Phase equilibria in the Al–Fe–Mo system

Abstract

This paper presents the results of our comprehensive study of phase equilibria in the Al–Fe–Mo system using DTA, X-ray diffraction, SEM and electron probe microanalysis. The liquidus and solidus projections were constructed. It is shown that the ternary compound Al₈FeMo₃ (τ) melts congruently at >1500 °C and has a homogeneity range from 2 to 9 at.% Fe and from 23.5 to 25 at.% Mo. Isostructural phases (Mo), (αFe) and AlMo (W-type structure, cI2-Im-3m) at solidus temperatures form a continuous solid solution (αFe,Mo,AlMo). Unlike the AlMo binary phase, which is not retained by quenching, the ternary (αFe,Mo,AlMo) phase is easily quenched due to its lower decomposition temperature. Among the binary based phases, the Al₈Fe₅ (ε) phase has the widest homogeneity region, which extends up to 24 at.% Mo at solidus temperatures. It is shown that the addition of Mo stabilizes the Al₈Fe₅ (ε) phase, and the temperature of its formation in the ternary system increases to 1331 °C in contrast to 1234 °C in the binary. Moreover, according to XRD data, the Al₈Fe₅ (ε) phase in the ternary system has a rhombohedral structure of the Al₈Cr₅-type (hR78-R-3m), rather than cubic Cu₅Zn₈-type structure (cI52-I-43m), like the binary one. A new binary compound Al₄₅Mo₇ was identified for the first time. Its crystal structure is established as monoclinic Al₄₅V₇-type (mS104-C2/m) with the lattice parameters a = 20.534, b = 7.561, c = 10.910 Å, β = 107.33. It was shown to form by peritectic reaction L + Al₅Mo ⇄ Al₄₅Mo₇ at ∼800 °C. Iron additions stabilize the Al₄Mo phase, which in the binary system is stable above 942 °C.