Thermal Stabilization and Mechanical Properties of Nanocrystalline Al and Al-Li Alloys

Abstract

Aluminum alloys in the conventional grain size state have had their share of research and are implemented in a variety of applications in our everyday life such as being the main structural materials in automobiles, aircrafts, solar panels supports and stations. Hence, it is long overdue to design light-weight aluminum alloys for the best performance in their nanostructured state. However, the strength associated with the reduced grain size of nanomaterials is associated with a penalty that leads to microstructural instabilities that result in grain growth, limiting their service temperatures and expected lifetime. Two mechanisms have been suggested to suppress grain growth: The first is a thermodynamic approach concerned with reducing the overall free energy of the system by lowering the grain boundary energy through solute segregation, and the second is kinetically by reducing grain boundary mobility. In this thesis, nanocrystalline (nc) Al and Al-2 at.% Li alloy are synthesized by mechanical cryomilling. It is proved that adding 1 at.% strontium (Sr) thermally stabilized the nc Al and its alloy up to 873 K, which represents 94% of its homologous temperature along with maintaining its exceptionally high hardness and good ductility, resulting in an alloy that has a specific strength higher than that of steel. Mechanical properties and thermal stability were studied through several characterization techniques such as XRD, TEM, micro-hardness, sheer punch tests, and SEM. It is concluded that thermal stabilization has occurred through both thermodynamic and kinetic mechanisms. Hence we anticipate that the results of this research will highly motivate the development of thermally stabilized, super-light and ultra-tough nanostructured materials for technological and structural applications.