GRAIN SIZE REFINEMENT THROUGH PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY

Abstract

Grain refinement in shape memory alloys (SMA) could enhance the stress required to introduce martensitic phase during deformation. Such an enhancement in stress is required for applications involving the use of thin sheets or wires to avoid the damage incurred through irrecoverable deformation. Therefore, the present study aims at generating ultra-fine and/or nanostructured grains by a combination of cold rolling and annealing. A Ni-rich NiTi alloying having nominal composition Ni50.3 at% and Ti49.7 at% was cold rolled to 50 % thickness reduction and the annealed at temperatures 450°C, 500°C, 550°C for different durations. All the samples were characterized by XRD and SEM for phase analysis and microstructure. Selected samples were characterized further with TEM and nano indentation for obtaining the grain refinement and mechanical properties. The XRD of cold rolled sample exhibited overlapping peaks between austenite and martensite phases. All the annealed samples were characterized by the austenite phase alone. SEM analyses showed presence of Ti2Ni phase, mostly along the grain boundaries. Formation of Ti2Ni in a Ni-rich matrix is attributed to the presence of oxygen in the sample. TEM investigations of sample annealed at 500° C for 20 min displayed presence of nanostructured grains with nano twins. Having grain size of 2.8μm while sample annealed at 550°C for 20 min have grain size of 3.5μm. Further, Ti2Ni type precipitates were also noticed within the grain interior at few locations. .The nano indentation derived hardness showed a decreasing trend with annealing time for samples annealed at .550°C, while samples annealed at 500°C showed an increasing trend. Such differences are attributed to the combined effects of grain growth, precipitate formation, and concomitant changes in the deformation behavior. Pole figure analyses were carried out for all the annealed samples to understand the formation of preferred orientation. The {111} and {110} fibers parallel to the rolling plane were observed in all the samples.