Microstructural evolution and interfacial crystallography in Cr–Cr2Ta

The microstructural features of a Cr–Ta alloy containing 10at% Ta have been studied in the as-cast and stress-relieved condition, and after long-term annealing at 1300 °C and 1500 °C. In the as-cast and stress-relieved state, the microstructure consisted of dendrites of the Cr2Ta Laves phase and a lamellar eutectic mixture of Cr(Ta) solid solution and Cr2Ta. After protracted annealing at 1300 °C, the lamellar eutectic was observed to spherodise, producing a microstructure consisting of a Cr(Ta) matrix, within which fine particles of Cr2Ta were embedded. This was accompanied by solid state precipitation of fine Cr2Ta particles within the Cr(Ta) matrix. In addition, the Cr2Ta phase transformed progressively from the hexagonal C14 polytype, found in the as-cast and stress-relieved condition, to either the cubic C15 polytype or intermediate, faulted, hexagonal-based structures. The cubic Laves phase was seen to contain either coarse or fine twin bands. An orientation relationship between the cubic Laves phase and the Cr(Ta) matrix, clearly different from ones previously reported elsewhere was observed. A significant density of dislocations was also observed in the matrix, which persisted even after annealing.