Mechanical Behaviors of Multi-Principal Element Alloys with Chemical Inhomogeneity 


Student thesis: Doctoral Thesis

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Awarding Institution
Award date8 Aug 2023


Multi-principal element alloys (MPEAs) are a new type of material characterized by a solid solution composed of many main elements. Their flexible preparation process, controllable preparation costs, and excellent performance lead to considerable applications in the aerospace industry, industrial equipment, national defense, energy, and extreme environments. Due to the vast compositional field, the development strategies of multi-principal element alloys provide a challenging opportunity to design alloys with improved combined performances. The differences in the atomic sizes and chemical properties of the constituent components create unique microstructures, resulting in the inherent existence of atomic-scale chemical heterogeneity, namely, local chemical order. More generally, the scale of chemical heterogeneity ranges from the atomic to the macroscopic, including atomic-level chemical short/medium range order, nano-scale concentration fluctuations, segregation, larger-scale concentration gradients or fluctuations, etc. Much experimental and computational research has been devoted to exploring the development of chemical inhomogeneity and its influence on dislocation behavior and deformation mechanisms, though the associated internal microstructures and properties have not been fully understood.

In this thesis, a series of comprehensive atomistic simulations on chemically inhomogeneous face-centered cubic (fcc) CoCrNi medium entropy alloys (MEAs) were carried out to investigate the effect of the compositional gradient on the tensile behaviors of MEAs and the corresponding atomic-scale mechanisms. The simulation results on cubic single crystalline CoCrNi MEAs reveal that the local absolute concentration of each element is the primary factor affecting their tensile properties. Dislocation nucleation occurs in the Cr-rich region due to the localized shear deformation induced by chemical inhomogeneity, thus resulting in a decrease in the tensile strength. Subsequently, based on a series of simulations on equiatomic CoCrNi nanopillars with different compositional periodicities, we found that the tensile strength and plastic deformation ability of the alloys can be improved owing to the plastic strain delocalization stemming from the compositional periodicity.

Using hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations, we also investigated the effect of compositional periodicity and chemical short-range order (SRO) on the tensile behavior of CoCrNi MEAs. Specifically, the differences between MEAs with and without compositional undulation in the evolution of defects, the local stress–strain state, dislocation nucleation, slip, and the interaction with planar defects were explored. The results reveal that compositional undulation in MEAs can improve the elastic limit and lead to a strong lattice distortion effect, which can enhance the dislocation glide resistance. In addition, strain hardening and stress/strain delocalization were notably observed in the MEAs with compositional periodicity, which promotes strength–plasticity synergy. More notable was the formation of many hcp microbands consisting of stacking faults, hcp-like structures, and nanotwins due to compositional undulation. Such structures induce delocalized activation of the slip systems, which improves the strength/plasticity and strain hardening ability. The proposed compositional design can further optimize the comprehensive mechanical properties in addition to the SRO, which provides a feasible approach to designing novel high-performance MPEAs.

In crystal solids, due to the chemical and mechanical interactions between atoms and defects, solute atoms tend to segregate to the defects, such as dislocations, grain boundaries, stacking faults, and others, causing local concentration changes. MPEAs usually attain excellent high-temperature mechanical properties through the generation of coherent fcc γ’ precipitates. Therefore, segregation-induced chemical inhomogeneity and its effect on the mechanical properties of the alloys were also investigated. The results indicate that Co segregation leads to local compositional fluctuations, which promote dislocation nucleation and reduce the yield stress and strain of the alloy. It is worth noting that Co segregation increases the plastic flow stress. The decrease in the local stacking fault energy and the enhanced bcc phase transitions caused by segregation contribute to the dislocation blocking mechanism, thus increasing the plastic flow stress. The investigation on the effects of segregation can provide useful insights for the rational compositional and structural design of MPEAs.

Finally, we attempted to develop a new computational approach to investigate the fracturing of polycrystals. Based on the traction–separation (T–S) constitutive relations extracted from MD simulations, a peridynamics (PD) model was proposed to investigate the crack propagation behavior of polycrystals under the mode-I loading condition. The MD simulations provided insights into the cracking process and fracture mechanism based on the analysis of atomic configurations and stress distribution. The atomic stress at the crack tip with respect to the opening distance was tracked during the steady cracking stage to provide a stable T–S relation. The fracture parameters of single crystals were obtained via MD simulations, based on which the PD parameters were determined via an energy equivalent method. Subsequently, a PD approach combined with a cohesive zone model (CZM) was proposed to investigate the mode-I fracture in polycrystals; the proposed PD model agreed well with the classical CZM based on a quasi-static splitting test of a single crystal. One can conclude that the T–S relation originating from classical cohesive theory can be regarded as an effective bridge between MD and PD. This work provides a new approach to studying the fracture behavior of polycrystals from the atomic deformation mechanism to the microfracture descriptions.