COMPARATIVE INTERNAL-FRICTION AND MODULUS EVOLUTIONS IN NI-TI AND NI-TI-CU SHAPE MEMORY ALLOYS

Internal friction and modulus measurements are performed for the purpose of a comparative study between the structural evolutions occurring, under the effect of the temperature, in two kinds of shape memory alloys : Ni-Ti and Ni-Ti-Cu. Modulus evolutions giving information about the changes in the relative percentages of martensitic and B2 high temperature phases, are useful for the determination of << start >> abd << finish >> transformation temperatures. Internal friction measurements, specially sensitive to structural changes occurring on atomic scale, allow the detection of atomic reorganizations preceding or following the transformation : the R-phase occurrence, on cooling or on heating, the reorientation of martensitic variants on heating. The comparison between the internal friction background levels of each alloy reveals the effect of structural instabilities, specific of Ni-Ti-Cu alloys, and which can be associated with the Cu content.