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Thermomechanical loading paths involving a simultaneous increase of stress and decrease of temperature (i.e., out-of-phase paths) were investigated for a NiTiHf High-Temperature Shape Memory Alloy (HTSMA). Isothermal and isobaric loadings were first performed to characterize the fundamental shape memory properties and establish the stress-temperature phase diagram. Fully-transforming out-of-phase loadings were then performed for different maximum stress levels. The obtained mechanical responses exhibited significant recoverable strains, indicating reversible martensitic transformations, contrary to the mechanical responses under pure isothermal mechanical loading. The out-of-phase responses were compared to those under isobaric paths to analyze the phase-transformation characteristics and identify the role of loading paths on the transformation reversibility and the possible interactions between deformation modes. The out-of-phase paths produce strain responses similar to the ones obtained from isobaric actuation tests. However, the strain recovery can be observed from both strain-temperature and stress–strain perspectives. Since recovery can occur from a stress–strain perspective, it is denominated as ”non-isothermal superelasticity”. The transformation temperatures obtained for these paths showed similar values to the ones corresponding to isobaric loading. A general definition of the work output is proposed to capture it under varying stresses, as opposed to the classical definition under constant stress levels in isobaric actuation experiments. An analysis of the work inputs and outputs, using this new definition, revealed that out-of-phase loadings produce a lower but relatively constant work output as a function of the stress level for a significantly lower work input, enabling new possibilities for HTSMA actuators in environments with limited work input available. 

This study investigated the evolution of strain and temperature fields of a NiTiHf High-Temperature Shape Memory Alloy (HTSMA) undergoing complex thermomechanical loads involving simultaneous stress and temperature changes. In situ Digital Image Correlation (DIC) was used to track strain due to combined phase transformation and plastic yielding. Two types of loadings were performed: out-of-phase (OP) involving a simultaneous increase of stress and decrease of temperature and in-phase (IP) involving a simultaneous increase of stress and temperature. Five cycles were performed for each path. Due to a temperature gradient in the samples during the experiments, transformation nucleated in regions with the adequate temperature at a given stress level. Results showed a smooth transformation behavior for out-of-phase loadings along with a concentration of residual strain where transformation fronts meet one another. Due to the simultaneous high stress and high temperature for in-phase paths, creep was identified as a significant deformation mechanism for those paths. Furthermore, DIC allowed us to identify twinning/detwinning regions within the samples. Partial phase diagrams of the alloy were generated using the test results. Those experiments demonstrated the feasibility of characterizing HTSMAs using thermo-mechanical cycling to gain insights into the couplings between thermally- and mechanically-induced solid-state phase transformations for combined loading paths. 

The present work investigates fracture toughness, and actuation and mechanical fatigue crack growth responses of Ni50.3Ti29.7Hf20 HTSMAs across martensitic transformation with two different microstructures, one with H-phase nanoprecipitates and one without. H-phase precipitation is known to stabilize the actuation cycling response of NiTiHf HTSMAs and notably impacts transformation-induced plasticity. The fracture toughness tests performed reveal that precipitate-free NiTiHf has a higher fracture toughness and undergoes significantly more inelastic deformation than the one with the precipitates resulting in toughness enhancement, i.e., stable crack advance during fracture toughness experiments, which is not observed in the precipitated NiTiHf for the crack configuration and loading conditions tested. Furthermore, the precipitate free NiTiHf has higher actuation and mechanical fatigue crack growth resistance than the precipitation-hardened microstructure. This is attributed to plasticity buildup, which exacerbates the manifestation of retained martensite upon repeated transformations. The fatigue crack growth rates obtained from both actuation and mechanical fatigue experiments align to a single Paris Law Curve for the precipitation-hardened NiTiHf. This work aims to determine if unified Paris Law curves can be generated from mechanical and actuation fatigue experiments, irrespective of composition and microstructure, to estimate actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.