Stress-induced orientation relationship variation for phase transformation of a-Ti to ß-Ti during high speed machining Ti-6Al-4V

Intense thermo-mechanical loads can destroy the classical Burgers orientation relationship (OR) between α-Ti and β-Ti during the phase transformation of α-Ti to β-Ti. This paper seeks to investigate the effect of local stress on the OR variation of α-Ti and β-Ti during high speed machining (HSM) of Ti-6Al-4V. Firstly, phase transformation of α-Ti to β-Ti with the OR variation from (0001)α//(110)β, [11-20]α//[1-11]β to (01-10)α//(110)β, [0001]α//[111]β is observed during HSM of Ti-6Al-4V. Secondly, the stress distributions in the machining primary deformation zone are analyzed via finite element method. Combined loads of constant shear stress and compressive/tensile stress are imposed on the material. At last, a phase transformation model with considering the stress state is proposed to describe the relationship between the OR variation and the stress state during HSM. The OR variation is elucidated by a compressive/shear and tensile/shear stress state. The stress-induced OR variation provides a new perspective for understanding the transformation process of α-Ti to β-Ti during HSM Ti-6Al-4V, in which a shear martensitic transformation is generated. It also proves that the OR variation between α-Ti and β-Ti is favor for the subsequent rapid cooling phase transformation.