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Assessing the Effects of Chemical Composition and Short-Range Ordering on the Tensile Deformation Behavior of Nanocrystalline High-Entropy Alloys Nb–Ta–Hf–Zr: A Combined Study on Molecular Dynamics and Monte Carlo Simulation
In the work, simulations using Monte Carlo (MC) and molecular dynamics (MD) methods are carried out to investigate the Nb–Ta–Hf–Zr system refractory high-entropy alloys (HEAs). In particular, the effects of chemical composition and grain boundary (GB) network on short-range ordering (SRO) during combined MC/MD modeling of relaxation mimicking diffusion process are investigated on single-crystalline and nanocrystalline (NC) HEAs. In addition, using MD modeling, the influence of SRO on the mechanical behavior of NC HEAs (NbTa)50–Hf25–Zr25, (NbTa)90–Hf5–Zr5, (NbTa)50–Hf45–Zr5, and (NbTa)50–Hf5–Zr45 subjected to the high-temperature (1000 K) uniaxial tensile loading is studied in comparison with corresponding alloys without preliminary MC/MD relaxation. It is revealed that the level of SRO of certain elements is dictated by their content in a material; the lower the number of atoms forming ordered clusters, the higher the tendency to form such clusters. Along with some nano-conglomerates formed by solely Nb, Ta, Hf, and Zr atoms, the MC/MD relaxation results in SRO of Zr with Hf and Nb with Ta in coherent nanoclusters having the B2 lattice. The presence of a dense GB network can significantly affect the SRO process; Zr, Nb, and Hf segregate to GBs, but they are depleted of Ta atoms. Such distribution of atoms in NC samples is mainly dictated by the difference in atomic sizes and chemical affinity between constituent elements. Results of MD modeling of tensile deformation for the relaxed and non-relaxed NC HEAs show that SRO strengthens the alloys, increasing the yield strength, especially in equiatomic alloy and in (NbTa)50–Hf45–Zr5. This is explained by both the formation of GB segregations and the enrichment of grains by Ta atoms, enhancing the body-centered cubic structure stability and, therefore, inhibiting preliminary phase transition and dislocation nucleation. Meanwhile, coherent B2 particles of NbTa can lead to additional strengthening of the HEAs through the load transfer mechanism. The work sheds light on the high-temperature mechanical behavior of Nb–Ti–Hf–Zr-based HEAs and can be used as a guide in the development of advanced HEAs for high-performance applications.