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

Nonequilibrium AlSi solid solutions have been prepared by means of the rmal annealing (T similar to 1000 K) at high pressure (P similar to 4 GPa). This synthesis is possible due to the metallic character of sili con under high pressure which greatly enhances the solubility of silic on in aluminum. The study of these supersaturated AlSi solid solutions has enabled us to investigate the chemical destabilization of a cryst alline lattice. We experimentally show by means of inelastic neutron s cattering and low-temperature specific heat that, as the silicon conce ntration in the aluminum lattice is enhanced, transverse acoustic phon on modes are becoming softer. This corresponds to a dramatic decrease of the shear modulus. Using differential scanning calorimetry, the pos itive 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 aluminu m compared to its usual covalent bonding after demixing. This large he at of dissolution appears as the major reason for the observed lattice instability. Comparison of our experimental results with correspondin g effects observed in usual amorphous metals suggests that further inc rease of the silicon concentration would induce a structural transform ation of this crystal if the silicon segregation could be prevented.