Multiscale Modeling of Twins and Dislocations Nucleation in High Entropy Alloys

高熵合金中位錯及孿晶形核的多尺度計算

Student thesis: Doctoral Thesis

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Award date9 Aug 2019

Abstract

The NiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In this thesis, face-centered-cubic (FCC) single-crystal NiCoCr, FeNiCoCrAl0.36 and FeNiCoCrSi0.36 HEAs were studied. First of all, a new approach named “self-optimized random structure generator” based on Markov Chain Monte Carlo (MCMC) or called Metropolis Monte Carlo, and quasi-simulated-annealing algorithm is developed to determine the stable structure of HEAs with desired components at appointed concentration. This approach uses chemical short-range order (CSRO) in HEAs as parameters in the global optimization of weight parameters in the cost functions and is able to effectively evolve given multi-component alloy structures out of their metastable states without high throughput calculations.

Then the multiscale Modeling and analysis were used: (1) the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of various structures. (2) kinetic Monte Carlo (kMC) based on first principle results to predict the substructures evolution under various circumstances, e.g. thermal activation. (3) Adaptive-boost molecular dynamics (ABMD): a time-accelerated MD method based on hyper-dynamics, to study the time evolution of dislocation nucleation and propagation in HEAs.

We proposed two different formation mechanisms of nano-twins formation and found that short-range hexagonal-close-packed (HCP) substructures could occur in this FeNiCoCrAl HEA. DFT calculations suggest that this HEA has the negative stacking fault energy, HCP formation energy, and twin-formation energy at 0 K. Phonon calculations represent that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning makes it possible to form HCP-like twin boundaries. The HCP-like twin boundaries and twin formation could be effectively controlled by tuning the Al contents.

With the great agreement of results from kinetic Monte Carlo simulations and experiments, this twin-formation mechanism offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which results in better mechanical properties of HEAs. On the other hand, the ABMD simulations of nucleation of screw and edge dislocations suggest that in the HEAs considered, the nucleation frequencies of both dislocations are strongly influenced by the local environment of species. Thus, these combined effects of substructures interfaces and impede of dislocation motion provide strength and ductility simultaneously in HEAs.

    Research areas

  • High entropy alloy, Multiscale modeling, Density functional theory