Aluminum Cast Alloys Based on the Al-Ce-Si-Mg System: An Influence of Silicon on Crystallization, Phase Composition, and Tensile Properties

An attempt at strengthening the aluminum-cerium-based alloys through additions of silicon was assessed using the experimental Al5Ce3Si0.5Mg (wt. pct) cast hypoeutectic composition, designed based on the commercial A356 (Al-7Si-0.3Mg, wt pct) grade by substituting a portion of Si with Ce. To determine a role of Si, the Al5Ce0.5Mg (wt. pct) reference was cast and tested under identical conditions. An addition of 3 wt. pct Si to the Al5Ce0.5Mg base increased the room temperature yield stress almost three times, from 47 to 135 MPa, but reduced its elongation by an order of magnitude from 8 pct to that below 1 pct. A presence of Si led also to essential changes in the alloy crystallization with the melting range widened substantially from 10 degrees C to 91 degrees C mainly due to a reduction in the solidus level. As-cast microstructure of the Al5Ce0.5Mg base consisting the primary alpha Al along with 44 pct of the alpha Al + Al11Ce3 eutectic was replaced in the Al5Ce3Si0.5Mg alloy with the alpha Al + AlCeSi2 coarse lamellae ternary eutectic, bulky compounds having the Ce-rich core of AlCe2Si with external shell of Al2CeSi2 along with the quaternary eutectic of fine alpha Al, Si, Al2MgSi/Al2Mg2Si, and Al-Si-Mg-Ce phases. The fractographic analysis revealed that additions of Si caused a transition from largely ductile fracture to the predominantly brittle mode with crack paths propagating mainly along the interface between the coarse AlCeSi2 lamellae and Al(Si, Mg) solid solution, which explains the measured drastic reduction in the alloy elongation.

Automated summary

An attempt to strengthen aluminum-cerium-based alloys through silicon additions resulted in significant increases in strength but reduced ductility. Silicon also caused changes in alloy crystallization and a shift from ductile to brittle fracture behavior.