Analysis of the Effect of Hydrostatic Pressure on the Nonvariant Eutectic Transformation in Al–Si, Al–Cu, and Al–Cu–Si Systems

Differential barothermal analysis (DBA) is used to analyze the effect of hydrostatic pressure of 100 MPa on characteristic temperatures of a number of eutectic alloys (wt %), such as Al–10Si, Al–12Si, Al‒22Cu, Al–33Cu, and Al–7Cu–7Si. According to DBA data, the increase in pressure resulted in an increase in the temperatures of phase transformation; in practice, the most important of them is the nonvariant eutectic transformation temperature that determines the solidus of alloys. It was found that, for the binary systems, the temperatures of the nonvariant eutectic transformation L → (Al) + Si and L → (Al) + Al₂Cu increase by 6 and 11°C (from 577 to 583°C and from 548 to 559°C, respectively); for the ternary system, the temperature of transformation L → (Al) +Al₂Cu + Si increases by 6°C (from 520 to 526°C). Theoretical analysis, performed using thermodynamic models and Thermo-Calc software, shows that the increase in the eutectic transformation temperature with increasing pressure is directly dependent on the relative decrease in the molar volume of system upon associated eutectic transformation. In this case, the excess dissolution of silicon in (Al) under high pressure can lead to an additional decrease in the molar volume of the system, whereas the increase in the copper solubility is thermodynamically unfavorable.