Structural and Thermomechanical Characterization of Shape Memory Alloys

Shape memory alloys take place in a class of functional materials due to the response to the variation of temperature, and they are used as shape memory element in devices which operates again temperature changes. Shape memory effect is facilitated by martensitic transformation which is a solid state phase transformation and occurs in thermal manner in material on cooling from high temperature parent phase region. This transformation is governed by changes in the crystalline structure of the material. Thermal induced martensite occurs as twinned martensite on cooling, and this martensite is called multivariant martensite. Twinned martensitic structure proceeds through martensite variant reorientation with mechanically deformation in martensitic state. The reoriented martensites turn into ordered parent structure in crystallographic level and recover the original shape in bulk level on heating. Martensitic transformation occurs with lattice invariant shears which occur in two opposite directions, <110 > -type directions on the {110}-type plane of austenite matrix which is basal plane of martensite. Copper based alloys exhibit this property in metastable β- phase region, which has bcc-based structures at high temperature parent phase field and these structures martensiticaly turn into layered complex structures following two ordered reactions on cooling. Lattice invariant shear is not uniform in noble metal copper based shape memory alloys and gives rise to the formation of layered structures, like 3R, 9R or 18R depending on the stacking sequences on the close-packed planes of the ordered lattice.In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction patterns reveal that both alloys exhibit super lattice reflections inherited from parent phase due to the displacive character of martensitic transformation.