Three-dimensional morphology of tri-modal microstructure and evolution mechanisms of constitute phases in dual heat treated near-α titanium alloy

Dual heat treatment, the combination of near-β heat treatment and two-phase field heat treatment, provides an effective approach for near-α titanium alloy to obtain a tri-modal microstructure with excellent comprehensive performance. In this paper, the three-dimensional morphology of a tri-modal microstructure in TA15 Ti-alloy and the evolution mechanisms of equiaxed α and lamellar α were revealed by means of interrupted quenching experiments and three-dimensional reconstruction experiments. It is found that the equiaxed α and the lamellar α showed irregular shapes in three-dimensional space, and the α phases were in contact with each other except for some small and fine lamellar α. During the near-β heat treatment, the decomposition of equiaxed α was caused by β phase wedging at grooves. Grooves formed on the surface of the equiaxed α, extending inwards and further the equiaxed α being separated. In addition, a prediction model of equiaxed α decomposition was proposed in this paper. During two-phase field heat treatment, the increase of equiaxed α size was caused by the merging of adjacent α grains or the adhering growth of fine lamellar α. Some lamellar α decomposed through the mechanism of deflection fracture proposed in this paper, i.e., grooves formed at the substructures of the lamellar α and expanded inwards. The lamellar α on both sides of the grooves deflected at small angles to reduce the interface energy and finally the fracture occurred.