanosubgrained structure forms as a result of PDA. After CR of moderate-to-high intensity (e = 0.5–1.0) followed by PDA, the structure is mixed (nanosubgrained+nanocrystalline). After high-intensity CR (e = 1.2–1.9) and PDA, the structure is mainly nanocrystalline. This nanostructure identification allows adequate analysis of the nature of the parent phase boundaries in the thermomechanically processed Ti–Ni alloys and of their effect on the transformation and mechanical behaviors.