Understanding crystal structure and morphology evolution of Fe, Mn, Cr-containing phases in Al-Si cast alloy

Abstract

As a common detrimental element in Al-Si cast alloy, Fe can be effectively removed by Fe-containing intermetallic separation in the alloy melt. However, the thermodynamic formation mechanism and growth morphology evolution of high separable Fe-containing intermetallic have not been investigated and clarified in detail until now. In this study, thermodynamic calculation, three-dimensional (3D) growth morphology and crystal structure were characterized to investigate the effect of Mn and Cr on morphology evolution, crystal structure and precipitation temperature of Fe-containing intermetallics. It is demonstrated that the addition of Mn and Cr can increase the precipitation temperatures of Fe-containing intermetallics in melt, and also result in the variation of crystal structures from monoclinic γ-AlFeSi to cubic α-AlFeMnCrSi phases. Moreover, the addition of Mn and Cr significantly changes the morphology of Fe-containing intermetallics from plate-like to cubic shapes. α-AlFeMnCrSi phase precipitated at high temperature exhibits the same structural basis as α-Al9Mn2Si2. The Fe-containing phase of cubic structure provides higher thermo stability which is in favor of its sufficient precipitation and separation.