Projections of the liquidus and solidus surfaces of the Al2O3-HfO2-Gd2O3 phase diagram

Abstract

The phase equilibria during crystallization of alloys in the Al 2 O 3 -HfO 2 -Gd 2 O 3 system are studied for the first time by physicochemical methods. The liquidus and solidus surfaces on the composition triangle as well as the melting diagram and crystallization scheme are constructed. No new ternary compounds or appreciable ternary solid solution regions are found in the system. The maximum temperature in the system is 2820°C and corresponds to the AL + F + GA (E 3 ) three-phase eutectic.Since crystallization in the system finishes with eutectic reactions, it allows materials of the quasiternary system to combine the unique properties of HfO 2 -based T and F solid solutions and the properties of other phases in composite materials.

Apatite and garnet stability in the Al–Ca–Mg–Si–(Gd/Y/Yb)–O systems and implications for T/EBC: CMAS reactions

“…Notably, in contradiction to the Gibbs phase rule, the Gd‐(0,0,0) sample contained three phases, namely alumina, GdAP (GdAlO 3 ), and GdAG, rather than forming single‐phase garnet. This observation is consistent with the mixed reports in the literature sources about whether GdAG is stable as a single phase at 1400°C, or if Al 2 O 3 + GdAP is the equilibrium assemblage 34,54–56 . Shorter (25 h) and longer (500 h) 1400°C heat treatments for the Gd‐(0,0,0) sample produced equivalent phase assemblages and similar grain size distributions to the 100 h heat treatment.…”

“…This observation is consistent with the mixed reports in the literature sources about whether GdAG is stable as a single phase at 1400 • C, or if Al 2 O 3 + GdAP is the equilibrium assemblage. 34,[54][55][56] Shorter (25 h) and longer (500 h) 1400 • C heat treatments for the Gd-(0,0,0) sample produced equivalent phase assemblages and similar grain size distributions to the 100 h heat treatment. No cooling effects were noted for any of the heat-treated samples.…”

“…These have been studied in a variety of contexts including nuclear waste immobilization, 35 solid oxide fuel cells and sensors, phosphors, and laser technology, [36][37][38][39] and for their petrologic and geochronologic importance. 40,41 Data from the REO 1.5 -SiO 2 [42][43][44][45] and AlO 1.5 -REO 1.5 34,[46][47][48][49][50][51][52][53][54][55][56] binary systems, studies into apatite crystal chemistry 13,15,35,36,57,58 for calcium-containing rare earth apatite stability, and the literature on T/EBC-CMAS interactions 6,7,12,18 were used to infer the stability of the endmembers at 1400 • C. The results are plotted in Figure 1 as a function of the 8-fold coordinated RE ion radii. 59 With increasing RE ion radius, the stability of the Ca-free apatite increases while that of the pure aluminate garnet endmember decreases.…”