Structure and strength of Al, Cu-Al3Ni directionally solidified composites

Abstract

A series of Al3Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 104 °C · s/mm2 (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve the Al-Al3Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility. Strength values of the order of 530 MN/m2 were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to the high work hardening capacity of the matrix especially in the presence of θ’ phase. Massive Al3Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength.