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

A new rationale is presented for various decomposition products obtain ed 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 a bout 8 vol% retained beta-phase, which is supersaturated with beta-sta bilizers to such an extent that the martensitic transformation has bee n suppressed. The metastable beta-phase decomposes by different modes during continuous cooling, depending on the actual composition of indi vidual beta-grains. Less enrichment of vanadium and iron favours the d irect formation of the equilibrium alpha-phase from the beta-matrix, w hile greater enrichment of vanadium and iron leads to a spinodal decom position of the metastable beta-phase, resulting in a beta + beta' two -phase structure. During further continuous cooling, the beta'-phase w hich is lean in beta-stabilizers will transform into isothermal omega- phase. In addition, an unknown phase has also been observed in the bet a-phase, which is typified by the appearance of 1/2{1 1 2}beta reflect ions in the SAD patterns.