Study of relaxation and crystallization kinetics of NiTi made amorphous by repeated cold rolling

Abstract

An amorphous structure of NiTi was obtained by repeated cold rolling. Relaxation and crystallization were studied applying a combination of sensitive calorimetric methods, X-ray diffraction and transmission electron microscopy. Upon deformation, a nanostructured mixture of crystalline and amorphous phase occurs. With increasing degree of RCR, the fraction of the crystalline phase decreases; at an equivalent strain of 16.8, an almost completely amorphous phase arises. Isothermal calorimetry yields a monotonously decreasing signal followed by a peak occurring by relaxation of the amorphous phase and nanocrystallization, respectively. The relaxation signal is well described by a Kohlrausch–Williams–Watts function. Crystallization kinetics agree with those expected from the Johnson–Mehl–Avrami model. The kinetic exponent agrees with three-dimensional growth and mixed nucleation or nucleation with a decreasing rate. The crystallization kinetics of NiTi made amorphous by severe plastic deformation differ from those of amorphous NiTi obtained by melt spinning or sputter deposition.