Hardness of high strength nanocomposite Al–X–Zr (X=Si,Cu,Ni) alloys

Nanocrystalline microstructures with a high fraction of intermetallic phases were synthesized in a series of Al based systems, Al–X–Zr (X=Si, Cu, Ni) by using rapid solidification processing. The intermetallic phases and the nano scaled grains both contribute to the very high hardness values, up to 5 GPa, for the as melt spun ternary alloys. The ternary Zr additions were found to achieve an increase in the hardness at least two times that of the corresponding binary eutectics, and nearly five times that of coarse grained Al. Several mechanisms of strengthening tend to operate simultaneously in such multiphase fine grained materials. An estimate of the different contributions to the strength via an analysis of their hardness, reveals that the strengthening due to grain refinement is the strongest mechanism. The Hall–Petch effect dominates over the Orowan strengthening by a factor of 4. However, the grain size effect is not a classical one, and is found to be substantially modified by the role played by the grain boundary precipitates.