Applications of Special Quasi-random Structures to High-Entropy Alloys

Special quasi-random structures (SQSs) are an important tool in modeling disordered alloys with atomic resolution. This chapter first presents the framework and the tools available to generate SQS for high-entropy alloys (HEAs). Examples of SQS in 4- and 5-component equiatomic alloys with face-centered cubic (FCC), hexagonal close-packed (HCP), and body-centered cubic (BCC) crystal structures, which are central to HEAs, are provided with different SQS cell sizes. Using SQS, the phase stability of known single-phase HEAs is examined, and the vibrational, electronic, and mechanical properties are predicted. Finally, the chapter compares the strength and limitations of SQS with hybrid Monte Carlo/molecular dynamics (MC/MD) simulations and coherent potential approximation (CPA) as introduced in prior chapters. First-principles calculations on selected single-phase HEAs show that the vibrational entropy of mixing is small, and the electronic entropy of mixing is truly negligible. Excellent agreement in the electronic density of states of HEAs was observed using SQS versus MC/MD. The accuracy of these SQS models is sensitive to the size of the cell, and larger cells produce more reliable results.