Validation of predicted precipitate compositions in Al-Si-Ge
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Validation of Predicted Precipitate Compositions in Al-Si-Ge B. DRACUP, P.E.A. TURCHI, V. RADMILOVIC, U. DAHMEN, and J.W. MORRIS, Jr. Aged alloys of Al-0.5Si-0.5Ge (at. pct) contain diamond-cubic-A4 precipitates in a dispersion that is much finer than is found in alloys with Si or Ge alone. To help understand this aging behavior, the present work was undertaken to determine alloy composition as a function of aging temperature. The composition was estimated theoretically using a CALPHAD approach, and measured experimentally with energy-dispersive spectroscopy (EDS) in a high-resolution electron microscope. Theory and experiment are in reasonable agreement. As the aging temperature rises, the precipitates become enriched in Si, changing from 50 at. pct in the low-temperature limit to about 80 at. pct Si as the temperature approaches 433 °C, the high-temperature limit of the precipitate field.
I. INTRODUCTION
PRIOR work on precipitation in Al-Si-Ge alloys has shown that the precipitate dispersion in the ternary alloy is almost an order of magnitude finer than that in the binary AlSi and Al-Ge alloys. Moreover, the precipitates are multiply twinned, which results in the elimination of the high-symmetry cube-cube interface between the precipitates and the matrix[10] and lowers the interfacial energy. As the surface energy is reduced, the interface becomes more rounded, resulting in a precipitate morphology that is much more equiaxed than that in the binary systems. The dense distribution of fine, equiaxed Si-Ge precipitates has also been shown to serve as a template for the nucleation of other matrix-hardening precipitate structures. Mitlin et al.[11,12,13] described the precipitation of AlCu-Si-Ge alloys as consisting of a secondary nucleation of precipitates off of the densely distributed Si-Ge precipitate template. These quaternary alloys demonstrated a Rockwell hardness comparable to that of many commercially available 2000 series aluminum alloys. To understand the aging behavior of these alloys, it is important to know the composition as well as the crystallography of the Si-Ge precipitates. The work reported here combined thermodynamic computations[14,15] with in situ experimental analyses to investigate this issue. [1–9]
II. MODELING AND EXPERIMENTAL PROCEDURES A. Thermodynamic Modeling
All calculations were performed with the thermodynamic application software Thermo-Calc (version N).[16] The Gibbs free energies of the unary phases were expressed in the form suggested by the Scientific Group Thermodata Europe (SGTE),[17] as follows: G i (T) a bT cT ln T d T 2 eT 3 f T 1 p
The left-hand side of Eq. [1] is the Gibbs free energy of the element i in the structure , relative to a standard element reference state with an enthalpy of the element i in its stable state at 298.15 K. The coefficients that enter Eq. [1] for the pure elements have been compiled by SGTE and are reported in Reference 18. In the following, we adopted the SGTE description of Al, Ge, and Si in
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