Abstract Iron-based shape memory alloys (SMAs) are promising alternatives to conventional SMA candidates such as Ni–Ti regarding cost-efficiency. The present study investigates cold-rolled Fe–Ni–Co–Al–Ti–B samples subjected to different recrystallization heat treatments. The impact on microstructure and thermomechanical behavior is discussed in detail. Two distinct recrystallization heat treatment (HT) procedures were conducted. After both HT processes, the samples were aged at 600 ℃ for 4 h to precipitate the γ'-phase, which is crucial for the thermomechanical behavior of the material. The cold-rolled material exhibits β-phase, which fully dissolves during a solution annealing above 1200 ℃. In addition, a Goss-type texture has formed with the main component in {hkl} < 100 > γ. During the HT process, both recrystallization and grain growth occurred. Thermomechanical experiments demonstrate reversible shape memory behavior and transformation strains up to 5 % under external load during heating and cooling experiments.

Abstract This study investigates the bending fatigue performance of Vacuum Arc Remelted/Electron Beam Refined (VAR/EBR) Nitinol for cardiovascular applications. Diamond-shaped fatigue specimens were manufactured from ultra-clean VAR/EBR Nitinol tubing with inclusion sizes below 10 μm and tested under physiologically relevant conditions to 100 million cycles. Testing included multiple combinations of mean strains (0–7%) and strain amplitudes (0.75–2.50%) to simulate in vivo conditions for cardiovascular devices. Results demonstrate that VAR/EBR Nitinol exhibits as high as 275% improvement in 10 8 -cycle Fatigue Strain Limit (FSL) compared to conventional VAR Nitinol at mean strains between 3 and 5%. The FSL behavior shows three distinct strain regimes: increasing FSL with mean strains from 0 to 3%, plateau between 3 and 5%, and decreasing FSL beyond 5%. Analysis confirms the critical impact of limiting inclusion size below the theoretical critical small crack length of 10 μm, where fatigue performance becomes dictated by cyclic stress/strain rather than flaw size. Advanced (S)TEM imaging of the fatigue specimens provides evidence that advanced thermomechanical processing leads to stable microstructures to enhance fatigue response up to 10 8 fatigue cycles.

Abstract Iron-based shape memory alloys (Fe-SMAs) are emerging as smart, high-performance materials with transformative potential for structural applications in civil engineering. Leveraging their unique thermomechanical properties (shape memory effect and superelasticity) Fe-SMAs enable innovative solutions for prestressing, strengthening, and retrofitting structural components. This review presents a comprehensive overview of recent advances in Fe-SMA development and their integration into strengthening systems for concrete and steel structures, particularly within the field of construction. Various installation methods are examined, including adhesive bonding, mechanical clamping, bolting, and nailing, highlighting their practicality and effectiveness across construction applications. Special attention is given to the fatigue performance of Fe-SMAs, particularly transformation-induced stress relaxation and its thermal reactivation to restore lost prestress. The review covers applications in concrete, steel, composite, and glass structures, supported by real-world implementations in buildings and bridges for retrofitting, strengthening, and fatigue crack repair. Emerging uses such as Fe-SMA couplers and their role in 4D printing for adaptive infrastructure are also discussed. In addition to summarizing current knowledge, the review identifies key research gaps and outlines directions for future study—including long-term durability, prestress retention, environmental performance, and corrosion resistance. This work aims to support both academic research and industrial adoption of Fe-SMAs as next-generation materials for smart, resilient, and sustainable construction.

Abstract 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

Abstract The Ni concentration and temperature dependence of the lattice parameters of the B2 and B19′ phases in binary Ti–Ni alloys was systematically clarified near the martensitic transformation temperatures, using samples with finely varied Ni concentrations ranging from 50.00 to 51.00 at.%. The differences between the present data and previously reported values were quantitatively explained and reinterpreted in terms of uncertainties in Ni concentration, measurement conditions, thermal history, and extrapolation methods. Furthermore, based on the lattice parameters obtained in this study, the effect of Ni concentration on the suppression of transformation-induced dislocations and the stabilization of thermal cycling behavior was discussed in terms of geometrical incompatibility. From the perspectives of composition increment, temperature steps, consistency in thermal history, and transparency of extrapolation procedure, the present dataset is considered to be among the most accurate, precise, and consistent lattice parameter datasets currently available. This dataset provides a highly reliable basis for evaluating the variation in lattice parameters among literature and serves as a foundation for future investigations into phase transformation behavior in Ti–Ni alloys.

Abstract The present work takes an overarching look on how particles can alter the functional and structural properties of shape memory alloys (SMAs). We consider precipitates which form in binary and ternary NiTi-based alloys, including the high-temperature SMAs NiTiHf and NiTiZr. We also take a look at precipitates which form in Cu–Al–Zn, Cu–Al–Ni, and Co–Ni–Ga–SMAs. We take these alloy systems as examples to review different effects of particles on local alloy chemistry and local stress and strain states at particle/matrix interfaces. Their influence on the nucleation and growth of martensite and on the propagation of a martensitic transformation front is discussed, and the consequences for macroscopic functional and structural properties like phase transformation temperatures and widths of thermal hysteresis are highlighted. Emphasis is also placed on particles which form because elements like C and O are picked up during alloy production and affect functional and structural properties. Finally, it is suggested to take a look at additive manufactured SMAs with added inert nano/micro-particles, which would facilitate the investigation of the mechanical constraints, and which may lead to SMAs with improved structural and functional properties.