Direct consolidation of γ-TiAl-Mn-Mo from elemental powder mixtures and control of porosity through a basic study of powder reactions

A gamma titanium aluminide was made by elemental powder metallurgy. For consolidation of the alloy from powder blending, either hot extrusion or hot forging was used. A good combination of tensile yield strength and ductility was obtained by hot extrusion that produced a grain size of 50 mu m. Consolidation by forging, however, resulted in a porous microstructure. On the basis of an investigation of the cause of the porosity by an Al/Ti diffusion couple experiment and by characterization of the temperature peaks due to an exothermic reaction among elemental powder particles, it was concluded that a transient phase such as TiAl3 was the culprit. Being the source of Al diffusion, the transient phase leaves behind Kirkendall voids when it forms prior to the major exothermic reaction among elemental powder particles. From this study, two processing techniques to circumvent the porosity were proposed and verified: a fast heating to the consolidation temperature or sufficient Soaking above the reaction temperature prior to consolidation. A sound, fully lamellar, beta-phase controlled microstructure was obtained by these methods.