Shape memory and superelasticity of nanograined Ti-51.2 at.% Ni alloy processed by severe plastic deformation via high-ratio differential speed rolling

초록

A novel method of producing nanograined Ni-rich superelastic NiTi alloys in sheet form was proposed using a combination of severe plastic deformation via high-ratio differential speed rolling (HRDSR) and post-deformation annealing. The HRDSR-processed microstructure was composed of heavily deformed austenite and martensite grains, and amorphous phases. After annealing at 673 K, the severely deformed microstructure with no functional properties evolved to the nanograined structure (20-70 nm) composed of austenite and martensite nanograins and sub-nanograins through static recovery or continuous static recrystallization process. The nanograined microstructure had a high resistance to martensitic transformation upon cooling and slip deformation during straining. As a result, the HRDSR-processed alloy annealed at 673K exhibited superior superelasticity compared to the alloys with coarse grains. At the higher annealing temperature of 873 K, the micron-sized recrystallized grains with low dislocations developed through discontinuous static recrystallization process. In this case, deformation during straining was governed by the detwinning of twinned martensite, and as a result, shape memory effect was more significantly pronounced than superelasticity.

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