An assessment of the thermal stability of refractory high entropy superalloys

Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

“…The EDX elemental distribution maps indicated that the dark contrast phase was enriched in Al and Zr and quantitative compositional analysis is presented in Table 1 . The composition, morphology and location of these features are very similar to those reported in previous work and, therefore, it is believed that this is the Al 4 Zr 5 hexagonal intermetallic (P6 3 /mcm, a = 8.31 Å, c = 5.52 Å) identified in references [ 12 , 20 ]. A layer of a bright contrast phase can be seen along the surface of the dark contrast phase, which did not occur in the previous reports of this alloy, but similar features were observed in AlMo 0.5 NbTa 0.5 TiZr 0.5 at temperatures where the refractory metal rich phase was present [ 20 ].…”

“…The composition, morphology and location of these features are very similar to those reported in previous work and, therefore, it is believed that this is the Al 4 Zr 5 hexagonal intermetallic (P6 3 /mcm, a = 8.31 Å, c = 5.52 Å) identified in references [ 12 , 20 ]. A layer of a bright contrast phase can be seen along the surface of the dark contrast phase, which did not occur in the previous reports of this alloy, but similar features were observed in AlMo 0.5 NbTa 0.5 TiZr 0.5 at temperatures where the refractory metal rich phase was present [ 20 ]. Given the slow furnace cool experienced by AlMo 0.5 NbTa 0.5 TiZr following homogenisation, this bright contrast layer is likely to have formed on the surface of the Al-Zr rich phase during cooling.…”

“…The XRD data, Figure 3 c, indicated the presence of a bcc phase and a P6 3 /mcm phase, with refined lattice parameters of a = 8.06 and c = 5.44 Å. Given the compositional and structural similarities, it is believed that Phase B is the same hexagonal P6 3 /mcm Al-Zr intermetallic reported in similar RSA studies [ 10 , 12 , 20 ].…”

“…Recently, we reported on the microstructural evolution of two RSAs, AlMo 0.5 NbTa 0.5 TiZr 0.5 and AlNbTa 0.5 TiZr 0.5 , following 1000 h exposures at 1200, 1000 and 800 °C [ 20 ]. The initial microstructures, which comprised homogeneous B2 grains delineated by an Al-Zr-rich grain boundary intermetallic phase, evolved during the exposures.…”

“…Significant coarsening of the Al-Zr phase on the grain boundaries occurred, along with precipitation of the same phase within the B2 grains. Furthermore, a number of additional phases also formed in AlNbTa 0.5 TiZr 0.5 , which would further alter in-service performance [ 20 ]. However, rather surprisingly, there are very few data relating to the microstructural stability of the archetypal B2 + bcc RSA, AlMo 0.5 NbTa 0.5 TiZr.…”