Effect of copper on the creep behaviour of gravity-cast zinc-aluminium-based ZA-8 alloy

Abstract

The effect of copper addition in the range from 0 to 3 % on the tensile creep behaviour of gravity cast alloy ZA-8 has been investigated at a temperature of 120 degrees C and an applied stress of 40 MPa. It was found that copper additions up to approximately 2 % had a profound effect in decreasing the amount of primary creep and secondary creep rate, and therefore substantially increased the creep performance measured as times to attain 1 % creep strain, but copper contents above 2% produced very little further creep resistance. Transmission electron microscopy work carried out to examine the microstructure of the alloys suggested that substantial improvement in the creep resistance of the alloys brought about by addition of copper was due to the presence of uniformly distributed, fine and dense dispersion of epsilon (CuZn4) phase precipitates in the eutectic eta matrix. The E phase was also present as discrete particles, much bigger in size and irregularly distributed among the eutectic cell boundaries in the alloys containing high copper content. These bigger particles probably did not have much effect in increasing the creep resistance of the alloys, and therefore increasing the copper content above 2 % produced a very little further creep resistance.