Developing an unknown alloy requires a lot of trial and error.
However, using material simulation together can noticeably reduce the number of trials and errors.
High entropy alloys is one of the metal materials that has been an issue in recent years. These alloys are characterized by having a microstructure consisting of single or dual phase without intermetallic compounds, even if the various alloy elements are composed of similar fractions.
Until now, these alloys are known to have high strength and good high-temperature strength, structural stability and also have high creep resistance and ductility.
Therefore, high entropy alloys have attracted the attention of many researchers as next generation structures and functional materials.
However, when developing these alloys, experimentally evaluating the change in microstructure and mechanical properties with changes in alloying elements requires considerable time and costs.
Materials simulations can solve these problems and help the alloy designs.
Senkov, O. N. et al. 2010. “Refractory High-Entropy Alloys.” Intermetallics 18(9): 1758–65.
Yeh, Jien Wei. 2015. “Physical Metallurgy of High-Entropy Alloys.” JOM 67(10): 2254–61.
1. First-principles Calculation
· Calculation of Strength of HEAs
In general, it is well known that the strength of the alloy is affected by the lattice distortion and the elastic energy.
Based on this theory, there are a lot of related empirical models. In these models, if the information on lattice and elastic energy is defined, the strength of alloys can be predicted.
One study showed that the strength of alloys is calculated based on this concept by using first-principles calculations.
Yin, Binglun, and William A. Curtin. 2019. “First-Principles-Based Prediction of Yield Strength in the RhIrPdPtNiCu High-Entropy Alloy.” npj Computational Materials 5(1).
Stacking fault energy is a very significant factor in understanding the deformation mechanism in fcc structure. There are many ways to get stacking fault energy in the fcc structure, but the most known way is to get the energy difference between the hcp structure and the fcc structure.
Based on this method, stacking fault energy can be estimated using first-principle calculations. Practically, one study discussed the relation between deformation mechanism and calculated stacking fault energy.
Jo, Yong Hee et al. 2019. “Cryogenic-Temperature Fracture Toughness Analysis of Non-Equi-Atomic V10Cr10Fe45Co20Ni15 High-Entropy Alloy.” Journal of Alloys and Compounds 809: 151864.
2. Molecular Dynamics
· Calculation of Strength of an Alloy and HEA Designs
Previously, it is mentioned here that the strength of a HEA alloy is obtained using empirical models and information on lattice and elastic energy estimated from first-principles calculations.
From a microstructural point of view, the strength is significantly affected by the energy required to cause movement of dislocations.
Accordingly, one study showed that the strength is evaluated through molecular simulations for the dislocation movement. Specifically, from the result obtained using this simulation, a model to estimate CRSS of alloys was developed. Practically, using this model, it is known that there is a case of developing new HEAs with improved strength.
Choi, Won-Mi et al. 2018. “Understanding the Physical Metallurgy of the CoCrFeMnNi High-Entropy Alloy: An Atomistic Simulation Study.” npj Computational Materials 4(1): 1-9.
Moon, Jongun et al. 2018. “Microstructure and Mechanical Properties of High-Entropy Alloy Co20Cr26Fe20Mn20Ni14 Processed by High-Pressure Torsion at 77 K and 300 K.” Scientific Reports 8(1): 11074
· Radiation Resistance of HEA
Mostly, it is known that HEAs have high radiation resistance due to sluggish diffusion. However, since some studies noticed that diffusion velocity in HEAs is not very slow, the debate for sluggish diffusion in HEAs continues.
One study mentioned that the molecular dynamics can evaluate the radiation resistance of a HEA.
To do this, firstly, the migration energy of both vacancy and interstitial defect is calculated using molecular dynamics. Secondly, the behavior of the arbitrary formed Frenkel defects over time is simulated using molecular dynamics.
From the migration energy point of view, the vacancy migration energy between the HEA and the other alloys is not different, but the interstitial defect migration energy of the HEA is higher than that of the other alloys.
From the behavior of Frenkel defects point of view, the HEA does not present many formations of the defect clusters and dislocations, compared to other alloys.
Therefore, this study breaks the hypothesis that HEAs is known to be resistant to the radiation damage because of the sluggish diffusion, and is a good example that reveals a more practical mechanism.
Do, Hyeon Seok, and Byeong Joo Lee. 2018. “Origin of Radiation Resistance in Multi-Principal Element Alloys.” Scientific Reports 8(1): 1–9.
Thermodynamic calculations are a very useful calculation technique for predicting the alloy's microstructure at the target temperature.
To use thermodynamic calculations, a well-defined database for a relevant alloy system is required.
However, there are two studies on the HEA designs using thermodynamic calculations without a well-defined database of the HEA systems.
One study is to find optimal mixture of alloy compositions by investigating the phase diagram of binary and ternary systems using previously defined thermodynamic database of various alloy systems.
The other study is to find optimal mixture of alloy compositions by using some models defined from some characteristics of HEAs, whose model parameters are estimated using thermodynamic calculations with previously defined thermodynamic database of various binary alloy systems.
Practically, there are some examples for the HEA designs utilizing these studies.
Globally, many researchers design the HEAs using thermodynamic calculations in their own ways. In addition, thermodynamic databases of the HEAs are being developed now.
Zhang, Chuan, Fan Zhang, Shuanglin Chen, and Weisheng Cao. 2012. “Computational Thermodynamics Aided High-Entropy Alloy Design.” JOM 64(7): 839–45.
Tapia, Antonio João Seco Ferreira, Dami Yim, Hyoung Seop Kim, and Byeong Joo Lee. 2018. “An Approach for Screening Single Phase High-Entropy Alloys Using an in-House Thermodynamic Database.” Intermetallics 101(June): 56–63.
Choi, Won-Mi et al. 2019. “A Thermodynamic Description of the Co-Cr-Fe-Ni-V System for High-Entropy Alloy Design.” Calphad 66: 101624.
Young-Kwang Kim, Ph.d.