Microstructure and Mechanical Properties of NiTi Shape Memory Alloys by In Situ Laser Directed Energy DepositionEI北大核心CSCD

以Ni粉与Ti粉为原料,采用激光定向能量沉积(LDED)技术制备NiTi形状记忆合金。利用XRD、物相拟合、SEM、EDS和DSC等测试方法,对NiTi合金的显微组织、物相含量和物相转变进行分析,随后采用压缩圆柱样品进行形状记忆效应测试,并评估其形状记忆效应。激光能量密度较低时,NiTi合金中产生大量Ni_(4)Ti_(3)相沉淀,随着激光能量密度增加,Ni_(4)Ti_(3)相消失。激光能量密度为20.0 J/mm^(2)时,NiTi合金具有2878 MPa的压缩断裂强度与34.9%的压缩失效应变,且样品在循环20 cyc后具有88.2%形状记忆恢复率。     

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.     

NiTi形状记忆合金激光增材制造研究进展

NiTi形状记忆合金卓越的功能特性,生物相容性,高阻尼及低刚度,耐腐蚀等性能,深受广泛关注。NiTi形状记忆合金制备、加工困难,成为阻碍NiTi形状记忆合金应用的关键。近20年发展起来的增材制造技术,能直接成形制造复杂的NiTi形状记忆合金结构,在航空航天、医疗设备等领域具有巨大应用价值和发展前景。本文结合国内外NiTi形状记忆合金激光增材制造研究中面临的主要问题和解决方法作综合评述。具体包括NiTi形状记忆合金传统制造和激光增材制造对比;激光能量输入和热处理工艺对激光增材制造NiTi形状记忆合金微观组织、机械力学性能和功能特性的影响;及展望未来激光增材制造NiTi形状记忆合金研究方向。     

LOW DENSITY AND HIGH STRENGTH NiTi MEMORY ALLOYS WITH CONTROLLABLE PORE CHARATERISTICS AND THEIR SUPERELASTICITY

采用球形尿素颗粒预造孔工艺结合梯级粉末烧结方法制备出孔形规则、孔径大的轻质高 强多孔NiTi形状记忆合金. 通过控制造孔剂(尿素)的含量可有效调节多孔合金的孔隙特征, 获得孔径均匀、孔形圆整和孔隙率可控的样品, 其中孔径为296-732 um, 孔隙率为31%-61.6%; 尿素形状和尺寸对多孔合金的孔隙特征有决定性影响, 具 有几何形态遗传性; 尿素对多孔NiTi合金的组成相影响很小, 相变仍具有马氏体相变特征; 合金具有优良的力学性能和稳定的线性超弹性.     

CuAlPd alloys for sensor and actuator applications

Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115–370°C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180°C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modified Bridgeman technique.     

Effect of laser microcutting on thermo-mechanical properties of NiTiCu shape memory alloy

The machining of shape memory alloys (SMAs), such as NiTi based alloys, is a very interesting and relevant topic for several industrial applications in the biomedical, sensor and actuator fields. Laser technology is one of the most suitable methods for the manufacturing of products in the aforementioned fields, mainly when small and precise features have to be included. Due to the thermal nature of this process, study of its effect on the functional properties of these materials is needed. Except for binary NiTi, few results on the laser machining of NiTi based alloys are available in the literature. In this work, thin sheets of Ni40Ti50Cu10 (at.%) were processed by a fibre laser and the effect of process speed on the material properties was analysed. Scanning electronic microscopy was adopted for observation of the laser cut edges’ morphology. Chemical composition of the processed material was evaluated by energy dispersion spectroscopy and nanohardness measurements were used to estimate the heat affected zone. SMA functional properties were studied on both base and laser machined material. These characteristics are affected by laser machining for the presence of melted material; this effect can be minimised by increasing the laser process speed.     

高熵形状记忆合金的研究进展

高熵形状记忆合金是在等原子比NiTi合金的基础上,结合高熵合金的概念,逐渐发展起来的一种新型高温形状记忆合金。近年来,已开发出了综合性能优异的(TiZrHf)50(NiCoCu)50系和(TiZrHf)50(NiCuPd)50系高熵形状记忆合金,引起了广泛的关注和研究兴趣。本文从物相组成、微观组织、马氏体相变行为、形状记忆效应和超弹性等角度出发,综述了高熵形状记忆合金的研究进展,并对高熵形状记忆合金未来的研究重点进行了展望。     

钛镍多孔材料的制备及其显微组织与形状记忆性能

采用粉末烧结法制备了高孔隙率的Ti-50.8 at%Ni形状记忆合金(SMA),并对其显微组织和形状记忆性能进行了研究。结果表明,用该种方法制备的多孔合金样品具有孔隙分布均匀以及开孔率高的特点,其平均孔径在200μm左右。合金组织主要由奥氏体NiTi相(B2)和单斜马氏体NiTi相(B19’)组成。循环加载-卸载实验结果表明,该多孔材料能表现出一定的形状记忆效应,而且形状记忆效应随孔隙率的增加而降低。     

(NiTi)_(50-0．5x)Nb_x形状记忆合金的阻尼性能及力学性能

通过加入Nb制备了具有双相组织的(NiTi)50-0.5xNbx(x=5,10,15,20)形状记忆合金,合金兼具高阻尼性能和高屈服强度.随着Nb含量x的增大,合金中(NiTi+β-Nb)共晶组织比例含量增加,合金轧制样品在马氏体状态自协作屈服强度随之升高,在x=15时达到最高(289 MPa);同时,合金轧制样品保持高阻尼性能,本征阻尼性能tan δ＞0.01,并随x增大而升高.根据形状记忆合金阻尼理论以及NiTiNb形状记忆合金的阻尼性能随温度的变化规律,探讨了β-Nb和NiTi相界面阻尼对合金阻尼性能的影响.     

Reproducibility Study of NiTi Parts Made by Metal Injection Molding

Powder metallurgy (P/M) is an attractive manufacturing process for net-shaped NiTi parts considering the limited machinability of NiTi alloys. Nevertheless, the industrial implementation of P/M processing for NiTi alloys is not trivial. To become competitive to manufacturing of NiTi alloys based on established ingot metallurgy, combination of fully pronounced shape memory behavior with sufficient mechanical properties is required. Successful use of P/M technology is strongly influenced by high affinity of NiTi alloys for uptake of oxygen and carbon, which leads to the formation of oxygen-containing Ti₂Ni and TiC phases coupled with increase of Ni content in the matrix. In the case of Ni-rich NiTi alloys, this increase leads to a shift of phase transformation temperatures to lower values. Furthermore, precipitation of Ni₄Ti₃ during cooling from sintering temperature is difficult to avoid. Even if these precipitates might be used to decrease the Ni:Ti ratio of the matrix balancing oxygen and carbon uptake, significant loss of ductility arises, especially in the case of finely dispersed Ni₄Ti₃ precipitates. In the present work, each step of P/M manufacturing is discussed regarding its influence on the specific properties of NiTi alloys. The work is based on the application of prealloyed, gas atomized NiTi powders. Metal injection molding was used for net-shaped manufacturing of tensile samples, which enabled detailed study of sintering behavior combined with investigation of shape memory and mechanical properties depending on particle size, oxygen and carbon content as well as precipitation of Ni₄Ti₃ phase.     

A predictive model and validation of laser cutting of nitinol with a novel moving volumetric pulsed heat flux

Nitinol alloys are widely used in manufacturing of cardiovascular stents due to excellent biomechanical properties. Laser cutting is the predominant process for stent manufacturing. However, laser cutting induces thermal damage such as heat affected zone (HAZ), micro cracks, and tensile residual stress, which detrimentally affect product performance. Laser cutting induced temperature distribution, stress development, and HAZ formation are critical process characteristics. However, they are difficult to measure experimentally due to the highly transient process. To better understand the process mechanics in laser cutting of nitinol, a three-dimensional finite element model of pulsed laser cutting was developed to incorporate a novel moving volumetric pulsed heat flux model with high spatial accuracy. A material subroutine was also incorporated to model superelasticity and shape memory of nitinol. The predicted kerf geometry and dimensions agreed well with the experimental data. Also, the effects of cutting speed, pulse power, and pulse width on kerf profile, temperature, and heat affected zone (HAZ) were investigated.     

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.     

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.     

镁造孔多孔NiTi合金的微波烧结制备与表征

为了获得轻质、高强和高阻尼的多孔NiTi合金,采用微波烧结协同镁造孔技术制备多孔NiTi合金.考察多孔NiTi合金的显微组织、力学性能、相变行为、超弹性和阻尼性能.结果表明:当烧结温度低于或等于900℃时,多孔NiTi合金主要由B2 NiTi相和少量B19'NiTi相组成.随着烧结温度的升高,多孔NiTi合金的孔隙率逐...     

航天轴承新贵:60NiTi合金

在NiTi系合金大家族中,等原子比的NiTi合金具有形状记忆效应和超弹性。相比于等原子比NiTi合金,60NiTi合金不会发生热弹性马氏体相变,不具有形状记忆效应和超弹性,但是它在硬度、耐腐蚀性和弹性性能等方面具有出众的潜能,在航天领域上展现了良好的应用前景。固溶态60NiTi合金同时具备结构稳定性、超强的耐腐蚀性、出色的耐磨性,已成为航天轴承候选新材料。     

颗粒尺寸对微波烧结多孔NiTi合金的显微结构及力学性能的影响

以不同颗粒尺寸的Ni/Ti粉末为原料,采用微波烧结技术制备了多孔NiTi合金,并系统考察了颗粒尺寸对多孔NiTi合金的显微结构和力学性能的影响。结果表明,随着颗粒尺寸的减小,多孔NiTi合金中的Ti2Ni和Ni3Ti第二相减少而单质Ni相消失。同时,多孔NiTi合金的孔隙形貌由带尖角的不规则形状向近球形转变。此外,多孔NiTi合金的孔隙率和孔径随着颗粒尺寸的增大而增大,而洛氏硬度、抗压强度和抗弯强度均下降。因此,减小颗粒尺寸有利于获得理想的显微结构(纯净的物相和均匀的孔隙结构)和提高微波烧结多孔NiTi合金的力学性能。     

纳米SiC颗粒增强NiTi形状记忆复合材料制备及其力学性能和阻尼行为

采用粉末冶金梯级烧结法成功制备出轻质、高强且具有一定孔隙率的纳米SiC颗粒增强NiTi合金基形状记忆复合材料(SiC/NiTi).研究发现,所制备的SiC/NiTi复合材料具有稳定的线性超弹性;SiC颗粒的引入使SiC/NiTi复合材料具有较高的压缩强度和等效压缩强度,且强度随SiC含量的增加而提高.研究还表明,SiC...     

Mechanical behavior and microstructural analysis of NiTi-40Au shape memory alloys exhibiting work output above 400 °C

Substituting Ni with Au in NiTi leads to dramatic increases in transformation temperatures, meeting one of the requirements for a viable high temperature actuator material. Consequently, four alloys containing between 49 and 51 at.% Ti, a fixed 40 at.% Au, and balance Ni, were prepared and investigated in detail using load-biased thermal cycling (LBTC), scanning electron microscopy (SEM), aberration corrected scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS). LBTC experiments demonstrated work output well above 400 °C, with full recovery up to 100 MPa. The alloys exhibit minimal variation in shape memory properties despite the relatively large composition range from Ti-lean to Ti-rich, in stark contrast to most other NiTi-based systems, which demonstrate extreme compositional sensitivity. Electron beam analysis revealed the presence of two types of secondary phases present in all compositions, which are subsequently characterized. Differences in secondary phase content as a function of alloy composition is shown to have a moderating effect on the transforming matrix composition - an important asset for this alloy system - potentially easing processing requirements and opening up shape memory alloys to new fabrication techniques. Unrecovered strain during cycling at higher loads is analyzed from a theoretical perspective to gain insight into the mechanisms of defect formation responsible for functional fatigue.     

Phase stability and transformations in NiTi from density functional theory calculations

We used density functional theory to characterize various crystalline phases of NiTi alloys: (i) high-temperature austenite phase B2; (ii) orthorhombic B19; (iii) the monoclinic martensite phase B19′; and (iv) a body-centered orthorhombic phase (BCO), theoretically predicted to be the ground state. We also investigated possible transition pathways between the various phases and the energetics involved. We found B19 to be metastable with a 1 meV energy barrier separating it from B19′. Interestingly, we predicted a new phase of NiTi, denoted B19′′, that is involved in the transition between B19′ and BCO. B19′′ is monoclinic and can exhibit shape memory; furthermore, its presence reduces the internal stress required to stabilize the experimentally observed B19′ structure, and it consequently plays a key role in NiTi’s properties.     

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.