Enhanced superelasticity and reversible elastocaloric effect in nano-grained NiTi alloys with low stress hysteresis

Solid-state cooling technologies have been considered as potential alternatives for vapor compression cooling systems.The search for refrigeration materials displaying a unique combination of pronounced caloric effect,low hysteresis,and high reversibility on phase transformation was very active in recent years.Here,we achieved increase in the elastocaloric reversibility and decrease in the friction dissipation of martensite transformations in the superelastic nano-grained NiTi alloys obtained by cold rolling and annealing treatment,with very low stress hysteresis(6.3 MPa)under a large applied strain(5%).Large adiabatic temperature changes(△T_(max)=16.3 K atε=5%)and moderate COP_(mater)values(maximum COP_(mater)=11.8 atε=2%)were achieved.The present nano-grained NiTi alloys exhibited great potential for applications as a highly efficient elastocaloric material.

Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing

Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni50Ti50 shape memory alloy were studied.Several specimens were produced by copper boat vacuum induction melting.The homogenized specimens were hot rolled and annealed at 900°C.Thereafter,annealed specimens were subjected to cold rolling with different thickness reductions up to 70%.Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni50Ti50 alloy.After annealing at 400°C for 1 h,the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed.Results showed that with increasing thickness reduction during cold rolling,the recoverable strain of Ni50Ti50 alloy was increased during superelastic experiments such that the 70%cold rolled-annealed specimen exhibited about 12%of recoverable strain.Moreover,with increasing thickness reduction,the critical stress for stress-induced martensitic transformation was increased.It is noteworthy that in the 70%cold rolled-annealed specimen,the damping capacity was measured to be 28 J/cm3 that is significantly higher than that of commercial NiTi alloys.

Microstructure and superelastic properties of free forged Ti-Ni shape-memory alloy

Elemental titanium(Ti)and nickel(Ni)powders were consolidated by spark plasma sintering(SPS)to fabricate Ti-51%Ni(mole fraction)shape-memory alloys(SMAs).The objective of this study is to enhance the superelasticity of SPS produced Ti-Ni alloy using free forging as a secondary process.Products from two processes(with and without free forging)were compared in terms of microstructure,transformation temperature and superelasticity.The results showed that,free forging effectively improved the tensile and shape-memory properties.Ductility increased from 6.8%to 9.2%after forging.The maximum strain during superelasticity increased from 5%to 7.5%and the strain recovery rate increased from 72%to 92%.The microstructure of produced Ti-51%Ni SMA consists of the cubic austenite(B2)matrix,monoclinic martensite(B19′),secondary phases(Ti3Ni4,Ti2Ni and TiNi3)and oxides(Ti4Ni2O and Ti3O5).There was a shift towards higher temperatures in the martensitic transformation of free forged specimen(aged at 500°C)due to the decrease in Ni content of B2 matrix.This is related to the presence of Ti3Ni4 precipitates,which were observed using transmission electron microscope(TEM).In conclusion,free forging could improve superelasticity and mechanical properties of Ti-51%Ni SMA.

Superelastic behavior of nanostructured Ti_(50)Ni_(48)Co_2 shape memory alloy with cold rolling processing

Effects of cold rolling followed by annealing on microstructural evolution and superelastic properties of the Ti50Ni48Co2 shape memory alloy were investigated. Results showed that during cold rolling, the alloy microstructure evolved through six basic stages including stress-induced martensite transformation and plastic deformation of martensite, deformation twinning, accumulation of dislocations along twin and variant boundaries in martensite, nanocrystallization, amorphization and reverse transformation of martensite to austenite. After annealing at 400 ℃ for 1 h, the amorphous phase formed in the cold-rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. The value of stress level of the upper plateau in this nanocrystalline alloy was measured as high as 730 MPa which was significantly higher than that of the coarse-grained Ni50Ti50 and Ti50Ni48Co2 alloys. Moreover, the nanocrystalline Ti50Ni48Co2 alloy had a high damping capacity and considerable efficiency for energy storage.

Superelasticity of NiTi Shape Memory Alloy Thin Films

The superelastic properties of NiTi thin films prepared with sputtering were studied. To characterize their superelasticity, tensile and bulging and indentation tests were performed. The measured mechanisms using these three methods were compared, and the factors that influence superelasticity were described.

Effects of aging treatment on the microstructure and superelasticity of columnar-grained Cu71Al18Mn11 shape memory alloy

The effect of aging treatment on the superelasticity and martensitic transformation critical stress in columnar-grained Cu_（71）Al_（18）Mn_（11） shape memory alloy（SMA） at the temperature ranging from 250°C to 400°C was investigated. The microstructure evolution during the aging treatment was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that the plate-like bainite precipitates distribute homogeneously within austenitic grains and at grain boundaries. The volume fraction of bainite increases with the increase in aging temperature and aging time, which substantially improves the martensitic transformation critical stress of the alloy, whereas the bainite only slightly affects the superelasticity. This behavior is attributed to a coherent relationship between the bainite and the austenite, as well as to the bainite and the martensite exhibiting the same crystal structure. The variations of the martensitic transformation critical stress and the superelasticity of columnar-grained Cu_（71）Al_（18）Mn_（11） SMA with aging-temperature and aging time are described by the Austin-Rickett equation, where the activation energy of bainite precipitation is 77.2 kJ ·mol1. Finally, a columnar-grained Cu_（71）Al_（18）Mn_（11） SMA with both excellent superelasticity（5%-9%） and high martensitic transformation critical stress（443-677 MPa） is obtained through the application of the appropriate aging treatments.

Superelastic damping of TiNi alloy wire under heavy mechanical shock in structural engineering

Based on superelastic damping application in structural engineering, the damping characteristics of commercial Ti-50.8Ni(mole fraction, %) alloy have been systematically studied by adjusting frequency of mechanical shock, temperature, stress, strain and number of cycling. The results show that at extremely low frequency mechanical shock at room temperature, the superelastic damping capacity increases with controlled strain, and such capacity of each cycle is greater than 50%. When the frequency of mechanical shock is 0.10.3 Hz, the superelastic damping capacity above room temperature is relatively large at high strain; when the temperature approaches to M_d, the damping begins at low stress. For specimen cycled under 0.5 Hz, above 6% strain mechanical shock at relatively high temperature, further large-strain cycling exhibits more than 35% damping capacity. The superelastic damping of trained specimen is relatively stable at 2050 ℃ and 0.10.5 Hz frequency mechanical shock.

Influence of α Phase on Shape Memory Effect and Superelasticity of CuZnAI Alloy

The influence of a small amount of α phase in β′ matrix on shape memory effect and superelasticity of CuZnAl shape memory alloy has been studied systematically.It has been found that transformation temperature can be adjusted in a large scale by controlling the amount of α phase, meanwhile,shape memory effect and superelasticity do not decrease obviously when there exists a small amount of α phase.Based on the optical and trans- mission electron microscopy observation,the influ- ence of α phase on shape memory effect and superelasticity has been discussed.

Superelasticity of TiPdNi Alloys with and without Rare Earth Ce Addition

Ti50.6Pd30Ni19.4 and Ti51Pd28Ni21 (Ce) alloys have been prepared under various temperatures for long time annealing. Differential scanning calorimetery (DSC), X-ray diffraction (XRD) and tensile test were employed to investigate the phase transformation behavior and superelasticity of the alloys. It has been found that the phase transformation temperature of Ti50.6Pd30Ni19.4 is about 40C higher than that of Ti51Pd28Ni21(Ce), and do not change much with different annealed temperature. Obvious superelasticity is retained in Ti50.6Pd30Ni19.4 alloy annealed at 400C for 18 h, and annealing at higher temperature shows a deterioration of this property. The Ce addition in TisiPd28Ni2i alloy significantly delays recrystallization, increases yied strength and elastic modulus, but the superelasticity is poor.

热型连铸柱状晶Cu-Al-Be合金丝力学性能试验研究

采用循环加载试验,研究了热型连铸柱状晶Cu-Al-Be合金丝的超弹性力学性能,重点考察了合金丝循环加载次数及加载速率的影响,并基于试验结果提出了二种本构模型。结果表明:Cu-Al-Be合金丝具有很好的塑性变形能力和可恢复变形能力;经过循环加载训练后的Cu-Al-Be合金丝具有稳定的滞回性能,且残余变形较小;Cu-Al-Be合金丝的力学性能具有很强的率相关性;修正Wilde模型和FS模型均具有较高的分析精度,且后者计算效率更高。

Cu-Al-Be超弹性合金拉伸过程马氏体相变的原位观察

对Cu-Al-Be超弹性合金做拉伸试验,用金相显微镜原位观察马氏体生长特点和组织变化。在拉伸过程中,β母相转变为马氏体,其后马氏体开始出现了内部生长、末端开叉、内部二次细分等多种方式生成的马氏体变体。在每一个新马氏体形成的过程中,出现了一些应力的突变,导致应力-应变曲线上出现应力波谷。

Characteristics of shape memory and superelasticity for TiNi thin films

TiNi shape memory alloy thin films were deposited by using a RF magnetron sputtering apparatus. The transformation and shape memory characteristics of the thin films have been investigated by using DSC and tensile tests. After aging, perfect shape memory effect and superelasticity were achieved in TiNi thin films.

Superelasticity of Cu–Ni–Al shape-memory fibers prepared by melt extraction technique

In the paper, a melt extraction method was used to fabricate Cu–4Ni–14Al（wt%） fiber materials with diameters between 50 and 200 μm. The fibers exhibited superelasticity and temperature-induced martensitic transformation. The microstructures and superelasticity behavior of the fibers were studied via scanning electron microscopy（SEM） and a dynamic mechanical analyzer（DMA）, respectively. Appropriate heat treatment further improves the plasticity of Cu-based alloys. The serration behavior observed during the loading process is due to the multiple martensite phase transformation.

Non-negligible role of gradient porous structure in superelasticity deterioration and improvement of NiTi shape memory alloys

Bone-mimicking gradient porous NiTi shape memory alloys(SMAs)are promising for orthopedic im-plants due to their distinctive superelastic functional properties.However,premature plastic deformation in weak areas such as thinner struts,nodes,and sharp corners severely deteriorates the superelasticity of gradient porous NiTi SMAs.In this work,we prepared gradient porous NiTi SMAs with a porosity of 50%by additive manufacturing(AM)and achieved a remarkable improvement of superelasticity by a simple solution treatment regime.After solution treatment,phase transformation temperatures dropped signif-icantly,the dislocation density decreased,and partial intergranular Ti-rich precipitates were transferred into the grain.Compared to as-built samples,the strain recovery rate of solution-treated samples was nearly doubled at a pre-strain of 6%(up to 90%),and all obtained a stable recoverable strain of more than 4%.The remarkable superelasticity improvement was attributed to lower phase transformation tem-peratures,fewer dislocations,and the synergistic strengthening effect of intragranular multi-scale Ti-Ni precipitates.Notably,the gradient porous structure played a non-negligible role in both superelasticity deterioration and improvement.The microstructure evolution of the solution-treated central strut after constant 10 cycles and the origin of the stable superelastic response of gradient porous NiTi SMAs were revealed.This work provides an accessible strategy for improving the superelastic performance of gra-dient porous NiTi SMAs and proposes a key strategy for achieving such high-performance architectured materials.

Inverse gradient nanostructure through gradient cold rolling demonstrated with superelasticity improvement in Ti-50.3Ni shape memory alloy

Gradient nanostructured(GNS)metallic materials are commonly achieved by gradient severe plastic de-formation with a gradient of nano-to micro-sized structural units from the surface/boundaries to the center.Certainly,such GNS can be inversely positioned,which however has not yet been reported.The present work reports a facile method in deformation gradient control to attain inverse gradient nanostructured(iGNS),i.e.,tailoring the cross-section shape,successfully demonstrated in Ti-50.3Ni shape memory alloy(SMA)wire through cold rolling.The microstructure of the rolled wire is characterized by a macroscopic inverse gradient from boundaries to the center—the average sizes of grain and martensite domain evolve from micrometer to nanometer scale.The iGNS leads to a gradient martensitic transforma-tion upon stress,which has been proved to be effectively reversible via in-situ bending scanning electron microscopy(SEM)observations.The iGNS Ti-50.3Ni SMA exhibits quasi-linear superelasticity(SE)in a wide temperature range from 173 to 423 K.Compared to uniform cold rolling,the gradient cold rolling with less overall plasticity further improves SE strain(up to 4.8%)and SE efficiency.In-situ tensiling synchrotron X-ray diffraction(SXRD)analysis reveals the underlying mechanisms of the unique SE in the iGNS SMAs.It provides a new design strategy to realize excellent SE in SMAs and sheds light on the advanced GNS metallic materials.

Gyroid Triply Periodic Minimal Surface Lattice Structure Enables Improved Superelasticity of CuAlMn Shape Memory Alloy

Improving the shape memory effect and superelasticity of Cu-based shape memory alloys(SMAs)has always been a research hotspot in many countries.This work systematically investigates the effects of Gyroid triply periodic minimal surface(TPMS)lattice structures with different unit sizes and volume fractions on the manufacturing viability,compressive mechanical response,superelasticity and heating recovery properties of CuAlMn SMAs.The results show that the increased specific surface area of the lattice structure leads to increased powder adhesion,making the manufacturability proportional to the unit size and volume fraction.The compressive response of the CuAlMn SMAs Gyroid TPMS lattice structure is negatively correlated with the unit size and positively correlated with the volume fraction.The superelastic recovery of all CuAlMn SMAs with Gyroid TPMS lattice structures is within 5%when the cyclic cumulative strain is set to be 10%.The lattice structure shows the maximum superelasticity when the unit size is 3.00 mm and the volume fraction is 12%,and after heating recovery,the total recovery strain increases as the volume fraction increases.This study introduces a new strategy to enhance the superelastic properties and expand the applications of CuAlMn SMAs in soft robotics,medical equipment,aerospace and other fields.

Passive smart self-repairing concrete beams by using shape memory alloy wires and fibers containing adhesives

An innovative approach to increase structural survivability of concrete and maintain structural durability of concrete was developed in case of earthquakes and typhoons. This approach takes advantage of the superelastic effect of shape memory alloy(SMA) and the cohering characteristic of repairing adhesive. These SMA wires and brittle fibers containing adhesives were embedded into concrete beams during concrete casting to form smart reinforced concrete beams. The self-repairing capacity of smart concrete beams was investigated by three-point bending tests. The experimental results show that SMA wires add self-restoration capacity,the concrete beams recover almost completely after incurring an extremely large deflection and the cracks are closed almost completely by the recovery forces of SMA wires. The number or areas of SMA wires has no influence on the tendency of deformation during loading and the tendency of reversion by the superelasticity. The adhesives released from the broken-open fibers fill voids and cracks. The repaired damage enables continued function and prevents further degradation.

Preparation and characterization of porous NiTi alloys synthesized by microwave sintering using Mg space holder

To obtain the lightweight,high strength,and high damping capacity porous NiTi alloys,the microwave sintering coupled with the Mg space holder technique was employed to prepare the porous NiTi alloys.The microstructure,mechanical properties,phase transformation behavior,superelasticity,and damping capacity of the porous NiTi alloys were investigated.The results show that the porous NiTi alloys are mainly composed of the B2 NiTi phase with a few B19'NiTi phase as the sintering temperature is lower than or equal to 900℃.With increasing the sintering temperature,the porosities of the porous NiTi alloys gradually decrease and the compressive strength increases first,reaching the maximum value at 900℃,and then decreases.With increasing the Mg content from 1 wt.%to 7 wt.%,the porosities of the porous NiTi alloys increase from 37.8%to 47.1%,while the compressive strength decreases from 2058 to 1146 MPa.Compared with the NH4HCO3 space holder,the phase transformation behavior of the porous NiTi alloys prepared with Mg space holder changes,and all of the compressive strength,superelasticity,shape memory effect and damping capacity are greatly improved.

Bio-inspired Actuating System for Swimming Using Shape Memory Alloy Composites

The paper addresses the designs of a caudal peduncle actuator, which is able to furnish a thrust for swimming of a robotic fish. The caudal peduncle actuator is based on concepts of ferromagnetic shape memory alloy （FSMA） composite and hybrid mechanism that can provide a fast response and a strong thrust. The caudal peduncle actuator was inspired by Scomber Scombrus which utilises thunniform mode swimming, which is the most efficient locomotion mode evolved in the aquatic environment, where the thrust is generated by the lift-based method, allowing high cruising speeds to be maintained for a long period of time. The morphology of an average size Scomber Scombrus （length in 310 mm） was investigated, and a 1：1 scale caudal peduncle actuator prototype was modelled and fabricated. The propulsive wave characteristics of the fish at steady speeds were employed as initial design objectives. Some key design parameters are investigated, i.e. aspect ratio （AR） （AR = 3.49）, Reynolds number （Re = 429 649）, reduced frequency （σ = 1.03）, Strouhal number （St = 0.306） and the maximum strain of the bent tail was estimated at ε = 1.11% which is in the range of superelasticity. The experimental test of the actuator was carried out in a water tank. By applying 7 V and 2.5 A, the actuator can reach the tip-to-tip rotational angle of 85° at 4 Hz.

Three dimensional large deformation analysis of phase transformation in shape memory alloys

Shape memory alloys（SMAs）have been explored as smart materials and used as dampers,actuator elements,and smart sensors.An important character of SMAs is its ability to recover all of its large deformations in mechanical loading-unloading cycles without showing permanent deformation.This paper presents a stress-induced phenomenological constitutive equation for SMAs,which can be used to describe the superelastic hysteresis loops and phase transformation between Martensite and Austenite.The Martensite fraction of SMAs is assumed to be dependent on deviatoric stress tensor.Therefore,phase transformation of SMAs is volume preserving during the phase transformation.The model is implemented in large deformation finite element code and cast in the updated Lagrangian scheme.In order to use the Cauchy stress and the linear strain in constitutive laws,a frame indifferent stress objective rate has to be used.In this paper,the Jaumann stress rate is used.Results of the numerical experiments conducted in this study show that the superelastic hysteresis loops arising with the phase transformation can be effectively captured.