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.

Surface modification of NiTi alloy by sol-gel derived porous TiO_(2)film

Titania films with nano-sized pores were prepared on the NaOH？HCl pretreated NiTi alloy substrate by sol？gel method.A crack-free film is obtained for the sample with a dense inner layer and a porous outside layer（sample TC1＋1）.The X-ray diffraction shows that the titania films are composed of anatase,and a little Ni4Ti3 phase in the heat treated substrate is also detected.The X-ray photoelectron spectroscopy results indicate that the titania film completely covered the NiTi substrate for sample TC1＋1.The sample TC1＋1 is hydrophilic with a contact angle about 20°,and UV illumination treatment for 15 min further decreases the contact angle to（9.2±3.2）°.The potentiodynamic polarization test in 0.9% NaCl solution reveals a better corrosion resistance of sample TC1＋1 than the polished NiTi sample.

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.

Deposition of Bioactive Layer on NiTi Alloy by Chemical Treatment

A simple chemical method was developed for inducing bioactivity on NiTi alloys (50 at. pct by Ni/Ti). A layer of calcium phosphate was deposited on the surface to improve biocompatibility of the alloy. NiTi alloys were first etched in HNO3 aqueous solution, and then treated with boiling diluted NaOH solution. A rough surface was created and a thin TiO2 layer was formed on the surface. Pre-calcification was then introduced by immersing the treated NiTi alloys in supersaturated Na2HPO4 solution and supersaturated Ca(OH)2 solution in turn before calcification in simulated body fluid (SBF). A dense and uniform bonelike calcium phosphate (Ca-P) bioactive layer was formed on the surfaces of the specimen, which would improve their biocompatibility. Morphology and element analysis on NiTi surfaces during the treatments were investigated in detail by means of environment scanning electron microscopy (ESEM), energy dispersion X-ray spectroscopy (EDXS), and X-ray diffraction (XRD).

Investigation on the Mechanical Properties and Shape Memory Effect of Landing Buffer Structure Based on NiTi Alloy Printing

With the deepening of human research on deep space exploration,our research on the soft landing methods of landers has gradually deepened.Adding a buffer and energy-absorbing structure to the leg structure of the lander has become an effective design solution.Based on the energy-absorbing structure of the leg of the interstellar lander,this paper studies the appearance characteristics of the predatory feet of the Odontodactylus scyllarus.The predatory feet of the Odontodactylus scyllarus can not only hit the prey highly when preying,but also can easily withstand the huge counter-impact force.The predatory feet structure of the Odontodactylus scyllarus,like a symmetrical cone,shows excellent rigidity and energy absorption capacity.Inspired by this discovery,we used SLM technology to design and manufacture two nickel-titanium samples,which respectively show high elasticity,shape memory,and get better energy absorption capacity.This research provides an effective way to design and manufacture high-mechanical energy-absorbing buffer structures using bionic 3D printing technology and nickel-titanium alloys.

Analysis of phase transformation from austenite to martensite in NiTi alloy strips under uniaxial tension

Phase transformation from austenite to martensite in NiTi alloy strips under the uniaxial tension has been observed in experiments and numerically simulated as a localized deformation. This work presents an analysis using the theory of phase transformation. The jump of deformation gradient across the interface between two phases and the Maxwell relation are considered. Governing equations for the phase transformation are derived. The analysis is reduced to finding the minimum value of the loading at which the governing equations have a unique, real and physically acceptable solution. The equations are solved numerically and it is verified that the unique solution exists definitely. The Maxwell stress, the stresses and strains inside both austenite and martensite phases, and the transformation-front orientation angle are determined to be in reasonably good agreement with experimental observations.

Characterization of PEG-Like Macromolecular Coatings on Plasma Modified NiTi Alloy

A poly（ethylene glycol） （PEG-like） coating was developed to improve the biocom- patibility of Nickel-Titanium （NiTi） alloy implants. The PEG-like macromolecular coatings were deposited on NiTi substrates at a room temperature of 298 K through a ECR （electron-cyclotron resonance） cold-plasma enhanced chemical vapor deposition method using tetraglyme （CH3-O- （CH2-CH2-O）4-CH3） as a precursor. A power supply with a frequency of 2.45 GHz was applied to ignite the plasma with Ar（argon） used as the carrier gas. Based on the atomic force microscopy （AFM） studies, a thin smooth coating on NiTi substrates with highly amorphous functional groups on the modified NiTi surfaces were mainly the same accumulated stoichiometric ratio of C and O with PEG. The vitro studies showed that platelet-rich plasma （PRP） adsorption on the modified NiTi alloy surface was significantly reduced. This study indicated that plasma surface modification changes the surface components of NiTi alloy and subsequently improves its biocompatibility.

Electrochemical Corrosion Behavior of the DLC Coated and Uncoated NiTi Alloys

A dense and well-adhered diamond-like carbon （DLC） coating was prepared on the nickel-titanium （NiTi） alloys by plasma immersion ion implantation and deposition （PIIID）. Potentiodynamic polarization tests indicated the corrosion resistance of the NiTi alloys was markedly improved by the DLC coating. The Ni ions release of the NiTi alloys was effectively blocked by the DLC coating.

Simulation research on heat-affected zone of the NiTi alloy plasma arc welded joints

In order to study the compositions and properties of NiTi alloy plasma arc welded joints, different temperatures of the heat-affected zone（HAZ） are simulated. Temperatures are set at 20 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃ and 1 000 ℃. X-ray diffraction results of the thermal simulated specimens show that NiTi2 phase emerges in the alloy when the temperature is between 500 ℃ and 800 ℃, while NiTi2 and Ni3 Ti phases emerge when the temperature is between 800 ℃ and 1 000 ℃. Tensile strengths of specimens at 600 ℃ and 900 ℃ are only 68. 0% and 61.3% of the strength at the room temperature respectively due to the emergence of NiTi2 and Ni3 Ti phases.

Influence of Ti Powder Characteristics on Combustion Synthesis of Porous NiTi Alloy

Porous NiTi shape memory alloy (SMA) is a novel biomedical material used for human hard tissue implant. The influence of elemental titanium powder characteristics such as powder morphology, particle size and specific surface area (SSA) on the minimal ignition temperature, combustion temperature and final product of porous NiTi SMA fabricated by combustion synthesis method was investigated in this paper by scanning electron microscopy (SEM) and laser diffraction. The preliminary data indicated that the titanium powder characteristics had a strong effect on combustion synthesis of porous NiTi SMA.

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.

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.

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.

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.

Finite element simulation of ball spinning of NiTi shape memory alloy tube based on variable temperature field

As a new attempt,ball spinning was used to manufacture the nickel-titanium shape memory alloy（NiTi SMA） tube at elevated temperature.The NiTi bar with a nominal composition of Ni50.9Ti49.1（mole fraction,%） was solution treated and was used as the original tube blank for ball spinning.Based on the variable temperature field and the constitutive equation,rigid-viscoplastic finite element method（FEM） was applied in order to simulate the ball spinning of NiTi SMA tube.The temperature field,the stress field,the strain field and the load prediction were obtained by means of FEM.FEM results reveal that there is a temperature increase of about 160 ℃ in the principal deformation zone of the spun part.It can be found from the stress fields and the strain fields that the outer wall of NiTi SMA tube is easier to meet the plastic yield criterion than the inner wall,and the plastic deformation zone is caused to be in a three-dimensional compressive stress state.The radial strain and the tangential strain are characterized by the compressive strain,while the axial strain belongs to the tensile strain.The variation of spinning loads with the progression of the ball is of great importance in predicting the stable flow of the spun part.

Sol–gel derived Ta-containing TiO_2 films on surface roughened NiTi alloy

The surface of NiTi alloy was roughened by NaOH-HC1 treatment, and the Ta-containing TiO2 films were coated on the pretreated NiTi alloy by the sol-gel method. Thermal analyses indicate that the evaporation temperature of the organics decreases with the addition of tantalum ethoxide in the TiO2 sol, but the crystallization temperature of anatase increases. The NaOH-HC1 pre- treatment improves the film integrity, but cracks still form in the films at high Ta contents （_〉20 %, molar ratio） owing to the increasing film thickness. X-ray diffraction （XRD） confirms that the addition of Ta suppresses the crystalli- zation of anatase. X-ray photoelectron spectroscopy （XPS） reveals that Ta exists as Ta2Os in the film. With the increase of Ta content, the hydrophilic conversion of the films under UV illumination is impeded, but their corrosion resistance in 0.9 % NaC1 solution increases, tested by the potentiodynamic polarization. The coating samples have acceptable hemolysis ratios for biomaterials （〈5）. The introduction of Ta improves the anti-aggregating function of the TiO2 film in the platelet adhesion test.

Effects of Anodic Voltages on Microstructure and Properties of Plasma Electrolytic Oxidation Coatings on Biomedical NiTi Alloy

Plasma electrolytic oxidation （PEO） coatings, formed under various anodic voltages （320-440 V） on biomedical NiTi alloy, are mainly composed of γ-AI203 crystal phase. The evolution of discharging sparks during the PEO process under different anodic voltages was observed. The surface and cross-sectional morphologies, composition, bonding strength, wear resistance and corrosion resistance of the coatings were investigated by scanning electron microscopy （SEM）, thin-film X-ray diffraction （TF-XRD）, energy dispersive X-ray spectrometry （EDS）, surface roughness, direct pull-off test, ball-on-disk friction and wear test and potentiodynamic polarization test, respectively. The results showed that the evolution of discharging sparks during the PEO process directly influenced the microstructure of the PEO coatings and further influences the properties. When the anodic voltage increased from 320 V to 400 V, the corrosion resistance and wear resistance of the coatings slowly increased, and all the bonding strength was higher than 60 MPa; further increasing the anodic voltages, especially up to 440 V, although the thickness and γ-AI203 crystallinity of the coatings further increased, the microstructure and properties of the coatings were obviously deteriorated.

Constitutive equation and processing map of equiatomic NiTi shape memory alloy under hot plastic deformation

In order to describe the deformation behavior and the hot workability of equiatomic NiTi shape memory alloy (SMA) during hot deformation, Arrhenius-type constitutive equation and hot processing map of the alloy were developed by hot compression tests at temperatures ranging from 500 to 1100 °C and strain rates ranging from 0.0005 to 0.5 s?1. The results show that the instability region of the hot processing map increases with the increase of deformation extent. The instability occurs in the low and high temperature regions. The instability region presents the adiabatic shear bands at low temperatures, but it exhibits the abnormal growth of the grains at high temperatures. Consequently, it is necessary to avoid processing the equiatomic NiTi SMA in these regions. It is preferable to process the NiTi SMA at the temperatures ranging from 750 to 900 °C.

On the Martensitic Transformation Temperatures and Mechanical Properties of NiTi Alloy Manufactured by Selective Laser Melting: Effect of Remelting

In this study, the infl uence of laser remelting on the relative density, martensitic transformation temperatures(MTTs), and mechanical properties of a NiTi alloy fabricated by selective laser melting(SLM) at a laser power between 15 and 75 W were investigated. A relative alloy density of approximately 99% was achieved in the power range of 45–60 W corresponding to the forming energy density range of 65.45–87.27 J/mm3. The MTTs increased with the increase in the energy density;thus, the initial contents of the B2 and B19′ phases of the SLM-produced NiTi alloy can be tailored by the utilized technique. However, the number of defects such as metallurgical pores and microcracks considerably increased at higher energy densities(> 87.27 J/mm3). Interestingly, the concentration of these defects was reduced by remelting in the energy density range of 21.82–65.45 J/mm3, while the alloy relative density increased to 99.7% ± 0.1% at a remelting energy density of 65.45 J/mm3. The results of tensile testing revealed that when the remelting energy was 75% or 100% of the forming energy input, the ultimate tensile strength and elongation of the alloy significantly increased. Therefore, the remelting strategy represents a promising route for manufacturing NiTi alloys with desired MTT ranges and mechanical properties.