Research progress in Zr-Ti containing high-entropymatellic glasses

Abstract

<p indent="0mm">High-entropy bulk metallic glasses (HE-BMGs) are alloys that contain five or more principal elements in equal or near-equal atomic ratios. HE-BMGs have the composition of a high-entropy alloy (HEA), structure of a bulk metallic glass, and satisfy the thermodynamic, kinetic, and structural conditions for amorphous formation. They have excellent comprehensive properties and potential as structural and functional materials. This paper summarizes the composition design, crystallization behaviors and the physical, mechanical, and thermal stability properties of Zr-Ti containing HE-BMGs. The phase formation rules for HEAs can be used to determine compositions region for bulk glass formation. However, the developed HE-BMGs have poor glass-forming ability (GFA) compared with other BMGs in the same system. In order to obtain HE-BMGs with high GFA, attempts have been made to design near-equiatomic HE-BMGs. An effective strategy for developing HE-BMGs with high GFA is to replace elements in the HE-BMGs at equal atomic ratios. This strategy has enabled the preparation of a Ti₂₀Zr₂₀Hf₂₀Be(Cu_7.5Ni_12.5) high-entropy amorphous alloy with a critical diameter of <sc>30 mm.</sc> New near-equiatomic HE-BMGs with high GFA can also be obtained by similar element substitution based on selected ternary alloys. Because of the high-entropy effect, the crystallization behaviors of HE-BMGs differ from those of other BMGs in the same/similar alloy systems. The non-isothermal crystallization of Zr-Ti containing HE-BMGs involves multistage crystallization, and the nucleation rate decreases with increasing crystallization degree. Zr-Ti containing HE-BMGs have higher activation energies than Vit-1, which means better thermodynamic stabilities. In addition to having a high-entropy effect, near equatomic Zr-Ti containing HE-BMGs are affected by a single principal element. For example, the nucleation rate of Zr₃₁Ti₂₇Be₂₆Cu₁₀Ni₆ increases gradually during the initial stage of crystallization; this is consistent with the behavior of Zr-based metallic glass. At the next stage, high entropy effect plays the major role. In other words, the effect induced by single principal elements has a stronger influence on the crystallization process of the near equiatomic Zr-Ti containing HE-BMGs. In terms of mechanical properties, a Zr-Ti containing HE-BMG has high strength, i.e., close to the theoretical prediction, high corrosion and wear resistance, and room-temperature brittleness. The room-temperature plasticity of the Zr-Ti containing HE-BMGs can be obviously improved by adjusting the composition, surface coating, cryogenic cycling treatment and cryothermal cycling treatment. Because of the high-entropy effects and sluggish diffusion effect due to the complexity of the ingredients, HE-BMGs have high creep resistance and corrosion resistance and give high thermal oxidation performances. The macroscopic deformation behaviors of the HE-BMGs show significant temperature and strain rate dependence. At the temperature of hot-embossing process, it was found that Zr-Ti containing HE-BMGs possess a relatively poor thermoplastic formability, especially under the reduced mould size to tens micrometers. Ultrasonic loading is an effective measure to conspicuously enhance the thermoplastic formability of Zr-Ti containing HE-BMGs. There are some expected goals that have not been reached in research on Zr-Ti containing HE-BMGs. At present, the alloy design strategies are mainly applicable to single-element, binary and ternary amorphous alloys. Multi-principal-element amorphous alloys have no appropriate measures for design. The compatibility of tensile ductility and high strength of HE-BMGs is still an overarching goal. New techniques such as machine learning and high throughput can be used to identify compositions with excellent properties. With the development and use of new experimental apparatus, the influence of a higher number of elements on the structures and properties of Zr-Ti HE-BMGs will able to be verified more accurately. In terms of hot-embossing process, the problem of the high viscosities of Zr-Ti HE-BMGs at their formation temperature needs to resolve by process optimization.