PHASE-FORMATION DURING BALL-MILLING AND SUBSEQUENT THERMAL-DECOMPOSITION OF TI-AL-SI POWDER BLENDS

With the aim of producing intermetallic composites for high-temperatur e structural applications, Ti-Al-Si alloys were prepared by mechanical alloying of elemental powder blends. The compositions were chosen to form intermetallic composites consisting of alpha(2)-Ti3Al, gamma-TiAl and Ti5Si3 intermetallic compounds in the equilibrium state. The allo ys were characterized by X-ray diffraction and differential scanning c alorimetry. The metastable phases prepared after milling decompose by a two-step process upon annealing. Thereby four intermetallic metastab le phases are formed in the composition range investigated. From the r esults of the phase analysis, partial Ti-Al-Si phase diagrams for the as-milled state, the intermediate states and the final state are deriv ed. The stability of the composite microstructure is investigated by X -ray diffraction and transmission electron microscopy.