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 (transfornation plasticity or defects that could triger 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. The specific functional properties of shape memory alloys, necessitate an extension of the usual definition of fatigue. Three different types of fatigue have to be considered in the case of shape memory alloys. 1. Failure by fracture due to stress or strain cycling at constant temperature. Three different situations are possible : the material is martensitic during cycling; the material remains beta during cycling: the martensite is stress induced during cycling above the Ar temperature. This type of fatigue has so far received the most attention in the literature / 1-20/. 2. Changes in physical, mechanical and functional properties such as the transformation temperatures, the transformation hysteresis, two way memory effect, . due to pure thermal cycling through the transformation /21-28/. 3. The degradation of the shape memory effect due to stress, strain or temperature cycling in or through the transformation region /30-37/. The three main parameters to be considered in the study of the global lifetime of a shape memory alloy are thus temperature, stress and the macroscopic shape strain during deformation. It might be Article published online by EDP Sciences and available at http://dx.doi. org/10.1051/jp4:1991429 C4-190 JOURNAL DE PHYSIQUE IV important for the latter to distinguish between the transformation induced plasticity. this is the shape strain under load during the forward transformation and the spontaneous shape strain due to the one or two way memory effect. The origin of the global lifetime or fatigue behaviour of shape memory alloys is due to accumulation of defects and structural changes like the change in order of the beta phase or martensite or the formation of other types of martensite. All types of changes will have an influence on the transformation temperatures, the transformation hysteresis, the reproducibility of the one and two way memory effect, and the amount of cycles before fracture. Generally, the main concern in the lifetime of a shape memory alloy is the stability of the required shapes at different temperatures or the stability of recovery stress or work output. Therefore information on the temperature and stress profile including the amount of cycles during the required lifetime is a prerequisite to make the proper choices in alloy composition, thermomechanical treatment and design. This paper will present some important results related to classic fatigue, thermal cycling and degradation of the shape memory effect.