A strain rate and temperature-dependent crystal plasticity model for hexagonal close-packed (HCP) materials: Application to α-titanium

The mechanical responses of hexagonal close-packed (HCP) materials are highly anisotropic, as well as strain rate and temperature-dependent, because of inherently asymmetric slip and twinning in constituent crystalline grains. How slip and twinning are affected by strain rate/temperature and how they alter the deformation response of HCP materials are still not well understood. To provide greater insights, a novel crystal plasticity model is proposed to describe the mechanical behavior and microstructure evolution of single-crystal and polycrystalline HCP materials under different loading conditions. A new flow rule is developed, which incorporates the effects of strain rate and temperature on the activation of specific deformation mechanisms. Both dislocation density-based and empirical hardening laws are adopted to describe the overall hardening contributed by slip, twinning, and their interaction, taking into consideration loading conditions and the difference between tension and compression twinning. The proposed model is implemented in the commercial FE package ABAQUS via a user-defined subroutine. The mechanical response and microstructure evolution of single-crystal titanium with different orientations, and polycrystalline titanium comprising grains with 350 random orientations, subjected to deformation at different strain rates and temperatures, are numerically explored. The numerical results display good agreement with published experimental data. The simulations indicate that an increase in strain rate or decrease in temperature will retard the activation of slip systems, thereby promoting twinning instead. A higher twinning volume fraction leads to a larger amount of lattice reorientation, more pronounced hardening, and higher stress levels. This effort has yielded a greater understanding of the influence of strain rate and temperature on the mechanical response of HCP materials and facilitated mesoscale simulations that couple slip and twinning.