Researchers from the Tokyo Institute of Technology have unveiled the formation mechanisms of various phases, including intermetallic compounds (IMCs), within nickel–tungsten (Ni–W) alloys. Their insights, published in the Journal of Alloys and Compounds, offer valuable insights for enhancing the longevity and effectiveness of Ni–W alloys, crucial for military applications and beyond.
The formation of Kirkendall vacancies depends on the difference between the nickel and tungsten diffusivities. Image Credit: Minho Oh, Tokyo Institute of Technology
A vital step in the creation of new materials is alloying. Scientists can create alloys with the right properties by combining metals that have the desired characteristics. For instance, stainless steel, which is created by combining iron with smaller amounts of chromium, nickel, and other elements, has a very high corrosion resistance. The nickel-tungsten alloy (Ni–W) class of alloys is particularly interesting for military applications. These alloys are good as coatings due to their high durability.
Diffusion and interfacial reactions, as well as other processes, create distinct layers at the joining interface between Ni and W, resulting in the formation of intermetallic compounds (IMCs) and DIR regions. When compared to the remainder of the alloy, these regions show notably different mechanical, thermal, and chemical behaviors. Thus, one of the key components of creating alloys with the right properties is knowing the characteristics of these interfaces.
The researchers were led by Assistant Professor Minho Oh from the Tokyo Institute of Technology and included Professor Hee-Soo Kim, who is currently at Chosun University, South Korea.
Insights from studies of IMCs and intermediate layers formed by diffusion at the Ni/W interface have the potential to significantly improve the effectiveness and longevity of important materials in various fields.
Minho Oh, Assistant Professor, Tokyo Institute of Technology
The researchers positioned a W sheet between two Ni plates to investigate the Ni/W interface. The sample was then heated to 1123 K for 112 hours to promote diffusion, and it was then annealed for 234.15 hours at the same temperature. The researchers then used experimental methods to examine the morphology and chemical compositions of the interface. The team examined the grain sizes of the areas formed at the interface, as well as the concentrations of Ni and W in each phase of the material’s cross-section.
To explain how these interface regions formed, the researchers also created a diffusion model that considered the rates of diffusion of Ni and W in both the bulk metal and various interface regions.
According to their analysis, an IMC layer of Ni4W is produced by the interdiffusion of Ni and W and grows in both directions toward the Ni and W plates. A diffusion-induced recrystallized region (DIR) is created between the Ni matrix and the IMC layer as a result of the W atoms' continued migration into the Ni matrix. Notably, a polycrystalline structure is visible in both the DIR region and the Ni4W IMC.
The DIR region is a solid-solution area inside the Ni phase rather than a separate phase. It is distinguished by the presence of long, columnar grains that help W atoms diffuse across grain boundaries. Kirkendall voids are irregularly shaped voids that form near the interface between Ni and DIR in the DIR region due to an imbalance in the diffusion rates of Ni and W. The material’s strength and thermal properties are significantly influenced by the interfaces made up of the voids, IMC, and DIR region.
These findings not only advance our comprehension of the DIR region resulting from IMC formation and diffusion at the Ni/W interface but also offer crucial insights into the phenomenon of Kirkendall void generation and the mechanism of defect formation within the DIR region of the metal system. This integrated approach enhances our understanding of thermodynamics and kinetics in the Ni–W diffusion couple, advancing knowledge crucial for high-temperature materials science.
Minho Oh, Assistant Professor, Tokyo Institute of Technology
Journal Reference:
Oh, M. et al. (2024) Understanding Kirkendall effect in Ni (W) diffusion-induced recrystallization region. Journal of Alloys and Compounds. doi.org/10.1016/j.jallcom.2024.174556