冷却速度对Ti-50.9%Ni形状记忆合金相变行为和组织的影响(英文)

为了探索冷却速度对镍钛形状记忆合金相变行为和组织的影响,通过差热扫描、X射线衍射、扫描电镜、X射线荧光分析等方法研究Ti-50.9%Ni(摩尔分数)合金的热处理。结果表明:当冷却速度非常低时(如炉冷)可诱发三级相变的产生;冷却速度对相变温度也有很大的影响,Ms和Mf都随冷却速度的降低而下降,降低冷却速度有利于提高M→A奥氏体相变温度,相变滞后宽度(Af–Mf)随着冷却速度的降低而增大;热处理不能消除镍钛合金热加工时所形成的织构,但能减弱其强度;冷却速度对晶粒大小影响不大。     

Comparative study on microstructure and homogeneity of NiTi shape memory alloy produced by copper boat induction melting and conventional vacuum arc melting

A comparative study was carried out on the microstructures of NiTi shape memory alloy produced by two techniques of copper boat induction melting and vacuum arc melting. The as-solidified and homogenized specimens were characterized by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with EDS, and differential scanning calorimetry (DSC). The results showed that the microstructure formed in the copper boat method was free of segregation (Ti or Ni-rich precipitates) with a low scatter (<5 K) in phase transition temperatures within the ingot at different positions. Unlike the multi-stage vacuum arc remelting process, a homogeneous microstructure was achieved only after one melting procedure.     

钙化处理引入电脉冲对 NiTi 形状记忆合金表面钙磷层的影响

采用纯化学处理的方法,在NiTi形状记忆合金表面沉积羟基磷灰石膜层,探讨了在钙化初期引入电脉冲对膜层的影响,分析了钙化液组分浓度以及脉冲参数对膜层生长的响应规律。结果表明:含Ca2+3.10mmol/L,K+4.64 mmol/L,Na+126.8 mmol/L,Cl-144.5 mmol/L及HPO42-1.86 mmol/L,且呈弱碱性的钙化液最适合钙磷层的生长,采用此钙化液,经适当处理后,试样表面生成了疏松、多孔的钙磷层,经检测,钙磷层为羟基磷灰石和其它钙磷盐的混合物;电脉冲的引入可在一定程度上加速钙磷层的沉积,并提高羟基磷灰石膜的纯度。     

基于变温度场的镍钛形状记忆合金管滚珠旋压有限元模拟(英文)

作为一种新的尝试,在高温下应用滚珠旋压制造镍钛形状记忆合金管。将名义成分为Ni50.9Ti49.1(摩尔分数)的镍钛形状记忆合金棒料进行固溶处理,制成用于滚珠旋压的镍钛形状记忆合金管坯。以变温度场和本构方程为基础,用刚粘塑性有限元法来模拟镍钛形状记忆合金管的滚珠旋压,获得了温度场、应力场和应变场,并进行了旋压载荷预测。有限元模拟结果表明,在旋压件的主变形区有大约160℃的温升。从应力场和应变场可以看出,镍钛形状记忆合金管的外壁比内壁更容易满足塑性屈服准则,塑性变形区处于三向压应力状态。径向应变和切向应变为压缩应变,轴向应变为伸长应变。旋压载荷伴随滚珠旋压行程的变化对于预测旋压件的稳定流动具有重要的意义。     

Equal channel angular extrusion of NiTi shape memory alloy tube

As a new attempt, equal channel angular extrusion (ECAE) of nickel–titanium shape memory alloy (NiTi SMA) tube was investigated by means of process experiment, finite element method (FEM) and microscopy. NiTi SMA tube with the steel core in it was inserted into the steel can during ECAE of NiTi SMA tube. Based on rigid-viscoplastic FEM, multiple coupled boundary conditions and multiple constitutive models were used for finite element simulation of ECAE of NiTi SMA tube, where the effective stress field, the effective strain field and the velocity field were obtained. Finite element simulation results are in good accordance with the experimental ones. Finite element simulation results reveal that the velocity field shows the minimum value in the corner of NiTi SMA tube, where severe shear deformation occurs. Microstructural observation results reveal that severe plastic deformation leads to a certain grain orientation as well as occurrence of substructures in the grain interior and dynamic recovery occurs during ECAE of NiTi SMA tube. ECAE of NiTi SMA tube provides a new approach to manufacturing ultrafine-grained NiTi SMA tube.     

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.     

Corrosion behaviour of NiTi endodontic instruments

Abstract

The present study investigates the influence of the sterilisation treatment conducted at 50°C in 5% sodium hypochlorite solution (NaClO) on the corrosion behaviour of endodontic instruments made of NiTi (GT Rotary and K3) and stainless steel (K file) by measuring potentiodynamic polarisation curves in the same environment at 37°C. For comparison, the electrochemical characterisation was also carried out on endodontic instruments that were not sterilised. The characterisation of the sample surface after the electrochemical tests were carried out by means of scanning electron microscopy coupled with X-ray energy dispersive spectroscopy. There was no significant influence of the sterilisation treatment on the corrosion behaviour of K file and GT Rotary endodontic instruments. However, a negative influence of the sterilisation treatment on the corrosion resistance of K3 endodontic instruments was observed, and the effect appears to be more dramatic for longer sterlisation treatment periods.     

Microstructural characteristics of directionally solidified Ni–43Ti–7Al alloys

Abstract

Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s^?1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s^?1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti₂Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti₂Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti₂Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s^?1. However, in the sample withdrawn at 2 μm s^?1, Ti₂Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.     

NiTi 形状记忆合金环向预应力混凝土结构强化现象的研究

采用 NYL-80 型压力试验机进行抗压试验,用YJ-26型静态电阻应变仪测量各级荷载下对应的横竖向应变,同时观察和记录试件的破坏现象,研究了NiTi形状记忆合金环向预应力混凝土结构的强化现象.结果表明SMA环向预应力混凝土柱与同等条件的普通混凝土柱相比,其极限荷载和抗裂性能都有较大的提高.     

Structural, electrical and mechanical properties of magnetron sputtered NiTi/PZT/TiOx thin film heterostructures

Shape memory alloy (NiTi) thin films coupled to ferroelectric lead zirconate titanate (PZT) produce an intelligent material capable of performing both sensing and actuating functions. In the present study, we report on the in-situ growth of NiTi/PZT/TiO_x heterostructure on Pt/Ti/SiO₂/Si substrates using magnetron sputtering technique. Deposition processing, microstructure, surface morphology, electrical properties and mechanical properties of these heterostructures were systematically investigated. The top NiTi films exhibit austenitic B2 structure with preferred (110) orientation. The varying thickness of NiTi films had a significant influence on properties of NiTi/PZT/TiO_x heterostructure. The bottom TiO_x layer was observed to favor the growth of perovskite PZT films with (100) orientation. Nanoindentation tests of these heterostructures were performed at room temperature. The mechanical hardness of the top NiTi layer of lower thickness was found to be highly influenced by underneath PZT layer. The heterostructure exhibited an interesting martensite to austenite phase transformation and polarization-electric field hysteresis behavior with remanent polarization (P_r) and the coercive field (E_r) of 17.1 μC/cm² and 69.6 kV/cm, respectively. These heterostructures having a layer of SMA material coupled to a ferroelectric material with underneath TiO_x layer are of immense technological importance for MEMS devices.