Metastable phase evolution and hardness sf 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 DO22 structure (with lattice parameters, a = -3.986 Angstrom, c = 9.006 Angstrom) The latter has been identified to have a crystal structure related to the equilibrium orthorhombic Si2Zr. The formation of both metastable phases has been attributed to the accommodation of a third element in the respective binary equilibrium structures. These manocrystalline alloys displayed high values of hardness going up to 3 6Pa. The different mechanisms responsible fur 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 rained materials. (C) 2001 Elsevier Science B.V. All rights reserved.