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The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β2). At low temperatures the equilibrium phases for this alloy were β + α+ β2, whereas at high temperature (850° to 950°C) the Ti3Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti3Al ⇌ α + β2.

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti3Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti3Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti3Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip.

The phase diagram of the isopleth section of the Ti-7 at. pct Mo-Al system has been improved and expanded to include alloys up to 25 at. pct aluminum. The mechanical and thermal stability of alloys aged in the two-phase region, β +Ti3Al, was correlated to the microstructure. X-ray rocking-curve studies of the polycrystalline specimens showed that after 2 pct deformation of a Ti-7 Mo-16 Al alloy theβ matrix became preferentially plastically deformed, while the Ti3Al particles functioned as hard particles undergoing little lattice distortions.