Microstructure and texture evolution during thermo-mechanical processing of two phase titanium alloy Ti-6Al-4V

Understanding the relationship between high-temperature deformation and microstructure evolution during hot forging of aero-engine alloys is important in ensuring optimum material properties in the final component. Of particular interest, with respect to two-phase titanium alloys is the break-up, below the beta-transus temperature, of an initial transformed alpha-lamellar microstructure during thermo-mechanical processing; this plays a key role in the development of the equiaxed alpha microstructure desired for the final product. Significant research effort has been put into understanding the mechanism for dynamic globularization, formation of kinking and shear banding within the alpha lamellae that can lead to break-up, but no complete quantitative analysis of the evolution of microstructural parameters and crystallographic texture with deformation currently exists. Therefore, reliable quantitative microstructural data associated with this process is important for both informing and validating models describing high-temperature metal-forming. To investigate the influence of hot working parameters, strain, strain rate and temperature, on microstructure evolution in Ti-6Al-4V a series of hot isothermal axis-symmetric compression tests have been carried out at temperatures both low and high in the alpha + beta stability field (880°C and 950°C, respectively), using strain rates (0.01, 0.1 and 1/s) relevant to industrial press forging. The experimental results showed that the morphology of the secondary a phase transforms gradually from lamellar to equiaxed under the influence of the deformation parameters and that the a lath thickness appears to have little influence on flow behaviour. It was also found that, at lower strains, the alpha laths appeared to be undeformed or only partially distorted. As strain progressed the laths were further broken up by distortion, bending and kinking. The mean aspect ratio of the alpha laths was found to exhibit a gradual reduction with increased strain. The lath area, length and perimeter showed a tendency to decrease with increasing strain. Furthermore, orientation image mapping (by EBSD) and texture analysis (by neutron diffraction) of the alpha phase were used to study the textural evolution during the hot deformation of the specimens and the mechanism involved on development and evolution of crystallographic texture during the a-β-a phase transformation. The strengthening of the ~-phase texture is observed on heating the sample during a-β phase transformation, where it was observed that Burgers relationship was followed but no evidence of preferential transformation was detected. In contrast a definite deviation from the Burgers relationship was observed during hot-compression. During β-a phase transformation upon cooling, however, the Burgers relationship was followed, which is evidence for a texture memory effect due to the growth of the primary alpha phase present at high temperatures.