High-temperature phase equilibria with the bcc-type β (AlMo) phase in the binary Al-Mo system

•The β (AlMo) phase forms congruently upon solidification.•Diffraction methods confirmed the disordered nature of the high-temperature β phase.•Diffuse scattering indicates a soft phonon mode behavior of quenched bcc β alloys.

The binary system Al-Mo is a key system for the development of refractory aluminides. This development involves the thermodynamic modeling of multicomponent materials systems such as TiAl- and NiAl-based systems. Due to inconsistencies within the literature data, the phase equilibria with the high-temperature β (AlMo) phase were re-investigated with alloys in the as-solidified state and after heat-treatment at 1773 K using complementary experimental methods such as metallography, electron-probe microanalysis (EMPA), X-ray diffraction (XRD) and thermal analysis. The obtained results show the congruent formation of β at nearly equiatomic composition and a good agreement with the phase equilibria at 1773 K, which were determined in an earlier work. In addition, the structural disorder in this high-temperature phase was confirmed by means of XRD and transmission electron microscopy (TEM) measurements. The diffuse scattering observed in the electron diffraction patterns taken at quenched β (bcc) alloys indicate a soft phonon mode behavior of this phase, which differs substantially from the vibrational characteristics of the bcc Mo-rich solid solution.

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