Abstract
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 600~1200 °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 °C, producing significant precipitation hardening effect.
Highlights
High entropy alloys (HEAs) are a new class of metallic material that consist of multiprincipal components but typically exhibit simple phase structure [1,2,3]
Refractory high entropy alloys (RHEAs) normally form solid solution phase with single BCC structure when being prepared by casting method, because the solid solution phase can be stabilized by high entropy effect at high temperatures and possible phase transformation or precipitation
RHEAs normally form solid solution phase with single BCC structure when being prepared by casting method, because the solid solution phase can be stabilized by high entropy effect at high temperatures and possible phase transformation or precipitation could be kinetically hindered during rapid solidification process [8,23]
Summary
High entropy alloys (HEAs) are a new class of metallic material that consist of multiprincipal components but typically exhibit simple phase structure [1,2,3]. Refractory high entropy alloys (RHEAs) are a specific category of HEAs, which are primarily made up of high-melting-point elements, typically from group IV–VI in the periodic table [7,8,9]. Most RHEAs possess high strength at high temperatures, making them a candidate for aerospace engineering, and have attracted extensive attention [10]. Some refractory metals, such as V and Mo, have poor high-temperature-oxidation resistance, making RHEAs difficult to work in atmosphere at high temperatures [12,13]. Addition of Al to RHEAs can effectively reduce density and improve oxidation resistance at the same time. Alloying with Al may be a feasible approach for RHEAs to apply in aerospace engineering
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