Fast cyclic solid phase transformations in shape memory alloys

Presented is a characterization of a novel phenomenon in shape-memory alloys experimentally observed in our laboratory. The observed phenomenon is a relatively fast cyclic solid phase transformation of elastically restrained SMA Nitinol wires subjected to cyclic pulses of voltage showing a maximum vibration frequency of about 13 Hz at which the amplitudes of oscillations become vanishingly small. However, our conclusion is that wider bandwidth for such vibrational solid phase transformations is possible under different types of restraining forces and heat transfer conditions. In our case free convection in air was the case. The observed meso-phases may very well be different combinations of Martensite and Austenite solid phases of shape memory alloys such as Nitinol. Our system was in the form of a parallel assembly of 0.8 mm diameter, 238 mm long Nitinol wires acquired from Dynaloy, Inc., circumscribed inside a helical compression spring with flat heads, end-capped by two parallel circular plates with embedded electrodes to which the ends of the SMA wires are secured. Thus, the wires can be electrically heated and subsequently contracted to compress the restraining helical spring back and forth. The question answered is; how fast a cyclic voltage can be applied to the system to induce cyclic Martensite-Austenite solid phase transformations. The answer appears to depend on the voltage pulse amplitude, its frequency, stress level in the wire bundle, ambient temperature, aerodynamic environment, and initial wire temperature. Regardless of the initial wire temperature, the equilibrium temperature set in the wires appears to have a value midway between the Martensitic transition temperature Ms and Austenitic transition temperature As. These vibrations appear to have potential for micro-electro- mechanical actuations and micro-robotic applications for biotechnological and medical uses. A mathematical model is also presented to simulate the electro-thermo-mechanics of such vibrational solid phase transformations. The proposed model takes into account all pertinent variables such as the strain (epsilon) , the temperature of the fibers T(t) as a function of time t, the abient temperature T0, the Martensite fraction (xi) , the helical compression spring constant k, the frictional effect and the coefficient of friction (mu) and the overall heat transfer coefficient h. Numerical simulations are then carried out and the results are compared with experimental observations.