Temperature dependent evolution of anisotropy and asymmetry of α-Ti in thermomechanical working : characterization and modeling

Abstract

The strong anisotropy, asymmetry and distorted evolution of plasticity in thermomechanical processing of high performance metallic materials such as α-Ti make it difficult to identify the deformation mechanisms. In this research, a general characterization framework was established to determine the anisotropic and asymmetrical deformation behavior of α-Ti under different thermomechanical loading conditions. The temperature dependent discontinuous constitutive framework was developed for modeling of the distorted evolution of plasticity. The link between crystal plasticity simulation and continuum modeling considering the evolving yield surface of materials with temperature was then established. By taking the thermomechanical working of thin-walled α-Ti tube as a study case, the high temperature digital image correlation (HT-DIC) measurement approach, and the electron backscattered diffraction (EBSD) based physical measurement of arc and small-column samples were proposed to obtain the fundamental tension/compression properties and texture evolution of the alloy at different working temperatures. A viscoplastic self-consistent (VPSC) crystal plasticity based numerical simulation was conducted to determine the temperature dependent anisotropy and asymmetry in plastic deformation. A multi-objective optimization method was used to calibrate the VPSC model parameters and an interpolation approach was employed to smoothly identify the nonlinear evolution of yield loci with strain and temperature. By considering five main deformation mechanisms, viz., prismatic slip, basal slip, pyramidal <c+a> slip, {10-12}<10-1-1> tension twinning and {11-22}<11-2-3> compression twinning, the interactive relationships among flow stress, R-value, yield locus, deformation mechanism, texture evolution and working temperature were established and elaborated. The major findings include: 1) α-Ti tube behaves a significant anisotropy and asymmetry in terms of flow stress, R-value, yield locus, and deformed texture at room temperature, but the anisotropy and asymmetry are significantly reduced at elevated temperatures due to the decrease of twinning activity; 2) The temperature dependency of twinning activity in α-Ti depends on the decrease of the critical resolved shear stress (CRSS) value of pyramidal <c+a> slip with temperature; 3) For the materials with the c-axes of grains towards to normal direction (ND), the increasing activities of pyramidal <c+a> slip, tension twinning and compression twinning decrease the R-value; 4) The activation of twinning generates significant anisotropy and asymmetry and leads to an irregular evolution of yield loci. The developed theoretical framework of characterization and modeling is efficient for producing the above findings and provides a basis for generating a whole spectrum of knowledge of the anisotropy and asymmetry in plastic deformation of the high performance materials that are generally difficult to characterize.