The creep fracture of copper and magnesium

Abstract

The creep life of copper and magnesium has been measured over a range of stress levels and the cavity population and resultant density changes determined at intervals during creep with the aim of assessing the build up of damage that eventually leads to fracture.For both metals, it is shown that the density change is initally approximately proportional to εtσ3 where ε is the creep strain after time t under a stress σ. This relationship holds throughout the most of the creep life for copper but, for magnesium in the later stages of creep, the density change tends to become proportional to (εtσ3)1.5. For both metals the number of cavities is roughly proportional to the creep strain and the creep life varies as σ−4. Crack propagation between cavities appears difficult and the observations show that a high fraction of the grain boundary area transverse to the applied stress is cavitated before fracture takes place.The similarity in behaviour of copper and magnesium implies that detailed crystal structure is relatively unimportant in cavitation failure, although the cavities are frequently of a crystallographic form.