Effect of Cr Incorporation on the Mechanical Properties of HfMoTiZrCr, HfMoNbZrCr, and HfMoNbTiCr Refractory High-Entropy Alloys

Cr was used to replace Nb, Ti, and Zr in a precursor alloy HfMoNbTiZr, and its effect on the microstructure, phase constitution, and mechanical properties of the newly formed alloys were investigated. Microstructural evolutions revealed that a cubic Laves phase emerged regardless of which element was substituted by Cr. The original single-phase BCC structure of the precursor alloy was thus deteriorated and replaced by two BCC and one cubic Laves phases. The formation of the Laves phase and BCC phases increased the strength to 2480 ± 40 MPa in the HfMoNbTiCr alloy, while the net-like structure (Laves phase) impeded the crack extension and compensated for the decreased plasticity due to the introduction of the brittle-stiff Laves phase. The other two alloys, HfMoTiZrCr and HfMoNbZrCr, possessed lower strengths of 1400 ± 20 and 1642 ± 23 MPa and exhibited a completely brittle fracture character, which was attributed to the high volume fraction and the morphologies of the Laves phase.