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.     

热爆反应合成多孔NiTi形状记忆合金的性能

利用热爆方法来制备了多孔NiTi形状记忆合金.研究了在不同热爆温度下制备出的样品与其机械性能之间的关系.结果表明:在1 223 K下热爆反应制备的NiTi合金,具有大的孔隙度,高开孔率和基本各向同性,同时表现出较好的超弹性.对断口分析发现,断裂为脆性断裂和韧性断裂的复合.这表明改善孔洞分布和形态,可以极大地提高多孔NiTi形状记忆合金的机械性能和超弹性.     

Chemical and mechanical treatments to improve the surface properties of shape memory NiTi wires

In this paper the results of an experimental study concerning the effect of different surface treatments on NiTi shape memory alloy wires are presented. These treatments were conducted in order to improve the adhesion properties between the NiTi wires and an epoxy resin, acting as the matrix of a composite material.Mechanical and chemical surface treatments (immersion in acid and alkaline solutions), and different combinations of the above surface preparation procedures were studied.For the characterisation of the resulting alloy surface conditions electrochemical impedance spectroscopy, polarisation curves and potential versus time measurements were carried out.The alloy wire/epoxy matrix adhesion was characterised through pull out tests. The results proved that all adopted treatments can remarkably influence the electrochemical properties of the wires. The acid treatments favour the formation of a surface passivation layer, while the alkaline treatments are effective in producing a rougher surface morphology. Moreover, these basic treatments significantly reduce corrosion resistance of the alloys, another material property that has been incidentally investigated in the present context. The main effect of the mechanical surface treatment, consisting in abrading the alloy wires using an emery paper, was to increase the homogeneity of surface roughness.From the experimental results clear indications on the most promising surface treatments can be inferred.     

Phase transformation behavior of pseudoelastic NiTi shape memory alloys under large strain

Although it is known that the plastic deformation after transformation could stabilize martensite and make the transformation irreversible, there lacks a systematic research on the effect of plasticity on phase transformation behavior of NiTi shape memory alloys (SMAs). Therefore, the present study focuses on this aspect of NiTi SMAs. A series of tensile cycling experiments are performed on a NiTi SMA at room temperature. Attention has been paid to the characteristics of the phase transformation stresses, the residual and recoverable strain and the dissipated and recoverable energy density as functions of deformation cycles and maximum strain amplitude. With the increasing of plastic strain amplitude at the first loading cycle, the stress–strain curves reach a stable state sooner during cycling. It is concluded that a small amount of plastic strain at the first loading cycle is helpful to get good stable mechanical properties.     

Laser surface alloying fabricated porous coating on NiTi shape memory alloy

Laser surface alloying technique was applied to fabricate a metallic porous coating on a solid NiTi shape memory alloy. By laser surface alloying a 40%TiH2-60%NiTi powder mixture on the surface of NiTi alloy using optimized laser process parameters, a porous but crack-free NiTi layer can be fabricated on the NiTi substrate. The porous coating is metallurgically bonded to the substrate NiTi alloy. The pores are uniformly distributed and are interconnected with each other in the coating. An average pore size of less than 10μm is achieved. The Ni content of the porous layer is much less than that of the original NiTi surface. The existence of the porous coating on the NiTi alloy causes a 37% reduction of the tensile strength and 55% reduction of the strain as compared with the NiTi alloy. Possible biomedical or other applications for this porous surface with good mechanical strength provided by the substrate are prospective.     

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.     

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.     

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect(TWSME) of polycrystalline Ni_(56)Mn_(25-x)Co_xGa_(18.9)Gd_(0.1) alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in Ni_(56)Mn_(21)Co_4Ga_(18.9)Gd_(0.1) alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.     

Strain mapping of crack extension in pseudoelastic NiTi shape memory alloys during static loading

Crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined during static loading. The material composition of 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using high-energy synchrotron X-ray diffraction. A miniature CT specimen was developed, which is small enough to allow in situ testing in a synchrotron beam line to identify phases, textures and lattice strains in front of a crack tip. Stress-induced martensite in pseudoelastic NiTi SMAs was mapped in front of the crack of a CT specimen during static loading using synchrotron radiation. The phase volume fraction and lattice microstrain results are discussed and compared with results from thermographic measurements. The Poisson effect is observed by comparing the lattice strains in the loading direction and transverse to the loading direction.     

Challenges in using waterjet machining of NiTi shape memory alloys: An analysis of controlled-depth milling

The nickel-titanium shape memory alloys (NiTi SMAs) have a very high potential for a wide variety of applications thanks to their unique mechanical properties: shape memory effect and pseudoelasticity. However, they have been proved to be more challenging to cut than other advanced engineering materials because of their high ductility, crystal-oriented and stress-oriented mechanical properties. In stark contrast to the extensive work on the metallurgical/microstructural properties of the SMA, there is limited research regarding non-conventional machining of this group of special alloys.Waterjet technology is well-known for cutting advanced difficult-to-cut materials owing to its benefits of reduced mechanical and thermal damages to workpiece surfaces. This paper reports for the first time the use of waterjet technology to mill the functional shape memory alloys and thus to open new avenues for the utilisation of these alloys for advanced engineering applications (e.g. aerospace, medical fields). However, when it comes to NiTi SMAs (characterised by low temperature phase martensite and parent phase austenite), the insignificant waterjet temperatures become critical to the material behaviour as their crystal structures are sensitive to the variations in both temperature and mechanical compression. This makes the processing (particularly waterjet controlled-depth milling) a real challenging task.By taking into consideration both of the waterjet temperatures at different material removal conditions (i.e. with and without abrasives in the focussing tube) and the transformation temperatures of NiTi, three different working zones (100% martensite; mix of austenite and martensite; 100% austenite) under waterjet process have been proposed. In addition, a combined phase and stress-strain diagram for shape memory effect in martensitic phase and pseudoelasticity in austenite phase of NiTi has been suggested. In this paper, Ni_49.8Ti_50.2 shape memory alloy was considered in which its transition temperature range is overlapped with the waterjet operating temperature; two approaches of waterjet processes (plain and abrasive waterjet milling) were proposed so as to investigate the mechanical and metallurgical effect provoked by the relationship between operating temperatures and transformation temperatures. It was found that abrasive waterjetting is more viable than plain waterjetting for controlled-depth milling of NiTi shape memory alloys.     

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni₄Ti₃ precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni₄Ti₃ precipitates.     

Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips – Berkovich and spherical – are employed to examine the SMAs’ indentation responses as a function of the representative strain, ε_R. In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, ε_R is systematically varied and its effect on the depth recovery ratio, η_d, is examined. Results indicate that RTA has higher η_d than RTM, but the difference decreases with increasing ε_R such that η_d values for both the alloys would be similar in the fully plastic regime. The experimental trends in η_d vs. ε_R for both the alloys could be described well with a η_d ∝ (ε_R)^?1 type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum ε_R, below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found.     

Machinability and surface integrity of Nitinol shape memory alloy

Nitinol shape memory alloys have wide applications in medical devices and actuators. However, the unique mechanical properties including superelasticity, high ductility, and severe strain-hardening make Nitinol exceedingly difficult to cut. This work determines dynamic mechanical behaviors of Nitinol in cutting. It is found that the very high strength and specific heat are responsible for large flank wear and fast tribo-chemical crater wear, respectively. The austenitic white layer in cutting is caused by deformation, while the twinned martensitic white layer is caused by quenching in EDM. Alloying from tools is negligible in cutting but unavoidable even in finish EDM trim cut.     

Influence of the post-deformation annealing heat treatment on the low-cycle fatigue of NiTi shape memory alloys

Shape memory alloys (SMA) suffer from the same impairing mechanisms experienced during cycling loading by classic alloys. Moreover, SMA fatigue behavior is greatly influenced by thermomechanical cycling through the zone of thermoelastic phase transformation, which is the basis of shape memory and superelasticity effects. Since the fatigue resistance of any material can be improved by an appropriate thermomechanical treatment, in the present work combined differential scanning calorimetry and microhardness testing were used to determine an optimum annealing temperature for the cold-worked Ni-50.1%Ti alloy. The optimization is based on the assumption that latent heat of transformation is proportional to the mechanical work generated by SMA upon heating, while material hardness is related to the yield stress of the material. It is supposed that an optimum trade-off in these two properties guarantees the best dimensional and functional stability of SMA devices. The level and stability of the mechanical work generated by the material during low-cycle fatigue testing are considered criteria for the material performance and thus of the validity of the proposed optimization procedure.     

Superelastic tension and bending characteristics of shape memory alloys

The objective of this study was to develop a numerical model of the superelastic behavior of shape memory alloys (SMA) on a macro-scale level. Results from a study on this behavior under tension and pure bending tests are presented and discussed. Two SMA samples were used in the experimental work and subjected to various loading paths in tension and pure bending: a single crystalline CuZnAl alloy and polycrystalline NiTi wire. Bending tests were performed under a pure bending loading condition on a new testing apparatus designed for the specific needs of this study. The experimental part of this study focused mainly on the response of the SMA to the loading paths in a quasi-plastic domain where the deformation mechanism is dominantly governed by the stress-induced martensitic transformation. Experimental results obtained from the NiTi polycrystals by tensile tests indicate that the superelastic SMA exhibits sufficient repeatability useful enough for a modeling task, while similar results obtained from the single crystalline CuZnAl indicate that the same modeling approach is not easily feasible. The facts have been qualitatively verified by the experimental data from pure bending tests, and a further area as study is suggested.     

Surface mechanical attrition treatment induced phase transformation behavior in NiTi shape memory alloy

The phase constituents and transformation behavior of the martensite B19′ NiTi shape memory alloy after undergoing surface mechanical attrition treatment (SMAT) are investigated. SMAT is found to induce the formation of a parent B2 phase from the martensite B19′ in the top surface layer. By removing the surface layer-by-layer, X-ray diffraction reveals that the amount of the B2 phase decreases with depth. Differential scanning calorimetry (DSC) further indicates that the deformed martensite in the sub-surface layer up to 300 μm deep exhibits the martensite stabilization effect. The graded phase structure and transformation behavior in the SMATed NiTi specimen can be attributed to the gradient change in strain with depth.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.