Formation and coarsening of a nanodispersed microstructure in melt spun Al–Ni–Zr alloy

The paper reports a detailed investigation on the microstructure evolution during rapid solidification and subsequent heat treatment of an Al89Ni10Zr1 alloy and evaluates the role of Zr in the phase selection and the stability of the microstructure. It is shown that the as solidified alloy consists of grains of aluminium solid solution containing fine nickel rich clusters having an Al3Ni crystal structure. Low temperature heat treatments lead to further development of these Al3Ni particles yielding a uniform nanodispersed microstructure. The dispersed phase does not have any orientation relationship with the matrix. The composite displays very high hardness values (∼5 GPa). At higher temperatures, the super saturated solid solution precipitated a metastable Al3Zr phase, less than 5 nm in size, which was found to be very stable to coarsening even up to 425°C. The Al3Zr also precipitated inside the Al3Ni phase indicating the partition of Zr in both the phases. However, it coarsened at a higher rate inside this phase. The presence of Al3Zr was also found to be effective in resisting coarsening of the matrix grains (Al), due to the similarities in structure and lattice constants between Al and Al3Zr. To bring out the effect produced by the added Zr, a binary counterpart was studied. Zr was found to have an effect equivalent to that produced due to a higher quench.