A novel shock-induced multistage phase transformation and underlying mechanism in textured Nano-Twinned Cu

Abstract

Nano-Twinned (NT) Cu under shock loading parallel to twin planes was investigated via nonequilibrium molecular dynamics (NEMD) simulations, with particular attention to the shock response and intrinsic deformation mechanism. During the shock compression process, a novel multi-stage phase transformation (MPT) scenario, i.e., from FCC phase to BCC phase and then to HCP phase, happens due to shock-enhanced twin boundary (TB) sliding. Both increasing the compressive strain caused by shock and decreasing the twin lamellae in the NT Cu can promote the MPT, with which the yield strength retains but the spallation strength increases. The critical shock pressure triggering the MPT depends exponentially on lamellae twin thickness (T). These new results have great potentials for improving the impact resistance of metals by tailoring their internal nanostructures.

Introduction

In nuclear reactor, aerospace engineering, military industry and laser processing, some key components serve in harsh strain-rate condition, which brings a huge challenge for selecting appropriate structural materials. Nano-twinned metals (NTMs), which show excellent combination of strength, ductility, fatigue and fracture resistance, are becoming promising materials for extreme loading environment [1], [2], [3], [4], [5], [6], [7]. Synthesized by means of magnetron sputtering deposition or direct current electrodeposition, NTM often chooses a preferred growth orientation, resulting in textured microstructure [8], [9]. Therefore, textured NTMs shows strong anisotropic, i.e., the strength, strain hardening and underlying deformation mechanisms generally exhibit orientation-dependent [10], [11], [12], [13]. Due to the different loading directions relative to the twin boundary (TB) plane, different dislocation-mediated strengthening and softening mechanisms have been found, especially when the twin lamella thickness drops to several nanometers [1], [14]. Owing to the excellent combination of strength and ductility of NTMs, they are potential structural materials for high impact resistance [4].

Under the impact condition, metals inevitably suffer from extremely high pressure, temperature and strain rate. The shock induced plastic response relates closely to the loading path, impact velocity, crystalline orientation and microstructure [15], [16], [17], [18], [19], [20], [21]. Some abnormal behaviors and novel mechanisms that are absent under quasi-static loads may appear during the shock process [11]. As a consequence, the deep understanding of the plastic response under shock condition and revealing the underlying mechanisms are of great significance. As a matter of fact, the shock induced plastic response is time dependent, due to the propagation and reflection of the shock wave. Thus, the details and mechanisms of plastic deformation in the shocked metals cannot be fully identified through a posterior analysis of the recovered sample. Under the circumstances, large scale non-equilibrium molecular dynamic (NEMD) modeling, which has the same intrinsic high strain rate effect and screen time as the shock experiment, becomes a powerful tool to capture the dynamic details of plasticity and damage due to the shock wave induced ultra-high strain rate [22]. Using the NEMD method, the grain size effect, anisotropic response and phase transformation in metals were re-assessed [18], [23], [24], [25]. Thanks to the development of in situ X-ray diffraction measurement technique, details of 3D plastic relaxation, twinning and void evolution due to the shock wave were unveiled [26], [27], [28], which to some extent verified the reliability of NEMD simulations. With the support of experiments, some modeling were performed to study the shock response of randomly orientated NT Cu and hierarchical NT Cu, highlighting the effect of twin thickness on the shear strength and spallation strength [16], [17]. To our knowledge, the mechanical behaviors of NTMs under shock conditions have not been thoroughly investigated yet, considering the complexity of their strong anisotropy. To unleash the potential applications of NTMs under high impact loading, it is necessary to enrich the comprehensive shock response and underlying mechanism of NTMs.

Accordingly, based on the above aspects, large scale NEMD simulations were performed to study shock-induced plastic deformation of the [111] textured NT Cu, with a series of twin lamella thicknesses considered. We systematically analyzed the possible underlying deformation mechanisms and elucidated their dependence on shock strength. The yield and spall behavior corresponding to different deformation mechanisms were also discussed. The results could provide a promising prospect for the material application in extreme environment.