Effects of Co addition on Fe-bearing intermetallic compounds and mechanical properties of AlSi20Cu2Ni1Fe0.7-1 alloys

Cobalt is generally considered as the element that cannot only neutralize the effect of iron, but also promote the high-temperature mechanical properties of Al-Si alloys. The effects of Co on the size and morphology of Fe-bearing intermetallics have been studied in two alloys: Al-20Si-2Cu-1Ni-0.7Fe and Al-20Si-2Cu-1Ni-1.0Fe. The effect on tensile strength was also investigated. In all cases, Co is proved to be a very effective element in reducing the size and improving morphology while increasing volume fraction of Fe-bearing intermetallic compounds. The Fe-bearing compounds presented as long acicular beta-Al-5(Fe, Ni)Si phases when Co was 0% while turned into Chinese script, granular, rod-like or dendritic alpha-Al-15(Fe,Co, Ni) Si-3(2) phases when Co: Fe ratio was 1.0, 1.3, 1.5, respectively. Co is also a very effective element in promotion of tensile strength of both alloys. The maximum tensile strengths at room temperature and elevated temperature (350 degrees C) of alloys with 0.7% Fe were respectively 275 and 101 MPa, while the corresponding data were 296 and 120 MPa respectively for alloys with 1.0% Fe. Compared with the minimum tensile strengths that obtained when Co was 0%, they respectively increased by 13%, 28%, 24% and 35%. The optimum Co:Fe ratio in these two alloys studied is 1.3. (C) 2012 Elsevier B.V. All rights reserved.