Superelastic behavior of ultrafine-grain-structured Ti-35 wt%Nb alloy fabricated by equal-channel angular extrusion

The superelasticity of a Ti-35?wt%Nb alloy has been explored by fabricating an ultrafine-grain-structured specimen through equal-channel-angular-extrusion processing. A complete superelasticity of 3.5% was realized by refining the grain size down to about 0.25?µm and by inducing the precipitation of an ω-phase. The superelasticity was possible because the β-phase was largely stabilized at room temperature as a result of the severe grain refinement and Nb-enrichment in the matrix on account of the ω-precipitation.     

NiTi superelasticity via atomistic simulations

The NiTi shape memory alloys (SMAs) are promising candidates for the next-generation multifunctional materials. These materials are superelastic i.e. they can fully recover their original shape even after fairly large inelastic deformations once the mechanical forces are removed. The superelasticity reportedly stems from atomic scale crystal transformations. However, very few computer simulations have emerged, elucidating the transformation mechanisms at the discrete lattice level, which underlie the extraordinary strain recoverability. Here, we conduct breakthrough molecular dynamics modelling on the superelastic behaviour of the NiTi single crystals, and unravel the atomistic genesis thereof. The deformation recovery is clearly traced to the reversible transformation between austenite and martensite crystals through simulations. We examine the mechanistic origin of the tension–compression asymmetries and the effects of pressure/temperature/strain rate variation isolatedly. Hence, this work essentially brings a new dimension to probing the NiTi performance based on the mesoscale physics under more complicated thermo-mechanical loading scenarios.