# Interdiffusion in Zr-Mo/W Intermetallics
## 1 Introduction  
Zirconium and its alloys are extensively used in the nuclear industry, chemical industry, aerospace, and biomedical fields because of their comprehensive properties, such as small thermal neutron absorption cross-section, high radiation resistance, excellent corrosion resistance, and mechanical properties [1; 2; 3]. To enhance the high-temperature performance of Zr alloys for complex application environments, refractory elements with high melting points are often added to Zr alloys [4; 5; 6]. The composition design of the alloy requires large and accurate diffusion kinetic databases. Fundamental kinetic data such as diffusion coefficients, growth constants, and activation energies are significant references for the analysis and prediction of phase transformation, microstructure formation, and performance modulation in alloys [7]. In addition, due to the high melting point of Zr-based refractory alloys and the different shapes of the alloys used in various applications, the materials prepared by conventional melting methods have problems such as poor uniformity of microstructure and difficulty in secondary processing. To solve these problems, the technique of powder metallurgy for the preparation of refractory metals and alloys was gradually developed. Powder metallurgy is a preparation technique in which high-purity metal powder is used as raw material, and the final products are produced without secondary processing or with less processing through powder mixing, press molding, and high-temperature sintering. This technique has the advantages of short process flow and near-net forming, which can achieve low-costand high-efficiency production of Zr-based refractory alloys [8]. Diffusion is the only and irreversible form of material migration in the solid phase sintering process of powder metallurgy, where the atoms and molecules are constantly moving from one position to another due to thermal movement. The microstructure of the material is formed by the diffusion of elements. To achieve sintered densification of the Zr alloys, the crucial point is to promote interdiffusion between the elements [9; 10]. Therefore, knowledge of the diffusion behavior of elements is essential for the design of powder metallurgical processes. In the past decades, some studies have reported the interdiffusion of Zr with other refractory elements [11; 12; 13; 14; 15; 16; 17; 18; 19]. [PERSON] et al. [14] analyzed the interdiffusion behavior of Zr-Ti alloys in the \(\beta\)-phase region and obtained the activation energy of diffusion in the Zr-Ti system. [PERSON] et al. [18] prepared Zr-Nb, Zr-Hf, and Zr-Ta diffusion couples and calculated the interdiffusion coefficients of the three binary systems from 950 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C, the results showed that the variation of the interdiffusion coefficients with temperature for Zr-Nb, Zr-Hf, and Zr-Ta systems were in accordance with the Arrhenius relationship. [PERSON] et al. [19] calculated the interdiffusion coefficients of the Zr-Nb binary system at different elemental concentrations between 1000 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C, the results showed that the interdiffusion coefficients decreased with the increase of Nb content. These were all reports on diffusion kinetics in Zr-based binary solid solution systems. However, for the Zr-Mo and Zr-W binary systems, relevant reports were quite lacking, especially at high temperatures [20]. Zr-Mo/W alloys are commonly used as nuclear fuel cladding materials or reactive structural materials due to their high density, radiation resistance, high strength, and good oxidized reactivity with oxygen [5; 6; 21; 22]. The binary phase diagram indicated that the intermetallic compounds Mo\({}_{2}\)Zr and W\({}_{2}\)Zr were formed in Zr-Mo and Zr-W systems [23; 24]. The diffusion with the formation of new phases was called multiphase diffusion, which was typically characterized by the discontinuous concentration distribution in the diffused region.