Microstructural and mechanical behavior of second-phase hardened porous refractory Ti-Nb-Zr-Ta alloys

Second-phase hardening in refractory high-entropy alloys (RHEAs) has been recognized as one of the main routes to significantly improve their mechanical properties. However, their cost-benefit should not be compromised. Thus, we designed second-phase strengthened RHEAs by a low-cost powder metallurgy method. We induced the formation of FCC (face-centered cubic) ZrC second phase, which has not been reported in Ti-Nb-Zr-Ta alloys before and may be easily confused with the FCC-Zr due to the complexity of identifying carbon by conventional methods. We used neutron diffraction and electron backscatter diffraction for microstructural studies, as well as hardness, ultrasonic and compression techniques for assessing mechanical properties. Our investigation centered on equiatomic and equimassic Ti-Nb-Zr-Ta alloys, both displaying similar grain morphology and porosity but differing in grain sizes and phases. The variation in grain size was attributed to the influence of Ta as a grain growth pinning element. Our second-phase ZrC hardened alloys revealed higher stiffness and hardness, surpassing those documented in the existing literature.