Poster
Due to their superior material properties, shape memory alloys (SMAs) are promising candidates for damping and actuation applications. For robust application of SMAs, cyclically stable transformation temperatures need to be guaranteed. Dislocation activity and martensite stabilization can be accounted for a cyclic degradation of the functional properties. The basic mechanisms responsible for martensite stabilization have been intensively investigated within the last decades and it has been revealed that pinning of phase boundaries, detwinning of martensite and changes in symmetry-conforming short-range order (SC-SRO) have to be accounted for.
SMAs developed for application at elevated temperatures, i.e. HT SMAs, are still facing serious drawbacks. Established HT SMAs either suffer extremely high cost, poor workability or low transformation strain, all hindering wide spread use. The current work focuses a concept of martensite stabilization, which can also be used for development of a new group of HT SMAs. Stabilization of martensite imposed by aging under compressive and tensile load, coined SIM-aging, has been found to increase transformation temperatures by about 130 K in CoNiGa, which allows for qualification of this alloy for a high temperature actuator. Heating-cooling experiments revealed actuation strains up to 9.5 % under tensile load. Microstructure analyses revealed the dominating effect of SC-SRO and cyclic tests showed functional stability up to 30 cycles.
