Peculiarities of nanostructure formation upon post-deformation annealing of Ti-Ni shape memory alloys

Abstract

Nanostructures formed in Ti-Ni shape memory alloys as a result of post-deformation annealing (PDA) after cold rolling (CR) in a wide strain range are classified and quantitatively studied using TEM analysis. Two types of nanostructure form in B2-austenite as a result of PDA at 400 °C(1 hr) after CR in the e=0.3 to 1.9 true strain range: (a) a nanosubgrained structure, which consists of subgrains formed as a result of polygonization of the initially highly-dislocated substructure; (b) a nanograined structure, which is a combination of the deformation-induced nano-grains grown during PDA and new nano-grains formed during crystallization of the amorphous phase. PDA after moderate CR (e=0.3) results in formation of mainly nanosubgrained structure, after moderate-to-high CR (e=0.5-1.0), of a mixed (nanosubgrained+nanograined) structure, and after severe CR (e=1.2-1.9), of mainly nanograined structure. 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.