Microstructures and transformation behavior of Ti-Ni-Cu shape memory alloy powders fabricated by ball milling method

Ti-Ni and Ti-Ni-Cu alloy powders have been fabricated by ball milling after which their microstructures and transformation behaviors were investigated by means of scanning electron microscopy, X-ray diffraction, energy dispersive X-ray analysis, transmission electron microscopy and differential scanning calorimetry. The powders of as-milled Ti-Ni-Cu alloys whose Cu-content is less than 5 at.% were mixtures of crystal and amorphous, whereas those alloy powders whose Cu-content is more than 10 at.% were crystalline. The B19’ martensite is formed in the Ti-Ni-Cu alloy powders whose Cu-content is less than 10 at.%, whereas the B19 and B19’ martensites coexist in those whose Cu-content is more than 10 at.%. The martensitic transformation range became wider with increasing rotating speed, and so endothermic and exothermic peaks obtained from differential scanning calorimetry were indiscernible.     

Ti-Ni-Cu形状记忆合金的温度记忆效应

采用差示扫描量热仪研究了Ti50Ni35Cu15以及Ti50Ni45Cu5(摩尔分数,%)形状记忆合金的温度记忆效应.结果表明:温度记忆效应仅在Ti-Ni-Cu合金的逆转变加热过程出现,在单斜结构马氏体与母相逆相变(B19'→B2)及正交结构马氏体与母相逆相变(B19→B2)过程中均能发生温度记忆效应;在随后的完全循环过程中,温度记忆记忆效应不再出现,DSC相变曲线又"恢复"到其原始形态;而在马氏体相变冷却过程中未发现温度记忆效应.分析表明,不完全相变过程中的弹性能再分布是可能的温度记忆效应机制.     

Structure evolution of Cu-based shape memory powder during mechanical alloying

The microstructures of shape memory powders of Cu-AI-Ni prepared from pure powders of Cu, AI and Ni using the mechanical alloying（MA） technique were studied by means of hardness measurement, metallograph observation, XRD and SEM. The hardness reaches the peak as the increase in hardness due to plastic deformation and the decrease in hardness due to kinetic annealing reach a balance. The process of MA leads to the formation of a laminated structure, and the layer becomes thinner with an increase in milling time. The pre-alloyed shape memory powder can be formed by milling at 300 r/min for 50 h using a planetary ball mill.     

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.     

Martensitic transformation and mechanical properties of Ti-rich Ti-Ni-Cu melt-spun ribbon

Martensitic transformation, mechanical and thermomechanical properties of a Ti-rich Ti₅₂Ni₂₃Cu₂₅ melt spun ribbon annealed at a temperature below the crystallization temperature were studied by XRD, DSC and DMA. After annealing the initially amorphous ribbon at 400 °C for 10 h, the ribbon is fully crystallized and exhibits one-stage B2?B19 phase transformation with the temperature hysteresis of 14 °C. The annealed ribbon is composed of B2, B19 and B11-TiCu phase with (001) preferential orientation. On the stress―strain curves, the rearrangement of the martensite variants and stress-induced martensitic transformation are observed below the M_f temperature and above the A_f temperature, respectively. The annealed ribbon exhibits up to 1.6% superelastic shape recovery with small stress hysteresis of 25 MPa. No flat stress-plateau is associated with the superelasticity. The annealed ribbon shows a well-defined shape memory effect during thermal cycling from ?60 to 100 °C. The transformation strain and recovery strain increase with increasing the applied external stress. Under the external stress above 150 MPa, the shape recovery strain is not sensitive to it and keeps stable at about 1.74%.     

R-phase transformation of aged Ti-Ni shape memory alloy

Ti-50.6Ni（molar fraction, %） shape memory alloy solution treated at 850 ℃ for lh followed by ageing treatment at 450 ℃ for 3 h was studied with differential scanning calorimetry（DSC）, X-ray diffractometry（XRD） and transmission electron microscopy（TEM）. DSC measurement reveals two separate transformation peaks. XRD and TEM demonstrate that a three-stage transformation occurs. The Ti3 Ni4 precipitates are coherent with the R- phase. The crystal structure of R-phase was analyzed by two diffraction patterns method. The diffraction patterns of R-phase were obtained in detail from the same region.     

复合稀土对CuZnAl形状记忆合金力学性能的影响

采用定量金相、拉伸试验、电子探针和扫描电镜等方法，研究了复合稀土对CuZnAl合金晶粒尺寸、晶粒生长动力学、力学性能的影响。结果表明：复合稀土可明显细化合金晶粒，改善合金的力学性能，合金的拉伸断口形貌由未加复合稀土的沿晶断裂变为韧窝状塑性断口，同时保持合金的记忆性能。微观分析表明：复合稀土富集在CMZnAl合金的晶界上，阻碍晶粒长大。讨论了晶粒细化机制以及合金力学性能提高的原因。     

Microstructures and transformation characteristics of thin films of TiNiCu shape memory alloy

Both sputtering conditions and crystallizing temperatures have great influence on the microstructures and phase transformation characteristics for TislNi44Cus. By means of the resistance-temperature measurement, X-ray diffraction and atomic fore microscopic study, the results indicate that the transformation temperatures of the thin films increase and the “rock candy“ martensitic relief is more easily obtained with promoting the sputtering Ar pressure, sputtering power, orcrystallizing temperature. However, when sputtering Ar pressure, sputtering power, or crystallizing temperature are lower, a kind of “chrysanthemum“ relief, which is related with Ti-rich GP zones, is much easier to be observed. The reason is that during crystallization process, both of the inherent compressive stresses introduced under the condition of higher sputteringpressure or higher crystallizing temperature are helpful to the transition from GP zones to Ti2(NiCu) precipitates and the increase of the transformation temperatures. The addition of copper to substitute for 5 96 nickel in mole fraction can reduce the transformation hvsteresis width to about 10 - 15 ℃.     

Stress-induced martensite phase transformation of FeMnSiCrNi shape memory alloy subjected to mechanical vibrating polishing

Fe₆₆Mn₁₅Si₅Cr₉Ni₅ (wt.%) shape memory alloy (SMA) with γ austenite and ε martensite was subjected to mechanical vibrating polishing and consequently its surface suffered from plastic deformation in the case of compressive stress. Almost complete ε martensite transformation is found to occur in FeMnSiCrNi sample subjected to mechanical vibrating polishing, where stress-induced martensite transformation plays a predominant role. Stress- induced martensite transformation of FeMnSiCrNi SMA is closely related to the orientation of external stress. The complicated compressive stress which results from the mechanical vibrating polishing contributes to ε martensite transformation from γ austenite of FeMnSiCrNi SMA. Mechanical vibrating polishing has a certain influence on the surface texture of ε martensite of FeMnSiCrNi SMA, where texture appears in the polished FeMnSiCrNi SMA.     

Crystallization of NiTi shape memory alloy sputtering-deposition film

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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.     

机械合金化法制备Al—Cu—Fe纳米非晶合金

采用行星式高能球磨机制备了Al80-xCuxFe20(x=20-40）三元非晶纳米合金粉末,分析了不同球磨时间及热处理工艺对粉末结构、颗粒大小等的影响。结果表明：成分为Al40Cu40Fe20的粉末球磨时逐步非晶化,球磨33h后,非晶化程度最大,最小颗粒尺寸达到5．6nm;进一步球磨,非晶晶化,颗粒尺寸增大;成分为Al80-xCuxFe20(x=20,25,30)的粉末球磨90h后,得到非晶,最小颗粒尺寸为3．4nm。球磨制备的Al-Cu-Fe非晶粉末具有铁磁性。用DSC测量了其晶化温度（Tc),Tc≈873℃。     

Kinetic arrest behavior in martensitic transformation of NiCoMnSn metamagnetic shape memory alloy

High-field magnetic measurements were carried out in order to investigate behaviors of field-induced reverse martensitic transformation and kinetic arrest of NiCoMnSn metamagnetic shape memory alloy. In the thermomagnetization curves, it was confirmed that the reverse martensitic transformation temperature decreases 67 K by applying magnetic field of 5 T, while in the magnetic field cooling process under 5 T, martensitic transformation does not occur down to low temperatures. Equilibrium magnetic field, defined from the critical magnetic fields of the metamagnetic evidence in the magnetization curves, exhibits almost constant below about 100 K, suggesting that the entropy change becomes zero, which is considered to cause kinetic arrest behavior.     

The suppression and recovery of martensitic transformation in a Ni-Co-Mn-In magnetic shape memory alloy

The intrinsic mechanism of the martensitic transformation (MT) suppression observed in Ni-Co-Mn-In alloys fabricated under non-equilibrium conditions still remains mysterious. Here, we used the undercooling technique to obtain a solidified microstructure in non-equilibrium state, subsequently leading to MT suppression even further cooling to 10 K. It was found that primary dendrite-like In-depleted precipitates occurred during solidification under a large undercooling. After a prolonged annealing, the MT interestingly appeared again due to the dissolution of the precipitates and the recovery of equilibrium chemical composition in the matrix.     

Shape memory behavior and tension-compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Incomplete transformation of Ti-Ni-X ternary shape memory alloy

If the reverse transformation of a shape memory alloy is arrested, a kinetic stop will appear in the next complete transformation. The kinetic stop temperature has a close relation with the previous arrest temperature. This kinetic stop can be regarded as a ＂memory＂ of the previous arrest temperature. This phenomenon is named temperature memory effect（TME）. The TME induced by incomplete cycling in Ti-Ni-Nb and Ti-Ni-Cu alloys was systematically investigated by performing either a single incomplete cycle, or a sequence of incomplete cycles with different arrested temperatures. The results indicate that TME only exists in the heating process, and TME can occur both in B 19＇→B2 and B 19→B2 reverse transformation during heating process. But, there is no evidence of TME during cooling in the Ti-Ni-X ternary alloys. And the reverse transformation temperature interval （At-As） of the Ti44-Ni47-Nb9 alloy induced by TME can be significantly enlarged compared with that of the Ti-Ni-Cu alloy by performing multi-times incomplete transformation cycling with a decreasing order arrested temperature.     

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.     

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。     

Effect of texture on phase-transformation strain in CuZnAl shape memory sheets

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