Solidification in the system Al-Ge-Si: The phase diagram, coring patterns, eutectic growth, and modification

Abstract

In the ternary system Al-Ge-Si, the binary eutectic reactions, L (liquid) ⇔ (Al) + (Si) and L ⇔ (Al) + (Ge) are connected by a monovariant valley, L ⇔ (Al) + (SiGe), falling from 578 °C at 12.7 wt pct Si to 424 °C at 53 wt pct Ge. The binary, solid eutectic surface extends from the Al corner of the ternary phase diagram (1.65 wt pct Si to 5.2 wt pct Ge) across to the continuous (SiGe) solid solution which contains very little Al: several tie triangles, L-(A1)-(SiGe), have been determined using thermal analysis and electron microprobe analysis (EMPA). Optical and scanning electron microscopy (SEM) examination reveals that coring in the normal (SiGe) phase is discontinuous, showing composition banding, which indicates that stationary 111 facets of the solid solution were exposed to the liquid for extended periods up to 500 seconds; details of this interrupted coring were quantified by EMPA. Similar, smaller, and more gradual variations could also be detected in the (Al) matrix, and these compositional fluctuations are considered to reflect discontinuities in the local eutectic growth rates. Modification and twin-ning, induced by Na, are observed in both Al-Si and Al-Ge but decrease progressively with Ge content; coring in the modified ternary alloys is more continuous.