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.

Using shape-memory alloy staples to treat comminuted manubrium sterni fractures: A case report

BACKGROUND Comminuted manubrium sterni fractures are rare,and internal fixation methods are limited.This report explored a practical and feasible method of internal fixation for comminuted manubrium sterni fractures.CASE SUMMARY A 17-year-old female was injured in a car accident for which she underwent debridement and suturing of her head and anterior chest wounds in another hospital.Eight days later,the patient was transferred to our hospital for surgical treatment.The manubrium sterni was found intraoperatively to be split into three irregular fragments with obvious overlap and separation displacement.Meanwhile,a manubriosternal joint dislocation and left first rib cartilage fracture were observed.The retraction force of the shape-memory alloy staples was used to pull the fracture fragments together.Two more titanium locking plates were then used to fix the manubrium sterni and corpus sterni longitudinally,and the left first rib cartilage fracture was repositioned and fixed with a titanium locking plate.A postoperative computed tomography scan showed reduced and rigid fixation of the comminuted manubrium sterni fractures.The patient recovered well with no significant complaints of discomfort.The patient was discharged 10 days postoperatively after the stitches had been removed.CONCLUSION Shape-memory alloy staples had the advantage of being safe and effective during the repositioning and internal fixation of comminuted manubrium sterni fractures.Therefore,they provided a new surgical option for comminuted manubrium sterni fractures.

Giant Magneto-Impedance of Co50Ni22Ga28 Alloy with High Chemical Ordering

The microstructure of CosoNi22Ga28 ribbon with the L10 structure is examined. The band-like morphology is observed. These bands with the width in a range of 40-200 nm appear along the transverse direction of the ribbon. The giant magnetoimpedance （GMI） effect in this alloy is measured. The results show that Co5oNi22Ga28 exhibits a sharp peak of the GAI effect. The maximum GAH ratio up to 360% is detected. The GMI effect measured versus temperature shows large jumps of the magnetoimpedance amplitude at the reversal martensitic transformation temperature 240℃ and Curie temperature 375℃C respectively. The jump ratios of the magnetoimpedance amplitude examined at these temperatures are about 5 and 10, respectively.

Magnetic-Field-Induced Strains of Bonded Ni-Mn-Ga Melt-Spun Ribbons

Non-stoichiometric Ni50Mn27 Ga23 polycrystalline ribbons are prepared by melt-spinning technique. The magneticfield-induced strain （MFIS） of Ni-Mn-Ga bulk alloy prepared by bonding the melt-spun ribbons is obtained. The experimental results show that Ni50Mn27Ga23 bonded ribbons exhibit a typical thermal-elastic shape memory effect in the thickness direction. The martensitic transformation strain of bonded ribbons is an expansive strain of about 0.3% without the magnetic field and a contractive strain of about -0.46% at the magnetic field of 1 T. The field can not only enhance the value of the martensitic transformation strain of the bonded ribbons, but can also change the direction of the strain. The bonded ribbons alloy presents negative MFIS and obtains a larger value of the strain though influenced by the adhesive between the ribbons. Therefore, the preparation technique of the Ni-Mn-Ga bulk alloy by bonding melt-spun ribbons is helpful to get rid of the size restriction of the ribbon and to broaden the applications of the ribbons.

Microstructures and mechanical properties of nanocrystalline NiTi intermetallics formed by mechanosynthesis

The formulation of nanocrystallinc NiTi shape memory alloys has potential effects in mechanical stimulation and medical im- plantology. The present work elucidates the effect of milling time on the product＇s structural characteristics, chemical composition, and mi- crohardness for NiTi synthesized by mechanical alloying for different milling durations. Increasing the milling duration led to the formation of a nanocrystalline NiTi intermetallic at a higher level. The formation of nanocrystalline materials was directed through cold fusion, fractur- ing, and the development of a steady state, which were influenced by the accumulation of strain energy. In the morphological study, uninter- rupted cold diffusion and fracturing were visualized using transmission electron microscopy. Particle size analysis revealed that the mean particle size was reduced to -93 μm after 20 h of milling. The mechanical strength was enhanced by the formation of a nanocrystalline in- termetallic phase at longer milling time, which was confirmed by the results of Vickers hardness analyses.

A fully solid-state cold thermal energy storage device for car seats using shape-memory alloys

Thermal energy storage has been a pivotal technology to fill the gap between energy demands and energy supplies.As a solid-solid phase change material,shape-memory alloys(SMAs)have the inherent advantages of leakage free,no encapsulation,negligible volume variation,as well as superior energy storage properties such as high thermal conductivity(compared with ice and paraffin)and volumetric energy density,making them excellent thermal energy storage materials.Considering these characteristics,the design of the shape-memory alloy based the cold thermal energy storage system for precooling car seat application is introduced in this paper based on the proposed shape-memory alloy-based cold thermal energy storage cycle.The simulation results show that the minimum temperature of the metal boss under the seat reaches 26.2°C at 9.85 s,which is reduced by 9.8°C,and the energy storage efficiency of the device is 66%.The influence of initial temperature,elastocaloric materials,and the shape-memory alloy geometry scheme on the performance of car seat cold thermal energy storage devices is also discussed.Since SMAs are both solid-state refrigerants and thermal energy storage materials,hopefully the proposed concept can promote the development of more promising shape-memory alloy-based cold and hot thermal energy storage devices.

Improving exposure of anodically ordered Ni-Ti-O and corrosion resistance and biological properties of NiTi alloys by substrate electropolishing

Although Ni-Ti-O nanopores(NPs) can be fabricated by anodization of mechanically polished NiTi alloys, the top disordered layer is difficult to remove thus hindering the functionality of the Ni-Ti-O NPs. In this work, an electropolishing(EP) pretreatment was performed on the NiTi substrate prior to anodization to thoroughly expose the NPs. Our results show that the EP pretreatment for 5 min perfectly removes the top disordered layer on the Ni-Ti-O NPs to expose the underlying NPs and consequently, the corrosion resistance and antibacterial ability are enhanced. The exposed NPs can elongate bone marrow mesenchymal stem cells, which may be responsible for the upregulated alkaline phosphatase activity, secretion of Type I collagen, and extracellular matrix mineralization. These results suggest that EP is a desirable pretreatment before anodization of the NiTi alloys because the irregular surface layer on the Ni-Ti-O NPs can be removed to enhance the corrosion resistance and biological functions.

Self-assembled nanosheets on NiTi alloy facilitate endothelial cell function and manipulate macrophage immune response

Stenting has been widely adopted for the treatment of cardiovascular diseases,but the complications such as in-stent restenosis and late stent thrombosis cannot be completely avoided,which are closely related to endothelial dysfunction and inflammatory response.In the present work,oxide nanosheets were grown on the surface of nearly equiatomic Ni Ti alloy by alkaline corrosion(AC),aiming at yielding favorable endothelial functionality and immune microenvironment.The results show nanosheets mainly composed of TiO_(2),Ni(OH)_(2),and K_(2)TiO_(3)can be grown on the alloy in KOH solution of 2.5–15 M at room temperature.The AC-treated samples significantly promote endothelial cell(EC)functionality such as proliferation,migration,NO production,VEGF secretion,and angiogenesis.In addition,the sample grown in KOH of 15 M can switch macrophages to an anti-inflammatory M2 phenotype and up-regulate the gene expression of VEGF to facilitate EC functionality.These results demonstrate that the nanosheets can directly and indirectly up-regulate EC functionality,possibly leading to rapid re-endothelialization of the stents thus addressing the stent-related complications.

Porous nickel–titanium alloy prepared by gel-casting

To explore the preparation of porous nickel- titanium alloy with excellent properties, larger size and complex shape, the premixed powder of Ni and Ti with atomic ratio of 1：1 was shaped by gel-casting. The effects of solids loading and the content of dispersant on flow ability of nickel-titanium slurry and the mechanical properties of nickel-titanium sintered body were studied. The drying models under different solids loading were also discussed. The results show that the viscosity of slurries significantly increases with an increase in solids loading. After a proper process of drying, the green body with complex shape is obtained. The sintered body with porosity rate reaching up to 49.5 % and compression strength reaching to 364.74 MPa could meet the basic demands of implant materials.

Corrosion behavior,antibacterial ability,and osteogenic activity of Zn-incorporated Ni-Ti-O nanopore layers on NiTi alloy

Development of bone fixation devices with excellent corrosion resistance,antibacterial ability,and osteogenic activity is critical for promoting fracture healing.In this study,Zn-incorporated nanopore(NP)layers were prepared on the NiTi alloy through anodization and hydrothermal treatment.Results show that Zn can be evenly incorporated into the NP layers in the form of ZnTiO_(2).The Zn-incorporated samples exhibit good corrosion resistance and significantly reduce Ni^(2+)release.Meanwhile,the samples can continuously release Zn^(2+),which is responsible for excellent long-term antibacterial ability.Furthermore,the synergetic effect of Zn^(2+) release and nanoporous structure of the NP layers endues the NiTi alloy excellent osteogenic activity,as verified by upregulated alkaline phosphatase activity,secretion of type I collagen,and extracellular matrix mineralization.Therefore,Zn-incorporated Ni-Ti-O NP layers have great potential as biomedical coatings of NiTi-based implant materials.

Electrically Actuated Shape Recovery of NiTi Components Processed by Laser Powder Bed Fusion after Regulating the Dimensional Accuracy and Phase Transformation Behavior

To develop self-recovery intelligent components based on resistance heating and obtain satisfactory performance in practical applications,this study optimized the forming quality,dimensional accuracy,and phase transformation temperatures of Nickel-titanium(NiTi)alloys by controlling the process parameters.The tensile properties and shape-memory effects of the NiTi alloys prepared using the optimized process were clarified.The relationship between the change in temperature and the shape recovery process of the deformed structure under electrical excitation was investigated.The results show that the suitable processing window for ensuring the forming quality without noticeable distortion and macro cracks depends on the laser parameters.In both the X and Y directions,the measured dimensions increased with an increase in laser power and first decreased and then stabilized with an increase in scanning speed.The XRD results showed that all the as-built samples consisted of B2 austenite and B19’martensite phases and Ni3Ti.Mechanical tests suggested that excellent tensile properties with a tensile strength of 753.28 MPa and elongation of 6.81%could be obtained under the optimal parameters of 250 W and 1200 mm/s.An excellent shape-recovery rate of 88.23%was achieved under the optimal parameters.Subsequently,chiral lattice structures were successfully fabricated by laser powder bed fusion(LPBF)under the optimal parameters,and a shape-recovery rate of 96.7%was achieved under electrical actuation for a structure with a pre-compressed strain of 20%.This study also found that the temperatures at the grasp regions were always higher than those at other positions because of the generation of contact resistance at the grasp regions.This facilitates the rapid recovery of the structure at the grasp regions,which has important implications for the design iteration of NiTi smart components.