NONEQUILIBRIUM STATE AND LATTICE INSTABILITY IN SUPERSATURATED ALUMINUM-SILICON SOLID-SOLUTIONS

Nonequilibrium AlSi solid solutions have been prepared by means of thermal annealing (T similar to 1000 K) at high pressure (P similar to 4 GPa). This synthesis is possible due to the metallic character of silicon under high pressure which greatly enhances the solubility of silicon in aluminum. The study of these supersaturated AlSi solid solutions has enabled us to investigate the chemical destabilization of a crystalline lattice. We experimentally show by means of inelastic neutron scattering and low-temperature specific heat that, as the silicon concentration in the aluminum lattice is enhanced, transverse acoustic phonon modes are becoming softer. This corresponds to a dramatic decrease of the shear modulus. Using differential scanning calorimetry, the positive heat of silicon dissolution in aluminum is measured under normal pressure: Delta H=38 kJ/mole. We interpret it as being mainly due to the energy difference between a metallic bonding of silicon in aluminum compared to its usual covalent bonding after demixing. This large heat of dissolution appears as the major reason for the observed lattice instability. Comparison of our experimental results with corresponding effects observed in usual amorphous metals suggests that further increase of the silicon concentration would induce a structural transformation of this crystal if the silicon segregation could be prevented.