This paper reports an anisotropic superelasticity and two-way shape memory effect of NiTi fabricated by selective laser melting (SLM). It was found that SLM-fabricated NiTi exhibited a compressive superelasticity of 6.1

Triply periodic minimal surface (TPMS) metamaterials possess exceptional properties not commonly found in natural materials. TPMS metamaterials are used in lightweight structures and impact energy absorption structures due to their surface geometry and mechanical properties. The quasi-static mechanic properties of resin-based homogeneous and gradient TPMS structures manufactured by stereolithography are investigated in this study. The results of both experimental and numerical simulations reveal that the gradient TPMS structures have superior energy absorption abilities compared to the homogeneous TPMS structures. Furthermore, the benefits of gradient TPMS structures can be further enhanced by changing the gradient variation interval of the relative density and cell thickness of TPMS. If the slope and intercept of the C value function of the TPMS structures remain constant, selecting a design where the gradient direction of the cell aligns with the direction of the load on the material can enhance the energy absorption capability of the TPMS structures.

Transformation from a body -centered cubic (BCC) to a face -centered cubic (FCC) phase has garnered substantial attention in materials design due to its capacity for adjustable properties. In this study, we investigated the magnetic behavior of stoichiometric Fe2MnGa during the BCC -FCC transformation. Our findings unveil that the Fe2MnGa alloy undergoes a gradual transformation from BCC to FCC phase. During this transformation, the proportion of FCC phase gradually increases, resulting in a dual -phase structure, until it ultimately reaches a pure FCC phase. Furthermore, as the cooling processes, a magnetic transition from ferromagnetic to weak magnetic state was observed within the FCC phase. Following this magnetic transition, the sample exhibits an exchange bias effect, indicating a mixed ferromagnetic/antiferromagnetic state rather than a pure antiferromagnetic state. The magnitude of the exchange bias field is directly proportional to the proportion of the FCC phase, suggesting a negligible contribution of the BCC phase to the exchange bias effect. The controllable dual -phase nature of FeMn-Ga alloys holds significant potential for material design.

The Triply Periodic Minimal Surface (TPMS) structure is regarded as a porous structure used for impact protection due to its lightweight, high strength, and high specific energy absorption (SEA). This paper investigates the dynamic mechanical properties of the TPMS structure by Split Hopkinson Bar (SHPB) experiments and finite element analysis (FEA). We compare stress peak, SEA, strain rate effect, and deformation pattern for functionally graded Primitive (P), uniform P, and uniform Gyroid (G) structures. It is found that the increasing magnitude of peak stress in the TPMS structures becomes more evident under high-impact loads. As the relative density increases, the strain rate effect becomes more prominent. The uniform G structures exhibit higher plateau stress and SEA than uniform P structures. Besides, graded P structures exhibit a climbing phenomenon compared to uniform P structures in the platform section. The FEA results also demonstrate that higher SEA and plateau stresses are exhibited in the graded P structure. The graded design minimizes the strain rate effect compared to the corresponding uniform structures. It is essential to set a properly graded index to enhance the dynamic energy absorption capacity of the TPMS structure, as more fluctuations occur when the graded index increases.

This study investigates the crystal structural, magnetic properties, and phase transformation of melt-spun Fe -MnGa alloys. We found that melt-spun Fe 50 Mn 25 Ga 25 alloy crystallizes into a gamma phase, which exhibits ferromagnetism at room temperature, and undergoes a magnetic transition at 257 K. On the other hand, melt-spun Fe 49 Mn 24 Ga 27 , Fe 43 Mn 28 Ga 29 and Fe 44 Mn 28 Ga 28 alloys exhibit a single body-centered cubic phase, with the latter two alloys undergo martensitic transformation during cooling. Additionally, a magnetic-field-induced martensitic transformation was observed within the temperature range of 180 to 160 K. Notably, the martensitic transformation temperatures of these two melt-spun Fe -Mn -Ga alloys are significantly lower compared to other fabrication methods, potentially attributed to the atomic disorder, microstructure and internal stresses that arise during the melt-spun fabrication process. These findings expand the available preparation methods for FeMn -Ga alloys and offer new possibilities for future applications.

The prestressing applications of shape memory alloys (SMAs) employs the recovery stress of a constrained SMA element that originates from its attempt to revert a pre-deformation to original shape upon heating. As the most readily available SMAs, NiTi alloys have shown poor performance of recovery stress compared to NiTiNb and FeMnSiCrNi due to their relatively small transformation hysteresis. This work investigated the effects of heat treatment and prestraining on the martensitic phase transformation characteristics and mechanical properties. It is found that suitably selected heat treatment and prestraining treatment can effectively tune the transformation hysteresis to greater than 100 °C while retaining reasonably high strength, which greatly elevate the recovery stress of NiTi–SMAs. A large recovery stresses of approximately 300 MPa with a wide temperature window of 180 °C of stable performance can be achieved via a simple one-step heat treatment and prestraining treatment using commercial superelastic NiTi wire products.

The structure of the all-d-metal alloy Ni 50– x Co x Mn 25 V 25 ( x = 0–50) is investigated by using theoretical and experimental methods. The first-principles calculations indicate that the most stable structure of the Ni 2 MnV alloy is face-centered cubic (fcc) type structure with ferrimagnetic state and the equilibrium lattice constant is 3.60 Å, which is in agreement with the experimental result. It is remarkable that replacing partial Ni with Co can turn the alloy from the fcc structure to the B2-type Heusler structure as Co content x > 37 by using the melting spinning method, implying that the d–d hybridization between Co/Mn elements and low-valent elements V stabilizes the Heusler structure. The Curie temperature T C of all-d-metal Heuser alloy Ni 50– x Co x Mn 25 V 25 ( x > 37) increases almost linearly with the increase of Co due to that the interaction of Co–Mn is stronger than that of Ni–Mn. A magnetic transition from ferromagnetic state to weak magnetic state accompanying with grinding stress induced transformation from B2 to the dual-phase of B2 and fcc has been observed in these all-d-metal Heusler alloys. This phase transformation and magnetic change provide a guide to overcome the brittleness and make the all-d-metal Heusler alloy interesting in stress and magnetic driving structural transition.

NiTiNb shape memory alloy has attracted much attention for its use in prestressing and remedying structural steel and concrete members due to its unique shape memory effect. However, the recovery stress values reported in the literature vary significantly, as the interplay between recovery stress and treatment parameters is not fully understood. In this study, we carefully investigated the effects of annealing and prestraining on the martensitic transformation behavior and mechanical properties of NiTiNb alloy and clarified their correlations to the recovery stress. Our results showed that the yield strength and superelasticity decreased while the ductility and retained stress-induced martensite increased with increasing annealing temperature. We found that high recovery stress can be achieved by annealing the sample above the recrystallization temperature while keeping the annealing temperature as low as possible to maintain high strength. Prestraining the sample to beyond the stress plateau but before yielding was found to be most effective for enhancing the recovery stress. By using these criteria, we were able to obtain large recovery stresses of greater than 550 MPa from 20°C to 200°C through a simple one-step annealing at 700°C and a 10% prestrain using commercial superelastic NiTiNb wire products.

Inclined fatigue cracks of U-ribs and diaphragms in orthotropic steel-box girders are challenging to be repaired using prestressing techniques. Integrating shape memory alloys (SMAs) and carbon fibre reinforced polymers (CFRPs) can achieve a hybrid self-prestressing strengthening approach for such fatigue cracks. To examine the strengthening effect of this approach on inclined cracks, 39 specimens were prepared in four groups and tested under static and fatigue tensioning in this study. Both static and fatigue test results showed that SMA/CFRP composite strengthening effectively retard steel cracking. Compared to control specimens, SMA/CFRP composite strengthening enhanced the notch tip yield load by 128% and 111% for specimens with a notch inclination of 0° and 30°, and the corresponding fatigue life by 7.62 and 8.69 times, respectively. Besides, the strengthening effect of combining SMA and CFRP (SMA/CFRP composite) on cracked steel plates was greater than that of either SMA or CFRP alone, particularly for the steel plates with an inclined crack. Finally, the calculation of mixed-mode I/II effective stress intensity factors confirmed the superior strengthening effect of SMA/CFRP composite on steel plates with an inclined crack. The findings of this study can provide a reference for the reinforcement of orthotropic steel-box girders.

NiTi shape memory alloys with quasilinear and slim pseudoelasticity are desirable for high fatigue life and easy controllability. This paper reports that the selective laser melting (SLM) technique can encourage formation of quasilinear pseudoplasticity with small hysteresis of NiTi. The NiTi parts are found to present a high density of dislocations in the as-fabricated state. Deformation cycling of the defect-containing SLM NiTi parts promotes the formation of a unique microstructure consisting of nanocrystalline and amorphous-like phases. Such microstructure leads to a continuous and quasilinear type pseudoelasticity with small hysteresis. It is found that the scanning speed of laser affects the dislocation density and transformation behaviour, thus influencing the pseudoelastic behaviour of the SLM-fabricated NiTi. The transformation temperatures are shown to decrease while the transformation intervals to increase with increasing the scanning speed due to the increased dislocation density. The mechanically cycled SLM fabricated NiTi with a scanning speed of 850 mm/s exhibits a stable quasilinear pseudoelasticity of 5.8% and a small energy dissipation of 1.4 MJ/m3.(c) 2022 Elsevier B.V. All rights reserved.

Fully compensated ferrimagnets do not create any magnetic stray field and allow for a completely polarized current of charges. As a result, these alloys show promising prospects for applications as spintronic devices. In this paper, we investigated the phase stability, the site preference, the tetragonal distortion and the influence of symmetry from the crystal structure and chemical environments of magnetic ions on the magnetic properties of Cr2YZ and Mn2YZ (Y = void, Ni, Cu, and Zn; Z = Ga, Ge, and As) full Heusler alloys by first-principles calculations. We found that the selected Cr2-based alloys, except for Cr2NiGa and Cr2NiGe, prefer to crystallize in the centrosymmetric L21-type structure, while the selected Mn2-based alloys, except for Mn2CuAs, Mn2ZnGe and Mn2ZnAs, tend to crystallize in the non-centrosymmetric XA-type structure. Due to the symmetry, the antiferromagnetism of the selected L21-type alloys is very stable, and no spin-polarized density of states could be generated. In contrast, the magnetic moment of the selected XA-type alloys depends heavily on the number of valence electrons and tetragonal distortion, and spin-polarized density of states is generated. Therefore, the selected alloys with L21-type structures and their tetragonal-distorted structure are potential candidates for conventional antiferromagnets, while those with XA-type structure and their tetragonal-distorted structure are promising candidates for (fully) compensated ferrimagnets.

In this paper, the effects of ambient temperature on the tensile response and superelastic fatigue behavior of the shape memory alloy (SMA) cable are investigated. The tested SMA cable is made of Nickel-Titanium and has an outer diameter of 8 mm with a 7 x 7 configuration. The SMA cables are subjected to an incrementally increasing loading protocol up to 14% strain amplitude under ambient temperatures varying from-20 ?C to 60 ?C. The effects of deformation amplitude and test temperature on stress-induced martensitic phase transformations are assessed. SMA cable specimens are also subjected to cyclic loading at a constant strain amplitude of 6% up to 1000 loading cycles at different test temperatures. The evolution of superelastic response with loading cycles is revealed by analyzing the maximum stress, residual deformation, hysteretic energy, and equivalent viscous damping ratio extracted from hysteretic curves. The results indicate that the ambient temperature considerably alters the superelastic behavior of SMA cables by shifting the stress-strain loops upward and narrowing them. When the ambient temperature is close to the reverse phase transformation temperature, the superelastic response degrades more progressively with both increasing loading amplitude and number of loading cycles. The least degradation in functional properties of SMAs takes place when the cable is tested at a temperature close to its austenitic transformation finish temperature, while increasing temperature decreases low cycle fatigue life of SMA cables when it is deformed up to the end of phase transformation. Overall, it is important to consider this temperature dependent response of SMA cables in applications where the cables will be subjected to varying outdoor temperatures.

This paper investigates the deformation mechanism and energy absorption behaviour of 316 L triply periodic minimal surface (TPMS) structures with uniform and graded wall thicknesses fabricated by the selective laser melting technique. The uniform P-surface TPMS structure presents a single-level stress plateau for energy absorption and a localized diagonal shear cell failure. A graded strategy was employed to break such localized geometrical deformation to improve the overall energy absorption and to provide a double-level function. Two segments with different wall thicknesses separated by a barrier layer were designed along the compression direction while keeping the same relative density as the uniform structure. The results show that the crushing of the cells of the graded P-surface TPMS structure occurs first within the thin segment and then propagates to the thick segment. The stress–strain response shows apparent double stress plateaus. The stress level and length of each plateau can be adjusted by changing the wall thickness and position of the barrier layer between the two segments. The total energy absorption of the gradient TPMS structure was also found slightly higher than that of the uniform TPMS counterparts. The gradient design of TPMS structures may find applications where the energy absorption requires a double-level feature or a warning function.

The influence of Cr doping for Mn on the structural, martensitic transformation and mechanical properties of Ni43Co5Mn44-xCrxSn8(x = 0, 1.5, 3, 5, 6 and 6.5) alloys has been systematically investigated. The replacement of Mn by Cr changed the alloy microstructure from a single-phase martensite to a refined eutectic dual-phase microstructure consisting of a martensitic/austenitic matrix and a y phase. The volume fraction of the y phase increased progressively with increasing the Cr doping. The y phase formation altered the composition and e/a ratio of the matrix phase. As a result, the martensitic temperatures and the entropy change of the transformation showed a decrease in these alloys. The Cr-doped eutectic Ni-Co-Mn-Sn alloy exhibited superior mechanical properties attributed to the well dispersed y phase lamellar grains for effective crack retardation while retaining the metamagnetic phase transformation. The compressive strength and ductility of alloys are notably enhanced from 290 MPa to 1420 MPa and from 6.7% to 15.8% respectively with Cr doping from 0 at% to 6.5 at%.

We reported a new atomic site preference occupation for Vanadium-based X(2)YZ full Heusler alloys. It tells that alloys with less 24 valence electrons form the L2(1)-type structure, while others form the XA-type structure. This fact means that the site preference rule for Vanadium-based alloys depends on the total number of valence electrons instead of the electropositive between X and Y atoms. The magnetic moments of XA-type alloys follow the Slater-Pauling rules, with two forms, M-t = N-V - 24 and M-t = N-V - 18 corresponding to different origins of the band gaps. XA-type V(2)YZ (Y = Cr, Mn and Fe; Z = Ga, Ge and As), V2CoGa, V2CoGe and V2NiGa alloys have half-metallic property. L21 -type V(2)YZ (Y = Cu and Zn; Z = Ga, Ge and As) alloys show no spin polarization and no half-metallic property due to the structural symmetry and the non-magnetism of copper and zinc atoms.

This article investigates the energy absorption performance and deformation mechanism of 316L stainless steel (SS316L) triply periodic minimal surface (TPMS) cellular structures fabricated by a selective laser melting (SLM) technique. The as‐built specimens are subjected to abrasive blasting treatment to improve the surface quality of the printed parts, in order to reveal the true surface and mechanical characteristics of the TPMS structures. It is found that the P‐type structure outperforms the G‐type structure with a higher energy absorption capability at low relative densities (<0.35). The macroscopic examination of these micro‐architectures reveals that the P‐type structure develops a rapid local cell deformation following the diagonal shear geometry on the face sheet, whereas the G‐type structure experiences continuous strain hardening along the stress plateau and deforms in a gradual manner during compression. The apparent strain hardening effect of the G‐type structure is caused by the development of many macro‐localities with extreme geometry distortion and cell wall self‐contacting during compression. The findings in this study may provide valuable insight into design, fabrication, and post‐fabrication treatment of metallic TPMS structures for the applications of high compression performance.

Two-dimensional nanomaterials are able to sustain ultra-large elastic strains, which in turn hold potential to alter the many functional properties. However, to achieve such large elastic strains in macro-forms suitable for applications has been a challenge. This paper reports an innovative approach to overcome this challenge by using a martensitic transforming substrate to induce ultra-large elastic lattice strains in metallic thin films deposited on it, as demonstrated in a Nb film-on-NiTi substrate system. This design is based on a novel concept of “lattice strain matching” between the uniform elastic lattice strain of the Nb film and the uniform crystallographic lattice strain of the martensitic transformation of the NiTi substrate. By this principle, the Nb film was able to exhibit reversible elastic lattice strains between −3.66% in compression and+3.74% in tension, for a total elastic strain span of +7.40% (the maximum in one loading deformation) by mechanical deformation of the substrate. These elastic lattice strains are 10–20 times of what are possible for bulk Nb or metallic thin films on conventional substrates. The findings of this work offer a unique opportunity to use ultra-large elastic strains as a means to engineer and improve functional properties of thin film materials.

Poor mechanical properties severely impede the practical application of ferromagnetic shape memory alloys. In this Letter, we report cold deformation-induced B2-γ phase transformation in polycrystalline FeMnGa alloys. Due to this property, the alloy achieved a high strength of ∼1000 MPa and a ductility of ∼75% in a compressive test, and a 90% deformation in thickness in a cold rolling experiment, indicating excellent cold-workability. The recrystallization annealing transformed the cold rolled γ phase to a textured B2 phase, which exhibited magnetic-field-induced phase transformation and anisotropic transformation strains. These properties indicate that FeMnGa is a promising candidate for future applications in actuator devices.

Isothermal annealing of a eutectic dual phase Ni–Mn–Sn–Fe alloy was carried out to encourage grain growth and investigate the effects of grain size of the γ phase on the martensitic transformation behaviour and mechanical properties of the alloy. It is found that with the increase of the annealing time, the grain size and volume fraction of the γ phase both increased with the annealing time predominantly by the inter-diffusion of Fe and Sn elements between the γ phase and the Heusler matrix. The isothermal anneals resulted in the decrease of the e/a ratio and suppression of the martensitic transformation of the matrix phase. The fine γ phase microstructure with an average grain size of 0.31 μm showed higher fracture strength and ductility values by 28% and 77% compared to the coarse-grained counterpart with an average grain size of 3.31 μm. The fine dual phase microstructure shows a quasi-linear superelasticity of 4.2% and very small stress hysteresis during cyclic loading, while the coarse dual phase counterpart presents degraded superelasticity of 2.6% and large stress hysteresis. These findings indicate that grain size refinement of the γ phase is an effective approach in improving the mechanical and transformation properties of dual phase Heusler alloys.

This paper investigates the effects of post-deposition annealing on the evolution of phase structure and magnetic properties of magnetron sputtered Ni2FeGa/Si (001) thin films. The results revealed that the as-deposited film was partially crystallized in an fcc structure, i.e. [Formula: see text] phase. Crystallization of the amorphous structure into the [Formula: see text] phase was greatly encouraged following annealing at 723 K for 1 h. Annealing at higher temperatures for the same period triggered the formation of the bcc austenitic phase, which competed with the [Formula: see text] phase simultaneously for crystallization and grain growth. The evolution of phase structure and grain size also influenced the nanomechanical properties of the films according to the nanoindentation measurement. The film annealed at 873 K for 1 h showed high hardness and elastic modulus values of 11.1 GPa and 156 GPa. The [Formula: see text] phase showed stronger ferromagnetic characteristics relative to the bcc austenite due to the richer Fe content. This leads to the saturation magnetization to be maximized at 80 emu/g when annealed at 773 K for 1 h attributed to the enhanced film crystallinity and dominant volume fraction of [Formula: see text] phase in the thin film.