Cheol-am Yu

Transformation behavior and mechanical properties of a nanostructured Ti−50.0Ni(at.%) alloy

Transformation behavior, the shape memory effect and the superelasticity of an equiatomic 40% cold worked Ti−Ni alloy followed by annealing at various temperatures have been investigated by means of differential scanning calorimetry (DSC), transmission electron microscopy, thermal cycling tests under constant load and tensile tests. The B2-R-B19 transformation occurred in all samples, and the R-B19’ transformation occurred in a wide temperature range. The B2-R transformation start temperature kept constant with raising annealing temperature. The average grain size increased from 23 to 80 nm with raising annealing temperature from 523 to 673 K. Transformation hysteresis increased rapidly with raising annealing temperature up to 623 K, above which they almost keep constant, which was ascribed to the small grain size and large constraints of grain boundaries.

Solid state amorphization and nanocrystallization of a Ti−50.0 Ni(at.%) alloy

Amorphization and crystallization behavior of an equiatomic Ti−Ni alloy have been investigated by means of differential scanning calorimetry, X-ray diffraction, optical microscopy and transmission electron microscopy. Amorphous phases were induced by cold working, and their morphologies changed from domain like shape to band shape with increasing the amount of cold working. Volume fraction of amorphous phase increased from 17 to 43% with increasing working ratio from 30 to 70%. With increasing the amount of cold rolling from 30 to 70%, the crystallization temperature increased from 552 to 539 K and enthalpy changes associated with crystallization increased from 170 to 447 J/mol. The average grain size of the 40% cold rolled alloy increased from 27 to 80 nm with increasing annealing temperature from 573 to 673 K.