Martensitic transformation of partially irradiated Ni-Ti films, resulting in a new technique for designing micro-actuators

The current work has been an investigation of a novel processing technique that uses ion irradiation to develop a two-way thin film micro-actuator. Conceptually, the irradiation of prestrained SMA thin film can suppress the martensitic transformation (MT) in a damage layer, whose thickness is on the order of 1/2 to 1/3 the films thickness. Large differential strains are created between the still transforming undamaged and damage layer upon heating and a reverse MT, causing the film to curl. The partial energy stored in the damaged layer by the prior heating is available to deform the martensite on subsequent cooling, which causes an uncurling and thereby creates a two-way motion during thermal cycling. TEM, thermomechanical tests, and DSC were used to study the phase transformations of irradiated microstructure. The film motion was found to be sensitive to the irradiation fluence, which was related to the percentage of martensitic transformation suppression in the damage layer. The imposed high recovery stresses, relatively small thickness of damage layer, and the presence of nanocrystalline phase in an amorphous matrix may led to the plastic deformation which limited the two-reversible motion. (orig.)