铸态镍钛形状记忆合金的断裂行为及显微组织(英文)

通过真空自耗电极熔炼法制备等原子比镍钛形状记忆合金。为了研究其断裂力学性能,进行铸态镍钛形状记忆合金的拉伸和压缩实验。为了更好地理解镍钛形状记忆合金的组织演变及断裂行为,分析铸态镍钛形状记忆合金及其断裂样品的显微组织。在拉伸加载下,镍钛形状记忆合金在750°C时具有较高的塑性,表现为韧性断裂,但在室温和-100°C时表现出较差的塑性,具有解理断裂和穿晶断裂的特征。在-100°C的压缩加载下,铸态镍钛形状记忆合金发生剪切断裂,剪切断裂面法线与压缩轴呈45°,具有解理断裂的特征,裂纹经由穿晶断裂而扩展。     

激光熔凝处理对NiTi形状记忆合金微动磨损性能的影响

采用高功率连续波固体Nd:YAG激光器,对NiTi形状记忆合金进行激光表面熔凝处理,利用SRVⅢ摩擦磨损试验机考察激光熔凝处理对NiTi形状记忆合金微动磨损性能的影响.采用扫描电子显微镜及能谱仪分析NiTi形状记忆合金表面磨痕形貌及磨损产物成分;用形貌仪测量样品表面磨痕深度,并对磨损体积进行计算.结果表明,与NiTi合金相比,熔凝层摩擦系数,磨损体积均显著降低,表明激光熔凝处理提高了Ni-Ti合金的耐微动磨损性能.NiTi合金微动磨损机理主要表现为氧化和磨粒磨损,而Ni-Ti合金激光熔凝层磨损机制主要以疲劳剥层理论及磨粒磨损为主.     

Electrochemical behavior of Ti-Ni shape memory alloy in fibrinogen solution

Electrochemical behaviors of Ti-Ni shape memory alloy in fibrinogen solution were studied by electrochemical techniques. The results indicate that the addition of the fibrinogen has no obvious effect on the corrosion potential, but decreases the pitting potential markedly and increases the passive current densities. The analysis of energy-dispersive X-ray for samples adsorbing fibrinogen exhibits that the elements of O, C and N exist on the surface of Ti-Ni alloy. Furthermore, the scanning electron microscope micrographs confirm that the configuration of the adsorbing fibrinogen concentrating on surface defects is like cluster and the fibrinogen adsorption concentration is 96.67 mg/m^2 through ultroviolet ray absorption method. Fibrinogen combined with Ti-Ni alloy surface by complex band and its electrochemical transfer accelerated the corrosion of alloy.     

铸态镍钛形状记忆合金压缩变形行为及组织演变(英文)

通过真空自耗电极电弧熔炼法制备等原子比镍钛形状记忆合金铸锭。应用差示扫描量热分析、X射线衍射分析、能谱分析和显微分析等技术研究铸态镍钛形状记忆合金在不同温度和不同应变速率压缩加载下的组织演变和变形行为。铸态镍钛形状记忆合金的显微组织由树枝晶和等轴晶组成,成分不均匀,存在偏析。在室温下,铸态镍钛形状记忆合金是B19′马氏体、B2奥氏体和Ti2Ni的混合物。铸态镍钛形状记忆合金在高温压缩变形下对应变速率敏感,铸态镍钛形状记忆合金在0.1s-1和0.01s-1应变速率下具有动态再结晶的特征,而在0.001s-1应变速率下则表现出动态回复的特征。在室温和-100°C时,应变速率对铸态镍钛形状记忆合金的显微组织和真实应力应变曲线影响不大。     

Influence of Fe addition on phase transformation behavior of NiTi shape memory alloy

Three different NiTi-based alloys, whose nominal compositions were Ni₅₀Ti₅₀, Ni₄₉Ti₄₉Fe₂, Ni₄₅Ti_51.8Fe_3.2 (mole fraction, %), respectively, were used in the current research to understand the influence of Fe addition on phase transformation behavior in NiTi shape memory alloy (SMA). The microstructure and phase transformation behavior of the alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results show that the matrix of the Ni₅₀Ti₅₀ alloy consists of both B19′ (martensite) phase and B2 (austenite) phase. Moreover, the substructures of twins could be observed in the B19′ phase. However, the ternary alloys of NiTiFe exhibit B2 phase in the microstructures. Such microstructures were also characterized by large presence of Ti₂Ni precipitates dispersed homogenously in the matrix of the two kinds of alloys. The addition of Fe to the NiTi SMA results in the decrease in phase transformation temperatures in the ternary alloys. Based on mechanism analysis, it can be concluded that this phenomenon is primarily attributed to atom relaxation of the distorted lattice induced by Ni-antisite defects and Fe substitutions during phase transformation, which enables stabilization of B2 phase during phase transformation.     

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.     

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.     

NiTi形状记忆合金的应变补偿物理本构模型

采用Gleeble-3500热模拟试验机对NiTi形状记忆合金进行了等温恒应变速率压缩试验,研究其在变形温度为700～900℃、应变速率为0.001～10 s-1和最大高度下压量为70％下的高温热变形行为;建立了引入物理参量的应变补偿本构模型.结果 表明:合金的流变应力具有负温度相关性和正应变速率敏感性,在应变速率为0...     

镍钛形状记忆合金在局部包套压缩下的塑性屈服

作为一种崭新的尝试,局部包套压缩被应用于实现镍钛形状记忆合金在室温下的大塑性变形。基于主应力法和塑性屈服准则,分析了镍钛形状记忆合金局部包套的压缩塑性力学。采用透射电镜、高分辨透射电镜和扫描电镜研究镍钛形状记忆合金在局部包套压缩下的显微组织演变和变形行为。静水压力随着包套外径的增加而增加,有效地抑制了显微裂纹的萌生和扩展,有助于提高镍钛形状记忆合金的塑性,避免了脆性断裂的发生。在0．15～0．50的真实应变范围内,镍钛形状记忆合金在三向压应力状态下的塑性变形满足密席斯塑性屈服准则。在更大的塑性应变下,由于非晶相的出现,镍钛合金不能满足密席斯塑性屈服准则。     

Crystallization of amorphous NiTi shape memory alloy fabricated by severe plastic deformation

Based on the local canning compression, severe plastic deformation (SPD) is able to lead to the almost complete amorphous nickel-titanium shape memory alloy (NiTi SMA), in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix. Crystallization of amorphous NiTi alloy annealed at 573, 723 and 873 K was investigated, respectively. The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K, where martensite phase transformation is suppressed due to the constraint of the grain boundaries. Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample, where (001) martensite compound twin is observed at room temperature. It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains. SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.     

Microstructures and mechanical properties of laser-welded TiNi shape memory alloy and stainless steel wires

The Nd:YAG laser welding was used to join the TiNi shape memory alloy and AISI304 stainless steel wires. The microstructural features of the dissimilar material joint were analyzed. The tensile and hardness tests were carried out to examine the mechanical properties and microhardness distribution of the welded joint. The results show that the joint has the non-homogeneous microstructure and element distribution. The brittle phases such as Fe2Ti, FeTi, Cr2Ti, Ti3Ni4, Fe0.2Ni4.8Ti5 and TiN mainly segregate in rich Ti region of fusion zone. The laser-welded joint has the tensile strength of 298 MPa with the elongation of 3.72% and exhibits the brittle fracture features on the fracture surfaces. The reasons for low joint strength were discussed in this investigation.     

TiNi形状记忆合金与不锈钢激光焊接头裂纹分析及防止措施

采用激光焊对TiNi形状记忆合金与不锈钢异种材料进行焊接,用扫描电镜和激光共聚焦显微镜研究了TiNi合金/不锈钢接头裂纹及断口特征,分析了焊缝裂纹的形成机理,并提出了防止裂纹的措施.结果表明,裂纹多以微裂纹的形式出现于焊缝中心和TiNi合金侧熔合区.焊缝中存在大量的脆性化合物是产生裂纹的内在原因,接头受到拉伸应力是产生裂纹的必要条件,焊缝裂纹是二者共同作用的结果.通过焊接区加镍和钴中间层材料、改变激光光斑位置、焊接区施加轴向力及优化激光焊接参数的方法均能在一定程度上改善焊缝金属的裂纹敏感性,其中加金属中间层效果更为明显,加镍和钴中间层后,接头抗拉强度分别达到372和347 MPa,比未加中间层的接头的抗拉强度分别提高98.9%和85.6%.     

固溶处理及时效对镍钛形状记忆合金组织演化及力学性能的影响

将镍钛形状记忆合金Ni50.9Ti49.1（摩尔分数）在1123 K固溶处理2 h,然后分别在573、723和873 K时效2h。采用透射电镜、高分辨率透射电镜、扫描电镜和压缩实验,系统研究固溶处理和时效对镍钛合金组织演化及力学性能的影响。结果表明：固溶处理有助于消除原始镍钛样品中的Ti2Ni相,但不能消除TiC相。固溶处理导致镍钛合金中原子排列的有序畴界。在所有时效镍钛样品中,Ni4Ti3析出相、R相和B2奥氏体相共存于室温下的镍钛基体上,然而在873 K时效的镍钛样品中,可以观察到马氏体孪晶。在573和723 K时效的镍钛样品中,细小密集的Ni4Ti3相均匀分布在镍钛基体上,而且与B2基体保持共格关系。然而,在873 K时效的镍钛样品中, Ni4Ti3相尺寸非常不均匀,和B2基体保持共格、半共格和非共格关系。在723 K时效的条件下,细小均匀的Ni4Ti3相阻碍位错运动,导致最大的位错滑移临界分切应力,因此镍钛样品表现出最高的屈服强度。     

添加铜箔中间层的NiTi形状记忆合金超声波焊接

采用超声波焊接方法对添加铜箔中间层的NiTi形状记忆合金进行搭接焊,研究接头的表面形貌、界面形貌、相变行为、抗拉性能和断裂特点. 结果表明,超声波焊接能实现添加铜箔中间层的薄片状NiTi形状记忆合金的有效连接. NiTi焊缝表面存在明显压痕,界面处结合良好,无金属间化合物生成. NiTi形状记忆合金超声波焊缝呈可逆单相转变过程,接头强度达到母材的65%,断裂位置在母材区,断口形貌呈现韧性断裂特征. 超声波焊接方法将有效解决NiTi形状记忆合金采用传统熔焊方法时产生脆性金属间化合物的这一问题,为NiTi记忆合金的连接提供了新的解决途径.     

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。     

镍钛形状记忆合金在热压缩变形下的动态回复和动态再结晶

通过获得镍钛形状记忆合金在应变速率(0.001～1 s-1)和变形温度(600～1000℃)下的压缩真实应力—应变曲线,研究镍钛形状记忆合金在热变形下的力学行为.通过显微组织演变研究镍钛形状记忆合金的动态回复和动态再结晶,获得应变速率、变形温度和变形程度对镍钛形状记忆合金的动态回复和动态再结晶的影响规律.镍钛形状记忆合金在600℃和700℃下,动态回复和动态再结晶共存,但在其他温度下表现出完全动态再结晶.增加变形温度或降低应变速率,导致较大的等轴晶粒.变形程度对镍钛形状记忆合金的动态再结晶具有重要的影响.在镍钛形状记忆合金的动态再结晶中存在临界变形程度,当大于临界变形程度时,较大的变形程度有助于获得细小的等轴再结晶晶粒.     

薄片状TiNi形状记忆合金的激光对接焊

采用低功率脉冲激光对厚度为0.2mm的TiNi形状记忆合金进行了对接焊,研究了接头的抗拉强度、断裂应变、相变过程和形状记忆效应.试验发现,低功率脉冲激光能够实现薄片状TiNi合金的良好焊接.冷轧态TiNi合金焊后接头的强度达到母材的97%,断裂应变为母材的95%,对接头进行焊后退火处理其强度有所提高,高于冷轧态母材、达到退火态母材的63.6%,断裂应变为退火态母材的82.5%.冷轧态TiNi合金焊接接头经焊后退火处理,其相变过程和退火态母材接近,形状记忆效应基本和退火态母材相同.     

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.     

Surface treatment of NiTi shape memory alloy by modified advanced oxidation process

A modified advanced oxidation process(AOP) utilizing a UV/electrochemically-generated peroxide system was used to fabricate titania films on chemically polished NiTi shape memory alloy(SMA). The microstructure and biomedical properties of the film were characterized by scanning electron microscopy(SEM), X-ray photoelectron spectroscopy(XPS), inductively-coupled plasma mass spectrometry(ICPMS), hemolysis analysis, and blood platelet adhesion test. It is found that the modified AOP has a high processing effectiveness and can result in the formation of a dense titania film with a Ni-free zone near its top surface. In comparison, Ni can still be detected on the outer NiTi surface by the conventional AOP using the UV/H2O2 system. The depth profiles of O, Ni, Ti show that the film possesses a smooth graded interface structure next to the NiTi substrate and this structure enhances the mechanical stability of titania film. The titania film can dramatically reduce toxic Ni ion release and also improve the hemolysis resistance and thromboresistance of biomedical NiTi SMA.     

NiTi形状记忆合金的热电行为

研究了形状记忆合金线的热电行为。当合金线被加热到温度高于其相变温度时,由于相的转变而产生一个大的机械力。将 SMA 线用作驱动器,并研究了不同的参数及它们之间的关系。在不同的压力水平下,这些变化的参数分别是弹性应变(位移)、温度磁滞和电阻。通过传递热模型得到安全的加热电流并预测了相变温度。在自然空气对流的条件下,采用680 mA的电流加热、冷却合金线796 s。在43 MPa的应力水平下,应变恢复率为4.33%,相应的电阻变化为11.2%。在加热、冷却循环过程中,电阻变化与位移和电流分别呈线性关系。该研究有助于精确控制有、无外部传感器反馈的SMA线驱动器。