Stress dependent electrical resistivity of orthodontic wire from the shape memory alloy NiTi

The aim of this study was to compare and evaluate the stress induced martensitic phase under different loading conditions in orthodontic wire from the Shape Memory Alloy Nickel–Titanium (NiTi). For this purpose we investigated the phase transformation from austenite to martensitic due to the different loading conditions by measuring the electrical resistance which could show the type of deformation which occurs at the beginning of phase transformations. In this framework we developed two special devices for measurements of electrical resistance in different types of load and their combination on the orthodontic wire. These results were compared with the analytically calculated stresses in the orthodontic wire. It was shown that they caused complex or multi-axial stress state phase transformation rather than other more simple load such as uniaxial loading. Finally, the article presents the deformation which occurs at the change of phase that is nearly connected to the useful superelasticity effect of the Shape Memory Alloy NiTi.     

空心变截面杆的等效网络

设计换能器时一般忽略螺杆的影响,将前、后盖板和压电陶瓷晶堆作为均匀的实心杆件,这种方法有一定的近似性.将任意空心变截面杆等效为同种形状的实心杆件,并推导出了出几种常见形状空心杆件的等效几何参数及其网络传输系数,从而可以利用等效网络的方法方便准确地设计超声振动系统.     

Thermomechanical behavior of Ti-Ni shape memory alloy films deposited by DC magnetron sputtering

In this study, thermomechanical properties of titanium-nickel (Ti-Ni) shape memory alloy (SMA) films are investigated in order to derive constitutive relations. Ti-Ni SMA films, deposited by DC magnetron sputtering under controlled film composition, are characterized by uniaxial tensile tests. At room temperature (R.T.), Ti-Ni films having Ti contents less than 50 at% exhibit superelastic behavior, and those having Ti contents greater than 50 at% exhibit shape memory behavior. However, the Ni—53.2 at% Ti film fractured at a tensile strain of 0.8% because of an increase in brittleness with increasing Ti content. At elevated temperatures, Ti-Ni films having Ti contents of 50.2 to 52.6 at% undergo phase change from martensite to austenite. The Young's modulus of the Ti-Ni films depends on temperature at each phase, regardless of film composition. Film composition does, however, affect the measured material constants b_A, b_M, c_A, and c_M. Stress-strain curves calculated from the constructed constitutive equation closely agree with those obtained from tensile tests, for both the martensite and austenite phases. The constitutive equations are expected to find great utility in the design of Ti-Ni film-actuated microelectromechanical systems (MEMS).     

七通道PVC合金中空纤维超滤膜的创新之路

30年来,我国科技工作者及企业家研发及生产的聚氯乙烯PVC合金超滤膜已成为膜科技及膜工业领域中的一朵艳丽开放的奇葩,在水处理领域的应用中大放异彩.PVC合金超滤膜具有性能优异、价格最低廉等突出优点,可以广泛应用于市政、环保、石油化工、电力、电子、冶金、食品加工、医药等等行业,以及反渗透制水系统的预处理、自来水厂水质改善深度处理、小区分质供水处理、家用饮水机及矿泉水的净化处理、污水及废水处理、中水回用处理等领域的净化、浓缩、回收、除菌、除浊度等过程.文章概述了我国在PVC膜材料的研发、PVC合金超滤膜的发明、七通道PVC合金中空纤维超滤膜的产业化方面所走过的科技创新之路.     

导线阵列产生的微电磁力操纵磁粒子

当磁粒子包裹相应的外层物质时,可以与细胞产生选择性黏附,该特性可用于细胞分离、分选、药物运输等.在硅片上制做导线阵列,通过对相应导线阵列的通断电控制,可以控制微磁粒子运动,也就控制了与其相联的细胞运动.细胞运动到指定位置,借助工具对细胞进行操作,研究细胞特性.讨论导线阵列的MEMS工艺,对通电导线产生的磁场、温度场进行了仿真,了解电磁力大小的影响因素.     

Optimizing dimensional accuracy of titanium alloy features produced by wire electrical discharge machining

This study investigates geometrical errors such as cylindricity, circularity and diametral errors of a feature (a hole) produced from wire electrical discharge machining of Ti6Al4V alloy where tension in wire, pulse on time, and flushing pressure are varied. Pareto analysis of variance (ANOVA), Taguchi design of experiment (DoE), and traditional analysis estimate the influence of variables on errors of holes. It was noted that flushing pressure is the most significant factor with individual contributions of 31.02%, 49.5% and 37.84% to circularity, cylindricity, and diametral errors, respectively. The circularity error of holes decreases as the flushing pressure and tension in wire rise, but decreases with the rise of pulse on time. The cylindricity error decreased with the increase of wire tension, flushing pressure and pulse on time. The absolute diametral error reduced as the pulse on time and tension in wire raised, but it raised with the rise of pulse on time. All these trends are associated with the influence of tension in wire on the flexibility of wire, the dependence of heat generation and dissipation on pulse on time, and ability of the flushing pressure to control the cooling, as well as debris removal from the machining zone.     

空心玻璃微珠表面化学镀Ni-P合金磁性涂层的研究

采用化学镀工艺在空心玻璃微珠表面包覆了一层磁性的Ni-P合金涂层,对其进行了表面金属化改性.分别用X射线衍射仪(XRD)、傅立叶变换红外光谱仪(FT-IR)、扫描电子显微镜(SEM)以及振动样品磁强计(VSM)对化学镀前后空心玻璃微珠的形貌、组成、结构以及磁性能进行了表征.结果表明:通过化学镀工艺制备的Ni-P合金涂层由原子团簇组成;涂层为非晶结构并具有较好的磁性能;化学镀后空心玻璃微珠的X射线衍射强度和红外透射强度均降低.     

Microstructural characteristics and deformation of magnesium alloy AZ31 produced by continuous variable cross-section direct extrusion

Magnesium(Mg) alloy AZ31 was produced by continuous variable cross-section direct extrusion(CVCDE)to study its deformation behavior. Metallographic microscopy(OM), transmission electron microscopy(TEM), and scanning electron microscopy(SEM) were used to observe the variations in microstructure and fracture morphology of Mg alloy AZ31 as a function of processing methods. The results reveal that grains of Mg alloy AZ31 were refined and their microstructure was homogenized by CVCDE. The recrystallization in Mg alloy AZ31 produced by CVCDE with 2 interim dies was more complete than that produced by conventional extrusion(CE) and CVCDE with 1 interim die, and the grains were finer and more uniform.Plasticity of the AZ31 alloy was improved. Fracture mode was evolved from a combination of ductility and brittleness to a sole ductile form. In summary, a CVCDE mold structure with 2 interim dies can improve microstructure, plasticity, and toughness of Mg alloy AZ31.     

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureT _C has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.     

Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observed by high-speed atomic force microscopy

The diffusion of individual point defects in a two-dimensional streptavidin crystal formed on biotin-containing supported lipid bilayers was observed by high-speed atomic force microscopy. The two-dimensional diffusion of monovacancy defects exhibited anisotropy correlated with the two crystallographic axes in the orthorhombic C 222 crystal; in the 2D plane, one axis (the a-axis) is comprised of contiguous biotin-bound subunit pairs whereas the other axis (the b-axis) is comprised of contiguous biotin-unbound subunit pairs. The diffusivity along the b-axis is approximately 2.4?times larger than that along the a-axis. This anisotropy is ascribed to the difference in the association free energy between the biotin-bound subunit-subunit interaction and the biotin-unbound subunit-subunit interaction. The preferred intermolecular contact occurs between the biotin-unbound subunits. The difference in the intermolecular binding energy between the two types of subunit pair is estimated to be approximately 0.52?kcal?mol(-1). Another observed dynamic behavior of point defects was fusion of two point defects into a larger defect, which occurred much more frequently than the fission of a point defect into smaller defects. The diffusivity of point defects increased with increasing defect size. The fusion and the higher diffusivity of larger defects are suggested to be involved in the mechanism for the formation of defect-free crystals.     

Experimental investigation on the debonding strength in shape memory alloy wire reinforced polymers

The interfacial shear strength between the shape memory alloy (SMA) wire and epoxy matrix was evaluated experimentally using a single wire pull-out test. Moreover, the effect of pre-strain in SMA wires on the interfacial behavior was studied by pre-straining the SMA wires to 2% and 4% pre-strains. Experiments were conducted in both martensite and austenite phases of SMA. Results showed that pre-straining SMA wire in the martensite phase caused enhancement in interfacial shear strength due to recovery force generation. Further, 9.7% and 33% improvements in the interfacial shear strength were achieved at 2% and 4% of SMA pre-strain, respectively. However, the enhancement of interface behavior did not occur, when the SMA wires were subjected to pre-strain in the austenite phase.     

Mechanical properties of Nb based multiphase alloy studied by nanoindentation and atomic force microscopy

Abstract

The mechanical properties and deformation behaviours of as cast and heat treated Nb–21Ti–4C–xAl (x: 0, 5, 10 and 15 at-%) alloys are comprehensively investigated using nanoindentation and atomic force microscopy. For the Nb–21Ti–4C alloy, nanoindentation tests are performed for the Nb solid solution (Nbss) matrix, carbide and the interface between them. The results show that the hard carbide, which has a strong bonding with the Nbss matrix, can enhance the alloy before and after heat treatment, and the eutectoid transformation of the large sized carbide after heat treatment leads to less possibility for the forming of cracks on the carbide surface, which in turn improves the toughness. For the Nb–21Ti–4C–(0, 5, 10, 15)Al alloys, the hardness of the Nbss matrix increases significantly with increasing Al fraction for both as cast and heat treated alloys. However, deviations of the elastic modulus are inconspicuous with the Al fraction for the as cast and heat treated alloys.     

Influence of heat treatment on the fatigue life of a laser-welded NiTi alloy wire

The influence of heat treatment on the rotating–bending fatigue of a laser-welded superelastic NiTi alloy wire was investigated. In comparison with the as-welded and annealed at 500 °C for 1 h specimens, the specimen annealed at 400 °C for 1 h shows the best fatigue resistance. The difference of fatigue life in the heat-treated materials is attributed to the different Ti₃Ni₄ precipitate size. Heat treatment to produce smaller coherent precipitates (10 nm) improves the fatigue resistance of the laser-welded NiTi alloy wire. When the Ti₃Ni₄ precipitates become coarse, the fatigue resistance decreases.     

控制压边力改善铝合金板成形性能的研究

以5052H32铝合金板为研究对象,以有限元数值模拟和基于计算机控制多点变压边力液压压力机的实验为手段,研究随位置变化的压边力对铝合金板成形性能的影响.研究结果表明:合理地控制随位置变化的变压边力可以显著提高铝合金板的成形性能,增加盒形件的拉深深度(最大拉深高度提高约12%);随位置变化的压边力对铝合金成形性能改进的主要原因是减少破裂危险区域壁厚减薄和应变的峰值;采用Barlat 96屈服准则描述铝合金的屈服行为具有较高精度,数值模拟和实验的偏差较小.     

Study of precipitates formed in a wrought aluminum alloy by means of atomic force microscopy

A study of the particles precipitated during heat treating a Type 6061 wrought aluminum alloy was carried out by means of atomic force microscopy and scanning electron microscopy (SEM). The specimens were cut from hot-extruded bars and were heat-treated to peak hardness and overaged stages to obtain precipitates of different sizes. The samples were prepared by standard metallographic techniques before being examined with either type of microscope. Observations with the atomic force microscope (AFM) were made in the topographic and lateral force modes, as each method yields different information. The former mode is issued to identify the particles and measure their size and shape, whereas the latter allows for data related to superficial characteristics. It is concluded that both types of microscopic examinations complement each other and that they can be used in conjunction to obtain a deeper understanding of precipitation behaviour and kinetics.     

On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy

Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material. The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format. Three loading amplitudes (/2=0.10%, 0.15, 0.20%) and three load ratios (R=–1, 0, 0.5) are considered. From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, _max, with respect to all possible shear strain planes. For certain inhomogeneities, the _max was as high as ten times the far field strains. Bonded inclusions have _max values two orders of magnitude smaller than voids, cracked, or debonded inclusions. A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle. Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation. Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases _max by about a factor of two. At low R ratios (–1) equivalent sized voids and debonded inclusions have comparable _max values. At higher R ratios (0, 0.5) debonded inclusions have _max values twice that of voids.