Variant Selection during Alpha Precipitation in Titanium Alloys: A Simulation Study

Variant selection of alpha phase during its precipitation from beta matrix plays a key role in determining transformation texture and final mechanical properties of alpha/beta and beta titanium alloys. In this study we develop a three-dimensional quantitative phase field model (PFM) to predict variant selection and microstructure evolution during beta to alpha transformation in polycrystalline Ti-6Al-4V under the influence of different processing variables. The model links its inputs directly to thermodynamic and mobility databases, and incorporates crystallography of BCC to HCP transformation, elastic anisotropy, defects within semi-coherent alpha/beta interfaces and elastic inhomogeneities among different beta grains. In particular, microstructure and transformation texture evolution are treated simultaneously via orientation distribution function (ODF) modeling of alpha/beta two-phase microstructure in beta polycrystalline obtained by PFM. It is found that, for a given undercooling, the development of transformation texture of the alpha phase due to variant selection during precipitation depends on both externally applied stress or strain, initial texture state of parent beta sample and internal stress generated by the precipitation reaction itself. Moreover, the growth of pre-existing widmanstatten alpha precipitates is accompanied by selective nucleation and growth of secondary alpha plates of preferred variants.;We further develop a crystallographic model based on the ideal Burgers orientation relationship (BOR) between GBalpha and one of the two adjacent beta grains to investigate how a prior beta grain boundary contributes to variant selection of grain boundary allotriomorph (GBalpha). The model is able to predict all possible special beta grain boundaries where GBalpha is able to maintain BOR with two neighboring grain. In particular, the model has been used to evaluate the validity of all current empirical variant selection rules to obtain more insight of how all grain boundary parameters (misorientation and grain boundary plane inclination) contribute to variant selection behavior titanium alloys. This work could shed light on how to control processing conditions to reduce microtexture at both the individual beta grain level and the overall polycrystalline sample level.