Influence of the β → α + β Transformation on the Stresses and Strains Evolutions during Quenching of Ti17 and TA6V Alloys from the β Phase Field

The evolution of the internal stresses and strains during quenching of Ti17 and TA6V alloys from the β phase field has been simulated numerically by taking into account the coupled thermal, mechanical and microstructural evolution phenomena. Emphasis was put on the influence of the β → α+β phase transformation on the internal stresses evolutions during quenching, which is rarely discussed in the case of titanium alloys as compared to steel [S. Denis et al., J. of Mat. Eng. and Perf. 2002;11(1):92]. The flow stress was calculated by using an isotropic visco-elasto-plastic law depending on temperature and microstructure. The parameters were determined experimentally as a function of temperature and the microstructural state: either only β or α+β. In the latter case, the amount of the α phase as well as its morphology were accounted for. As for the prediction of phase transformation kinetics, the model developed in [J. Teixeira et al., Mat. Sci. Eng A 2007;448:135] was used. It is based on a JMAK rule and an additivity hypothesis and the parameters were deduced from isothermal kinetics determination. The effects of stress on the phase transformation (kinetics modification and transformation plasticity) were examined as well as the small volume change due to the transformation. The coupled calculation of the thermal, microstructural and mechanical evolutions was set up in the finite element code ZeBuLoN. Cylindrical geometries were considered with a diameter sufficiently large to obtain significant thermal and microstructural gradients. The calculation results show that the β → α+β phase transformation has a significant effect on the level of the residual stresses and strains. The effect of the transformation kinetics and of the relationship between the α microstructure and the mechanical behavior are analyzed and discussed.