| Shape memory materials, which have unique shape memory effect and superelasticity, have been used in many areas. To further harness their special properties, we need to study physical mechanism of the materials. This thesis studies one source of hysteresis which plays an important role in applications. A geometrically nonlinear theory of martensitic transformations is introduced. The theory suggests that there exists minimum hysteresis in materials with special lattice parameters. A set of rules governing the relation between the hysteresis of shape memory alloys and their lattice parameters of both austenite and martensite is derived. Experiments on TiNi-based and Cu-based systems are conducted to search for alloys with special lattice parameters that are predicted by the theory to minimize hysteresis. The experimental procedures with details are described. The systems are systematically explored by the arc melting method, and lattice parameters are measured by powder X-ray method. Alloys satisfying the conditions of special lattice parameters are found in the systems. The results of this experimental study prove that the theory is a good guide to develop new shape memory alloys with low hysteresis. It potentially opens a door to improve crack resistance and fatigue life. |
