On the tensile behaviour of coarse and ultrafine grained NiTi

Ni50.7Ti49.3 alloy of coarse grained (CG) and ultrafine grained (UFG) microstructures were subjected to tensile tests at different strain rates of 10(-1) s(-1), 10(-2) s(-1), 10(-3) s(-1) and temperatures of 25 degrees C, 50 degrees C and 200 degrees C. The CG state was attained by quenching and the UFG state by electroplastic rolling (EPR). Transmission electron microscopy of the UFG Ni50.7Ti49.3 alloy revealed the presence of shear bands and sub -grains. The microstructure after tensile testing revealed the presence of grains and twin planes of the B19' martensite phase, which indicates that twinning also plays a role in the deformation of the alloy, in addition to crystallographic slip. The effects of grain size, strain rate and temperature on the stress-induced martensite transformation present during the tensile tests are discussed. Serrated flow was observed intermittently in the stress-strain curves of both CG and UFG Ni50.7Ti49.3 alloy for some specific combinations of strain rate and temperature. Serrations observed were not limited to the stress plateau region, which is typically seen in the stress-strain curves of NiTi alloys as a result of phase transformation, but were noticed rather randomly through the entire strain range. This unsteady flow behaviour is also discussed.