Phase transition in shock compressed high-entropy alloy FeNiCrCoCu

The concept of “high-entropy”, realized by deliberately incorporating five or more elemental species with equal or nearly equal-atomic components into a single lattice, first succeeded in metallic alloys, incubating a novel class of alloys called high-entropy alloys. This novel and appealing multi-component design paradigm allows for a near-infinite space of elemental compositions, subverting our inherent cognition with respect to alloying concepts beginning from the edge of a phase diagram with only one or rarely two primary elements. Derived from the cooperation effects of multi-principal components, HEAs render four salient characteristics: high entropy effects, sluggish diffusion effects, lattice distortion effects, and cocktail effects. In particular, owing to high mixing configurational entropy, the diluted enthalpic interplay between the component elements in high-entropy alloys gives rise to a restrained formation for intergranular compounds, which enables the formation of simple single-phase solid solutions rather than intermetallic compounds, regardless of chemically complicated composition. A combination of unique intrinsic natures and simple crystallographic structures enables the high-entropy alloys with advantages that traditional alloys cannot match, such as high strength, ductility, and fracture toughness within a more extensive spectrum of temperatures, which enables them to be expected as potential candidates for next-generation structural applications.

FeNiCrCoCu high-entropy alloys are a subclass of high-entropy alloys that consist of iron (Fe), nickel (Ni), chromium (Cr), cobalt (Co), and copper (Cu). These alloys, a single-face-centered cubic structure, exhibit aroused increasing concern over the past couple of years by virtue of their outstanding properties, such as high hardness, superior wear resistance, and exceptional elevated-temperature stability. For instance, the enhancement in hardness and wear resistance can be attained for FeNiCrCoCu HEAs by adding Cu elements properly. A synergistic improvement of both strength and plasticity of FeNiCrCoCu HEAs was attainable through nanoscale phase transformation induced by ion irradiation. This synergistic improvement can also be given by either a moderate annealing treatment or combining it with a hydrogel film by means of magnetron sputtering. Although a significant amount of research has been conducted on the mechanical properties of high-entropy FeNiCrCoCu alloys when they are subjected to quasi-static or static deformation, a fundamental understanding is still unavailable on the deformation behavior of these alloys under dynamic loading conditions. When structural materials are put through demanding conditions, their mechanical responses become extremely sophisticated. Because of this, having a comprehensive understanding of the dynamic properties of high-entropy alloys is essential for the application of these materials in shock engineering.

Shock loading is considered to be an irreversible and non-equilibrium energy-dissipation process in nature, where various microstructural evolutions occur inside metallic materials, involving phase transition, dislocation activity, twinning, spall damage, amorphization, and so on. The ultrafast evolution of microstructure typically involves picosecond timescales when shock waves propagate into materials. Meanwhile, due to the limitations of conventional experimental techniques in achieving atomic resolution and capturing the evolution of microstructure in real time, unraveling an elaborate dynamic characterization upon shock compression remains a challenge at the atomic scale. Fortunately, nonequilibrium molecular dynamics simulation provides a three-dimensional visualization method for simulating the propagation process of shock waves at the atom level, which allows each atom in a system to move in a microcanonical ensemble based on the rules of the Newton motion equation without artificial control of temperature and pressure. The system is in an adiabatic state, consistent with the basic assumptions of the shock wave theory. More importantly, it can real-timely explore the evolution processes of phase transitions and dislocation behaviors at nonequilibrium states. Furthermore, some studies have successfully revealed the dynamic deformation behavior and corresponding mechanisms of metal systems by means of this method. Meanwhile, with the improvement of computing power, the lack of potential faced by the multi-principal component system has been addressed gradually, which makes it possible to reveal the deformation behavior of high-entropy alloys by means of nonequilibrium molecular simulation.

In a new study published in the peer-reviewed International Journal of Mechanical Sciences, Ph.D. candidate Hongcai Xie, Professor Zhichao Ma, Ph.D. candidate Wei Zhang, Professor Hongwei Zhao, and Professor Luquan Ren from Jilin University investigated the shock-induced phase transition for the equiatomic FeNiCrCoCu high-entropy alloys in terms of the crystallographic direction and shock velocity, through nonequilibrium molecular dynamics simulations. They demonstrated that the face-centered cubic to body-centered cubic phase transition due to uniaxial compression and lattice rotation was prone to appearing for shock along the [100] orientation, which was conducive to activating dislocation nucleation to release shear stress. More importantly, they found that a shift from a dislocation-dominated deformation to a phase-transition-dominated one with the increase in shock velocity facilitates the swift stress relaxation at higher strains, contributing to the attenuation of the shock wave and thereby weakening the shock damage.

In a nutshell, the new study rendered valuable insights into how the FeNiCrCoCu high-entropy alloy behaves under harsh conditions, and they can guide future studies on the creation and innovative use of these alloys in the future, including nuclear power and aerospace engineering.