Kockar, Benat (2007-12). Shape memory behavior of ultrafine grained NiTi and TiNiPd shape memory alloys. Doctoral Dissertation.
The cyclic instability in shape memory characteristics of NiTi-based shape memory alloys (SMAs), such as transformation temperatures, transformation and irrecoverable strains and transformation hysteresis upon thermal and mechanical cycling limits their applications requiring high number of cycles. The main reasons for these instabilities are lattice incompatibility between transforming phases and relatively low lattice resistance against dislocation motion. The objective of this study is to increase the slip resistance and thus, to minimize the plastic accommodation upon phase transformation in NiTi and TiNiPd SMAs. The effects of grain refinement down to submicron to nanorange through Equal Channel Angular Extrusion (ECAE) on the cyclic stability were investigated as potential remedies. The influence of ECAE parameters, such as processing temperature and strain path on the microstructural refinement was explored as well as the corresponding evolution in the stress differential between the yield strength of martensite and the critical stress to induce martensite and SMA characteristics of Ni49.7Ti50.3, Ti50Ni30Pd20, and Ti50.3Ni33.7Pd16 SMAs. Severe plastic deformation via ECAE at temperatures from 300?C up to 450?C refined the grains from 50?m down to a range between 0.03?m and 0.3?m in Ni49.7Ti50.3 and 0.5?m and 1?m in TiNiPd alloys. Regardless of the material, the lower the ECAE temperature and the higher the ECAE strain path, the better the cyclic stability. ECAE led to an increase in the stress differential between the yield strength of martensite and critical stress to induce martensite due to observed microstructural refinement and this increase is responsible for the improvement in the cyclic stability of the aforementioned SMA characteristics in all investigated materials. Addition of Pd to the NiTi alloy reduced the thermal hysteresis from 36?C down to 11?C, and enhanced the cyclic stability of the SMA characteristics. In additions to positive influence of ECAE on cyclic stability, it also led to an increase in the fracture stress levels of the TiNiPd alloys due to the fragmentation or dissolution of the precipitates responsible for the premature failures. ECAE caused a slight reduction in the work output; however it was possible to obtain large stable work outputs under higher stress levels than unprocessed materials.