STABILITY AND DECOMPOSITION MECHANISMS OF SUPERSATURATED SOLID-SOLUTIONS IN RAPIDLY SOLIDIFIED ALUMINUM TRANSITION-METAL ALLOYS

The annealing of supersaturated solid solutions has been studied in a number of rapidly solidified Al-transition metal alloys using transmis sion electron microscopy. Two very different decomposition process hav e been found. The first involves the generation and migration of dislo cation networks which provide shore circuity diffusion paths to the gr ain boundaries for the solute atoms, where they precipitate as the equ ilibrium phase. Typical examples are Al-Mo and Al-V. The second mechan ism involves the precipitation of metastable quasicrystals in the grai ns before the equilibrium phase is formed, also within the grains. Thi s process has been observed in Al-Cr and Al-Fe alloys. The different m echanisms are discussed in terms of the ability to generate dislocatio ns and the stability of quasicrystal precipitates in Al-transition met al alloys. Stable supersaturated solid solutions containing quasicryst als exhibit strong strengthening behaviour. The significance of these observations is discussed in terms of the development of advanced Al a lloys for high performance applications. (C) 1996 The Institute of Mat erials.