Achievement of nearly fully amorphous structure from NiTi alloys via differential speed rolling at 268 K and effect of annealing on superelasticity

Abstract

High-ratio differential speed rolling (HRDSR) technique was applied to a Ni-rich NiTi alloy at a subzero temperature of 268 K and a nearly fully amorphous microstructure was obtained after a single HRDSR pass with a thickness reduction of 50%. This result was attributed to the severe plastic deformation under low roll pressure at low temperatures where high-density of dislocations and vacancies were rapidly generated while dynamic recovery was suppressed. Subsequent annealing at 673 K for 2 min resulted in the formation of homogeneous nanograins with sizes of 20-30 nm. With increasing annealing time, grain size continued to increase and the volume fraction of remaining amorphous phase continued to decrease. Compared to that for the sample with the ultrafine grains of 100-200 nm, the alloy with the nanograins of 20-30 nm exhibited the superelastic strain larger by a factor of 35 (3.6% vs. 0.1%), indicating that the nanograin refinement through amorphization and subsequent annealing is an effective method of improving superelasticity of NiTi alloys. The homogeneous refinement to nanosized grains contributes to enhancement of superelasticity by considerably increasing the resistance of austenite matrix to plastic deformation. The presence of some amount of amorphous phase that remained after heat treatment may also contribute to superelasticity through the load transfer effect or/and particle strengthening effect. The currently proposed technique provides a solution for the easy and continuous production of (nearly) amorphous NiTi alloys in sheet form.