Plastic deformation and strengthening mechanism of FCC/HCP nano-laminated dual-phase CoCrFeMnNi high entropy alloy

Abstract

FCC-structured CoCrFeMnNi high entropy alloy (HEA) has attracted abroad interests for years because of its excellent mechanical properties, except for strength. Recent experiments have reported a kind of nano-laminated dual-phase (NLDP) FCC/HCP structure that can strengthen the HEA. However, it is still unknown why the HEA can be strengthened by this kind of NLDP structure. Here, we employ molecular dynamics simulations to study the atomistic strengthening mechanism of the NLDP HEA. Dislocation-assisted multiple plastic deformation mechanisms in both FCC and HCP single phase HEAs are observed, and amorphization is also found in the plasticity of HCP phase, which are consistent with the previous experimental characterizations. The HCP phase possesses higher strength because of its higher stacking fault energy, higher Peierls–Nabarro stress and less active dislocation slip systems. It is also found that the introduction of HCP phase can enhance the mechanical properties, including yield stress, yield strain and plastic flow stress, of the NLDP HEAs, which also show volume fraction dependence. And the phase boundary plays crucial roles in the deformation and strengthening of the NLDP HEAs. The plastic deformation of the NLDP HEAs can be divided into two stages, i.e. stage I (plasticity only appears in FCC lamella) and stage II (plasticity in both FCC and HCP lamellas). With the increase of volume fraction, the lamella thickness of FCC matrix phase decreases, leading to continuous strengthening of yield properties and flow stress of stage I because of suppressed dislocation nucleation and confined dislocation motion in FCC matrix phase by the phase boundary. While there is no monotonous relationship between the flow stresses of stage II and the increasing volume fraction of HCP phase, which can be attributed to the competitive mechanisms between strengthening effect of phase boundary on the dislocation motion in FCC phase and softening effect of phase boundary on the dislocation motion in HCP phase. The results should be helpful for understanding the underlying physical mechanism of strengthening of HEAs with NLDP structure.

Molecular dynamics simulation of the heterostructure of the CoCrFeMnNi high entropy alloy under an impact load

“…In the y-direction, the lattice orientation is [1 1 0] for the FCC phase and [1 0 −1 0] for the HCP phase. Lastly, in the zdirection, the lattice orientation is [−1 1 −1] for the FCC phase and [0 0 0 1] for the HCP phase [27]. Detailed dimensions and orientation information of the structures are indicated in figure 1(g).…”

“…The HCP structure has higher strength due to its higher stacking fault energy, higher critical shear stress, and fewer active dislocation slip systems. Therefore, the introduction of the HCP phase can strengthen the material by reducing the dislocation density when it is subjected to external loading [27]. Therefore, the phase boundary of the FCC structure has a strengthening effect on dislocation motion, while the phase boundary of the HCP structure has a suppressing effect.…”

Molecular dynamics analysis of the low-temperature shock behavior of the CoCrFeMnNi high-entropy alloy

“…The interactions of atoms in CoCrFeMnNi HEA are described by the second nearestneighbor modified embedded-atom method potential developed by Choi et al [28] This potential has been used to study the cyclic deformation, indentation, temperature and strain rate effects in CoCrFeMnNi HEA [29][30][31][32]. It has also been successfully applied in a recent MD studied on the mechanical behavior of dual-phase CoCrFeMnNi [33].…”

Phase Volume Fraction-Dependent Strengthening in a Nano-Laminated Dual-Phase High-Entropy Alloy

“…In addition to the abovementioned strengthening and toughening strategies, the recently reported dual-phase (DP) structure can make HEAs reach good strength–ductility synergy. − For instance, the synthesized Fe 50 Mn 30 Co 10 Cr 10 HEA exhibits a DP structure with ∼28% HCP phase and ∼72% FCC matrix phase, which leads to an apparent increase of both tensile strength and ductility. Similarly, the Co 20 Cr 20 Fe 40– x Mn 20 Ni x HEAs with x = 0–20% with a DP structure are designed .…”