On the rate of microstructural degradation of Al-Ta-Ti-Zr refractory metal high entropy superalloys

Refractory metal high entropy superalloys (RSA) are being developed for high temperature structural applications. However, recently the microstructures of several RSA have been shown to be unstable, precipitating potentially deleterious intermetallic phases during prolonged exposure at elevated temperatures. For such instability to be acceptable it is necessary for the intermetallic phase formation to be sufficiently sluggish so as to not compromise alloy performance over the time scales likely to be experienced in service. However, the rate of intermetallic phase formation in RSA are not yet known. To address this issue, here, two quaternary Al-Ta-Ti-Zr refractory superalloys that contain all of the key microstructural features of more compositionally complex RSA have been studied following exposures of 1, 10 and 100 h at temperatures between 700 and 900 °C and compared to those previously obtained following 1000 h exposures. Critically, needles of an Al-Zr-rich intermetallic phase were observed following only 1 h exposure at 900 °C, demonstrating rapid formation kinetics. Furthermore, the microstructures continued to coarsen from 100 to 1000 h, indicating low morphological stability. Higher Al content increased both the prevalence of the Al-Zr-rich intermetallic phase and raised the solvus of the desirable B2 phase. Consequently, this work highlights additional challenges in RSA development for elevated temperature applications.