Effect of Cr2O3 purity on the microstructure and hardness of AlTiVCr high entropy alloy prepared by self-propagating high-temperature synthesis

The present study reports the preparation of a hard AlTiVCr high-entropy alloy (HEA) by the self-propagating high-temperature synthesis (SHS) in the Cr₂O₃–V₂O₅–TiO₂–Al system. To reduce the production costs, various laboratory-grade and commercial-grade Cr₂O₃ were used as starting materials. Results demonstrated that the alloy was composed of a solid solution phase with a BCC crystal structure, while Al₂O₃ was the main phase in the slag. Microstructure characterization confirmed the development of a Ti₃Al intermediate phase in the microstructure of the alloys prepared using the laboratory-grade Cr₂O₃. However, utilizing the commercial-grade Cr₂O₃ with an S impurity altered the type of secondary phase to TiS₂ and resulted in a significant grain refinement of the alloy. The roasting heat treatment of the primary Cr₂O₃, as well as the addition of 5 wt% CaO flux to the reactants did not eliminate the S impurity. The hardness and the density of the prepared alloy were evaluated between 608 and 863 HV and 4660–5059 kg/m³, respectively. These results confirmed that the SHS reaction is a facile method to obtain a lightweight AlTiVCr alloy with an extreme hardness.