Polytypic transformations of the HfCr2 Laves phase – Part I: Structural evolution as a function of temperature, time and composition

Abstract

HfCr2-Laves phase alloys of different composition have been produced by arc-melting followed by homogenization by annealing at temperatures at which the C14-high temperature modification is stable. Polytypic phase transformations have been induced by subsequent annealing at different, lower temperatures for various times. The resulting (micro-) structure of the Laves phase has been characterized by X-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). At temperatures between 1300 °C and 1225 °C, a microstructure forms, which can be regarded as composed of randomly alternating C36 and C15 lamellae. At lower annealing temperatures (1050 °C-1150 °C), the C36 stacking sequences do not occur. Instead a lamellar C14/C15 microstructure develops, representing an incomplete C14 → C15 transformation. A phase map for the Laves-phase part of the Hf-Cr system has been constructed. The XRPD patterns are not a superposition of the diffraction patterns corresponding to the different occurring Laves-phase polytypes; instead a common, average diffraction pattern can be discerned which can be ascribed to coherent diffraction of the polytypes constituting the lamellar microstructure. This coherent diffraction by the irregular layer-stacking sequence induces broadening of the reflections corresponding to the specific polytypes, whereas the “fundamental” reflections, occurring for all polytypes, remain unbroadened. From the diffraction patterns two scalar parameters can be obtained, which describe the stage of phase transformation: one based on the intensity of a selected polytype reflection and the other one based on the average unit cell parameter ratio, c/a, of the lamellar microstructure.