Effects of Al on Precipitation Behavior of Ti-Nb-Ta-Zr Refractory High Entropy Alloys

Addition of Al can decrease density and improve oxidation resistance of refractory high entropy alloys (RHEAs), but may cause complicated precipitation and further affect mechanical properties. The present work studied the microstructural evolution of Al-contained RHEAs at elevated temperatures. The effects of Al on precipitation behavior were discussed. Results show that, TiNbTa0.5ZrAlx alloys (x <= 0.5) have single BCC (Body Centered Cubic) structure, but the primary BCC phase is supersaturated. Precipitation of BCC2((Nb,Ta)-rich) solid solution phase, HCP(Zr,Al)-rich intermetallic phase, and ordered B2 phase can occur during heat treatment at 600 similar to 1200 degrees C. The precipitation of BCC2 phase mainly exists in RHEAs with low content of Al, while HCP (Hexagonal Close Packed) precipitates prefer to form in RHEAs with high content of Al. Interestingly, ordered B2 precipitates with fine and basket-weave structure can form in TiNbTa0.5ZrAl0.5 alloy after annealing at 800 degrees C, producing significant precipitation hardening effect.

Automated summary

The addition of aluminum in refractory high entropy alloys can lead to complex precipitation behaviors, affecting the mechanical properties of the alloys. The study found that the type of precipitates formed in Al-contained RHEAs varies with the aluminum content, with different phases forming at different levels of aluminum.