In order to explore whether the tensile actuation performance of Ni50.13 Ti29.22 Hf20.65 (at.%) high temperature shape memory alloy (HTSMA) fabricated by laser powder bed fusion (LPBF) can be further improved, three post-processing heat treatments commonly used in NiTiHf research were applied (650 degrees C/3 h; 1100 degrees C/3 h; 1100 degrees C/3 h + 650 degrees C/3 h). Microstructures, textures, and secondary phase oxide particles in LPBF-fabricated and heat-treated alloys were characterized, and functional thermomechanical properties were evaluated. It was found that post-processing heat treatments modify: (i) the austenitic grain size, shape and texture, (ii) the size, morphology, surface area fractions of fine homogeneously dispersed oxide nanoparticles created during the LPBF fabrication, (iii) the chemical composition of the matrix and oxide particles, (iv) increase the size of martensite domains, (v) the transformation temperatures, (vi) the stability of transformation response upon thermal cycling, (vii) do not affect significantly the strength but improve ductility of LPBF-fabricated alloy in isothermal tensile and compression tests, and (viii) increase actuation strain and decrease cyclic stability in tensile actuation tests. It is concluded that transformation temperatures and tensile actuation performance of LPBF-fabricated NiTiHf alloy containing finely dispersed oxide nanoparticles can be manipulated by post-processing heat treatments, which promote diffusion across oxide/matrix interfaces and affect the chemical composition of the matrix, oxide nanoparticles, and ultimately the size of domains in martensite variant microstructures. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & ( http://creativecommons.org/licenses/by/4.0/ )

In this study, the mechanical properties of a superelastic Nitinol alloy synthesized by Spark Plasma Sintering (SPS) were investigated using nanoindentation, a technique that enables the evaluation of mechanical properties at the local scale. It was shown that using a spherical indenter allows better assessment of the material’s superelastic properties compared to a Berkovich tip because of the gradual increase of the strain under the spherical indenter. The elastic recovery measured during the single and multicycle indentations was compared with the results of the cyclic tensile tests, and both methods proved to be consistent. For the indentation loads not surpassing the superelastic plateau (below 100 mN), the recovery of the SPS sample was not load sensitive. It decreased from approximately 95% in the first cycle to 80% at the 20th cycle. For higher loads the recovery decreased, pointing to the development of the plasticity. For the commercial sample, the recovery was load dependent at all loads indicating reduced superelastic capacity. It was demonstrated that the superelastic properties of the SPS sample can be superior to those of the commercial sample, proving the SPS method to be very promising for the preparation of superelastic materials. This study highlights nanoindentation as an effective and reliable method for characterizing superelastic materials, making it a valuable tool for the rapid development of superelastic alloys produced via powder metallurgy techniques.

Nitinol technology, besides utilizing the functional thermomechanical properties derived from the B2 cubic to B19' monoclinic martensitic transformation, also exploits the excellent plastic deformability of NiTi in the martensite state. It originates from the unique mechanism of plastic deformation of the B19' martensite by kwinking involving dislocation slip based kinking assisted by deformation twinning. Although the mechanism of plastic deformation of martensite by kwinking was revealed only very recently, various unusual phenomena that can only be rationalized by kwinking, have been reported in literature in the last 50 years. These phenomena include: 1) cold working with a high degree of reduction without introducing cracks, 2) excellent plastic deformability in the martensite state (plastic deformation up to 80

The Mg-Y-Zn magnesium alloy system is known for the presence of Long-Period Stacking Ordered (LPSO) phases that improves strength and ductility with minimal amounts of alloying elements. Even better improvements are associated with the specific microstructure known as the Mille-Feuille (MF) structure that can occur in this alloy as well after proper heat treatment. This study systematically compares the traditional ingot metallurgy method with the Bridgman method (slow cooling), coupled with diverse heat treatments and extrusion process. Microscopic analyses reveal variations in the presence of LPSO phases, MF structure, and especially grain size, leading to divergent mechanical and corrosion properties. The Bridgman approach surprisingly stands out, ensuring superior mechanical properties due to kink and texture strengthening.

NiTiHf high-temperature shape memory alloys (HTSMAs) fabricated via laser powder bed fusion (LPBF) suffer from poor tensile strength preventing their successful use in engineering applications. We managed to fabricate NiTiHf HTSMA showing tensile actuation under stresses up to 500 MPa in the temperature range 250-350 degrees C. A tensile strength of 821 MPa, an actuation strain 2.34 % under 300 MPa tensile stress with negligible irrecoverable plastic strain, and fracture upon cooling under 600 MPa tensile stress are the best values reported so far for LPBFfabricated NiTiHf HTSMAs in the literature. The enhanced tensile strength reaching half of the strength of the conventional cast and thermomechanically processed NiTiHf HTSMAs was attributed to the lack of micropores (porosity 0.02 %). The tensile actuation performance is claimed to be facilitated by the strengthening effect from homogeneously distributed oxide nanoparticles introduced naturally by the LPBF fabrication. The oxide nano-particles decrease the size of domains of (001) compound twinned martensite created by the forward martensitic transformation upon cooling under stress, which reduces actuation strain but also suppresses dislocation slip in martensite. This dimensional strengthening mechanism lowers actuation strain but increases cyclic stability of the actuation response of the LPBF NiTiHf HTSMA.

The origin of cyclic instability of stress-strain-temperature responses of NiTi shape memory alloy (SMA) in cyclic thermomechanical loads (functional fatigue) represents one of the unsolved problems in the SMA field. In particular, the mechanism by which incremental plastic strains and permanent lattice defects are generated during thermomechanical cycling is not known. To reveal it, we evaluated recoverable strain, plastic strain and permanent lattice defects in austenite created by forward and reverse martensitic transformations (MT) proceeding under wide range of external tensile stress by closed loop thermal cycling under constant stress and transmission electron microscopy (TEM) analysis of permanent lattice defects in austenite consisting of slip dislocations and deformation bands, variation of crystal lattice orientation and elastic strain field in grains. We propose that forward MT proceeding upon cooling under external stress generates plastic strain via slip of [100](001) dislocations that nucleate at habit plane interfaces, glide in martensite across entire grains and disappear within grain boundaries. Reverse MT upon heating under external stress generates plastic strain also via [100](001) dislocation slip but on multiple slip systems within interfacial microstructure layers created by reorientation of martensite that propagate prior habit plane interfaces transforming the oriented martensite into plastically deformed austenite. Both forward and reverse MT proceed via cooperative transformation/twinning/ slipping within large number of grains enabling shape strains in grains to remain compatible at grain boundaries. When NiTi wire is subjected to cyclic thermomechanical loading, incremental plastic strains and permanent lattice defects are generated by the proposed deformation mechanisms anytime the forward and reverse MTs occur under external stress above certain thresholds characteristic for each NiTi wire. The plastic strains and permanent lattice defects accumulate upon thermomechanical cycling which brings about functional fatigue. Assuming dislocation slip in martensite as the origin of functional fatigue, it is discussed how NiTi-based alloys can be strengthened against it.

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.

Stochastic geometry provides a powerful framework for modelling complex random structures, with applications in physics, materials science, biology, and other fields. The three-dimensional microstructure of polycrystalline materials is usually modeled by a randomly marked tessellation, where the marks correspond to crystallographic orientations. The purpose of this study is to extend the modelling approach to a finer scale, focusing on the subcells that emerge when a material specimen is exposed to mechanical loading. Specifically, the deformation twinning gives rise to nested tessellation, where the subcells are parallel twin lamellae and their complement is embedded within the original mother cells. The aim of this study is to develop a parametric mathematical model of marked nested tessellation and to realize it using stochastic simulations. We were able to deal with this model using computational tools. The sensitivity of the model to selected key parameters was investigated using statistical methods. As an application, a numerical simulation of the stress and strain fields resulting from deformation twinning is provided, and the contribution of the subcells to the total strain energy density under varying initial conditions was evaluated. This study highlights the dynamic capabilities of stochastic geometry in modelling a phenomenon that changes the microstructure.

NiTi-20Hf high-temperature shape memory alloys (HTSMAs) were fabricated using laser powder bed fusion (LPBF) additive manufacturing with low laser power and medium scanning speeds. Two alloys S1 (60 W, 120 mm/s) and S2 (70 W, 100 mm/s) were prepared using slightly different processing parameters. Both alloys displayed high strength 2 GPa, partial shape memory effect 1

The Mg-Y-Zn alloy system is well known for its outstanding combination of high strength and ductility, even at relatively low concentrations of alloying elements. This exceptional performance is primarily attributed to its characteristic microstructure, which features Long-Period Stacking Ordered (LPSO) phases and the distinctive Mille-Feuille Structure (MFS). Kink-induced strengthening, developed during thermomechanical processing, has emerged as a promising strategy to simultaneously enhance strength and ductility. In this study, the beneficial effect of pre-deformation aimed at introducing additional kinks into the microstructure prior to extrusion is demonstrated. The subsequent extrusion process promotes dynamic recrystallization (DRX), generating fine DRX grains while preserving kink structures in the non-DRX regions. As a result, the yield strength is enhanced by approximately 80 MPa, accompanied by a slight improvement in ductility.

To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 degrees C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 degrees C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress.

Abstract Deformation-transformation mechanisms in NiTi shape memory alloys (SMAs) wires subjected to tension can be investigated using various methods and techniques, often through either isothermal tensile tests or isostress thermal loading. Excluding the R-phase, there are essentially five deformation-transformation processes (elastic deformation, B2 ⇔ B19’ martensitic transformation, martensite reorientation, plastic deformation of martensite, and plastic deformation of austenite) that can be studied under both isothermal and isostress loads.

Whenever the forward and/or reverse B2 cubic to B19' monoclinic martensitic transformation in NiTi shape memory alloy wire proceeds under external stress above certain threshold, it generates incremental plastic strains which accumulate during thermomechanical cyclic loading and lead to functional fatigue preventing many promising engineering applications from realization. In this work, unique thermomechanical loading experiments were performed on NiTi shape memory wire with the aim to reveal the mechanism by which the forward martensitic transformation upon cooling under external stress generates plastic strain. Recoverable transformation strains and plastic strains generated by the forward transformation on cooling under various tensile stresses were evaluated, martensite variant microstructures in grains were reconstructed by nanoscale orientation mapping in TEM, martensite textures after cooling at room temperature were evaluated by in-situ synchrotron x-ray diffraction and permanent dislocation defects in martensite were analyzed by TEM. Based on the obtained experimental evidence, it is proposed that the forward martensitic transformation on cooling under stress proceeds via habit plane interfaces between austenite and second order laminate of (001) compound twins in martensite. Depending on the magnitude of the applied stress, the induced martensite reorients, partially detwins and deforms plastically via [100](001) dislocation slip in martensite in extent permitted by the requirement for compatible deformation of grains in nanocrystalline NiTi wire via a single deformation system. During the forward MT upon cooling under highest stresses 600 MPa, the martensite deforms via kwinking deformation enabling generation of large plastic strains 8 %.

The cubic B2 to monoclinic B19’ martensitic transformation in NiTi shape memory alloys proceeds by the nucleation and propagation of habit plane interfaces that remain undistorted and unrotated during the transformation. Due to incompatibility of austenite and martensite lattices in NiTi, habit plane interfaces are assumed to form between austenite and twinned martensite as described by Phenomenological Theory of Martensite Crystallography (PTMC). This view is currently widely spread in the literature in spite of the fact that it contradicts experimental observations of interfaces between austenite and single martensite lattice by TEM and EBSD methods frequently reported in the literature. On this account, we propose a different solution for the formation of strain compatible habit planes propagating during stress induced martensitic transformation by considering the effect of elastic deformation of both lattices caused by the external stress. Using modified PTMC theory, we evaluate the magnitudes of the uniaxial stress in tension and compression along a general austenite crystal direction, for which strain compatible habit plane interfaces can be established between austenite and single variant of martensite. The stress calculated in such a way is, however, too high. Nevertheless, considering the pre-transformation softening of austenite, the critical stress decreases to 500 MPa commonly observed in experiments. Three scenarios are simulated assuming softening of elastic constants of austenite. Orientation dependence and tension-compression asymmetry of predicted habit planes of stress induced martensitic transformation is compared with available experimental results.

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Abstract Compared to conventional engineering materials, NiTi shape memory alloys deform via a wide range of deformation mechanisms owing to the B2⇔B19’ martensitic transformation, including twinning in martensite and plastic deformation by dislocation slip. A detailed understanding of the functional properties of NiTi requires comprehensive knowledge of all deformation processes possibly activated in thermomechanical loads. A stress-temperature diagram (Fig. 1c), constructed from the results of isothermal (Fig. 1a) and isostress (Fig. 1b) tensile tests on superelastic NiTi wire (Fig. 1a,b), provides basic information on the critical stress and temperature conditions at which individual deformation/transformation processes are activated in thermomechanical loads. The σ-T diagram is a very useful tool in NiTi research since it defines stress and temperature conditions under which martensitic transformation occurs and plastic deformation is avoided. Problems arise when multiple deformation processes are activated simultaneously, and one cannot be sure which deformation mechanism is activated. In such cases, in-situ experimental methods (e.g., in- situ electric resistivity, in-situ ultrasonic methods, in-situ x-ray diffraction) are beneficially employed. In this work, we report on the application of in-situ Dynamic Mechanical Analysis (DMA) to detect and distinguish the activation of various deformation/transformation processes during the tensile thermomechanical loading of nanocrystalline NiTi wires, particularly upon isostress heating under a wide range of tensile stresses up to fracture (Fig. 1b,d).

Abstract The stress-strain-temperature thermomechanical responses of NiTi shape memory alloys due to B2-B19′ martensitic transformation (MT) should ideally be phase and strain reversible in closed-loop thermomechanical load cycles, where the austenite and martensite phases do not undergo plastic deformation. However, this ideal behavior is only observed when MT occurs under zero or very low externally applied stresses. When MT occurs under higher externally applied stresses, it generates small plastic strains. These strains accumulate whenever MT proceeds under external stress, leading to the accumulation of residual plastic strains, internal stress, and lattice defects during cyclic thermomechanical loads. This accumulation results in the instability of cyclic thermomechanical responses, a phenomenon known as “functional fatigue.”

Tensile deformation of superelastic NiTi shape memory alloy wires at temperatures above austenite finish temperature proceeds via stress-induced martensitic transformation followed by plastic deformation of oriented martensite. While superelastic deformation tends to proceed in localized manner, plastic deformation of martensite is considered to be homogeneous. In this work, we have investigated strain localization patterns in tensile tests on superelastic NiTi wires deformed until fracture in wide temperature range from 10 to 400 °C using in situ digital image correlation analysis of local strains and analyzed lattice defects created during the deformation in TEM. We have found that plastic deformation of oriented martensite can be either homogeneous or localized, depending on the yield stress and strain hardening rate (on the Considere criterion for stability of tensile deformation). Plastic deformation of martensite proceeds via peculiar deformation mode involving combination of deformation twinning and dislocation-based kinking. Strain localization takes the form of either necking leading to wire fracture at 13–15% strain or via propagation of macroscopic deformation band fronts at constant stress. Regardless the deformation is homogeneous or localized, plastic strains at fracture reach ~ 50%. Strain localized within the propagating band front as large as ~ 40% was observed in tensile tests test on NiTi wires having specific microstructures (grain size ~ 230 nm) in a narrow temperature range (~ 10–60 °C).

In this work, effects of temperature and microstructure on tensile behavior of superelastic NiTi wires were analyzed. We analyzed cold-drawn NiTi wires of 1.78 mm that received four different final heat treatments using short direct electrical current pulses. Final coarse-grained microstructures ranged from recovered up to fully recrystallized microstructure with grain size of 25 µm. Effects of temperature in the range 30–130 °C were analyzed using monotonous tensile tests where a temperature gradient was induced along the sample axis. Deformation processes related to martensitic transformation and plastic deformation were characterized by temperature resolved stress–strain responses, deformation work, released heat, volume fraction of martensite. Deformation-induced microstructure changes were analyzed post-mortem using Electron Backscatter Diffraction and x-ray diffraction. Except the least recovered microstructure, the samples did not follow linear temperature trends according to Clausius-Clapeyron. With increasing temperature, the plateau stress of localized martensitic transformation stabilized at a constant stress while volume fraction of martensite decreased down to zero. Localized strains induced by martensitic transformation and ductility evolved non-monotonously with temperature, reaching maxima at temperatures that decreased with increasing microstructure recovery or recrystallization. Large ductility was positively correlated with occurrence of austenite deformation twins in deformed microstructure.