Tailoring a Refractory High Entropy Alloy by Powder Metallurgy Process Optimization

This paper reports the microstructural evolution and mechanical properties of a low-density Al_0.3NbTa_0.8Ti_1.5V_0.2Zr refractory high-entropy alloy (RHEA) prepared by means of a combination of mechanical alloying and spark plasma sintering (SPS). Prior to sintering, the morphology, chemical homogeneity and crystal structures of the powders were thoroughly investigated by varying the milling times to find optimal conditions for densification. The sintered bulk RHEAs were produced with diverse feedstock powder conditions. The microstructural development of the materials was analyzed in terms of phase composition and constitution, chemical homogeneity, and crystallographic properties. Hardness and elastic constants also were measured. The calculation of phase diagrams (CALPHAD) was performed to predict the phase changes in the alloy, and the results were compared with the experiments. Milling time seems to play a significant role in the contamination level of the sintered materials. Even though a protective atmosphere was used in the entire manufacturing process, carbide formation was detected in the sintered bulks as early as after 3 h of powder milling. Oxides were observed after 30 h due to wear of the high-carbon steel milling media and SPS consolidation. Ten hours of milling seems sufficient for achieving an optimal equilibrium between microstructural homogeneity and refinement, high hardness and minimal contamination.