Solidification behavior of Al particles embedded in a Zr aluminide matrix

A hyperperitectic Al-50 wt% Zr alloy has been manufactured by melt spinning, and the resulting microstructure has been examined by transmission electron microscopy. As-melt spun and annealed hyperperitectic Al-50 wt% Zr consist of a Zr aluminide matrix and an Al rich phase distributed in the form of small and large particles with sizes ∼15 and ∼100 nm, and as an irregular layer at the cell and grain boundaries. Diffraction analysis of the Zr aluminide matrix is consistent with the aluminide having a tetragonal unit cell with a = 4.014 angstrom and c = 17.32 angstrom, similar to equilibrium D023 tetragonal ZrAl3 but with a different stoichiometry and different atomic ordering on alternate (004) planes. The Al rich particles show a (001)Al parallel(001)Matrix; [100]Al parallel[100]Matrix orientation relationship with the Zr aluminide matrix. The solidification nucleation kinetics of the Al rich particles have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Solidification of Al rich particles is nucleated catalytically on the Zr aluminide matrix at an undercooling in the range 0--5 K. Analysis of the solidification nucleation kinetics of the Al rich particles supports the hypothesis that the classical spherical cap model of heterogeneous nucleation breaks down at low undercoolings and small contact angles