Al-Hf-Sc (Aluminum-Hafnium-Scandium)

Recently, [2010Rok] determined the phase equilibria between 600 and 400 °C in Al-rich alloys of this ternary system.

Binary Systems

The Al-Hf system [1998Mur] depicts a number of intermediate phases: Hf₂Al (C16, CuAl₂-type tetragonal), Hf₃Al₂ (Zr₃Al₂-type tetragonal), Hf₄Al₃ (Zr₄Al₃-type hexagonal), HfAl (B _f , CrB-type orthorhombic), Hf₂Al₃ (Zr₂Al₃-type orthorhombic), HfAl₂ (C14, MgZn₂-type hexagonal), $\upbeta\hbox{HfAl}_{3}$ (D0₂₃, ZrAl₃-type tetragonal), and $\upalpha \hbox{HfAl}_{3}$ (D0₂₂, TiAl₃-type tetragonal). The Al-Sc phase diagram [Massalski2] depicts the following intermetallic compounds: ScAl₃ (L1₂, AuCu₃-type cubic), ScAl₂ (C15, MgCu₂-type cubic), ScAl (B2, CsCl-type cubic), and Sc₂Al (B8₂, Ni₂In-type hexagonal). In the Hf-Sc system, continuous solid solutions form between $\upbeta$Hf and $\upbeta$Sc, and between $\upalpha$Hf and $\upalpha$Sc.

Phase Equilibria in Al-Rich Alloys

With starting metals of 99.99% Al, 99.8% Hf and 99.875% Sc, [2010Rok] melted about 40 binary and ternary alloys in a resistance furnace, by adding Al-Hf and Al-Sc master alloys to an Al melt. The alloys were given a final anneal at 600, 500 and 400 °C for 30, 100 and 100 h respectively, followed by quenching in water. The phase equilibria were studied with electrical resistivity measurements, optical microscopy and scanning electron microscopy with energy dispersive x-ray analysis. The solubility limits of Hf and Sc in (Al) derived from electrical resisitivity measurements at 600, 500 and 400 °C are shown in Fig. 1. Figure 2 shows the isothermal section near the Al corner at 600 °C [2010Rok]. The binary compounds $\upalpha\hbox{HfAl}_{3}$ and ScAl₃ are in equilibrium with (Al). The isothermal sections at 500 and 400  °C were found by [2010Rok] to be similar to the one in Fig. 2.

Fig. 1

Al-Hf-Sc solubility limit in (Al) at 600, 500 and 400 °C [2010Rok]

Fig. 2

Al-Hf-Sc isothermal section at 600 °C near Al corner [2010Rok]