UCLA researchers in the Department of Mechanical and Aerospace engineering have fabricated bulk, thermally stable ultrafine grained/nanocrystalline metals using conventional casting techniques.
Ultrafine grained (UFG)/nanocrystalline metals, or metals with grain sizes smaller than one micrometer, exhibit extraordinary mechanical, physical, and chemical properties. These materials have generated interest in aerospace, transportation, and other applications where superior mechanical properties are critical. However, bulk UFG/nanocrystalline metals cannot be synthesized by conventional casting methods due to slow cooling (e.g., less than 100 K/s). Fast cooling methods, such as melt spinning, have been proposed to limit grain size but are limited in the size and shape of samples it can produce. Moreover, current UFG/nanocrystalline metals are not thermally stable, and nanocrystalline metals such as Cu, Al, Zn, and Mg exhibit extensive grain growth even at ambient temperatures. Thus, there remains a need for scalable manufacturing of bulk, thermally stable UFG/nanocrystalline metals using regular casting techniques.
UCLA researchers have fabricated bulk, thermally stable UFG nanocrystalline metals through a conventional casting process. Nanoparticles are incorporated to refine metal grains down to ultrafine (even nanoscale) grain by instilling a continuous nucleation and growth control mechanism during slow solidification. As-solidified bulk UFG/nanocrystalline Cu reveals high thermal stability up to 750°C (0.75 melting point of Cu) and little grain growth even at 850°C. Furthermore, this newly revealed grain control mechanism has also been successfully applied in other materials systems such as Al-TiB2 and Zn-WC for ultrafine grains via slow cooling. Overall, this new microstructure refinement method overcomes grain refinement limitations and may be extended to other processes that involve cooling, nucleation, and phase growth.
Successfully cast UFG/nanocrystalline copper (Cu), aluminum (Al) and zinc (Zn).
Ultrafine grained (UFG)/nanocrystalline metals, microstructure, casting, nucleation and growth, solidification, grain refinement, slow cooling