Characterization and Optimization of the Two-Way Effect in Melt-spun NiTi-based Shape Memory Alloys
Research output: Thesis › Doctoral Thesis
Shape memory alloys have been successfully introduced into a variety of technical areas over the past few years. A very promising field for their application in the near future is the microsensor and microactuator technology, since with a shape memory element a pre-determined response can be obtained very easily by thermal or electric stimulus. For long-term applications, however, it is very important to investigate and optimize the stability of the shape memory effect especially with respect to size and transformation temperatures, since the switching-temperatures should stay constant during the life-time of a shape memory element. From the several modes of using the shape memory effect, the intrinsic two-way shape memory effect (TWSME) is the most suitable to apply in actuators since no resetting force has to be considered in design. In order to get small-dimensioned shape memory alloys (SMAs) with good functional and mechanical properties, a rapid solidification technique was employed. NiTi-based alloy samples have been fabricated by melt-spinning and splat-cooling. The application of rapid solidification can change the microstructure drastically, improve the ductility and shape memory characteristics, and lead to small-dimensioned samples. As an initial step, different parameters of melt-spinning, such as ejection temperature and pressure, wheel speeds and various crucible materials, were investigated in order to obtain ductile ribbons showing shape memory effect. Cooling rates during melt-spinning are directly proportional to the wheel speed and inversely proportional to the square of the ribbon thickness. The influence of different solidification rates and crucibles on microstructure, properties and transformation temperatures was studied and compared with results of splat-cooled disks. The second step was to investigate the influence of copper (5-25 at.% Cu) and tungsten (2 at.% W) on the microstructure, and the functional and mechanical behavior of NiTi thin ribbons. All samples show a shape memory effect immediately after processing without further heat treatment. The stress-strain, strain-temperature, stress-temperature and cyclic properties of various ribbons were obtained, giving a better understanding of the behavior of SMA under different test conditions. The third step was aimed to study the influence of different thermomechanical training methods on the two-way shape memory effect of ribbons (magnitude and stability) and to examine its correlation with a stress-assisted two-way memory effect (SATWME), which is of particular interest for applications. The results displayed that the different training methods used in this work were effective in developing a useable two-way shape memory effect. Finally, when dealing with the weak intrinsic two-way shape memory effect, it is essential to elaborate the stability behavior thoroughly and to know how changes within the substructure affect the magnitude of the TWSME. Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the TWSME, substructures, mechanical properties and actuating temperatures. It was found that a good stability of the TWSME can be achieved by proper training process. Therefore, the trained material has the potential for interesting applications, e.g. as microsensors and microactuators.
|Translated title of the contribution||Charakterisierung und Optimierung der Zweiweg-Effekt in schmelzgesponnenen NiTi-Basis Formgedächtnislegierungen|
|Publication status||Published - 2011|