Surface characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi alloy

NiTi shape memory alloy has been modified by Nb implantation with different implantation parameters including incident dose and current. The surface morphology and chemical components are determined by atomic force microscopy (AFM), Auger Electron Spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results show that Nb implantation leads to the formation of compact Nb₂O₅/TiO₂ thin film about 30 nm in thickness on the surface of the NiTi alloy and decreases the surface concentration of Ni. A larger incident dose or incident current causes a higher surface roughness and a higher Nb content in the implantation layer of NiTi alloy. The nano-indentation measurements indicate the obvious reduction of both nano-hardness and Young's modulus of the Nb implanted NiTi alloy in the implantation layer and even in deeper NiTi matrix. The results of potentiodynamic polarization test show that the corrosion resistance of NiTi alloy in Hanks's solution has been evidently improved by Nb implantation. The NiTi alloy with a moderate implantation parameter of 1.5 × 10¹⁷ ions/cm² and 2 mA exhibits the best corrosion resistance ability.     

Nickel titanium and nickel titanium hafnium shape memory alloy thin films

Shape memory alloy (SMA) coatings of NiTi and NiTiHf have been deposited onto Si substrates using pulse DC sputtering. Coatings of NiTi with compositions containing 45 to 65 at.% Ti have been fabricated by co-sputtering NiTi with Ti. NiTiHf coatings with Hf compositions ranging from 2 to 30 at.% Hf have been fabricated by co-sputtering NiTi with Hf. XRD results reveal the as-deposited coatings as amorphous. A high temperature, 1100 °C anneal followed by a low temperature, 550 °C anneal was employed to crystallise the coatings. The XRD then shows the coatings to be martensitic at room temperature.Two sets of samples were produced for characterisation; one set was used for indentation studies and the other set used to prepare freestanding films required for differential scanning calorimetry (DSC) studies.Using the DSC, a NiTi coating containing 52 at.% Ti shows an endothermic austenite peak phase transformation, (A_p) at around 105 °C and an exothermic peak martensite phase transformation, (M_p) at 65 °C, resulting in a hysteresis of 40 °C. For a NiTi coating containing 65 at.% Ti the hysteresis remained unchanged at 40 °C, but there was a decrease in the phase transformation enthalpies when compared with the coatings containing 52 at.% Ti. Calculated phase transformation enthalpies in the NiTi coatings ranged from 6 to 13 J/g for the austenite phase and − 8 to − 11 J/g for the martensite phase.The NiTiHf coating shows SMA behaviour for a film containing 30 at.% Hf. DSC reveals an ‘R’ phase transition in this film. It is understood that this phase is present in films that have high internal stresses and is understood to nucleate near Ti₃Ni₄ precipitates. Phase transformation temperatures occur at 98 °C and 149 °C during heating and occur at 99 °C during cooling. Phase transformation enthalpies range between 2 and 3 J/g for the austenite phase and − 7 J/g for the martensite phase.A scratch tester equipped with a 5 mm spherical tip has been utilised with loads ranging from 1 to 5 N to determine the recovery properties of the films. The results in this study conclude that NiTi films containing 65 at.% Ti deform 3 times more than films containing 52 at.% Ti. For NiTiHf thin films, increasing the Hf composition from 2 at.% to 30 at.%, doubled the deformation measured in the coatings.     

Selective surface oxidation and nitridation of NiTi shape memory alloys by reduction annealing

The current investigation aims in understanding the effect of short-term (300 s) annealing of NiTi shape memory alloys (SMAs) in a reducing atmosphere of N₂-10% H₂. The influence of temperature on the resulting surface morphology and chemistry is elucidated. On annealing at 600 °C, the surface is covered with a thin layer of titanium oxide, which is 7.5 nm thick, while at 800 °C, the surface is covered with a golden-yellow layer of TiN of thickness more than 100 nm. The surface analysis carried out by XPS on the specimen annealed at 800 °C confirms the formation of TiN and more notably, the surface is devoid of Ni.     

Surface treatment of NiTi shape memory alloy and its influence on corrosion behavior

Influence of various surface treatments of a NiTi alloy on its surface chemistry and corrosion resistance was studied. NiTi (50.9 at.% Ni) alloy was subjected to mechanical polishing (MP), chemical etching (CHE) in an acidic bath, combination of mechanical polishing and oxidation at 530 °C/10 min in air (MPO) and combination of chemical etching and oxidation at 530 °C/10 min in air (CHEO). Scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectrometry were used for the surface examination. Corrosion behavior was determined by measuring potentiodynamic curves and nickel release into a model physiological solution with pH = 2. It was shown that oxidation at 530 °C has a slightly retarding effect on the nickel release in the case of MP material. However, in the case of CHE material, oxidation negatively influences corrosion, i.e. it strongly accelerates the nickel release, despite that it produces a relatively thick Ni-depleted oxide layer. Chemical etching was evidenced to produce the best corrosion performance in terms of nickel release. By comparing the MP and MPO sample, it was evidenced that oxidation significantly enhances susceptibility to the pitting corrosion. The findings obtained in our work were discussed in relation to variations in the surface chemistry and structure after different treatments.     

Corrosion loss of mild steel in high temperature hard freshwater

This paper presents an interpretation of corrosion loss data for mild steel coupons exposed for up to 2.6 years to freshwater under a range of dissolved oxygen levels and temperatures 50-60 °C. The water total alkalinity was 0.5-0.6 mmol/l and pH 8.7-9.2. It is shown herein that the data is consistent with the early stages of the corrosion loss model proposed earlier for steel exposed to seawater, brackish water or freshwaters in the usual environmental range of 0-30 °C. The new data was found to be consistent with the effect of dissolved oxygen and the effect of water hardness on the model.     

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers >10⁸) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys.     

Microstructure and mechanical properties of magnesium composites prepared by spark plasma sintering technology

Spark plasma sintering (SPS) technology was used to determine the appropriate conditions for SPS sintering of commercially pure magnesium as well as the magnesium alloy AZ31. It was found that the sintering temperatures of 585 °C and 552 °C were the most suitable sintering temperatures for the magnesium and the AZ31 alloy, respectively. Magnesium matrix and AZ31 alloy matrix composites reinforced with SiC particles were then successfully fabricated by the SPS method at sintering temperatures of 585 °C and 552 °C, respectively. A uniform distribution of SiC particles was observed along the boundary between matrix particles. The mechanical properties, i.e. hardness and tensile strength increased with increasing SiC content up to 10 wt%. However, when the SiC content was larger than 10 wt%, the tensile strength decreased due to the agglomeration of SiC particles. The agglomeration of SiC particles was found to lead to the degradation of the interfacial bonding strength between matrix and reinforcement.     

A comparative study on titania layers formed on Ti, Ti-6Al-4V and NiTi shape memory alloy through a low temperature oxidation process

For improving the bioactivity and biocompatibility of metals for medical applications, anatase titania layers were synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using the H₂O₂-oxidation and hot water aging treatment method at 80 °C. The thickness of the titania layers on Ti, Ti-6Al-4V and NiTi SMA was 7.43 ± 0.93 μm, 3.14 ± 0.38 μm and 4.04 ± 0.25 μm, respectively. X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis indicated that the titania layers formed were poorly crystalline anatase. Fourier transform infrared spectroscopy (FTIR) suggested that abundant Ti-OH functional groups were produced on titania, which could improve bioactivity of the metals. In addition, the titania layer formed on Ti substrate was shown to contain more molecularly chemisorbed water and Ti-OH functional groups than those on Ti-6Al-4V and NiTi SMA. Atomic force microscopic (AFM) results showed that the surface roughness values of metal samples depended on the scanning size and that surface roughness of samples significantly increased after the H₂O₂-oxidation and hot water aging treatment for all three metals. Compared to Ti-6Al-4V and NiTi SMA, the H₂O₂-treated and aged Ti samples exhibited the roughest surface. The wettability of samples was evaluated through water contact angle measurements. After the H₂O₂-oxidation treatment, the three metals exhibited high hydrophilicity. The bonding strength of titania layers on Ti, Ti-6Al-4V and NiTi was also investigated. Potentiodynamic polarization tests indicated that the corrosion resistance of H₂O₂-treated and aged Ti, Ti-6Al-4V and NiTi SMA was significantly improved due to the titania layer formation.     

Sn buffered by shape memory effect of NiTi alloys as high-performance anodes for lithium ion batteries

By applying the shape memory effect of the NiTi alloys to buffer the Sn anodes, we demonstrate a simple approach to overcome a long-standing challenge of Sn anode in the applications of Li-ion batteries – the capacity decay. By supporting the Sn anodes with NiTi shape memory alloys, the large volume change of Sn anodes due to lithiation and delithiation can be effectively accommodated, based on the stress-induced martensitic transformation and superelastic recovery of the NiTi matrix respectively, which leads to a decrease in the internal stress and closing of cracks in Sn anodes. Accordingly, stable cycleability (630 mA h g^?1 after 100 cycles at 0.7C) and excellent high-rate capabilities (478 mA h g^?1 at 6.7C) were attained with the NiTi/Sn/NiTi film electrode. These shape memory alloys can also combine with other high-capacity metallic anodes, such as Si, Sb, Al, and improve their cycle performance.     

Preparation and characterization of hydroxyapatite/titania composite coating on NiTi alloy by electrochemical deposition

NiTi alloy is used as biomaterial due to its unique properties, but the high content of Ni (about 50 at.%) in biomedical NiTi is of concern. Hydroxyapatite/titania composite coating was directly electrodeposited on the surface of NiTi alloy. The coated samples were characterized using X-ray diffraction, scanning electron microscopy, infrared spectroscopy, bonding strength test, polarization and electrochemical impedance spectroscopy (EIS). Results showed that addition of TiO₂ to the electrolyte changed the morphology of hydroxyapatite from thin flake-flower-like crystals to needle-flower-like crystals, and the coating was much denser. Besides, hydroxyapatite crystal grains in the coating were preferentially arranged in the [001] direction, which was perpendicular to the surface of NiTi alloy. The addition of TiO₂ improved the bonding strength between the coating and the substrate. Corrosion resistance of NiTi in the simulated body fluid at 37 °C was significantly improved by more than 50 times by electrodeposition of the hydroxyapatite/titania composite coating.     

Wear resistance of NiTi alloy after surface mechanical attrition treatment

An ultrafine grain layer consisting of nanocrystallites as well as submicrometer grains is produced on NiTi shape memory alloy by surface mechanical attrition treatment (SMAT) and the effects of the ultrafine grain layer on the tribological properties are investigated under dry sliding conditions. Compared to the coarse grain (CG) NiTi, the SMAT NiTi has smaller friction coefficients and improved wear resistance at applied loads from 5 to 15 N due to the grain refinement effect. Examination of the worn surfaces indicates that materials delamination and particles co-exist on both the CG and SMAT NiTi samples. Our results indicate that delamination is the main wear mechanism on CG NiTi whereas abrasive particles dominate the wear process on SMAT NiTi.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

医用封堵器中形状记忆合金与不锈钢的焊接

研究医用封堵器制造过程中的焊接工艺,首先利用钨极氩弧焊(TIG)将一束NiTi形状记忆合金丝焊接在一起,然后采用激光点焊将NiTi形状记忆合金丝接头与不锈钢管焊接起来.采用光学显微镜观察接头的显微结构,并利用电子探针(EPMA)研究接头中合金元素的分布.结果表明,在形状记忆合金TIG焊接头中弥散分布有TiC相,但TiC的含量在形状记忆合金与不锈钢的激光点焊焊缝处较少.在熔合线附近存在元素间的相互扩散,并且在熔合线附近产生金属间化合物Ni3Ti+(Fe,Ni)Ti.     

Phase formation of Al2O3/Ti(C,N)–NiTi composite

This study investigates the addition of NiTi shape memory alloy into Al₂O₃/Ti(C,N) ceramic as a toughening agent. The reported study is focused on the thermal–chemical interaction between the NiTi additive and the ceramic matrix. It is found that at the sintering temperature of 1673 K, the NiTi alloy is melted and the Ti content of the molten alloy react with the ceramic matrix to form TiC, leaving behind Ni which resolidifies into elliptical inclusions with distinctive mutlizone structures in the matrix of the ceramic. These findings provide useful guidance for NiTi–ceramic composite design.     

Erosion-corrosion of mild steel in hot caustic. Part I: NaOH solution

A novel apparatus, high-pressure/high-temperature nickel flow loop, was constructed to study the effect of the flow on the rate of erosion-corrosion of mild steel in hot caustic. It has been successfully used to measure the corrosion rate of 1020 steel in 2.75 M NaOH solution at a temperature of 160 °C and velocities of 0.32 and 2.5 m/s. In situ electrochemical methods were used to measure the corrosion rate such as the potentiodynamic sweep, the polarization resistance method, and electrochemical impedance spectroscopy (EIS). Also used were the weight-loss method and scanning electron microscopy (SEM).Eight electrodes/coupons were used to monitor the metal loss rate, four were placed at the low velocity section, while the other four were placed in the high velocity section. The first three coupons in each section were placed within the disturbed flow region, while the fourth was placed in a fully developed flow region.The corrosion rate of the coupons in the high velocity section was generally higher than that of the coupons in the low velocity section. One coupon in the disturbed flow region had a significantly higher corrosion rate than the others.     

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni shape memory alloy

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni (mass%) shape memory alloy at 25 °C in 0.5 M H₂SO₄ and 3.5% NaCl solutions has been studied using potentiodynamic polarization and electrochemical impedance techniques. Three different microstructures viz., single-phase γ, γ-δ and γ-Fe₅Ni₃Si₂, were produced by heat-treating the alloy in different equilibrium phase fields. The corrosion behaviour in 0.5 M H₂SO₄ solution is almost same for all three microstructures, barring a slight difference in the passivation range. Although, the passivation current in 0.5 M H₂SO₄, is in the same range as that of SS 304, the critical current required for onset of passivation is almost three orders higher and the passivation range is much shorter. In 3.5% NaCl solution the corrosion behaviour of all three microstructures of the Fe-15Mn-7Si-9Cr-5Ni shape memory alloy was that of general dissolution without passivity or localized attack (pitting). The best corrosion resistance in both H₂SO₄ and NaCl solutions is shown by the single-phase γ microstructure.     

Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a Cu-Sn-based active filler alloy

Brazing of diamonds is important in grinding technology. The brazing parameters can strongly influence the grinding tool's performance. In this work a Cu-Sn-based active filler alloy (73.9 Cu-14.4 Sn-10.2 Ti-1.5 Zr, wt.%) was applied to join monocrystalline block-shaped diamonds onto a stainless steel substrate using three different brazing temperatures (880, 930 and 980 °C) and two different dwell times (10 and 30 min), respectively. The characteristics of the joints were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (microstructure and phase composition), by Raman-spectroscopy (residual stress) as well as by shear testing (bond strength). The microstructural investigations revealed an intermetallic interlayer of type Fe₂Ti at the steel-filler alloy interface, which grew with increasing brazing temperatures and longer dwell durations. The brazing parameters strongly affected the residual stresses in the diamond. Compressive residual stresses with a maximum value of − 350 MPa were found in the samples brazed at 880 and 930 °C, whereas tensile stresses of maximum + 150 MPa were determined in samples joined at 980 °C. The effect of the brazing parameters on the shear strength is very pronounced. The shear strength decreased from (321 ± 107) MPa at 880 °C, 10 min to (78 ± 30) MPa at 980 °C, 30 min.     

Fatigue behavior of (graded) (Ti, Al)N-coated 1Cr11Ni2W2MoV stainless steel at high temperature

The high-cycle fatigue properties of graded (Ti, Al)N- and Ti_0.7Al_0.3N-coated 1Cr11Ni2W2MoV at 500 °C have been investigated using a rotating bending fatigue testing machine. The results show that fatigue strength and life of 1Cr11Ni2W2MoV stainless steel were apparently increased by the presence of Ti_0.7Al_0.3N coating. The fatigue life and strength was also improved to some extent by the presence of the graded coating at higher stress levels (475 MPa-525 MPa). The fracture morphologies were analyzed by SEM. It is concluded that the presence of the coatings, which were well adhered and have high compressive stress, restricted plastic deformation of the substrate, thus improved the fatigue properties of the steel.     

CuAlPd alloys for sensor and actuator applications

Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115–370°C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180°C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modified Bridgeman technique.     

The influence of age-hardening on turning and milling performance of Ti-Al-N coated inserts

Ti-Al-N coatings are well known for their excellent properties and age-hardening abilities. Here we show that the life-time of coated inserts during turning of stainless steel can be increased to 200% by post-deposition vacuum annealing at 900 °C combined with a ~ 1 K/min vacuum furnace cooling. During milling of 42CrMo steel an increase in tool life-time to 140% is only obtained if the cooling condition after annealing at 900 °C contains a fast segment with 50 K/min from 800 to 700 °C. Thereby, the Co-binder in cemented carbide exhibits a retarded phase transformation from cubic to hexagonal. Consequently, the fracture toughness of the cemented carbide is reduced only from ~ 10.8 to 10.4 MPa√m while the coating still has an adhesive strength of ~ 65 N.Our results indicate that best machining performances of coated inserts are obtained after annealing at 900 °C where the supersaturated Ti_0.34Al_0.66N coating undergoes spinodal decomposition to form nm-sized cubic TiN and AlN domains resulting in a hardness increase from 34.5 to 38.7 GPa. Additionally, we demonstrate that careful attention needs to be paid on the influence of annealing conditions on adhesive strength and fracture toughness of coated inserts.