Crystal plasticity simulation of strain aging phenomena in α-titanium at room temperature

Abstract

Strain aging phenomena are shown to affect the viscoplastic behavior of commercially pure α-titanium at room temperature. A yield stress anomaly corresponding to static strain aging was experimentally observed when the material was loaded in the transverse direction. At low strain rates small serrations on the stress-strain curves, typical for the Portevin-Le Chatelier effect are observed for the material loaded in the transverse and rolling directions at room temperature. The presence of a stress peak is attributed to the interaction between activated <c+a> slip systems with the atoms of interstitial oxygen. The Portevin-Le Chatelier effect is presumably due to the non-planar core of screw-type dislocations. A phenomenological strain aging model is combined with a comprehensive description of slip systems active in HCP crystals in order to take into account the role of crystal plasticity in static as well as dynamic strain aging. Finite element simulations are performed on polycrystalline aggregates with various numbers of grains accounting for the elastic and plastic anisotropy of α-titanium. The simulations of static strain aging do not show formation and propagation of macroscopic shear bands. Instead, a complex strain localization phenomenon is taking place within some grains and specific associations of grains, which leads to the formation of meso-Lüders bands. The results of dynamic strain aging simulations predict the initiation and propagation of macroscopic Portevin-Le Chatelier bands even in the presence of positive apparent strain rate sensitivity.