Study On The Mechanical Relaxation Behaviors Of Cu-based Shape Memory Alloys

Shape memory alloys have been a promising class of functional materials due to their outstanding shape memory effect, superelasticity and high damping capacity, which will find applications in smart structures, sensitive devices, high damping materials and active composite materials. Thermoelastic martensitic transformation is one of the most important features of these alloy systems and the behavior of the phase interface in the thermoelastic martensitic transformation will to a considerable degree determine their physical and mechanical properties. Therefore, the motion and variation of the phase interface during thermoelastic martensitic transformation have long been a focus for the studies in this area.It has been experimentally found that the internal friction peak measured in martensitic transformation of CuAlNiMnTi alloy consist actually of two peaks, which relate to different motion modes of the interface. The low-temperature peak corresponds to the minimum of relative dynamic modulus and is attributed to elastic modulus softening effect caused by the anelastic motion of the phase interface. The high-temperature peak corresponds to the inflexion point of relative dynamic modulus and the volume change produced by the normal motion of the phase interface is responsible for the peak. The maxima of low-temperature internal friction peaks plot a symmetrical peak-like curve at logarithmic frequency spectrum and the peak-like curve shifts toward high frequencies with the increase of heating rate, indicating that phase transition relaxation time decreases; the maxima of high-temperature internal friction peaks exhibit a linear relation to the reciprocal frequencies, and as the heating rate increases its slope rises, which implies that the heating rate drives the normal movement of phase interface further in low-frequency measurements.The alloy exhibits anomalous internal friction in reverse martensitic transformation in the case of lower frequency, more martensite plates and higher heating rate, i.e., the high-temperature internal friction peak is evolved into an "ordinary peak" and an "inverted peak", and values of the "inverted peak" can even be negative at relatively low frequencies. Based on the experimental results and theoretical analysis, it is reasonable to think that the physical mechanism on the internal friction anomalies in the reverse transformations arises from the interaction between two types of martensitic variants of positive and negative loss elastic modulus. A necessary criterion for the stability of the shape memory alloy of negative loss elastic modulus requires the absorption of heat from surroundings caused by a bigger increment of entropy.