Precipitation in an Al-1.78 wt % Hf alloy after rapid solidification

A metallographic study has been made of an Al-1.78 wt % Hf alloy after rapid solidification, and after high temperature annealing of the as-cast alloy. After solidification, Hf stays in supersaturated solid solution with a rather inhomogeneous distribution. On annealing, spheres and dendrites of an intermediate Al3Hf phase with LI(2) structure, and perfectly coherent with the matrix, are formed. The spheres form by a continuous, the dendrite by a discontinuous precipitation reaction. After continued annealing the LI(2) structure is replaced by a laminated structure of LI(2) and D0(22), where D0(22) is the equilibrium structure of Al3Hf. The precipitation reactions in the systems Al-Hf and Al-Zr are compared.