Electrochemical behavior of Ti-Ni shape memory alloy in fibrinogen solution

Electrochemical behaviors of Ti Ni shape memory alloy in fibrinogen solution were studied by electrochemical techniques. The results indicate that the addition of the fibrinogen has no obvious effect on the corrosion potential, but decreases the pitting potential markedly and increases the passive current densities. The analysis of energy dispersive X ray for samples adsorbing fibrinogen exhibits that the elements of O, C and N exist on the surface of Ti Ni alloy. Furthermore, the scanning electron microscope micrographs confirm that the configuration of the adsorbing fibrinogen concentrating on surface defects is like cluster and the fibrinogen adsorption concentration is 96.67 mg/m 2 through ultroviolet ray absorption method. Fibrinogen combined with Ti Ni alloy surface by complex band and its electrochemical transfer accelerated the corrosion of alloy.

R-phase transformation of aged Ti-Ni shape memory alloy

Ti-50.6Ni（molar fraction, %） shape memory alloy solution treated at 850℃ for 1h followed by ageing treatment at 450℃ for 3h was studied with differential scanning calorimetry（DSC）, X-ray diffractometry（XRD） and transmission electron microscopy（TEM）. DSC measurement reveals two separate transformation peaks. XRD and TEM demonstrate that a three-stage transformation occurs. The Ti3Ni4 precipitates are coherent with the R-phase. The crystal structure of R-phase was analyzed by two diffraction patterns method. The diffraction patterns of R-phase were obtained in detail from the same region.

Machine learning-assisted efficient design of Cu-based shape memory alloy with specific phase transition temperature

The martensitic transformation temperature is the basis for the application of shape memory alloys(SMAs),and the ability to quickly and accurately predict the transformation temperature of SMAs has very important practical significance.In this work,machine learning(ML)methods were utilized to accelerate the search for shape memory alloys with targeted properties(phase transition temperature).A group of component data was selected to design shape memory alloys using reverse design method from numerous unexplored data.Component modeling and feature modeling were used to predict the phase transition temperature of the shape memory alloys.The experimental results of the shape memory alloys were obtained to verify the effectiveness of the support vector regression(SVR)model.The results show that the machine learning model can obtain target materials more efficiently and pertinently,and realize the accurate and rapid design of shape memory alloys with specific target phase transition temperature.On this basis,the relationship between phase transition temperature and material descriptors is analyzed,and it is proved that the key factors affecting the phase transition temperature of shape memory alloys are based on the strength of the bond energy between atoms.This work provides new ideas for the controllable design and performance optimization of Cu-based shape memory alloys.

Effect of solution treatment on the microstructure,phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials.In this work,a ternary(NiTi)91Nb9(at.%)shape memory alloy was produced by laser powder bed fusion(L-PBF)using pre-alloyed NiTi and elemental Nb powders.The effect of solution treatment on the microstructure,phase transformation behavior and mechanical/functional performances was investigated.The in-situ alloyed(NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix.Upon high-temperature(1273 K)solution treatment,Nb-rich precipitates were precipitated from the supersaturated matrix.The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment,leading to a morphological transition from mesh-like into rod-like and sphere-like.Coarsening of theβ-Nb phases occurred with increasing holding time.The martensite transformation temperature increases after solution treatment,mainly attributed to:(i)reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi,and(ii)the Ti expulsion from theβ-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix,which resulted from the microstructure changes from non-equilibrium to equilibrium state.The thermal hysteresis of the solutionized alloys is around 145 K after 20%pre-deformation,which is comparable to the conventional NiTiNb alloys.A short-term solution treatment(i.e.at 1273 K for 30 min)enhances the ductility and strength of the as-printed specimen,with the increase of fracture stress from(613±19)MPa to(781±20)MPa and the increase of fracture strain from(7.6±0.1)%to(9.5±0.4)%.Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates>90%.This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

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.

Improving comprehensive properties of Cu-11.9Al-2.5Mn shape memory alloy by adding multi-layer graphene carried by Cu_(51)Zr_(14)inoculant particles

In order to improve the comprehensive properties of the Cu-11.9Al-2.5Mn shape memory alloy(SMA),multilayer graphene(MLG)carried by Cu_(51)Zr_(14)inoculant particles was incorporated and dispersed into this alloy through preparing the preform of the cold-pressed MLG-Cu_(51)Zr_(14)composite powders.In the resultant novel MLG/Cu-Al-Mn composites,MLG in fragmented or flocculent form has a good bonding with the Cu-Al-Mn matrix.MLG can prevent the coarsening of grains of the Cu-Al-Mn SMA and cause thermal mismatch dislocations near the MLG/Cu-Al-Mn interfaces.The damping and mechanical properties of the MLG/Cu-Al-Mn composites are significantly improved.When the content of MLG reaches 0.2 wt.%,the highest room temperature damping of 0.0558,tensile strength of 801.5 MPa,elongation of 10.8%,and hardness of HV 308 can be obtained.On the basis of in-depth observation of microstructures,combined with the theory of internal friction and strengthening and toughening theories of metals,the relevant mechanisms are discussed.

Phase Transformation and Thermal Stability of Aged Ti-Ni-Hf High Temperature Shape Memory Alloys

The use of Ni-rich TiNiHf alloys as high temperature shape memory alloys （SMAs） through aging has been presented. For Ni-rich Ti80-xNixHf20 alloys, their phase transformation temperatures are averagely increased more than 100 K by aging at 823 K for 2 h. Especially for the alloys with Ni-content of 50.4 at. pct and 50.6 at. pct, their martensitic transformation start temperatures （Ms） are more than 473 K after aging. TEM observation confirmed that some fine particles precipitate from the matrix during aging. The aged Ni-rich TiNiHf SMAs show the better thermal stability of phase transformation temperatures than the solutiontreated TiNiHf alloys. The fine particles precipitated during aging should be responsible for the increase of phase transformation temperatures and its high stability.

Effect of Training on Two-way Shape Memory Effect and Its Stability in a Ti-Ni-Hf High Temperature Shape Memory Alloy

The Effect of the thermal cycling training under constant strain on the two-way shape memory effect (TWSME) in a Ti36l\li49Hf15 high temperature shape memory alloy (SMA) has been investigated by bending tests. The results indicated that the training procedure is beneficial to get the better TWSME. The two-way shape memory strain increases with increasing the training strain. And it decreases with increasing the training temperature. The TWSME obtained in the present alloy shows poorer stability compared with that obtained in the TiNi alloys.

Two-way Shape Memory Effect in a Ti-Ni-Nb Shape Memory Alloy with Wide Hysteresis

A two-way shape memory effect (TWSM E) in the Ti46.3Ni44.7Nb9 alloy has been systematically investigated by means of bending test and transmission electron microscopy (TEM ) observations. Based on the analysis of the microstructure after training. the mechanism of TWSME in the Ti46.3 Ni44.7Nb9 alloy has been discussed.

Two-way shape memory effect and its stability in Ti-Ni-Hf high temperature shape memory alloy

The two-way shape memory effect（TWSME） in a Ti36Ni49Hf15 high temperature shape memory alloy（SMA） was systematically studied by bending tests. In the TiNiHf alloy, the martensite deformation is an effective method to get two-way shape memory effect even with a small deformation strain. The results indicate that the internal stress field formed by the bending deformation is in the direction of the preferentially oriented martensite variants formed during the bending deformation. Upon cooling the preferentially oriented martensite variants form under such an oriented stress field, which should be responsible for the generation of the two-way shape memory effect. Proper training process benefits the formation of the oriented stress field, resulting in the improvement of the two-way shape memory effect. A maximum TWSME of 0.88% is obtained in the present alloy.

Effect of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi shape memory alloy

As-received nickel-titanium （NiTi） shape memory alloy with a nominal composition of Ni50.9Ti49.1 （mole fraction,%） was subjected to solution treatment at 1123 K for 2 h and subsequent aging for 2 h at 573 K, 723 K and 873 K, respectively. The influence of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi alloy was systematically investigated by transmission electron microscopy （TEM）, high resolution transmission electron microscopy （HRTEM）, scanning electron microscopy （SEM） and compression test. Solution treatment contributes to eliminating the Ti2Ni phase in the as-received NiTi sample, in which the TiC phase is unable to be removed. Solution treatment leads to ordered domain of atomic arrangement in NiTi alloy. In all the aged NiTi samples, the Ni4Ti3 precipitates, the R phase and the B2 austenite coexist in the NiTi matrix at room temperature, while the martensitic twins can be observed in the NiTi samples aged at 873 K. In the NiTi samples aged at 573 and 723 K, the fine and dense Ni4Ti3 precipitates distribute uniformly in the NiTi matrix, and thus they are coherent with the B2 matrix. However, in the NiTi sample aged at 873 K, the Ni4Ti3 precipitates exhibit the very inhomogeneous size, and they are coherent, semi-coherent and incoherent with the B2 matrix. In the case of aging at 723 K, the NiTi sample exhibits the maximum yield strength, where the fine and homogeneous Ni4Ti3 precipitates act as the effective obstacles against the dislocation motion, which results in the maximum critical resolved shear stress for dislocation slip.

Crystallization of amorphous NiTi shape memory alloy fabricated by severe plastic deformation

Based on the local canning compression,severe plastic deformation（SPD） is able to lead to the almost complete amorphous nickel-titanium shape memory alloy（NiTi SMA）,in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix.Crystallization of amorphous NiTi alloy annealed at 573,723 and 873 K was investigated,respectively.The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-MehlAvrami-Kolmogorov（JMAK） equation.NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K,where martensite phase transformation is suppressed due to the constraint of the grain boundaries.Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample,where（001） martensite compound twin is observed at room temperature.It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains.SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.

Dynamic recovery and dynamic recrystallization of NiTi shape memory alloy under hot compression deformation

Mechanical behavior of nickel？titanium shape memory alloy（NiTi SMA） under hot deformation was investigated according to the true stress—strain curves of NiTi samples under compression at the strain rates of 0.001-1 s-1 and at the temperatures of 600？1000℃.Dynamic recovery and dynamic recrystallization of NiTi SMA were systematically investigated by microstructural evolution.The influence of the strain rates,the deformation temperatures and the deformation degree on the dynamic recovery and dynamic recrystallization of NiTi SMA was obtained as well.NiTi SMA was characterized by the combination of dynamic recovery and dynamic recrystallization at 600℃ and 700℃,but the complete dynamic recrystallization occurred at other deformation temperatures.Increasing the deformation temperatures or decreasing the stain rates leads to larger equiaxed grains.The deformation degree has an important influence on the dynamic recrystallization of NiTi SMA.There exists the critical deformation degree during the dynamic recrystallization of NiTi SMA,beyond which the larger deformation degree contributes to obtaining the finer equiaxed grains.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti-Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing （ECAP） at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti-Mo-Nb-V-Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Influence of Ni_4Ti_3 precipitates on phase transformation of NiTi shape memory alloy

Ni Ti shape memory alloy samples were aged for 2 h at 573, 723 and 873 K, respectively. Two R-phase variants are observed in the Ni Ti samples aged at 573 and 723 K, where the orientation relationship between the two R-phase variants and the B2 matrix is determined. In the Ni Ti samples aged at 573 and 723 K, fine and homogeneous Ni4Ti3 precipitates are coherent with the B2 austenite matrix. The Ni4Ti3 particles precipitate in the grain interior and at the grain boundaries, where the heterogeneous Ni4Ti3 precipitates are coherent, semi-coherent and incoherent with the B2 matrix in the Ni Ti sample aged at 873 K. As for the Ni Ti sample aged at 873 K, one-stage phase transformation from B19＇ martensite to B2 austenite occurs on heating, but two-stage phase transformation of B2-R-B19＇ arises on cooling. The Ni Ti sample aged at 723 K shows two-stage phase transformation of B2-R-B19＇ on cooling as well, but exhibits two-stage phase transformation of B19＇-R-B2 on heating. The Ni Ti sample aged at 573 K exhibits three-stage transformation on cooling due to local stress inhomogeneity and local composition inhomogeneity around the Ni4Ti3 precipitates.

Plastic yielding of NiTi shape memory alloy under local canning compression

As a new attempt, local canning compression was applied in order to implement large plastic deformation of nickel-titanium shape memory alloy （NiTi SMA） at room temperature. The plastic mechanics of local canning compression of NiTi SMA was analyzed according to the slab method as the well as plastic yield criterion. Transmission electron microscopy （TEM）, high resolution transmission electron microscopy （HRTEM） and scanning electron microscopy （SEM） were used to study the microstructural evolution as well as deformation behavior of NiTi samples under local canning compression. Increasing the hydrostatic pressure with the increase in the outer diameters of the steel cans is responsible for suppressing the initiation and growth of the micro-cracks, which contributes to enhancing the plasticity ofNiTi SMA and avoiding the occurrence of brittle fracture. Plastic deformation of NiTi SMA under a three-dimensional compressive stress state meets von-Mises yield criterion at the true strains ranging from about 0.15 to 0.50, while in the case of larger plastic strain, von-Mises yield criterion is unable to be met since the amorphous phase arises in the deformed NiTi sample.

Influence of cooling rate on phase transformation and microstructure of Ti-50.9%Ni shape memory alloy

Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature （Ms） and martensitic finish transformation temperature （Mf） decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis （Af-Mf） increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.

Microstructure and properties of laser micro welded joint of TiNi shape memory alloy

Butt welding of 0.2 mm-thick TiNi shape memory alloy sheet （SMA） was carried out using impulse laser, and tensile strength, fracture morphology, microstructure and phase change behaviour of welded joint were studied. The results show that using impulse laser can realize good butt welding of TiNi SMA sheet, tensile strength of welded joint is 683 MPa, which achieves 97% of that of cold rolled base metal, and the fracture mode of welded joint is ductile type as well as base metal. The welded joint can be divided into four zones according to grain size and microstructure. The microstructures of welded seam center zone are fine equiaxed crystals and the microstructures of both lower surface and upper surface edge zones are columnar crystals. When welded joint is vacuum annealed after welding, the phase transformation process is basically similar to the annealed base metal.

Microstructure,mechanical and shape memory properties of Ti-55Ta-xSi biomedical alloys

The effects of Si addition on microstructures, mechanical and shape memory properties of Ti-55Ta biomedical alloy were investigated. The results show that the microstructures consist of mainly α′′ martensite and a little β phase, and the grain size decreases obviously with increasing Si addition. When x = 0.2, small （Ti, Ta）3Si precipitates are formed at grain boundaries. With further increasing Si content, the amount of the precipitates gradually increases. The tensile and yield strength of Ti-55Ta-xSi alloys gradually increase with increasing Si addition, whereas elongation decreases. Ti-55Ta-0.1Si alloy exhibits the lowest elastic modulus and the best shape memory recoverable strain. It is revealed that the refinement of grain and the precipitation of （Ti, Ta）3Si phase are responsible to the changes of their mechanical and shape memory properties.