Structure and thermal expansion of (Crx,V1−x)n+1AlCn phases measured by X-ray diffraction

Abstract

MAX phases in the (Crx,V1−x)n+1AlCn system were synthesized by reactive sintering or hot isostatic pressing of elemental powders at temperatures between 1400°C and 1600°C. For n=1, a complete range (0≤x≤1) of solid solutions was found; for n=2 and 3 the solubility ranges were 0.25≤x≤0.75 and 0≤x≤0.5, respectively. Powder X-ray diffraction revealed that the lattice parameters of all (Crx,V1−x)n+1AlCn solid solutions followed Vegard’s law. The thermal expansion coefficients of the various compounds were determined from Rietveld refinements of X-Ray patterns obtained at temperatures between ambient and 800°C. For the n=1 and 3 phases the thermal expansion coefficients were almost isotropic; those for the n=2, however, were quite anisotropic with the expansion along the a-axis being significantly larger than along the c-axis. As a general trend, vanadium rich compounds have smaller thermal expansion coefficients than their Cr-rich counterparts.