Modeling the Mechanical Response in BCC Metals Under Extreme Thermo-Mechanical Loading: Atomistically Informed Multiscale Dislocation Dynamics Simulations

Abstract

The mechanical response of materials subjected to extreme thermo-mechanical loading is of supreme importance to satisfy the highly technological needs. In this work, a multiscale model of plasticity that couples three-dimensional discrete dislocation dynamics with finite element analysis are carried out to investigate the mechanical response and microstructure evolution of single crystal, micro-pillars and micro-polycrystalline BCC α-iron subjected to high strain rate compression over a wide range of high deformation temperatures. In the first part of this study, computers simulations are designed to mimic the bulk behavior of BCC α-iron subjected to quasi-static and shock loadings. For both types of loading, the applied deformation rate ranges between 102 to107 s-1 at a deformation temperature ranging between 300K and 900K. Multiscale dislocation dynamics plasticity (MDDP) based constitutive equations interrelating temperature and strain rate with the flow stress at high strain rate under quasi-static and shock loading conditions are proposed. Results show that there is a transition at 105s-1 of the dependency of flow stress on strain rate sensitivity under both types of loading. At strain rates less than 105s-1, the flow stress is less sensitive to the applied rate while it becomes highly sensitive as the strain rate exceeds 105s-1. Detailed investigations of the dislocation microstructure evolution show the formation of extended screw dislocation lines at temperatures below 340 K due to the large value of the lattice friction of the pure screw segments. Moreover, small sessile loops of radius in the order of few nanometers are formed. The formation of these sessile loops is facilitated by the easiness of multiple cross-slip on available slip planes. In the second part of this study, the high strain rate simulations were expanded to focus on the inter-relationship between the dislocation-velocity related strain rate sensitivity and the time dependent evolution of dislocation density. MDDP simulations were performed to calculate different rise times to mimic high deformation ramp loading for BCC α-iron and Tantalum. The simulation results are compared with the thermal activation strain rate analysis (TASRA) model proposed by Armstrong, and other phonon drag and dislocation generation models. Weak shock behavior was detected at very short rise time in the order of 0.2ns. In the third part of this study, MDDP simulations are carried out to investigate the mechanical response and microstructure evolution of BCC iron micropillars under combined high temperature and strain rate deformation. The simulations are conducted at sizes ranging between 0.25μm and 2μm under an applied deformation rate ranging between 103s-1 and 107s-1 and subjected to different deformation temperatures. MDDP based constitutive equation interrelating the size effect exponent to strain rate and temperature is also proposed indicating that the exponent is relatively sensitive to temperature and at a lesser degree to strain rate. Detailed investigation of the microstructure shows that self-multiplication of dislocations is responsible for the strengthening mechanism in BCC iron micropillars. At low temperatures and small sizes, screw dislocations have a weak effect on plasticity for a certain period of time but subsequently control the self-multiplication process. At larger sizes, the motion of screw dislocations is responsible for plasticity at low temperatures. Due to the large volume size, screw dislocations are entangled inside the sample leading to a self-multiplication of dislocations via cross-slip and other dislocation –dislocation interactions. At higher temperatures and for all sample sizes, mixed dislocations control plasticity via the multiplication of a complex network of dislocations. In the last part of this study, size effect for BCC α-iron is also investigated but for micro-polycrystalline samples. MDDP simulations were performed to mimic impenetrable grain boundaries at sizes ranging between 0.5μm and 2μm at strain rates ranging between 103s-1 and 105s-1 and at 300K, 600K and 900K temperatures. For the three deformation temperatures, the Hall-Petch effect is reproduced. A comprehensive study of the microstructure evolution shows that screw dislocations control the plastic deformation of the polycrystalline materials via the activation of cross-slip mechanisms. Hardening is seen at low sizes for all temperatures at low strain range due to the dislocations pile up inside the grains prior to cross-slip activation. Once cross-slip is thermally activated, self-multiplication of dislocations is detected resulting in strain softening.