TEM STUDY OF METASTABLE BETA-PHASE DECOMPOSITION IN RAPIDLY SOLIDIFIED TI-6AL-4V ALLOY

A new rationale is presented for various decomposition products obtained from the metastable beta-phase found in Ti-6Al-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an alpha-matrix with about 8 vol% retained beta-phase, which is supersaturated with beta-stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable beta-phase decomposes by different modes during continuous cooling, depending on the actual composition of individual beta-grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium alpha-phase from the beta-matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable beta-phase, resulting in a beta + beta' two-phase structure. During further continuous cooling, the beta'-phase which is lean in beta-stabilizers will transform into isothermal omega-phase. In addition, an unknown phase has also been observed in the beta-phase, which is typified by the appearance of 1/2{1 1 2}beta reflections in the SAD patterns.