ATOMISTIC SIMULATIONS TO STUDY STATIC AND DYNAMIC BEHAVIOUR OF MULTI-ELEMENTAL ALLOYS

Abstract

High entropy (HEAs) or multi-elemental alloys with exceptional mechanical properties are potential candidates to replace conventional structural material. HEAs have shown superior ductility and fracture toughness, even at cryogenic temperatures. These alloys are solid solutions of five or more principle metallic elements (usually upper limit is arbitrarily set to thirteen), resulting in high entropy of mixing. In HEAs, an equal percentage contribution of alloying elements is employed, unlike the traditional alloys with one parent principle metallic element, whereas other secondary elements are in small proportions. Due to extremely high configurational entropy and low heat of mixing, HEAs are stable in a single phase. HEAs are emerging as a potential candidate to replace conventional metals in aerospace due to their high strength and ductility at a broader range of temperatures. In addition to strength at such a wide range of temperatures, HEAs possess a good strength-to-weight ratio, excellent fatigue resistance, wear and creep resistance. Initially, molecular dynamics (MD) and statics-based simulations were carried out to study the effect of equi and non-equiatomic compositions of multi-elemental alloys on defect dynamics and tensile deformation. The 2NN-MEAM force field was employed in the MD-based calculations to capture the interatomic interactions. High and medium entropy alloy configurations were subjected to uniaxial tensile loading, and a deformation governing mechanism was identified with the help of a dislocation extraction algorithm (DXA) and common neighbor analysis (CNA). Due to the low or negative stacking fault energy, the phase transformation from FCC to HCP in conjunction with solid solution hardening was identified as the deformation governing mechanism in multi-elemental alloys. It was predicted from the MD simulations that alloys containing higher Cr content, twinning, and stacking fault are prominent modes of deformation. Nudged elastic band algorithm was used to predict the vacancy migration energies in each atomistic configuration. After capturing the defect dynamics, MD-based simulations were performed to study the fracture behavior in single crystal ternary, quaternary, and quinary alloy configurations. Simulations were performed after aligning the crack plane with three principal planes of FCC crystal. Deformation is primarily governed by dislocations, twinning, and stacking faults emanating in front of the crack tips or from the surface of the crack plane. Medium vii and high entropy alloys have shown higher resistance to fracture and were capable of retaining strength up to a more considerable extent than low entropy alloy and pure Ni crystal. In the next set of simulations, a single crystal configuration was replaced with polycrystalline arrangements of five elemental (Co-Cr-Cu-Fe-Ni) high entropy alloys. The crack was treated at inter and intragranular positions in the polycrystalline configuration of HEAs. In these simulations, the size of the grains varied in the range of 2.5 nm to 10 nm. The effect of lattice distortion on the crack tip behavior was captured with the help of the average atom (A-atom) configuration, and results were compared with random alloy configuration/HEA. Simulations revealed that A-atom possesses higher critical stress values, but early onset of dislocation emission from the crack tip in random alloys leads to crack tip blunting. The spatial positioning of the crack in the polycrystalline HEA, in conjunction with grain size, significantly affects the crack tip behavior. Nonequilibrium molecular dynamics (NEMD) based simulations were performed to study the effect of shock compression on the deformation governing mechanism in high entropy alloys. Quinary configuration of the alloy containing (Co-Cr-Cu-Fe-Ni) as primary elements were considered, and interaction between them was simulated with the help of embedded atom method potential. Single-crystal HEA was subjected to shock compression and ultra-short pulse at piston velocities above and below the Hugoniot elastic limit. To study the dynamics of the shock wave, spectra-temporal distribution of pressure and velocities were captured at the onset and propagation of the shock wave in single crystal HEA. It was predicted from the atomistic simulations that the insertion of voids in the path of the shock front helps in dispersing the energy and reducing the speed of shock propagation. Voids significantly affect the shock deformation governing mechanism in the single crystal of HEA with the early onset of plastic deformation, even at piston velocities below the Hugoniot elastic limit. It was revealed from the comparison between A-atom and random alloy configurations that the effect of lattice distortion helps in blunting the shock front and retarding the speed of propagation. The lattice distortion effect dominates at lower simulation temperatures and piston impact velocities. The shock resistance of HEAs was also studied for polycrystalline configurations of multi-elemental alloys. Similar to single crystal, A-atom configurations were also developed for polycrystalline compositions of multi-elemental alloys. It was predicted from the simulations that a higher value of lattice distortion component in CoCrCuFeNi alloy leads to provide superior resistance against the shock wave propagation as compared to ternary viii alloy CrFeNi. In the nanocrystalline, dislocations in conjunction with stacking faults govern the deformation. On the other hand, in monocrystalline configurations, only dislocations control the deformation mechanism. The simulations indicate that grain size significantly affects the rates of generation of secondary/partial dislocations, hence affecting the stresses and deformation mechanism of the structures. In the last section, molecular dynamics-based simulations were performed to study the effect of radiation-induced defects on the tensile and shock-induced deformation mechanisms of medium and high entropy alloys. Overall, simulations were divided into two broad sections; initially, a displacement cascade was generated with varying PKA parameters to study defect formation in multi-elemental alloys. Later, the same defected crystal was subjected to tensile and shock loading to study the deformation mechanism of multi-elemental alloys containing these radiation-induced defects. This work will help elucidate the defect dynamics and deformation governing mechanism in multi-elemental alloys under complex loading scenarios.