The substructural features of nanocrystalline materials such as grain boundaries, junction lines, and nodes can significantly dictate their properties. These materials are processed through grain refinery. Currently, bottom-up and top-down processes are the main grain refinement process used. Even though the bottom-up process is quite expensive, it cannot produce bulk samples due to low buildup rate and the small sample dimension. On the other hand, top-down is a cost-effective process especially for the production of nanocrystalline materials in large quantities. However, considering the increasing demand for advanced nanocrystalline materials, development of efficient top-down technologies for high-scale production of ultrafine-grained materials is highly desirable.
Recent studies have shown that understanding the structural decomposition mechanisms will significantly contribute to the development of advanced top-down processing techniques. As such, several severe plastic deformation processes have been developed. Among the available methods, cryomilling has attracted significant attention of researchers owing to its large-scale production capabilities. Unfortunately, grain growth occurring beyond the unstable nanocrystalline grains due to high consolidation temperatures may negatively affect the materials’’ properties. Presently, various measures have been taken to enhance the microstructure stability at elevated temperatures. For example, in Al and its alloys, nanoscale particles have been dispersed within the grain boundaries during the cryomilling process to enhance their thermal stability. Alternatively, the addition of diamantane has also proved effective for enhancing thermal stability. However, the high-temperature performance of nanocrystalline materials stabilized by the addition of diamantane has not been clarified.
To this note, researchers at University of California Irvine: Dr. Walid Hanna, Khinlay Maung, M. Enayati, Professor James C. Earthman and Professor Farghalli Mohamed assessed the feasibility of enhancing the stability of nanocrystalline aluminum powders at elevated temperature through the addition of diamantane during cryomilling process. The stability performance of Al 5083 powders cryomilled with 0.5wt% diamantane and nanocrystalline Al-5083 alloy without diamantane, produced through mechanical milling in liquid nitrogen medium, at elevated temperature were measured and compared to each other. The work is currently published in the journal, Materials Science and Engineering A.
Al 5083 alloy cryomilled with diamantane exhibited relatively greater thermal stability at elevated temperature as compared to that without diamantane. It was necessary to estimate the grain sizes and lattice strains of both the Al grades annealed at various temperatures. Interestingly, a similar trend was observed in both the grades but at elevated temperatures above 673K, a discrepancy that increased with increase in the temperature was observed. This was due to the difficulties in measuring grain seizes below 100nm.
In summary, University of California Irvine scientists confirmed that addition of diamantane during cryomilling stabilized the grain size beyond that witnessed in Al-5083 grade cryomilled without diamantane. This was attributed to the small diameter of the diamantane as compared to the natural dispersoids as well as the particle spacing less than the diameter of the grains. To actualize their study, they used Zener’s particle pinning concept to explain the results taking into consideration the nanostructure, strength and hardness especially when the powder is used in surface coating. The study will enhance the properties of different nanomaterials.
Hanna, W., Maung, K., Enayati, M., Earthman, J., & Mohamed, F. (2019). Grain size stability in a cryomilled nanocrystalline Al alloy powders containing diamantane nanoparticles. Materials Science and Engineering: A, 746, 290-299.Go To Materials Science and Engineering:A