ADIGUZEL Osman Firat University

Shape-memory alloys are functional materials and used as shape memory element in devices. Shape memory behavior is based on a solid state phase transformation called martensitic transformation which occurs in the material on cooling from high temperature parent phase field, and the material changes its internal crystalline structure with changing temperature. Martensitic transformation occurs mainly in two steps. First one is Bain distortion, and second one is lattice invariant shear. Copper based alloys exhibit this property in metastable beta- phase field which has bcc-based high symmetric structure at high temperature parent state. These structures turn into non-conventional stacking ordered structure with low symmetry following two ordered reactions on cooling from high temperatures. Lattice invariant shears occur in two opposite directions on {110}-type parent phase planes of copper based shape memory alloys, and these shears give rise the formation of unusual layered structures called as 3R, 9R or 18R martensites depending on the stacking sequences on the close-packed planes of the matrix. As a result of transformation, {110}-type close packed planes of austenite are subjected to hexagonal distortion. The hexagonal structure deviates from regular hexagon with aging the material in martensitic condition, this result is attributed to the rearrangement of atoms due to the metastable character of the alloy. This result relays on the differences atom sizes and can be a measure the degree of ordering. In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based alloys; CuZnAl and CuAlMn.

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