Crystallization studies of amorphous melt-spun Ti$\\mathsf{_{50}}$Ni$\\mathsf{_{25}}$Cu$\\mathsf{_{25}}$

Abstract

\nAmorphous ribbons of the ternary shape memory alloy Ti$_{50}$Ni$_{25}$Cu$_{25}$ crystallize polymorphously upon heating. The subsequent growth proceeds isotropic resulting in the formation of perfectly spherical grains in an amorphous matrix at intermediate stages of the transformation. These experimental results match exactly with the idealized preconditions that are employed in various models describing nucleation- and growth kinetics at deep undercooling. Therefore, amorphous Ti$_{50}$Ni$_{25}$Cu$_{25}$ is an ideal candidate to study the crystallization by calorimetry in conjunction with microstructure analyses and to compare the results with theoretical predictions. It was found, that the isothermal crystallization process of Ti$_{50}$Ni$_{25}$Cu$_{25}$ near the glass transition can be adequately described by Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetics using an Avrami exponent of $\\rm n =3$. Kissinger analysis of nonisothermal crystallization using different heating rates revealed an activation energy of $\\rm Q_a = 374$ kJ/mol.\n