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1. Complex Thermo-Mechanical Loading in High Temperature Shape Memory Alloys

   Cassagne; Adrien (February 2026, etd.tamu.edu)

   This research focuses on the understanding of complex thermomechanical loadings involving simultaneous stress and temperature cycling in high temperature shape memory alloys (HTSMA). Such alloys may encounter loading conditions more complex than the widely studied isobaric and isothermal conditions. Two new characterization loading paths are then investigated: out-of-phase (OP) involving a simultaneous increase in stress and decrease in temperature and in-phase (IP) involving a simultaneous increase in stress and temperature. This work may pave the way for new applications for HTSMA in various sectors, including aerospace but also oil and gas, electronics etc. Furthermore, as thermomechanical responses are strongly path dependent due to couplings (transformation, plasticity etc.), this work aims to identify pros and cons of different paths from an actuation performance perspective. All experiments and modeling are conducted with a Ti-rich Ni₄₉.₈Ti₃₀.₆Hf₁₉.₂ (at.% + traces of Zr) alloy. First, the present work explores how the transformation properties are affected by inelastic deformation created during different thermomechanical loading paths. Path dependency is studied by introducing inelastic strain through isothermal and OP compression. New transformation temperatures demonstrate a path dependency attributed to material’s ability to undergo some detwinning during loading. However, after recovery, path dependency is absent as transformation temperatures only depend on the remaining irrecoverable deformation. A phenomenological modeling of transformation temperatures shift associated to irrecoverable deformation post recovery is proposed. The second part highlights the different mechanical responses associated to 4 distinct paths: isothermal, isobaric, OP and IP. A comparison of actuation strains, residual strains and work output is discussed. A new efficiency metric for HTSMA is defined for any loading path and used to compare OP and isobaric loads. Analysis of the transformation temperatures encountered during different paths is discussed. Full-field investigation using Digital Image Correlation (DIC) of IP and OP paths is performed to identify mechanisms of deformation and propagation of martensitic transformation. The final chapter aims to use and extend a crystal plasticity model to better understand experimental observations for IP and OP paths. In order to predict accurately experimental responses, a mean field approach is developed. Two distinct scale transition rules to compute local grain stresses are implemented and compared. Good prediction capability was achieved but limitations were observed concerning the hysteresis width and low stress levels. A significant improvement of the computational performance was achieved with the mean-field method compared to regular finite element analysis. 

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2. Full-field investigation of in-phase and out-of-phase thermomechanical loading paths in high-temperature shape memory alloys

   https://doi.org/10.1016/j.jallcom.2025.183335  

   Cassagne, Adrien R; Mark, Joshua; Vedula, Ram; Benafan, Othmane; Lagoudas, Dimitris C; Graverend, Jean-Briac le (September 2025, Journal of Alloys and Compounds)

   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. 

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3. Phase-field description of fracture in NiTi single crystals

   https://doi.org/10.1016/j.cma.2023.116677  

   Kavvadias, D.; Baxevanis, Th. (February 2024, Computer Methods in Applied Mechanics and Engineering)

   A phase-field model for thermomechanically-induced fracture in NiTi at the single crystal level, i.e., fracture under loading paths that may take advantage of either of the functional properties of NiTi–superelasticity or shape memory effect–, is presented, formulated within the kinematically linear regime. The model accounts for reversible phase transformation from austenite to martensite habit plane variants and plastic deformation in the austenite phase. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interfaces and is accounted for by introducing an interaction term in the free energy derived based on the Mori–Tanaka and Kröner micromechanical assumptions and the hypothesis of martensite instantaneous growth within austenite. Based on experimental observations suggesting that NiTi fractures in a stress-controlled manner, damage is assumed to be driven by the elastic energy, i.e., phase transformation and plastic deformation are assumed to contribute in crack formation and growth indirectly through stress redistribution. The model is restricted to quasistatic mechanical loading (no latent heat effects), thermal loading sufficiently slow with respect to the time rate of heat transfer by conduction (no thermal gradients), and a temperature range below 𝑀𝑑, which is the temperature above which the austenite phase is stable, i.e., stress-induced martensitic transformation is suppressed. The numerical implementation of the model is based on an efficient scheme of viscous regularization in both phase transformation and plastic deformation, an explicit numerical integration via a tangent modulus method, and a staggered scheme for the coupling of the unknown fields. The model is shown able to capture transformation-induced toughening, i.e., stable crack advance attributed to the shielding effect of inelastic deformation left in the wake of the growing crack under nominal isothermal loading, actuation-induced fracture under a constant bias load, and crystallographic dependence on crack pattern. 

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4. Fracture toughness and fatigue crack growth resistance of precipitate-free and precipitation hardened NiTiHf shape memory alloys

   https://doi.org/10.1016/j.msea.2024.146443  

   Young, Benjamin; Orrostieta, Roberto; Haghgouyan, Behrouz; Lagoudas, Dimitris C; Baxevanis, T; Karaman, Ibrahim (May 2024, Materials Science and Engineering: A)

   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. 

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5. Simulations of multivariant Si I to Si II phase transformation in polycrystalline silicon with finite-strain scale-free phase-field approach

   https://doi.org/10.1016/j.actamat.2023.118996  

   Babaei, H; Pratoori, R; Levitas, V.I. (May 2023, Acta Materialia)

   Scale-free phase-field approach and corresponding finite element method simulations for multivariant martensitic phase transformation from cubic Si I to tetragonal Si II in a polycrystalline aggregate are presented. Important features of the model are large and very anisotropic transformation strain tensor 𝜺𝑡 = {0.1753; 0.1753; −0.447} and stress-tensor dependent athermal dissipative threshold for transformation, which produce essential challenges for computations. 3D polycrystals with stochastically oriented grains are subjected to uniaxial strain- and stress-controlled loadings under periodic boundary conditions and zero averaged lateral strains. Coupled evolution of discrete martensitic microstructure, volume fractions of martensitic variants and Si II, stress and transformation strain tensors, and texture are presented and analyzed. Macroscopic variables effectively representing multivariant transformational behavior are introduced. Macroscopic stress–strain and transformational behavior for 55 and 910 grains are close. Large transformation strains and grain boundaries lead to huge internal stresses of tens GPa, which affect microstructure evolution and macroscopic behavior. In contrast to a single crystal, the local mechanical instabilities due to phase transformation and negative local tangent modulus are stabilized at the macroscale by arresting/slowing the growth of Si II regions by the grain boundaries. This leads to increasing stress during transformation. The developed methodology can be used for studying similar phase transformations with large transformation strains and for further development by including plastic strain and strain-induced transformations. 

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