Effects of Sr content on the microstructure and mechanical properties of cast Al-12Si-4Cu-2Ni-0.8Mg alloys

The influence of Sr content on the microstructure and mechanical properties of gravity cast Al-12Si-4Cu-2Ni-0.8Mg alloy is reported. From the SEM, EDS and XRD results, five kinds of intermetallic phases including Al2Cu, Al7Cu4Ni, Al3CuNi, AlSiFeNiCu-Aluminide and Al5Cu2Mg8Si6 are detected. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes at different Sr contents. After Sr additions, the morphology of the eutectic silicon changing from acicular to fibrous, the eutectic cells formed and the morphology of the AlSiFeNiCu-Aluminide changing from block-like to strip-like. When the addition of Sr increased from 0 wt.% to 0.02 wt.%, the secondary dendrite arm spacing (SDAS) reduced from 18 μm to 13 μm, and there were no noticeable changes of the volume of AlSiFeNiCu-Aluminide phase. The changes results in the ultimate tensile strength increased from 196 MPa to 249 MPa, the yield strength increased from 176 MPa to 207 MPa, the elongation increase from 0.74% to 0.95% and the quality index (Q) increased from 176 MPa to 246 MPa. When the addition of Sr increased from 0.02 wt.% to 0.04 wt.%, the SDAS increased from 13 μm to 21 μm, and the volume of AlSiFeNiCu-Aluminide phase reduced from 1.4 vol.% to 0.8 vol.%. As a result, the ultimate tensile strength reduced from 249 MPa to 226 MPa, the yield strength reduced from 207 MPa to 165 MPa, and there were no noticeable changes of the elongation and the quality index (Q) reduced from 246 MPa to 219 MPa. A typical brittle fracture mode was observed on the tensile fractograph. Plate-like particles and interdendritic regions acted as an easy path for crack propagation.