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.

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.

Improvement of tensile superelasticity by aging treatment of NiTi shape memory alloys fabricated by electron beam wire-feed additive manufacturing

For the first time,this work comprehensively studied the effectiveness of precipitation hardening achieved by aging treatment in improving the tensile superelasticity of NiTi alloys fabricated by elec-tron beam wire-feed additive manufacturing(EBAM),which possesses inherent advantages in producing dense and oxidation-free structures.Aging treatments under three temperatures(450,350,and 250℃)and different durations were conducted,and the resultant performance of tensile superelasticity,together with the corresponding evolution of precipitation and phase transformation behavior were investigated for the EBAM-fabricated NiTi alloys.Results showed that by appropriate aging treatment,EBAM fabricated NiTi alloys could achieve excellent recovery rates of approximately 95%and 90%after the 1st and 10th load/unload cycle for a maximum tensile strain of 6%,which were almost the highest achieved so far by AM processed NiTi alloys and close to those of some conventional NiTi alloys.The improvement of tensile superelasticity benefited from the fine and dispersive Ni4Ti3 precipitates,which could be introduced by aging at 350℃ for 4 h or at 250℃ for 200 h.Moreover,the large amount of Ni4Ti3 precipitates would promote the intermediate R-phase transformation and bring a two-stage or three-stage transformation sequence,which depended on whether the distribution of the precipitation was homogeneous or not.This work could provide guidance for the production of NiTi alloys with good tensile superelasticity by EBAM or other additive manufacturing processes.

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.

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.

Additive manufacturing of Cu-AI-Mn shape memory alloy with enhanced superelasticity

The Cu-based shape memory alloy(SMA)with highly oriented columnar crystals is an ideal candidate for the commercial application,especially the ones obtained through rapid cooling via additive manufacturing method.In this work,Cu_(71)Al_(18)Mn_(11)(at%)shape memory alloy with strong<001>texture columnar grains was successfully prepared by selective laser melting(SLM).An L27(313)orthogonal array was designed to systematically investigate the effects of laser power,scanning speed,scanning spacing,layer thickness and their interactions on the forming quality of Cu_(71)Al_(18)Mn_(11)alloys.Cu_(71)Al_(18)Mn_(11)alloys with density of 7.3204 g·cm^(-3)and relative density of 99.18%were successfully prepared when the laser power,scanning speed,scanning distance and layer thickness were 240 W,1000 mm·s^(-1),0.11 mm and 25μm,the transformation onset temperature(Ms),martensite phase transformation termination temperature(Mf),austenite phase transformation onset temperature(AS)and austenite phase transformation termination temperature(Af)are-21.84,-26.04,-15.75 and-12.36℃,respectively.The compression strength and fracture strain along the building direction(BD)were significantly superior to the scanning direction(SD),while the superelasticity of compression along the SD reached 2.50%,which was better than that of2.32%along BD.The mechanical property and superelasticity anisotropy due to the formation of columnar grains and texture were discussed.This study shows that SLM is a proposed method for the preparation of Cu-Al-Mn SMAs with high superelasticity,which provides a new strategy for enhancing the shape memory alloy superelasticity.

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.

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.

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.

Microstructure evolution and superelasticity behavior of Ti-Ni-Hf shape memory alloy composite with multi-scale and heterogeneous reinforcements

In the present study,the in-situ TiB whisker was introduced into the Ti-Ni-Hf shape memory alloy composite by the in-situ reaction of the Ti-Ni-Hf alloy powder and TiB2 powders.The(Ti,Hf)2 Ni phase also precipitated,accompanied with the formation of TiB phase.Moreover,the residual TiB2 particles can be observed,as the TiB2 content was higher than 0.7 wt%.Thereinto,the larger scale reinforcements constituted the quasi-continuous network structure.The smaller scale reinforcements distributed in the interior of the network structure.The two-scale reinforcements showed the uniform distribution at macroscopic level and inhomogeneous distribution at microscopic level.The single stage B19?B2 martensitic transformation occurred in the Ti-Ni-Hf composites.In addition,the martensitic transformation temperatures continuously decreased with the increased TiB2 content owing to the compositional and mechanical effect.The moderate TiB2 addition not noly enhanced the matrix strength,but also significantly improved the superelasticity.The excellent superelaticity with the completely recoverable strain of 4%can be obtained in the Ti-Ni-Hf composite containing 0.7 wt%TiB2.

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.

Enhanced superelasticity of Cu-Al-Ni shape memory alloys with strong orientation prepared by horizontal continuous casting

The preparation of large-scale Cu-Al-Ni shape memory alloys with excellent microstructure and texture is a significant challenge in this field.In this study,large-scale Cu-Al-Ni shape memory alloy(SMA)slabs with good surface quality and strong orientation were prepared by the horizontal continuous casting(HCC).The microstructure and mechanical properties were compared with the ordinary casting(OC)Cu-Al-Ni alloy.The results showed that the microstructure of OC Cu-Al-Ni alloy was equiaxed grains with randomly orientation,which had no obvious superelasticity.The alloys producedby Hcchad herringbone grains withstrong orientation near(100)and the cumulative tensile superelasticity of 4.58%.The superelasticity of the alloy produced by HCC has been improved by 4-5 times.This work has preliminarily realized the production of large-scale Cu-Al-Ni SMA slab with good superelasticity,which lays a foundation forexpanding the industrial production and application of Cu-based SMAs.

Transformation behaviors and superelasticity of Ti_(50)Ni_(48)Fe_2 shape memory alloy subjected to cold-rolling and subsequent annealing

The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2→R→B19’two-stage martensitic transformation upon cooling and a B19’→B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress–strain curves show the critical stress for stress-induced martensite（rSIM）of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the rSIM decreases. Upon a cold-rolling and annealing at 623 K for30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.

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.

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.

Microstructural evolution and superelastic properties of ultrafine-graine d NiTi-base d shape memory alloy via sintering of amorphous ribbon precursor

NiTi-based shape memory alloys(SMAs)are considered as cutting-edge intelligent functional materials.However,it remains a great challenge to obtain ultrafine-grained(UFGed)bulk materials with mm-scale size as well as outstanding superelastic properties.Here,UFGed bulk Ti_(35)Zr_(15)Ni_(35)Cu_(15)NiTi-based SMA is successfully prepared via spark plasma sintering of amorphous ribbon precursor at different sintering temperatures,and microstructural evolution and superelastic properties are symmetrically investigated.It is found that its grain size ranges from UFG to micro-grain with increased sintering temperature regard-less of the predominant B2 matrix in all bulk samples.Interestingly,the orientation relationships between B2 matrix and nano-scale fcc(Ti,Zr)_(2)Ni precipitate evolve from coherent to incoherent.Consequently,the UFGed samples exhibit perfect superelasticity with the high recoverable strain of∼5.8%,the stable recov-ery rate above 99%,and the great critical stress inducing martensitic transformation higher than 1 GPa,far superior to the corresponding ones of suction-cast micro-grained TiZrNiCu SMAs.Fundamentally,the perfect superelasticity is attributed to the good resistance to dislocation slip or grain boundary slip by residual nano-scale amorphous phase or secondary phase of coherent and semi-coherent fcc(Ti,Zr)_(2)Ni precipitate.In addition,the gentle superelastic plateau is associated to the favorable transfer stress and the strong ability to accommodate dislocation movement,which is generated by the coherent interface between nano-scale fcc(Ti,Zr)_(2)Ni and UFGed B2 matrix.These results suggest that spark plasma sintering of amorphous alloy precursor is a feasible route to obtaining excellent superelasticity in NiTi-based SMAs.

Martensitic transformation,shape memory effect and superelasticity of Ti-xZr-(30-x)Nb-4Ta alloys

Martensitic transformations,mechanical properties,shape memory effect and superelasticity of Ti-xZr-(30-x)Nb-4Ta(x=15,16,17 and 18;at%) alloys were investigated.X-ray diffraction(XRD),optical microscopy(OM) and transmission electron microscopy(TEM) results indicated that the Ti-16Zr-14Nb-4Ta,Ti-17Zr-13Nb-4Ta and Ti-18Zr-12Nb4Ta alloys were mainly composed of α″-martensite,while the Ti-15Zr-15Nb-4Ta alloy was characterized by predominant p phase.The reverse martensitic transformation temperatures increased when Nb was replaced by Zr,indicating stronger p-stabilizing effect for the former.The Ti-15Zr-15Nb-4Ta alloy displayed superelasticity during tensile deformation with a recovery strain of 3.51%.For the other three alloys with higher Zr content,the martensitic reorientation occurred during tensile deformation,resulting in shape memory recovery upon subsequent heating.The maximum shape memory effect was 3.46% in the Ti-18Zr-12Nb-4Ta alloy.

Effect of cyclic heat treatment on abnormal grain growth in Fe-Mn-Al-based shape memory alloys with different Ni contents

Cyclic heat treatment that can continuously promote abnormal grain growth is widely used for the prepa-ration of single-crystal Fe-Mn-Al-based shape memory alloys.However,it takes a long time to prepare large-size Fe-Mn-Al-based alloy single crystals via the reported cyclic heat treatments.Meanwhile,the long-time cyclic heat treatment at high temperatures leads to the development of defects including oxidation and a decrease in Mn,which would deteriorate superelasticity in the Fe-Mn-Al-based shape memory alloys.To shorten the fabrication time of single crystals,the effect of the cyclic heat treatment process on the abnormal grain growth in the Fe-Mn-Al-based alloys with different Ni contents was systematically investigated.It is found that the abnormal grain growth of Fe-Mn-Al-based alloys was not significantly affected by the Ni contents(within 2.1 at.%-6.2 at.%).In addition,the abnormal grain growth could be promoted by 1-2℃ min^(-1) cooling rate,high solution temperature,and multiple cycles,while it was insensitive to other processes including heating rate,dual-phase time as well as long-time solution treat-ment.These findings can guide optimizing the fabrication process of single crystals by cyclic heat treat-ment.Finally,the Fe_(41.9)Mn_(37.8)Al_(14.1) Ni_(6.2) single crystal prepared by the optimized cyclic heat treatment showed a recoverable strain of about 4%.

Modeling of Superelastic–plastic Behavior of Porous Shape Memory Alloys Incorporating Void Shape Effects

A new constitutive model for describing the superelastic–plastic behavior of porous shape memory alloys(SMAs)is proposed.The model incorporates the influences of void shape and hydrostatic pressure as well as the elastic modulus mismatch between austenite and martensite.In addition,the interactions between plastic strain and transformation strain are considered via the plastic back stress.The porous SMAs are considered as two-phase composites with the dense SMA matrix and the second phase representing ellipsoidal voids.Based on Gurson’s formulation,the transformation and plastic flow potentials accounting for the transformation–plasticity coupling are developed.The numerical results present good agreement with available experimental data for various levels of porosity,which proves that the model is capable of capturing stress-induced phase transformation and plastic deformation of porous SMAs.Using the proposed model,the influence of plastic strain on reverse transformation and the effects of porosity and void shape on the pseudoelastic and plastic behavior of porous SMAs are investigated.

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.