Shape memory optimization in copper-based beta - phase alloys

The behaviour of shape memory alloys is evaluated by the structural changes in microscopic scale. Shape-memory effect is based on martensitic transformation, with which the material changes its internal crystalline structure, and the ordered structure or super lattice structure is essential for the shape memory quality of the material.Copper-based ternary alloys exhibit shape memory effect in metastable beta phase region, and martensitic transformations occur in a few steps. The first one is Bain distortion and the second one is lattice invariant shears which occurs on a {110} -type plane of austenite matrix, which is basal plane of martensite. With these distortions, 9R (or 18R)-type layered structures occur in the material.In the shape memory alloys, homogenization and releasing the external effect is obtained by ageing at beta- phase field for adequate duration, crystallization and formation of the ordered structure is also obtained by the quenching process in the suitable media. The heat treating and the quenching rate are also important for the formation of homogenous ordered structures and shape memory optimization.In order to make the material satisfactorily ordered and to delay the martensite stabilization, copper-based shape memory alloys are usually treated by step-quenching after homogenization. The material atoms move cooperatively by means of a shear-like mechanism with the transformation, and structural and fundamental properties of these alloys are altered by aging in the martensitic state. Therefore, the ageing gives rise to the structural changes in both long and short-range order in material.In the present contribution two alloys (CuZnAl and CuAlMn) were selected; x-ray and TEM studies were carried out on the quenched and post-quench heat treated samples.

Keywords:

Shape memory effect , martensitic transformation , shapes memory optimization , martensite stabilization