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<p>The instability of cyclic thermomechanical responses of NiTi (functional fatigue) represents one of the unsolved problems of NiTi technology. It has been intuitively understood that it originates from plastic deformation accompanying Martensitic Transformation (MT), but it is not known why and how it occurs. The mechanism by which thermomechanically loaded NiTi generates plastic strains has remained blurred for several decades despite its importance and research effort&#xa0;aimed at revealing the origin of functional fatigue. Recently, we investigated incremental plastic strains, martensite variant microstructures, martensite textures, and permanent lattice defects generated by forward and reverse MTs proceeding under tensile stress in experiments on superelastic (SE) and shape memory (SME) NiTi wires having recrystallized nanograin microstructure. In this work, based on the results of these earlier&#xa0;works, we propose the mechanism by which forward and reverse MTs proceeding under stresses above certain thresholds generate incremental plastic strains, the magnitudes of which are characteristic for stress–temperature conditions at which the MTs occurred. We claim that plastic strains are generated by [100](001) dislocation slip in (001) compound twinned martensite filling whole grains of nanocrystalline NiTi wires cooled and/or deformed at constant temperature under stress above certain stress thresholds. Dislocation slip in martensite is proposed to occur as a part of the cooperative transformation/twinning/slipping proceeding simultaneously&#xa0;within large number of grains allowing thus for strain compatibility to be achieved at grain boundaries of the nanocrystalline NiTi wire. The incremental plastic strains generated &#xa0;whenever the forward and/or reverse MTs occur above stress thresholds in cyclic thermomechanical loadings give rise to functional fatigue. It is discussed (i) how incremental plastic strains accumulating during cyclic thermomechanical loading cause functional fatigue of nanocrystalline NiTi wires, (ii) how stress–temperature diagrams updated with information on magnitudes of incremental plastic strains generated by forward and reverse MT under stress characterize functional fatigue performance of NiTi, and (iii) why SE wires show better functional fatigue performance than the SME wires. </p> Graphical Abstract <p></p>

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Generation of Plastic Strains by the Martensitic Transformations under Stress via Dislocation Slip in Martensite as the Origin of Functional Fatigue

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年7月23日

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  • P. Šittner,
  • L. Heller,
  • E. Iaparova,
  • L. Kadeřávek,
  • O. Molnárová,
  • O. Tyc

摘要

The instability of cyclic thermomechanical responses of NiTi (functional fatigue) represents one of the unsolved problems of NiTi technology. It has been intuitively understood that it originates from plastic deformation accompanying Martensitic Transformation (MT), but it is not known why and how it occurs. The mechanism by which thermomechanically loaded NiTi generates plastic strains has remained blurred for several decades despite its importance and research effort aimed at revealing the origin of functional fatigue. Recently, we investigated incremental plastic strains, martensite variant microstructures, martensite textures, and permanent lattice defects generated by forward and reverse MTs proceeding under tensile stress in experiments on superelastic (SE) and shape memory (SME) NiTi wires having recrystallized nanograin microstructure. In this work, based on the results of these earlier works, we propose the mechanism by which forward and reverse MTs proceeding under stresses above certain thresholds generate incremental plastic strains, the magnitudes of which are characteristic for stress–temperature conditions at which the MTs occurred. We claim that plastic strains are generated by [100](001) dislocation slip in (001) compound twinned martensite filling whole grains of nanocrystalline NiTi wires cooled and/or deformed at constant temperature under stress above certain stress thresholds. Dislocation slip in martensite is proposed to occur as a part of the cooperative transformation/twinning/slipping proceeding simultaneously within large number of grains allowing thus for strain compatibility to be achieved at grain boundaries of the nanocrystalline NiTi wire. The incremental plastic strains generated  whenever the forward and/or reverse MTs occur above stress thresholds in cyclic thermomechanical loadings give rise to functional fatigue. It is discussed (i) how incremental plastic strains accumulating during cyclic thermomechanical loading cause functional fatigue of nanocrystalline NiTi wires, (ii) how stress–temperature diagrams updated with information on magnitudes of incremental plastic strains generated by forward and reverse MT under stress characterize functional fatigue performance of NiTi, and (iii) why SE wires show better functional fatigue performance than the SME wires.

Graphical Abstract