STRESS STATE DEPENDENCE OF MARTENSITE FORMATION IN Cu-BASED SHAPE MEMORY ALLOYS

P. Šittner, V. Novák, V.Studnicka, and N.Zárubová

Institute of Physics, Academy of Sciences of the Czech Republic Na Slovance 2, 180 40, Prague 8, Czech Republic

Keywords:Cu-based Shape memory Alloys, Single Crystal, Anisotropy, Martensite Structures, Thermomechanical Loads

Unique thermomechanical properties of the shape memory alloys are associated with the thermoelastic martensitic transformation in solid state and depend thus sensitively on the crystal structure of the phases involved. It is well known that thermally-induced and stress-induced martensites in Cu-based shape memory alloys do not posses usually the same crystal structure. Different martensite phases are even known to be stress-induced in a single alloy depending on the magnitude of the uniaxial tensile stress applied. This variety causes difficulties in micromechanical simulation and prediction response of SMA's in general thermomechanical loads.

In this work, TEM diffraction, DSC calorimetry, thermomechanical cyclic loading tests and X-ray diffraction profile analysis of the martensitic structures in CuAlNi and CuAlZnMn single- and polycrystals were performed to gather systematic information about the structure-property relations controlling the shape memory phenomena in these materials.

It is shown that not only the magnitude of the uniaxial stress but, very likely, the applied stress state in general (i.e. in present uniaxial loading tests - the orientation of the load axis, tension vs. compression sense of loading) affects the martensite phase formation and, consequently, the associated stress-strain-temperature behaviour of Cu-based shape memory alloys.