Unraveling size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti by non-local crystal plasticity modeling

Abstract

Titanium alloy is prominent in manufacturing of high-performance miniature devices, while size-affected plastic heterogeneity and asymmetry in micro-scaled deformation are not well understood due to its complex deformation mechanism. How the interplay among dislocation slip, deformation twinning, and strain gradient affects the size-dependent plastic heterogeneity and asymmetry is an intractable and non-eluded issue that needs to be unraveled. In this research, a non-local crystal plasticity model (NLCPM) considering discrete twins was established and implemented into a finite element framework. The proposed NLCPM successfully represented the size-dependent asymmetric mechanical response, primary and secondary twinning, and texture evolution. The size-dependent fracture behavior, plastic heterogeneity, and T-C asymmetry during micro-scaled deformation of commercial pure titanium (CP-Ti) were explored by coupling micro-tension/compression test and full-field simulation. Results showed that T-C asymmetry in flow stress was enhanced in the coarse-grained and small-diameter samples. For the sample with D≥0.6 mm, the ductility of CP-Ti was improved with the increase of grain size as there are more prevailing twins and the concentrating stress could be relieved by the abundant slip rather than cracking. Grain-level strain is more homogeneous in compression when d was increased from 60 to 110 μm since more non-prismatic slips were activated that accommodated strain along various directions. Meanwhile, plastic strain was allocated within discrete twin bands and thus alleviated strain concentration. Furthermore, the hardening effect of twin boundaries cancelled out the softening effect near the free surface, resulting in a narrower softening layer in the coarse-grained samples compared to its grain size. With the decrease of the sample size, the proportion of the softening layer increased, while the volume fraction of twinning decreased, leading to a reduction in macroscopic stress and fracture strain. This work enhances the in-depth understanding of size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti and supports the forming of high-quality micro components under complex deformation path.