Hierarchal heterogeneity of microstructure via aging of Ti-6Al-4V alloy with α+α′ duplex initial microstructure and its effect on strengthening

In this study, with the aim of further improving the strength and ductility balance of industrial Ti alloys, we investigated the phase decomposition behaviors of a Ti-6Al-4 V alloy with a (α+α′martensite) duplex microstructure during low-temperature aging (at 500 °C). In addition, we examined its effect on strength and ductility. The (α+α′) duplex microstructure (for the as-solution treated and quenched specimens) demonstrates a strength-ductility balance that is at par or better than that of the equilibrium (α+β) bimodal structure (for 700 °C aged specimen). The strength of the (α+α') duplex microstructure that was aged at 500 °C for 1 h was significantly increased while retaining good ductility. Here, although no apparent structural changes in both nano- and micro scale level were observed in the primary α grains, three unique phase decomposition/structural evolutions in nano scale level that formed new domains distinguished into the Area 1, 2, 3 were observed in the fine acicular α' martensite region as follows. Under aging at 500 °C, fine acicular α′ martensite evolves into ultrafine globular α′ grains via enhanced recrystallization (corresponding to Area 1), followed by occurrences of β precipitation without elemental diffusional partitioning (corresponding to Area 2) and substitution from globular α' grains to ultrafine β subgrains (correponding to Area 3). Thus low temperature aging at 500 °C leads to complicated multimodal structural formation in the fine α' martensite region, and it contibutes to significantly improved strength.