Property Change During Fixtured Sintering of NiTi Memory Alloy

Fixtured sintering of up to 5 hours at 1223 to 1323 K is successfully used to diminish the dimensional change of NiTi memory alloys produced from mixtures of elemental Ni and Ti powders packed and pressed at room temperature under 400, 500, and 600 MPa pressure. The improvements obtained can help near-net-shape technology via powder metallurgy that overcomes the traditional casting problems such as oxygen, nitrogen, hydrogen, and carbon absorption and intermetallic compound precipitation, which lower the workability and the homogeneity of the final alloy. The effects of compaction pressure, sintering time, and sintering temperature on dimensional change, porosity, hardness, and morphology of the produced TiNi intermetallic alloy are investigated. Phase transformation temperatures and mechanical properties of the sintered samples were determined and compared for the influence of compaction pressure and sintering conditions. It was concluded that suitable thermoelastic and superelastic effects are achievable via appropriate selection of fixtured sintering conditions.     

Ti-content and annealing temperature dependence of transformation behavior of TiXNi(92-XCu8 shape memory alloys

Ti-content and annealing temperature dependence of the transformation behavior of Ti_XNi_(92-X)Cu_8.0 (at,%) (X = 49.0–5l.0) alloys was investigated by varying the annealing temperature from 573 to 1273 K. It was found that the peak temperature of B2–B19 transformation (O^*) increases with increasing annealing temperature from 673 to 873 K for all of the alloys. With annealing at temperatures above 873 K, the influence of annealing on O^* depends on Ti-content. In the range of 50.4–51.0 at.% Ti, O^* shows little dependence on annealing temperature. In the range of 49.3–50.2 at.% Ti, O^* firstly decreases and then keeps constant with increasing annealing temperature. For the alloy of 49.0 at.% Ti, O^* continuously decreases with increasing annealing temperature from 873 to 1273 K. On the basis of the above data, a partial phase diagram of Ti-Ni-8.0Cu (at.%) was proposed. The transformation hysteresis also showed unique Ti-content and annealing temperature dependence.     

Effect of the surface oxide layer on transformation behavior and shape memory characteristics of Ti-Ni and Ti-Ni-Mo alloys

Oxidation behaviors of Ti-49Ni and Ti-48.3Ni-0.7Mo (at.%) alloys in dry air from 723 K to 1273 K have been investigated by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, and then effects of oxidation on transformation behavior and shape memory characteristics of the alloys were studied by means of differential scanning calorimetry and thermal cycling tests under constant load. Three-layered surface scale was formed in both alloys oxidized at temperatures higher than 1023 K, consisting of an outer TiO₂ layer, an intermediate layer of mixture of TiO₂ and Ni and an inner TiNi₃ layer. Thickness of the surface oxide layer increased from 1 m to 50 m with raising oxidation temperature from 923 K to 1273 K. The surface oxide layer raised transformation temperatures associated with the B2-B19 and the R-B19 transformation, while it did not almost change transformation temperatures associated with the B2-R transformation. Recoverable elongation was not changed in the alloys oxidized at temperatures below 823 K with raising oxidation temperature, whereas it decreased in the alloys oxidized at temperatures above 923 K. Transformation hysteresis was not almost changed by oxidation in a Ti-49Ni alloy, but it decreased largely in a Ti-48.3Ni-0.7Mo alloy.     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

造孔剂法制备孔隙率可控的多孔镍钛合金

用常规粉末冶金烧结法结合造孔剂技术制备了多孔镍钛形状记忆合金.研究结果表明,孔隙特征主要取决于所用造孔剂的尺寸和形貌,而孔隙率可以通过加入造孔剂的含量来调控;所制备合金的主要成分是B2相和B19'相的NiTi,在经高温固溶处理后仍有极少量的Ti2Ni,Ni3Ti和Ni4Ti3等杂质相存在,而且合金杂质相的含量随造孔剂加入量的增加而相应提高.对合金的相变热分析表明,在从高温降至低温过程中多孔镍钛发生特殊的三阶段相变,这主要是由合金成分和相组成在晶界的不均匀造成的.最后,研究表明相变对多孔镍钛合金的阻尼性能(内耗)有独特的影响,而孔隙率对阻尼的影响则不明显.     

Mechanical activation of pre-alloyed NiTi<Subscript>2</Subscript> and elemental Ni for the synthesis of NiTi alloys

This work reports on an efficient powder metallurgy method for the synthesis of NiTi alloys, involving mechanical activation of pre-alloyed NiTi₂ and elemental Ni powders (NiTi₂–Ni) followed by a press-and-sinter step. The idea is to take advantage of the brittle nature of NiTi₂ to promote a better efficiency of the mechanical activation process. The conventional mechanical activation route using elemental Ti and Ni powders (Ti–Ni) was also used for comparative purposes. Starting with (NiTi₂–Ni) powder mixtures resulted in the formation of a predominant amorphous structure after mechanical activation at 300 rpm for 2 h. A sintered specimen consisting mainly of NiTi phase was obtained after vacuum sintering at 1050 °C for 0.5 h. The produced NiTi phase exhibited the martensitic transformation behavior. Using elemental Ti powders instead of pre-alloyed NiTi₂ powders, the structural homogenization of the synthesized NiTi alloys was delayed. Performing the mechanical activation at 300 rpm for the (Ti–Ni) powder mixtures gave rise to the formation of composite particles consisting in dense areas of alternate fine layers of Ni and Ti. However, no significant structural modification was observed even after 16 h of mechanical activation. Only after vacuum sintering at 1050 °C for 6 h, the NiTi phase was observed to be the predominant phase. The higher reactivity of the mechanically activated (NiTi₂–Ni) powder particles can explain the different sintering behavior of those powders compared with the mechanically activated (Ti–Ni) powders. It is demonstrated that this innovative approach allows an effective time reduction in the mechanical activation and of the vacuum sintering step.     

Phase Transformation and Thermal Stability of Aged Ti-Ni-Hf High Temperature Shape Memory Alloys

The use of Ni-rich TiNiHf alloys as high temperature shape memory alloys （SMAs） through aging has been presented. For Ni-rich Ti80-xNixHf20 alloys, their phase transformation temperatures are averagely increased more than 100 K by aging at 823 K for 2 h. Especially for the alloys with Ni-content of 50.4 at. pct and 50.6 at. pct, their martensitic transformation start temperatures （Ms） are more than 473 K after aging. TEM observation confirmed that some fine particles precipitate from the matrix during aging. The aged Ni-rich TiNiHf SMAs show the better thermal stability of phase transformation temperatures than the solutiontreated TiNiHf alloys. The fine particles precipitated during aging should be responsible for the increase of phase transformation temperatures and its high stability.     

Superelasticity of free‐standing NiTi films depending on the oxygen impurity of the used targets

Off‐stoichiometric NiTi sputter targets were manufactured by ingot metallurgy in order to produce Ni‐rich NiTi thin films. Crystalline samples, which undergo a martensitic transformation, were fabricated by magnetron sputtering and subsequent thermal treatment. The sacrificial layer technique was applied to obtain free‐standing NiTi films. The stoichiometry of the sputtered film is different to the stoichiometry of the target material. Sputtering targets with a Ti content higher than 49.5 at.% have a typical Ti‐loss rate in the order of 4.3 at.%, whereas at more Ni‐rich targets a Ti loss rate of only 0.5–1.5 at.% is observed. Due to this Ti loss rate target composites of Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% were taken. The NiTi films obtained from these two targets were compared. The main difference between the two targets is the impurity content of oxygen and carbon. Auger electron spectroscopy measurements were performed to determine qualitatively the content of oxygen in the free‐standing NiTi films. Transformation temperatures and hysteresis properties of the martensitic transformation were characterised by differential scanning calorimetry. Free‐standing films obtained from the Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% sputtering targets revealed superelastic properties at 45 °C as demonstrated by tensile testing. Tensile tests were taken at various temperatures of 40 °C, 45 °C, 50 °C, 55 °C and 60 °C, revealing e.g. a maximum superelastic strain of approximately 5 % at 45 °C.     

Interfacial microstructure and strength of partial transient liquid-phase bonding of silicon nitride with Ti/Ni multi-interlayer

Partial transient liquid-phase bonding (PTLP bonding) of silicon nitride (Si₃N₄) ceramic has been performed using Ti/Ni multi-interlayer in vacuum at 1273–1423 K. Interfacial microstructures were examined by scanning electron microscope, electron probe micro-analysis, and X-ray diffraction. The joint strength has been measured by four-point bending tests from room temperature up to 1000 °C. Interfacial structure of Si₃N₄/TiN/Ti₅Si₃ + Ti₅Si₄ + Ni₃Si/(NiTi)/Ni₃Ti/Ni is formed after bonding process. The NiTi layer is gradually consumed with simultaneous growth of the reaction layer and the Ni₃Ti layer. The room temperature joint strength is significantly affected by the reaction layer thickness, whereas the elevated temperature joint strength significantly depends on whether the low melting point NiTi layer exists in the joint. The joint strength of more than 100 MPa is retained up to 800 °C as the NiTi layer is completely consumed. A model is proposed to optimize the PTLP bonding parameters for optimizing joint strength at both room temperature and elevated temperature.     

Growth and characterization of shape memory alloy thin films for Si microactuator technologies

NiTi thin film alloys have been grown on Si (1 0 0) substrates by sequential multilayer deposition of Ni and Ti layers followed by metal interdiffusion via annealing. Short-time (5 min) annealing of the deposited layers at 500 °C leads to alloying of Ni and Ti, provided that preferential oxidation of Ti is avoided. This has been achieved by capping the layers with AlN, which constitutes a very efficient barrier preventing oxygen contamination of the film. For uncapped films, annealing has to be performed under high vacuum conditions. The structure of the processed films strongly depends on the temperature and time of the thermal process, obtaining Ni- and Ti-rich phases when the films are annealed at higher temperatures and/or during longer times. As-grown and processed films are analyzed by X-ray diffraction, in-depth Auger electron spectroscopy and X-ray photoelectron spectroscopy techniques. Finally, resistivity measurements show the existence of a phase transition in the films annealed at 600 °C without the capping layers (vacuum furnace). This contrasts with the behavior observed for AlN capped films, where cracking of the film occurs during cooling, which has been attributed to generation of stress in the buried film during the phase transition.     

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

Abstract

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni₅Si₂ phase and finely precipitated Ni₃Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni₃(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni₅Si₂ phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni₃Si and Ni₃(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni₃Si and Ni₃(Si,Ti) alloys.     

热处理对NiTi形状记忆合金的相变的影响

对含Ti 50.8%的NiTi形状记忆合金在300℃、400℃、500℃保温不同时间进行时效处理,然后通过差示扫描量热仪DSC对其相变温度点进行测定,发现经时效处理调节后,Af最大可下降20~30℃左右,而Ms变化不大。通过不同时效温度和保温时间的DSC曲线比较,得出热处理对其相变温度点的影响规律,即在300~500℃时效处理时,保温时间越短,Af降低越明显,在相同保温时间时,保温温度越低,Af降低越明显。     

Effects of Ta Addition on NiTi Shape Memory Alloys

1. IntroductionSince NiTi shape memory alloy have excellentshape memory effect, good mechanical properties, excellent biocompatibility and high corrosion resistance,they have been used for various applications, espysctally in the biomedical fi.ldll~4]. However, the lowX-ray visibility of NiTi shape memory alloys has limited some medical applicatinns. With low X-ray visibility, the detection of the very small implacable devices or tools which are made of NiTi alloys can bevery difficult. So…     

Superelasticity of TiPdNi Alloys with and without Rare Earth Ce Addition

Ti50.6Pd30Ni19.4 and Ti51Pd28Ni21 (Ce) alloys have been prepared under various temperatures for long time annealing. Differential scanning calorimetery (DSC), X-ray diffraction (XRD) and tensile test were employed to investigate the phase transformation behavior and superelasticity of the alloys. It has been found that the phase transformation temperature of Ti50.6Pd30Ni19.4 is about 40C higher than that of Ti51Pd28Ni21(Ce), and do not change much with different annealed temperature. Obvious superelasticity is retained in Ti50.6Pd30Ni19.4 alloy annealed at 400C for 18 h, and annealing at higher temperature shows a deterioration of this property. The Ce addition in TisiPd28Ni2i alloy significantly delays recrystallization, increases yied strength and elastic modulus, but the superelasticity is poor.     

Mg/NiTi复合材料的制备及阻尼性能

采用Mg粉的无压熔渗法制备Mg/NiTi复合材料以提高多孔NiTi合金的强度和阻尼性能。通过OM、SEM、EDS和XRD分析Mg/NiTi复合材料的显微组织结构,采用压缩实验分析其抗压强度、吸能能力,采用热机械分析仪分析其内耗和存储模量。结果表明:经Mg粉无压熔渗后,多孔NiTi合金的孔隙被Mg填充,其孔隙率由原来的50.38%下降至5.6%,且Mg与NiTi合金的界面结合良好。多孔NiTi合金主要由B2奥氏体相和B19'马氏体相及少量Ni₃Ti相和NiTi₂相组成;Mg/NiTi复合材料除增加了熔渗的Mg相外,还新生成了Mg₂Ni相。Mg的渗入未改变多孔NiTi合金相变行为,但提高了相变温度。Mg/NiTi复合材料的抗压强度可达554 MPa,较多孔NiTi合金提高了61%,压缩断裂方式也由多孔NiTi合金的孔壁崩塌断裂转变为Mg/NiTi复合材料的剪切断裂。Mg/NiTi复合材料的吸能较多孔NiTi合金有大幅提高。同时,Mg/NiTi复合材料的内耗值有所增加,而存储模量大幅提高,整体呈现出更佳的阻尼性能。      

表面多孔NiTi-羟基磷灰石/NiTi生物复合材料的制备与性能

利用放电等离子烧结技术制备了表面多孔NiTi-羟基磷灰石(HA)/NiTi生物复合材料,研究了烧结温度对复合材料宏观形貌、微观结构、表面孔隙特征、力学性能及体外生物活性的影响。结果表明:随着烧结温度从800℃提高到950℃,NiTi-HA/NiTi复合材料由复杂的Ti、Ni、Ti_2Ni、Ni_3Ti、HA混合相逐渐转变为单一的NiTi+HA相,内外层界面形成稳定的冶金结合且表面孔隙率与平均孔径呈缓慢减小趋势;同时抗压强度显著提高而弹性模量变化不明显。与传统NiTi、多孔NiTi及多孔NiTi-HA材料相比,950℃温度下制备的NiTi-HA/NiTi复合材料不仅具有良好的界面结合和表面孔隙特征(孔隙率45.6%、平均孔径393μm)、较高的抗压强度(1 301MPa)、较低的弹性模量(10.2GPa)以及优异的超弹性行为(超弹性恢复应变4%)的最佳匹配,而且还具有良好的体外生物活性。     

Porous NiTi shape memory alloys produced by SHS: microstructure and biocompatibility in comparison with Ti2Ni and TiNi3

Shape memory alloys based on NiTi have found their main applications in manufacturing of new biomedical devices mainly in surgery tools, stents and orthopedics. Porous NiTi can exhibit an engineering elastic modulus comparable to that of cortical bone (12–17 GPa). This condition, combined with proper pore size, allows good osteointegration. Open cells porous NiTi was produced by self propagating high temperature synthesis (SHS), starting from Ni and Ti mixed powders. The main NiTi phase is formed during SHS together with other Ni–Ti compounds. The biocompatibility of such material was investigated by single culture experiment and ionic release on small specimen. In particular, NiTi and porous NiTi were evaluated together with elemental Ti and Ni reference metals and the two intermetallic TiNi₃, Ti₂Ni phases. This approach permitted to clearly identify the influence of secondary phases in porous NiTi materials and relation with Ni-ion release. The results indicated, apart the well-known high toxicity of Ni, also toxicity of TiNi₃, whilst phases with higher Ti content showed high biocompatibility. A slightly reduced biocompatibility of porous NiTi was ascribed to combined effect of TiNi₃ presence and topography that requires higher effort for the cells to adapt to the surface.     

The low temperature magnetic properties of austentic FeCrNi alloys: 1. The effect of nickel concentration in FeNi-20 wt% Cr alloys

The magnetic properties of ternary FeCrNi alloys containing 20 wt% Cr and 11 – 32 wt% Ni have been studied over a range of temperatures from 10 – 300 K by both ac and dc techniques, in fields from 5 A m^?1 to 6 MA m^?1. Apart from small amounts of δ ferrite in some alloys, the matrix constituent at room temperature is the paramagnetic fcc austenite phase. On cooling to low temperatures all alloys exhibit peaks in the temperature dependence of ac susceptibility, but in the 20 Cr/11 Ni alloy this is superimposed on the effects of an irreversible martensitic transformation below 195 K. Although a quantitative analysis of the results enables appropriate Neel and Curie temperatures to be determined for the alloys, it also reveals that the changeover from antiferromagnet behaviour at 0 K, with increasing nickel content, is a very gradual process involving the formation of increasing concentrations of superparamagnetic clusters, the moments, numbers, and sizes of which have been determined in alloys containing 16, 18, and 20 wt% Ni. The practical consequences of these findings are discussed in terms of the application of stainless steels in cryogenic engineering and appropriate susceptibility values are tabulated for use as design data.     

Dilatometric study of martensitic transformation in NiTiCu and NiTi shape memory alloys

Dilatometric measurements have been carried out for the study of nature of martensitic transformation in the NiTiCu and NiTi shape memory alloy wire samples. Investigation has been done in the heat-treat temperature range 300–800°C. NiTiCu exhibits only single stage A M martensitic transformation in the entire heat-treat temperature range indicating the suppression of R-phase by Cu substitution. NiTi shows the two-stage A R M martensitic transformation in the heat-treat temperature range 340–410°C and the single-stage A M martensitic transformation above heat-treat temperature 410°C. The extent of dilation during phase transformation decreases with increasing heat-treat temperature in both the alloys. Effect of first 15 thermal cycles on transformation temperatures in both the alloys has been studied. It is found that transformation temperatures are unaffected with thermal cycles in NiTiCu whereas considerable decrease in transformation temperatures has been observed in the case of NiTi. The stability of transformation temperatures in NiTiCu during M A transformation against thermal cycling may be attributed to the associated smaller thermal hysteresis compared to NiTi.     

医用多孔NiTi合金表面微弧氧化改性研究

为解决多孔Ni Ti合金耐蚀性降低和Ni离子释放量增大而引起的使用安全性问题.本文采用微弧氧化技术对医用多孔Ni Ti合金进行表面改性处理,研究结果表明,微弧氧化处理并未改变多孔Ni Ti合金原有的孔隙结构和孔隙率,只在其外表面和孔隙内表面均形成了典型的微弧氧化多孔涂层.该涂层主要由氧化铝相组成,并含有少量的Ti和Ni元素,且外表面涂层的Ti和Ni含量要略低于孔隙内表面涂层.微弧氧化涂层提高了多孔Ni Ti合金的表面接触角,将原有的亲水表面转变成了疏水表面.经微弧处理后,多孔Ni Ti合金的耐蚀性较基体提高了1个数量级以上,Ni离子释放量也较基体降低了1个数量级以上.