Internal Friction Behaviour And Mechanism Of Phase Transformation In Cu-based Shape Memory Alloy

Thermoelastic martensitic transformation is the most important feature of shape memory alloy, which will to a considerable degree determine shape memory effect , superelasticity and high damping capacity in shape memory alloy. For internal friction technique has the special advantage in revealing changes in microscopic structures , so it becomes effective method in studying various defects and structures changes for its sensitivity to structural changes . By studying the relation between internal friction and external factors , it is helpful for us to learn regulation and mechanism of internal friction in phase transformation and realize microscopic processes of phase interface motion in transformation more intensively .During the complete martensitic transformation (MT),we have measured the spectra of internal friction versus temperature , internal friction versus frequency , internal friction versus temperature rate , internal friction versus stress amplitude under the isothermal and anisothermal condition and analyzed general features in thermoelastic martensitic transformation. Different from the linear relation in nonthermoelastic martensitic transformation, the experimental results show that the internal friction of Cu-Al-Ni-Mn-Ti shape memory alloy exhibits nonlinear relationship with the ratio of temperature rate to vibration frequency( f/f). This nonlinear relation is difficult to be explained by existed theories on internal friction in phase transformation.In order to study mechanism of nonlinear internal friction in the Cu-Al-Ni-Mn-Ti alloy. A new way (partial phase transformation method )in IF measurements was used in the present experiments to study the anisothermal spectra of internal friction versus temperature , internal friction versus frequency , internal friction versus temperature rate. The experiments exhibit that in the anisothermal measurements, an IF peak arisen at the complete reverse MT is actually composed of two independent peaks (low-temperature peak Pl ,high-temperature peak PH ), suggesting that the MT would not be a single process but associated with a process with multi-mechanisms. These two peaks correspond to the different mechanism seperately. The low-temperature peak Pl has a maximum in frequency spectra , Which is strongly dependent on time and relatesto the viscous motion of phase interface. The high-temperature peak Ph is inverse proportional to frequency, which corresponds to the variation of the phase volume transformed per unit time, and associated with the normal interface movement , It is just the complexity in phase transformation that leads to the different phenomena about either linear or nonlinear relationship.