Simulation of coupled temperature, microstructure and internal stresses evolutions during quenching of a β-metastable titanium alloy

Abstract

The evolution of the internal stresses and strains during quenching of Ti17 alloy 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 [Denis et al., J. Mat. Eng. Perf., 2002;11(1):92]. The simulation of these coupled phenomena was built on a thorough knowledge of internal stresses development during quenching of different metallic alloys, the solid phase transformations in titanium alloys, existing models of transformations kinetics and characterizations of the mechanical behavior.The flow stress was calculated by using an isotropic thermo–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, was accounted for. As for the prediction of phase transformation kinetics, the model developed in [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 (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 geometry was considered with a diameter sufficiently large to obtain significant thermal and microstructural gradients. The calculation results show that taking into account the relatively slow kinetics of the β→α+β phase transformation in Ti17 has a significant effect on the level of the residual stresses and strains.