Aging response of laser melting deposited Ti–6Al–2Zr–1Mo–1V alloy

Abstract

Laser melting deposited Ti–6Al–2Zr–1Mo–1V alloy was isothermally aged in the α+β phase region after a β solution treatment. A special microstructure consisting of crisscross α lamellas and dispersedly distributed β precipitates is obtained by aging below the Ms of the alloy. β solution treatment and water quenching result in hexagonal α′ martensite structure. During subsequent aging treatments, densely dispersed fine β precipitates nucleate heterogeneously inside α′ martensite plates and at plate-plate interfaces, with the variant formation is strongly restricted. The fine β precipitates grow and merge, leading to short rod-like morphology. Some α plates with relative big sizes and high aspect ratios intersect with each other, resulting in colonies or regions, within which the short rod-like β precipitates aligned parallel to only one crystallographic direction. The morphology of the precipitates is the result of the geometrical effect of martensite orientation upon the nucleation process. The kinetics of α′→α+β transformation at 550°C–600°C is sluggish, and the β phase precipitates remain appreciably fine both aging at 550°C for 360min and aging at 600°C for 120min. When aging at 750°C–800°C, α lamellas coarsen and the α lath spacing widen clearly through the coalescence and coarsening of β precipitates, and the morphologies of β precipitations become various and irregular. For practical applications, isothermal aging at temperatures between 650°C and 700°C in times of around 120min is applicable to improve the strength of the alloy.