|M.Sc Student||Abu-Salih Samy|
|Subject||Mechanical Properties of Shape Memory Alloys|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Eli Altus|
Shape memory alloys (SMA) are characterized by two unique properties: shape memory effect (SME) and superelasticity (SE). These two properties are related to the thermoelastic martensitic phase transformation of the material: from a high symmetry austenitic phase to a low symmetry martensitic phase and vice versa. This transformation occurs due to changes of temperature and/or stress. At low temperatures (below Mf) the SMA can be “plastically” deformed to a very large deformation, which can be recovered by relatively modest increase in temperature. This behavior is known as the shape memory effect. On the other hand, at high temperatures (above Af), the material exhibits large strain (up to 8%) when loaded and restoring it at unloading. This behavior is known as the superelasticity effect, despite the considerable hysteresis.
The most commercial alloy of the SMA materials is NiTi. Series of experiments on one-dimensional specimens (tubes of NiTi alloy (55.45% of Ni) with dimensions: diameter 5[mm], thickness 180[mm] and length 150[mm]) were conducted in order to examine its basic mechanical behavior. The special features of SMA were studied via two types of models: a macro model, based on the change of the free energy, which is the source of the driving force of the phase transformation. Based on the theory of Deformation Kinetics, the driving force is assumed to be exponential with the volume fractions of the phases. In contrast to other models, the difference between the elastic moduli of the phases is not neglected. The relation between the transformation stresses and temperature is found to be nonlinear. For examining the prediction capabilities of the proposed model, a comparison with two existing models is studied. The model is compared also with experimental results of other researchers and with our own set of experiments.
The second proposed model is of a microscopic type, which attempts to describe the micro-scale source of the superelasticity effect. The model is based on micro units, which consist of "atom like" elements, and provides a simple and qualitative explanation to the macro instability during the phase transformation process. The force-displacement curve of the assumed micro unit was found to be similar to the stress-strain curve of the shape memory materials, where macro instabilities are correlated with a sudden change in local geometry of the unit.