Correlation between macroscopic necking and microscopic kink bands during quasi-static tension for a selective laser melted β-Ti alloy

Abstract

Macroscopic necking is of significant importance as it limits the ductility of materials. At the microscopic level, deformation bands serve as indicators of strain localization. This study investigates the correlation between the two length-scale behaviors for a hardening-free β-Ti alloy prepared by selective laser melting, where necking is dominant under tension. The macro/micro features and the tensile flow stresses can be reproduced by a crystal plasticity finite element model (CPFEM) with explicit grain structures. Through the mechanism-based simulation, the identified correlation between microstructure and mechanical properties can be attributed to (1) limited strain rate sensitivity (SRS), (2) low strain hardening, and (3) the presence of hard particles. The first two favor strong necking, while the third is essential for the formation of deformation bands. Two kinds of deformation bands were identified: lenticular kink bands which only appeared in the necking region, and slim slip bands. The former can only be reproduced with a compact set of active slip systems, in conjunction with the abovementioned three conditions. Increasing SRS resulted in the suppression of necking, leading to the disappearance of the typical kink band morphology. The presence of hard particles induced an unloading yield effect, which can enhance ductility by slowing the progress of necking. The present results strengthen the fundamental understanding of (1) the effect of both SRS and hardening capability on necking, and (2) the necessity of necking, hard particles, and the depletion of active slip systems for the kink band formation. These findings are peculiarly relevant to the studied β-Ti alloy.