The kinetics of a polytypic Laves phase transformation in TiCr2

A systematic investigation of the kinetics of the polytypic C36 ⇄ C14 equilibrium phase (shear) transformation occurring at about 1545 K was performed. A recently developed calibration and desmearing procedure for a differential thermal analysis (DTA) apparatus [Baumann et al. Thermochim. Acta 2008; 472:50] enabled the investigation of the progress of this transformation by isochronal DTA measurements, both upon heating and cooling. A kinetic model was developed on basis of the modular approach, which considers overlapping nucleation, growth and impingement. It was shown that athermal nucleation prevailed, i.e. pre-existing nuclei (here: arrays of dislocations) become activated as function of undercooling/overheating. The model fitting led to values for the equilibrium transformation temperature, the C36/C14 interface energy and the activation energy for growth. Moreover, a quantitative explanation was given of the composition-dependent thermal hysteresis of the transformation.