Cyclic Degradation Mechanisms In Iron-Based Shape Memory Alloys

Iron-based shape memory alloys (Fe-based SMAs) attract a lot of attention due to their lower processing costs and better workability in comparison to traditional SMAs such as Ni-Ti. Furthermore Fe-based SMAs exhibit high theoretical transformation strains. This makes them very attractive for superelastic and damping applications. Superelastic strains of up to 13 % have been experimentally demonstrated for textured polycrystalline FeNiCoAlTa SMAs in single cycle tests. At the same time superelastic strains reported for single crystalline specimen are significantly lower.

Considering actual applications, cyclic stability is crucial, but still, data reporting on the mechanisms which govern functional degradation are extremely rare in open literature, and thus, reliable conclusions on the role of microstructural features are not possible, yet.

Consequently, this study focused on the functional degradation of [001]-oriented FeNiCoAlTa shape memory single crystals. In-Situ observations were conducted in order to evaluate the local strain evolution and interacting martensite variants on the samples surface up to 100 cycles at room temperature. Superelastic cycling up to 4.5 % strain resulted in an intensive strain accumulation caused by interacting martensite variants. Microstructure evolution has been thoroughly analyzed by high resolution electron microscopy. Precipitates introduced by heat treatment play a key role regarding cyclic stability. Clearly, finer particles following shorter aging times lead to a significant improvement, and the minimization of irreversible strains during cyclic loading will be demonstrated.