Microscale to Macroscale Analyses of Superplasticity of Ti-6242S Titanium Alloy

The superplastic performance of the dual-phase Ti-6242S titanium alloy makes it a good material for aerospace application to produce structural components using the advanced superplastic forming (SPF) process. The need to optimize the SPF process demands the understanding and quantifications of the influence of the different phase constituents - α and β on the global superplastic behavior. Numerical modelling has been useful to predict mechanical behavior for both one-level and multiscale approach. Multiscale approach: bottom-up (microscale to macroscale) has enabled to understand how the different microstructural parameters influence global material/structural mechanical response; which by large means the modelling approach depends on the material local properties. The identification of these local properties is non-trivial in polycrystal materials, particularly at superplastic (elevated) temperatures. We have developed a methodology that permit us to quantify the microstructural parameters of each of the constitutive phases of a polycrystal at a superplastic temperature using genetic algorithm optimization method on the data from in-situ high energy X-ray diffraction (synchrotron radiation), coupled with SEM (scanning electron microscope) and EBSD (electron backscattered diffraction). These identified local microstructural parameters were directly used in the finite strain crystal plasticity model to simulate the material global response. This approach enabled the quantification of the phase influences on global behavior with much accuracy. It was found that α phase planes have high critical resolved shearing stress (CRSS) at 730°C which is similar to its behaviours at room temperatures, while β phase slip planes have low CRSS that encourage slip shearing at low stress. However, more applied load is partitioned in β phase than in α phase, despite that β phase fraction is about 15% at 730°C. Keyword: Multiscale modelling, CPFE, optimization, HEXRD, dual-phase titanium alloy, superplasticity