Aging effect on microstructure of cold groove-rolled α′-type Ti-12 mass%V-2 mass% Al alloys studied by transmission electron microscopy

Microstructure and phase decomposition of hexagonal α′ martensite in cold groove rolled (CGR) Ti-12 mass% V-2 mass% Al alloys have been studied by transmission electron microscopy and electron diffraction. Acicular structure of the α′ martensite changes into equiaxed α′ grains by CGR with a cold reduction of 75%. After aging at 573K for 500 h, local Moiré fringes disappear and diffraction rings become sharp due to recovery. However, Vickers hardness also increased in spite of the recovery. Partitioning of solute atoms and/or formation of fine precipitates can be possible cause for the observed age-hardening. Although a CGR alloy is characterized by a microstructure with polycrystalline equiaxed grains, following orientation relationship is locally observed between β precipitates and α grains after aging at 673 K: (101)β // (011̄1̄)α, [010]β // (101̄1)α. Electron tomography revealed a heterogeneous nucleation and growth of β precipitates in the deformed α′ matrix.