CYCLING EFFECTS, FATIGUE AND DEGRADATION OF SHAPE MEMORY ALLOYS

The stability and lifetime of a shape memory device is characterised by the changes of its transformation temperatures. its cold shape, its hot shape, its global two way shape memory effect, its recovery stress. This global behaviour is influenced by a complex combination of internal and external parameters. Internal parameters are: the alloy system, the alloy composition, the type of transformation and the lattice structure including defects. External parameters are : the thermomechanical treatments, the way of training, the applied stress, the imposed shape memory strain, the amplitude of temperature cycling, the absolute temperature of the environment. The possible physical mechanisms which are at the origin of the limited lifetime of any shape memory element and which are, to a greater or lesser extent. controlled by the above mentioned internal and external parameters, are: the stabilisation of specific martensite variants, the creation of lattice defects during transformation-cycling (transformation plasticity or defects that could trigger unwanted variants). changes in the order of the lattice, changes in defect density and/or defect configuration as a result of ageing. This paper will present an overview of reported and new observations related to those aspects of shape memory alloys.