Polytypic transformations of the HfCr2 Laves phase – Part II: Kinetics of the polymorphic C14 → C15 transformation

The progress of the polymorphic phase transformation C14 → C15 of the HfCr2 Laves phase has been traced using two different parameters: (i) the normalized intensity of the 100 reflection of the C14 phase and, (ii) the ratio of certain lattice spacings common to all Laves phases. These tracing parameters allow determination of the degree of transformation. The effective activation energy of the phase transformation has been determined using the equivalent-time method on the basis of the isothermal annealing experiments for three alloys of different composition within the homogeneity range of the HfCr2 phase. For the stoichiometric alloy a value of 235–240 kJ/mol for the effective activation energy of the C14 → C15 transformation has been obtained. The corresponding values for the non-stoichiometric alloys vary from 180 kJ/mol to 300 kJ/mol; the activation energy increases with decreasing Hf content, adopting the kinetic data determined from the lattice spacings. The magnitude of the values obtained for the effective activation energy indicates that jumps of individual atoms at kinks of gliding synchro-Shockley partial dislocation determine the kinetics of the polytypic shear transformation C14 → C15 of the HfCr2 Laves phase.