New investigation of phase equilibria in the system Al-Cu-Si

The phase equilibria and invariant reactions in the system Al-Cu-Si were investigated by a combination of optical microscopy, powder X-ray diffraction (XRD), differential thermal analysis (DTA) and electron probe micro analysis (EPMA). Isothermal phase equilibria were investigated within two isothermal sections. The isothermal section at 500 degrees C covers the whole ternary composition range and largely confirms the findings of previous phase diagram investigations. The isothermal section at 700 degrees C describes phase equilibria only in the complex Cu-rich part of the phase diagram. A new ternary compound tau was found in the region between (Al,Cu)-gamma(1) and (Cu,Si)-gamma and its solubility range was determined. The solubility of Al in kappa-CuSi was found to be extremely high at 700 degrees C. In contrast, no ternary solubility in the beta-phase of Cu-Al was found, although this phase is supposed to form a complete solid solution according to previous phase diagram assessments. Two isopleths, at 10 and 40 at.% Si, were investigated by means of DTA and a partial ternary reaction scheme (Scheil diagram) was constructed, based on the current work and the latest findings in the binary systems Al-Cu and Cu-Si. The current study shows that the high temperature equilibria in the Cu-rich corner are still poorly understood and additional studies in this area would be favorable. (C) 2011 Elsevier B.V. All rights reserved.