Metastable phase evolution and hardness of nanocrystalline Al–Si–Zr alloys

Nanocrystalline microstructures were synthesised in the Al–Si–Zr system, by using a combination of rapid solidification processing and transition metal Zr additions. Two new metastable phases have been observed in this series. One forms during solidification, and the other, on annealing. The former is identified as a high temperature ternary ordered phase $\tau_1$, having a tetragonal $DO_{22}$ structure (with lattice parameters, a=3.986 Å, c=9.006 Å). The latter has been identified to have a crystal structure related to the equilibrium orthorhombic $Si_2Zr$. The formation of both metastable phases has been attributed to the accommodation of a third element in the respective binary equilibrium structures. These nanocrystalline alloys displayed high values of hardness going up to 3 GPa. The different mechanisms responsible for strengthening have been examined, via an analysis of their hardness. It has been concluded that, the impact due to grain refinement is the strongest, substantially supplemented by the role played by the grain boundary precipitates, in these multiphase, multicomponent, fine grained materials