Thermodynamic calculation of partial phase diagram of Al-Si alloy at high pressure

• DOI: 10.1007/bf00720763
• Publication Date: 2004-11-30T15:14:32Z
• Authors: H. -C. Wu
• Keywords: Partial pressure; CALPHAD

Generally phase diagrams represent the relationship between temperature and composition under atmospheric pressure. However, the effect of pressure on the phase diagram is less well understood. In particular, metastable phases may occur at high pressures. Most often these phases are detected by experimental examination. Theoretical prediction of metastable phases appearing under conditions of high pressure is, at best, difficult. Efforts towards determination of the phase diagram of A1-Si binary alloy at normal pressure are abundant and fruitful [1]; this alloy is of interest due to its ample applications in industry. In addition, rapid quenching processes were applied to achieve supersaturation in A1-Si alloys. The silicon contents of these metastable solid solutions were increased significantly from a maximum of 1.59 at % at equilibrium conditions to 11 at % [2, 3]. Subsequently, followed by an ageing treatment, very fine silicon precipitations possibly contribute to a dispersionstrengthened alloy [2]. Furthermore, a pressure effect on the solubility of Si in A1 was studied experimentally [4]. They found that the solubility limit was extended beyond 15 at % at 5.4 MPa in comparison with 1.59 at % at atmospheric pressure. A thermodynamic analysis of high-pressure phase equilibria was also conducted [5]. Similar conclusions were drawn about the significant increases of Si solubility due to pressure. Their approach was based on the approximation of regular solutions and the independence of volume, energy and entropy on temperature, pressure and composition. The latter, which neglects thermal expansion and compressibility, may be inadequate. Also, research by the author on the high-pressure effect in solidifying an AI-Si alloy revealed the potential importance of the rapid pressure solidification process over the more conventional quenching method [6]. The pressure effects on microstructures of the test alloy were found to be significant, whereas mechanical properties such as the yield strength and hardness showed only minor differences in samples solidified under atmospheric and pressurized conditions. This may be due to the competing effects of the dendritic and eutectic phases in the resultant structure [6]. In this study a partial phase diagram of AI-Si alloy at 0.69 MPa (105 lb in -2) was predicted on the basis of the Clapeyron equation and method of resolution into simple diagrams. In general, the condition for the coexistence of