Comparison for Grain Growth Dynamics of Severely Deformed Austenite and Martensite NiTi Shape Memory Alloys after Complete Crystallization of Amorphous Phase

Abstract

Severe plastic deformation (SPD) based on local canning compression is imposed on Ni-rich Ni50.7Ti49.3 (at.%) shape memory alloy (SMA) with B2 austenite phase and equiatomic Ni50Ti50 (at.%) SMA with B19′ martensite phase in order to induce the amorphization of the two alloys. SPD-processed Ni50.7Ti49.3 and Ni50Ti50 samples are annealed at 450°C for various times in order to comparatively investigate grain growth dynamics of severely deformed austenite and martensite NiTi SMAs after complete crystallization of amorphous phase. The grain growth exponent of B2 austenite Ni50.7Ti49.3 SMA is 2.78, whereas the grain growth exponent of B19′ martensite Ni50Ti50 SMA is 3.18. The results show that grain growth dynamics of amorphous NiTi samples is sensitive to the chemical composition. The grain growth of equiatomic B19′ martensite Ni50Ti50 sample is inhibited to a greater extent than that of Ni-rich B2 austenite Ni50.7Ti49.3 one. The mechanisms for grain growth of NiTi SMAs deal with formation of nano-clusters due to crystallization of amorphous phase, recovery of retained nanocrystalline grains containing dislocations under thermal driving force, growth of retained nanocrystalline grains as well as new nanocrystalline grains and incorporation of nano-clusters or smaller nanocrystalline grains into larger nanocrystalline grains. The grains of Ni50.7Ti49.3 sample absorb nano-clusters more readily than that Ni50Ti50 one during grain growth. Investigation on grain growth dynamics of severely deformed NiTi SMAs lays the foundation for fabricating nanocrystalline NiTi SMAs with tailored grain size.