The generation of microactuation using narrow thermal hysteresis Ti-Ni-Cu shape-memory alloy films deposited on non-metallic substrates as the active element is studied based on a model previously developed for Ni-Ti/Si bimorphs. To this end, the compositional range in which the B2 (monoclinic) → B19 (orthorhombic) martensitic phase transformation occurs was considered, and films were deposited by magnetron sputtering on heated Si and Kapton substrates. Ultra-fine grains were observed for the 550 °C deposition temperature. The selected composition was close to Ti50Ni35Cu15, so the narrowing of the thermal hysteresis is not associated with a significant reduction in shape recovery capability. The microstructure and composition of the target materials and as-deposited films used in our experiments were characterized by X-ray diffraction and scanning electron microscopy, whereas the temperature dependence of the volume fraction of the martensite phase was derived using differential scanning calorimetry records for the target materials and from the temperature dependence of the electrical resistance data for the films. An original model was used to predict the actuation of cantilever-type bimorphs with Kapton and Si substrates.
Shape memory alloy film-based micro-actuators have their behaviour controlled by a change in the thermomechanical stress that occurs in the bimorph - shape memory alloy film plus substrate assembly. The modification of the composition of the shape memory alloy leads to a change of the transformation temperature and implicitly of the temperature at which the stress change takes place in the bimorph. The design of micro-actuator blocks in which the composition and/or the temperature control mode of each micro-actuator in the block allows to obtain successive or sequential transformation sequences. The paper analyses the case of cantilever actuator modules with films of different compositions, deposited on the same substrate. It is highlighted how the composition of the alloy film with shape memory influences the modification of the curvature of bimorph cantilever type actuators in the studied block.
The welding of shape-memory alloys is a challenge due to the fact that there are numerous compositions and because the properties are greatly influenced by the temperature. Of the multitude of shape-memory alloy systems, the most popular and widely used is the Ni-Ti system. Pulsed laser welding is a solution used for joining NiTi shape-memory alloys, having the advantage of localized heating and the possibility to adjust the welding parameters to obtain optimal properties. NiTi wires were welded using a 500W pulsed laser equipment by varying the current intensity. Thus, laser melted spots were made on a NiTi wire, changing only the value of current intensity in 20A increments in the range of 100-200A. The results analyzed by scanning electron microscopy showed, as expected, that as the current intensity increased, the spot size and the heat affected zone (HAZ) increased for each spot. Differential scanning calorimetry revealed that the martensitic transformation is still present in all samples, but with a decrease in peaks related to the phase transformation, as well as an increase of the temperature (8-12 °C), compared to the base material.
Genome engineering is a powerful tool that enhances the accelerated innovation in materials development allowing both the discovery and optimization of functionalities based on a wide range of techniques. Thin films engineering is in the forefront of this new approach by allowing the generation of a wide range of compositions in a limited number of experiments and taking advantage out of the possibility to use high-throughput characterization techniques. The paper exemplifies the innovation via compositional spreads generated by magnetron sputtering and the mapping of the material’s manufactured libraries via electric resistivity and interfacial contact resistance measurements.
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