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 meta l Zr additions. Two new metastable phases have been observed in this series , one forms during solidification, and the other on annealing. The former i s 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 structur e related to the equilibrium orthorhombic Si2Zr. The formation of both meta stable phases has been attributed to the accommodation of a third element i n the respective binary equilibrium structures. These manocrystalline alloy s displayed high values of hardness going up to 3 6Pa. The different mechan isms responsible fur strengthening have been examined? via an analysis of t heir hardness. It has been concluded that, the impact due to grain refineme nt is the strongest, substantially supplemented by the role played by the g rain boundary precipitates, in these multiphase, multicomponent, fine raine d materials. (C) 2001 Elsevier Science B.V. All rights reserved.