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Nb–Zr Interdiffusion in the (Hf,Ta,Ti,Nb)B2–(Hf,Ta,Ti,Zr)B2 High-Entropy Boride System
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DOI：10.1111/jace.70958.png)
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Abstract
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Niobium (Nb) presents a challenge for achieving homogeneous solid solutions in high-entropy boride and carbide ceramics. In this work, we synthesized (Hf,Ta,Ti,Zr)B2 (Zr-HEB) and (Hf,Ta,Ti,Nb)B2 (Nb-HEB) ceramics by boro/carbothermal reduction, followed by densification via spark plasma sintering (SPS). Diffusion couples were prepared to investigate the interdiffusion behavior of Nb and zirconium (Zr) in these complex systems at temperatures between 2000°C and 2200°C. Aside from Nb and Zr, the other elements exhibited no significant concentration gradient across the diffusion interface. Nb exhibited lower diffusion coefficients ( D ∼ $\tilde{D}$ Nb = 1.7 × 10−17–1.2 × 10−16 m2/s) compared to Zr ( D ∼ $\tilde{D}$ Zr = 1.9 × 10−17–1.7 × 10−16 m2/s) over the temperature range of 2000°C–2200°C. However, at 2200°C, their interdiffusion coefficients were comparable. The calculated activation energy for diffusion of Nb into Zr-HEB (QNb = 1008 ± 76 kJ/mol) was nearly twice that of Zr into Nb-HEB (QZr = 565 ± 54 kJ/mol). The slower diffusion of Nb is derived from the electronegativity difference between Nb and boron (B) and the bonding characteristics of NbB2 compared to the other diborides.
Keywords:
diffusion couple
interdiffusion
Kirkendall effect
niobium
zirconium
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