P.F. Gao

Morphology development of elongated α phases in hot working of large-scale titanium alloy plate

Abstract The role of subtransus hot working on microstructure morphology of TA15 titanium alloy plate with elongated α phases was studied by quantitative metallography on different sections. The results show that the microstructure morphology is mainly affected by loading direction. When the sample is compressed along normal direction, microstructure on the section vertical to normal direction has equiaxed primary α phase but microstructure on the section vertical to rolling direction has strip primary α phase with long axis along tangential direction. When the sample is compressed along rolling direction, microstructure on the section vertical to normal direction has strip primary α phase elongated along tangential direction but microstructure on the section vertical to rolling direction consists of strip and irregular broad-band primary α phase. The strip primary α phase aspect ratio is smaller at lower temperature due to the dynamic break-down of α phase. The difference on primary α phase aspect ratio between different sections decreases after compression along distinct directions in two loading passes, suggesting the improvement of equiaxity of primary α phase.

Prediction of tri-modal microstructure under complex thermomechanical processing history in isothermal local loading forming of titanium alloy

Abstract To control the tri-modal microstructure and performance, a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed. The staged isothermal local loading experiment on TA15 alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure, i.e., the generation of lamellar α, content variation of equiaxed α, spatial orientation change of lamellar α and globularization of lamellar α. Considering the laws of these microstructure phenomena, the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions. Then, the developed microstructure model was integrated with finite element (FE) model to predict the tri-modal microstructure in the isothermal local loading forming. Its reliability and accuracy were verified by the microstructure observation at different locations of sample. Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15 alloy.

Morphology transformation of primary strip α phase in hot working of two-phase titanium alloy

Abstract Microstructural development in hot working of TA15 titanium alloy with primary strip α structure was investigated with the aim to globularize α strips. Results show that the mechanisms of morphology transformation are the same to the spheroidization mechanisms of lamellar structure. Boundary splitting and termination migration are more important than coarsening due to the large size of strip α . The α strips are stable in annealing due to the unfavorable geometrical orientation of intra- α boundaries, the large thickness of strip and the geometrical stability of α particles. Predeformation and low speed deformation accelerate globularization of α strips in the following ways: direct changing of particle shape, promotion of boundary splitting and termination migration by increasing high angle grain boundaries and interfacial area, promotion of coarsening by forming dislocation structures. Large predeformation combined with high temperature annealing is a feasible way to globularize strip α .

Deformation banding in β working of two-phase TA15 titanium alloy

Abstract To study deformation banding in β working of TA15 titanium alloy, hot simulation compression experiments were carried out on a Gleeble 3500 thermal simulator, and the microstructure was investigated by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It is found that in β working of TA15 titanium alloy, deformation banding is still an important grain refinement mechanism up to temperature as high as 0.7Tm (Tm is the melting temperature). Boundaries of deformation bands (DBBs) may be sharp or diffusive. Sharp DBBs retard discontinuous dynamic recrystallization (DDRX) by prohibiting nucleation, while the diffusive ones are sources of continuous dynamic recrystallization (CDRX). Deformation banding is more significant at high strain rate and large initial grain size. The average width of grain subdivisions is sensitive to strain rate but less affected by temperature and initial grain size. Multi-directional forging which produces crossing DDBs is potential to refine microstructure of small-size forgings.

Phase fraction evolution in hot working of a two-phase titanium alloy: experiment and modeling

In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the constituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determines the phase fraction for a certain initial structure. β–α–β transformation occurs during heating and holding. β to α transformation leads to a significant increase in content and size of lamellar α. The α to β transformation occurs simultaneously in equiaxed α and lamellar α. The thickness of lamellar α increases with temperature, which is caused by the vanishing of fine α lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.