Formation mechanism of β precipitate within α′ martensite in α+β titanium alloys

Abstract

The precipitation mechanism of β within α′ martensite was systematically investigated in a representative Ti-2 wt.%Mo α+β alloy using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The β precipitates consistently form on a perfect dislocations generated during the α′ martensitic transformation and exhibit a plate-like morphology with six variants. Their habit plane is oriented normal to the Δg vector (Δg = g ($01\bar{1}$)_β-g($1\bar{1}00$)_α′). The formation of β structure from α′ lattice involves a shear on the {$01\bar{1}0$}_α′ plane, accompanied by an atomic shuffle on alternate (0001)_α′ plane. This shear generates two a/2 partial dislocations with opposite Burgers vector directions and the a perfect dislocation within α′ matrix can react with one of these partials, such as -a/ 2 + a → a/2, thereby reducing the system energy. The thickening of β precipitates is achieved by the accumulation of opposite shear, which minimizes the shear strain induced by β precipitation.