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.

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.

Laser powder bed fusion additive manufacturing of NiTi shape memory alloys: a review

NiTi alloys have drawn significant attentions in biomedical and aerospace fields due to their unique shape memory effect(SME),superelasticity(SE),damping characteristics,high corrosion resistance,and good biocompatibility.Because of the unsatisfying processabilities and manufacturing requirements of complex NiTi components,additive manufacturing technology,especially laser powder bed fusion(LPBF),is appropriate for fabricating NiTi products.This paper comprehensively summarizes recent research on the NiTi alloys fabricated by LPBF,including printability,microstructural characteristics,phase transformation behaviors,lattice structures,and applications.Process parameters and microstructural features mainly influence the printability of LPBF-processed NiTi alloys.The phase transformation behaviors between austenite and martensite phases,phase transformation temperatures,and an overview of the influencing factors are summarized in this paper.This paper provides a comprehensive review of the mechanical properties with unique strain-stress responses,which comprise tensile mechanical properties,thermomechanical properties(e.g.critical stress to induce martensitic transformation,thermo-recoverable strain,and SE strain),damping properties and hardness.Moreover,several common structures(e.g.a negative Poisson’s ratio structure and a diamond-like structure)are considered,and the corresponding studies are summarized.It illustrates the various fields of application,including biological scaffolds,shock absorbers,and driving devices.In the end,the paper concludes with the main achievements from the recent studies and puts forward the limitations and development tendencies in the future.

Effects of Ta Addition on NiTi Shape Memory Alloys

The effect of Ta addition on the martensitic transformation characteristics and the X-ray visibility on NiTi shape memory alloy have been studied in (Ni51Ti49)1-xTax system. It was found that the transformation temperatures of the Ni51Ti49 binary alloy increased drastically by an addition of 0～4 at. pet Ta, but only slightly when the concentration exceeded 4 at. pct; the addition of Ta greatly decreases the sensitivity of the martensitic transformations to the variation in the Ni-Ti ratio. The addition of Ta to the NiTi binary alloy can improve its X-ray visibility.

Improving the Mechanical and Tribological Properties of NiTi Alloys by Combining Cryo-Rolling and Post-Annealing

By combining cryo-rolling and post-annealing treatments,the nanostructured NiTi alloy is produced.A diff erential scanning calorimetry measurement was used to test the eff ect of the preparation process on phase transformation.The cryo-rolling changes the tensile fracture of NiTi alloy to a ductile manner.Interestingly,the recovered structure exhibits signifi cant strength improvement,while the tensile plasticity is still comparable to that of the coarse-grained structure.This optimized mechanical performance is due to the strengthening eff ect of refi ned microstructure and the high work hardening capability rendered by moderate dislocation density.Ball-on-plate reciprocating dry-sliding wear test reveals that the nanostructured NiTi alloy also has enhanced wear resistance,which is primarily ascribed to the high content of residue martensite formed during cryo-rolling.These results provide an eff ective route to optimize the mechanical and wear properties of NiTi alloys.

Review on structural fatigue of NiTi shape memory alloys:Pure mechanical and thermo-mechanical ones

Structural fatigue of NiTi shape memory alloys is a key issue that should be solved in order to promote their engineering applications and utilize their unique shape memory effect and super-elasticity more sufficiently. In this paper, the latest progresses made in experimental and theoretical analyses for the structural fatigue features of NiTi shape memory alloys are reviewed. First, macroscopic experimental observations to the pure mechanical and thermo-mechanical fatigue features of the alloys are summarized; then the state-of-arts in the mechanism analysis of fatigue rupture are addressed; further, advances in the construction of fatigue failure models are provided; finally, summary and future topics are outlined.

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.

Effect of Texture on the Grain-Size-Dependent Functional Properties of NiTi Shape Memory Alloys and Texture Gradient Design:A Phase Field Study

Texture is inevitably introduced during the manufacturing of most NiTi shape memory alloys(SMAs),and the textured nanocrystalline NiTi has been extensively employed in engineering.However,the effect of texture,and the joint effect of grain size(GS)and texture on the functional properties of NiTi SMAs and the corresponding microscopic mechanisms have not been clearly understood yet.In this work,based on the phase field method,the effect of texture on the GS-dependent functional properties of NiTi SMAs,including super-elasticity(SE),one-way shape memory effect(OWSME),and stress-assisted two-way shape memory effect(SATWSME),is investigated,and the corresponding microscopic mechanisms are revealed.Moreover,the samples with discrete geometrical gradients and/or texture gradients are designed to achieve graded functional properties.The simulation results indicate that the dependence of functional properties on texture is due to the effect of crystallographic orientation on martensite transformation and reorientation,which can lead to different inelastic strains.In the designed samples with texture gradients,the stress–strain responses of sheets with various textures are different,allowing for the coordination of overall deformation of the sample by combining such sheets,with varying inelastic deformation degrees.Thus,the overall response of the sample differs from that without texture gradient,leading to the achievement of graded functional properties.The simulation results and new findings in this work contribute to a deeper understanding of the effects of texture,GS,and their interaction on the functional properties of SMAs,and provide valuable reference for the design and development of SMA-based devices with desired functional properties.

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.

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).

The Theoretical Study of O_2 Adsorption on NiTi (100) and (110) Surfaces

The discrete variational Xα method (DV-Xα) within the framework of density-functional theory was applied to study O 2 molecule adsorption on NiTi (100) and (110) surfaces.The bond order and charge distribution between Ti and O atoms for two possible O 2 molecule adsorption ways on NiTi(100) and (110) surfaces were calculated.It is found that the adsorption way for O-O bond perpendicular to NiTi surface is preferred to that for O-O bond parallel to NiTi surface,and O 2 molecule only interacted with one nearest surface titanium atom during the adsorption process.Mulliken population and the partial density of state analysis show that the interaction between Ti and O atoms is mainly donated by O 2p and Ti 4s electrons on NiTi(110) surface,O 2p and Ti 4s,4p electrons on NiTi(100) surface,respectively.The total density of state analysis shows that NiTi(100) surface is more favorable for O 2 molecule adsorption.

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.

Microstructural Evolution of Plastic Deformation of NiTi Shape Memory Alloy at Low Temperature

Nickel-titanium shape memory alloy （NiTi SMA） which possesses crystal structure of B2 austenite at room temperature was subjected to plastic deformation at low temperature （-150 ℃） by means of local canning compression. The microstructural evolution of NiTi SMA at the different deformation degree was investigated by transmission electron microscopy （TEM） and high resolution transmission electron microscopy （HRTEM）. At the deformation degree by 15%, a high density of dislocations occurs in the deformed NiTi sample. At the deformation degree by 25%, the deformed NiTi sample exhibits the martensite morphology due to the pinning of dislocations at the grain boundaries. At the deformation degree by 50%, a small amount of nanocrystalline phase arises in the deformed NiTi sample. At the deformation degree by 80%, severe plastic deformation （SPD） leads to the occurrence of a great deal of amorphous and nanocrystalline phase,

Electrolytic Passivation of Nitinol Shape Memory Alloy in Different Electrolytes

The corrosion behavior of the nitinol alloy was studied in various corrosion media of different Cl^- ion concentrations. The results demonstrate that the Cl^- ion concentration has significant influences on the corrosion behavior of the nitinol alloy. In order to enhance the corrosion resistance, protective films were generated on the surface of the nitinol alloy by means of the electrochemical passivation method, for which five different electrolytic solutions were investigated. The surface analysis indicates full growth of all samples passivated in the different electrolytic solutions with layers, however, showing different morphological features. Without any defects like micro-cracks and pores, the surface of the samples passivated in the molybdate solution turns out smoother and denser than those passivated in other solutions. It is shown that the electro-chemical passivation will reduce Ni content but increase Ti content in the surface, reaching the Mole ratio of Ti：Ni = 9.01：1 on the outermost surface. Potentiodynamic polarization test demonstrates that the samples electrochemically passivated in the molybdate solution present a significant increase in breakdown potential due to titanium enrichment on the outermost surface.

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.

Effect of Nitrogen Ion Implantation on the Structure and Corrosion Resistance of Equiatomic NiTi Shape Memory Alloy

To protect the surface of NiTi from corrosion, an ion implantation method was proposed. In the present work, a surface oxidized sample was implanted with nitrogen at energy of 100 keV. The corrosion resistarwe property was examined by the anodic polarization method in a simulated body fluid （SBF） at a temperature of 37 ℃ and contrasted to non-implanted NiTi samples. The composition and structure of the implanted layers were investigated by XPS. The experimental results from the electrochemical measurements provide an evidence that the nitrogen ion-implantation increases the corrosion resistance of NiTi shape memory alloy.

Novel Impact Absorption Device Using NiTi Shape Memory Alloy Strips

The aim is to propose and design a kind of novel impact absorption devices using constant-force elements made from Ni Ti shape memory alloy( SMA) strips for safety protection.The availability evaluation results indicate that the constant-force elements can absorb over one half of the impact energy for its martensite transformation and thus the maximum impact force is reduced by nearly 80%.Compared with the ordinary cylindrical compression spring,the device's maximum impact force is reduced by nearly 50%,otherwise it has a very compact structure and insensitivity to the varying impact,and thus it is especially suitable for narrow space and safety purpose.

Catastrophe Theory Models for Stress-dependent Behaviour in a NiTi Shape Memory Alloy

The measurements by Huibin XU et al of the stress-dependence ot hysteresis in a NiTi shape memo ry alloy are modeled by catastrophe theory. The cusp catastrophe is used with the strain as the behaviour variable and the control parameters being functions of the stress and the temperature. A two constant model is found to be preferred to a four constant model.

Deformation Mechanism of Hot Spinning of NiTi Shape Memory Alloy Tube Based on FEM

As a successively and locally plastic deformation process, ball spinning is applied to manufacturing thin-walled Nickel-Titanium shape memory alloy （NiTi SMA） tube at high temperature. NiTi SMA tube blank belongs to the as-cast state which consists of a lot of dendritic grains and a few equiaxed grains. The compression tests of NiTi SMA were carried out at various strain rates at high temperature in order to obtain the constitutive model of NiTi SMA. Because NiTi SMA is sensitive to the strain rates at high temperature, rigid-viscoplastic finite element method （FEM） is used to simulate ball spinning of thin-walled NiTi SMA tube in order to analyze the deformation behavior of ball spinning of NiTi SMA tube. Stress fields, strain fields as well as velocity fields is obtained by means of rigid-viscoplastic FEM, which lays the profound foundations for studying the metal flow rule in ball spinning and forming perfect spun NiTi SMA tube.