Cyclic deformation and fatigue behaviour of the magnesium alloy AZ91

In this report, the cyclic deformation behaviour of the die-casting magnesium alloy AZ91 was investigated at constant total strain amplitudes between 1.4 x 10 exp -3 and 2 x 10 exp -2 at room temperature and at 130 deg C. The fatigue life data at both temperatures can be described well by the laws of Manson-Coffin and Basquin. The microstructural investigations performed show the strong influence of several microstructural features on the initiation and propagation of fatigue cracks. In order to understand the fatigue crack propagation behaviour, fatigue tests were interrupted at certain numbers of cycles in order to make replicas of the surface of the samples. It could be verified that crack propagation occurs mainly by the coalescence of smaller cracks. Furthermore, unloading tests, performed within closed cycles, were carried out in order to capture the changes of stiffness during a closed cycle with the aim to ascertain the damage evolution occurring during the fatigue tests and to determine the stresses at which the cracks open and close. The results obtained in this study form the microstructural basis of a life-prediction concept, reported earlier.