Formation Process of Lamellar γ Phase in a Ti-40at.%Al Alloy

Abstract

Listen

Translate article

The formation process of L1 o γ lamellae in the hcp a phase of a Ti-4Oat.%Al alloy solution-treated at 1473 K for 86.4 ks followed by isothermal decomposition at 1273 K has been analysed in terms of morphology and crystallography. The α phase was ordered to D0 19 α 2 phase during cooling. The γ lamella with the orientation relationships, {111} γ // (0001)α 2 and a 2 , nucleated at a grain boundary γ allotriomorph / α 2 matrix interface and exhibited 90° domain relationship with the γ allotriomorph. Both sharp surface reliefs and the overlapping stacking faults with almost regularly spaced 1/6[112] γ partial dislocations on every two close-packed layers formed during the growth of the γ lamellae, inheritting the anti-phase domain boundaries (APBs) in the α 2 phase. The C-curve for the γ lamellae nucleation indicated the diffusional nature. The phase transition processes are as follows: homogeneous γ phase → homogeneous α 2 phase with thermal APBs → the formation of γ allotriomorph at grain boundary → the nucleation of γ lamella at γ allotriomorph / α 2 matrix interface. The formation of γ allotriomorph induces an elastic strain field around it due to the semicoherency at γ allotriomorph / α 2 matrix interface. The γ lamella with the 90° domain relationship to a γ allotriomorph nucleates at this interface to relax the elastic strain field. The growth of γ lamella is due to the glides of 1/3[1010] α 2 partial dislocations with short-range rearrangements between Ti and At atoms at an (0001)α 2 // {111}γ interphase boundary.