Two-way shape memory effect and its stability in Ti-Ni-Hf high temperature shape memory alloy

The two-way shape memory effect(TWSME) in a Ti36 Ni49 Hf15 high temperature shape memory alloy (SMA) was systematically studied by bending tests. In the TiNiHf alloy, the martensite deformation is an effective method to get two-way shape memory effect even with a small deformation strain. The results indicate that the internal stress field formed by the bending deformation is in the direction of the preferentially oriented martensite variants formed during the bending deformation. Upon cooling the preferentially oriented martensite variants form under such an oriented stress field, which should be responsible for the generation of the two-way shape memory effect.Proper training process benefits the formation of the oriented stress field, resulting in the improvement of the twoway shape memory effect. A maximum TWSME of 0.88% is obtained in the present alloy.     

Characterisation of melt spun Ni-Ti shape memory Ribbons’ microstructure

NiTi alloys are the most technologically important medical Shape Memory Alloys in a wide range of applications used in Orthopaedics, Neurology, Cardiology and interventional Radiology as guide-wires, self-expandable stents, stent grafts, inferior vena cava filters and clinical instruments. This paper discusses the use of rapid solidification by the melt spinning method for the preparation of thin NiTi ribbons for medical uses. Generally, the application of rapid solidification via melt-spinning can change the microstructure drastically, which improves ductility and shape memory characteristics and leads to samples with small dimensions. As the increase in the wheel speed led to a reduced ribbon thickness, the cooling rate increased and, therefore, the martensitic substructure became finer. Furthermore, no transition from the crystalline phase to the amorphous phase was obtained by increasing the cooling rate, even at a wheel speed of 30 m/s. Specimens for our metallographic investigation were cut from the longitudinal cross sections of melt-spun ribbons. Conventional TEM studies were carried out with an acceleration voltage of 120 kV. Additionally, the chemical composition of the samples was examined with a TEM equipped with an EDX analyser. The crystallographic structure was determined using Bragg-Brentano x-ray diffraction with Cu-K?? radiation at room temperature.     

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect(TWSME) of polycrystalline Ni_(56)Mn_(25-x)Co_xGa_(18.9)Gd_(0.1) alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in Ni_(56)Mn_(21)Co_4Ga_(18.9)Gd_(0.1) alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.     

Two-way shape memory effect and alternating current driving characteristics of a TiNi alloy spring

Two-way shape memory effect (TWSME) was induced into the TiNi shape memory alloys (SMAs) spring by thermomechanical training after annealing treatment, which has promising application in micro-actuating fields. The TWSME spring can contract upon heating and extend upon cooling. The results show that there is an increase of the recovery ratio up to a maximum TWSME of 45%. During the training procedure, transformation temperatures and hysteresis were measured by different scanning calorimetry (DSC). The results show that As (reverse transformation start temperature) and Af (reverse transformation fmish temperature) shift to lower temperature after training. The intervals of ArAs and Ms-Mf (Ms and Mf are the martensite start and finish temperatures, respectively) increase and the heat of transformation decreases after training. The electrothermal driving characteristics of the TWSME springs were also investigated with alternating current density of 3.2-14.7A/mm^2. It is found that the time response and the maximum contraction ratio greatly depend on the magnitude of the electrical current density.     

Ni50Mn27Ga23快淬薄带的马氏体相变和磁感生应变

采用快淬技术制各了非化学计量成分的Ni50Mn27Ga23多晶薄带，对快淬薄带的马氏体相变和磁感生应变进行了研究。结果表明，快淬薄带侄冷却和加热过程中仍然发牛热弹性马氏体相变及逆相变，具有典型的热弹性形状记忆效应，并且比铸态合金具有更大的应变，但是马氏体相变温度下降。快淬工艺在合金内部引入特定的内应力，使薄带形成织构，是合金获得更大相变应变和磁感卞应变的原因。热处理后，内应力大大降低，导致磁感牛应变降低。     

熔体旋淬法制备纳米Cu-13.2Al-5.1Ni形状记忆合金的结构和性能

比较了纳米Cu-13.2Al-5.1Ni形状记忆合金和初始粗晶结构合金的结构和性能。采用熔体旋淬技术,通过快速凝固制备纳米Cu-Al-Ni薄带。利用XRD、SEM、AFM和DSC等技术表征了纳米结构和粗晶样品结构及性能。结果表明,纳米结构薄带显示了单次形状记忆效应。另外,生成的纳米γ2（Cu9Al4）粒子和纳米晶粒导致马氏体？奥氏体相变温度显著降低。经过变形？回复循环后,所制备的纳米结构不仅导致回复变形量大幅增加,而且导致结构稳定性增加。     

Influence of Quenching Rates on Equiatomic NiTi Ribbons Fabricated by Melt-Spinning

Quenching rates, in terms of circumferential wheel speed in melt-spinning, play an important role in the resultant characteristics of the ribbons. In this study, the influence of various wheel speeds on ribbon dimensions, cross-sectional microstructures, and crystallographic phases within samples were investigated. The Ni-50.3at.%Ti melt-spun ribbons were produced under 200 mbar He atmosphere at wheel speeds of 5-30 ms^?1 using a quartz crucible coated internally with Y₂O₃. The different wheel speeds led to different sample dimensions and cooling rates. The microstructures of the samples were observed using optical microscopy and TEM, while the shape memory effect of samples and the existing phases were studied by differential scanning calorimetry, x-ray diffraction, and tensile tests.     

TiNiCu形状记忆合金薄带研究进展

TiNiCu形状记忆合金薄带凭借其窄相变滞后、优异的形状记忆效应与超弹性、良好的热循环稳定性成为一种很有前途的微驱动器材料。本文全面阐述了国内外在TiNiCu薄带研究方面的最新进展,主要包括晶化行为、显微组织、马氏体相变行为、形状记忆效应与超弹性,重点介绍了热处理工艺-显微组织-性能之间的内在联系,探讨了TiNiCu薄带今后的研究重点     

Effects of the spinning wheel velocity on the microstructure of Fe–Pd shape memory melt-spun ribbons

Fe–Pd with approximately 30 at.% of Pd shape memory ribbons have been manufactured by the free jet melt-spinning method with various wheel velocities. The microstructure of the as-spun ribbons have been investigated.     

Zr含量对(Al86Ni9La5)100-xZrx非晶合金热稳定性和腐蚀行为的影响

用单辊甩带法制备出组分为(Al86Ni9La5)100-xZrx(x=0,3,5,7,10)的合金薄带。用X射线衍射(XRD)仪和示差扫描量热计(DSC)对这些薄带的非晶特性和热稳定性进行分析;在0.01mol/L的碱性NaCl溶液中,对(Al86Ni9La5)100-xZrx(x=0,3,5,7)甩带试样的电化学腐蚀行为进行研究。结果表明,(Al86Ni9La5)100-xZrx(x=0,3,5,7,10)的合金薄带中,组分为(Al86Ni9La5)95Zr5样品的非晶热稳定性最好,随着Zr含量的增加,(Al86Ni9La5)100-xZrx的抗电化学腐蚀性能增强。     

热机械训练影响TiNiCu记忆合金DSC曲线特征的研究

利用示差扫描量热法(DSC)研究了加热伸长、冷却收缩和加热收缩、冷却伸长2种TiNiCu双向记忆效应弹簧训练过程中相变特征。研究结果表明，训练前升温时，DSC曲线表现出吸热峰由多个小峰组成，随着训练次数的增加，峰的个数减少，具有稳定双向记忆效应的试样中，升温时只出现1个锐利的峰。这是因为在训练过程中引入特定取向的位错，产生一定的应力场，使得马氏体变体的取向比较一致，相变点基本相同所导致。并由于位错的引入，引起相变点As和Af下降。     

Correlation between liquid structure and γ2-phase precipitation of Cu-Al-Ni shape memory alloys

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快淬参数对Nd9.5Fe84B6.5合金磁性能和微观结构的影响

本文采用直接快淬法制备了Nd_9.5Fe₈₄B_6.5合金,研究了腔室压力和甩带速度这两个快淬参数对其磁性能和微观结构的影响,建立了两者之间的关联性。研究发现：熔体快淬过程中在其它条件保持不变时,腔室压力和甩带速度之间存在着互相补偿的关系。腔室压力为0.03 MPa、快淬速度为19 m/s时制备的条带样品与腔室压力为0.05 MPa、快淬速度为15 m/s时制备的样品厚度和磁性能的数值相近。X 射线衍射和透射电子显微镜的结果表明：这两种条件下得到的条带样品具有相同的相组成和相似的微观结构。因此,通过调整腔室压力和甩带速度可以得到微观结构和磁性能相近的条带样品,这将为制备高性能的稀土永磁材料及其工业化生产提供指导。     

Microstructure and magnetocaloric properties of partially crystallized Gd60Co30Fe10 amorphous alloy prepared by different solidification cooling rates

The Gd₆₀Co₃₀Fe₁₀alloy ribbons with different solidification cooling rates were prepared by modifying the melt-spinning speed of 6.0,12.5,25.0 and 50.0 m·s^-1·With cooling rate decreasing,the(Fe,Co)5 Gd and hcp-Gd nanocrystalline was in situ precipitated among the amorphous matrix,which resulted in the composition change of the amorphous phase.Because of the only slight amount of crystalline phase in Gd₆₀Co₃₀Fe₁₀alloys,the magnetic and magnetocaloric properties mainly depend on the amorphous phase,and all the magnetic entropy change versus temperature(|Δ|SMI-T)curves are table-like,indicating the suitability for Ericsson cycle.The magnetic transition temperature of the Gd₆₀Co₃₀Fe₁₀alloy at a melt-spinning speed of 6.0 m·s^-1 shifted obviously to the lower value with the applied magnetic fields increasing.The peak value of magnetic entropy change(|ΔSMpk|)is 2.19 J·kg^-1·K^-1at217 K under the magnetic field change of 0-2 T,and the table-like region is 200-230 K.It was proved that the moderate reduction of the cooling rate will not deteriorate the magnetocaloric performance of the Gd₆₀Co₃₀Fe₁₀ribbons seriously.     

快速凝固法制备Mg-Al-Ca合金薄带

通过单辊快淬法制备了Mg-8.2Al-4.7Ca合金薄带,采用XRF、XRD、金相显微分析、显微硬度测量等分析方法研究了其凝固组织及相结构,以及转速对镁合金条带厚度和显微硬度的影响。研究结果表明,急冷快速凝固条件下,合金形成非晶相 细小hcp-Mg(Al,Ca)相;镁合金薄带显微组织沿厚度方向分为近辊面细晶区、内部柱状晶和自由面粗晶区;随着辊速的提高,晶粒不断细化,薄带的硬度不断提高。晶粒细化是显微硬度提高的主要原因。     

Synthesis and mechanical properties of cast quasicrystal-reinforced Al-alloys

We report on the preparation and mechanical properties of Al-based-composites with quasicrystalline particles as reinforcement. Al–Mn–Ce/Fe and Al–Mn–Pd bulk samples were synthesized by die casting into a copper mould at moderate cooling rate. Thin ribbons were prepared by melt-spinning as a reference state for the phase formation under most rapid quenching conditions. Microstructural analysis was done using X-ray diffraction, scanning and transmission electron microscopy as well as calorimetric methods. Significant differences in the phase formation, the composite microstructure and the thermal stability of the microstructure were found for different alloy compositions. The deformation behaviour was characterized by constant-rate compression tests at room temperature for a number of alloy variations yielding excellent properties compared to conventionally produced Al-alloys. The mechanical properties vary within a wide range of strength and ductility as a function of the quasicrystal volume fraction and their morphology. Also first tests at elevated temperatures were carried out, revealing a promising high temperature stability of the composite.     

非晶态及部分晶化态Al-Ni-Y合金的磁性

采用熔体急冷法制备了Al_(90)Ni_2Y_8和Al_(84)Ni_8Y_8合金条带,并用XRD进行了结构表征,用差示扫描量热仪分析了合金的热稳定性,使用超导量子干涉仪对Al_(90)Ni_2Y_8和Al_(84)Ni_8Y_8非晶态及部分晶化态合金的磁性进行了研究.结果表明,Al_(90)Ni_2Y_8和Al_(84)Ni_8Y_8非晶合金为抗磁性,而且随着Ni含量的增加,合金更容易被磁化.当磁场强度达到0.5T时,Al_(90)Ni_2Y_8合金对应的比磁化强度为-0.083 Am~2/kg,磁化率为-1.66×10~(-5),而Al_(84)Ni_8Y_8合金对应的比磁化强度为-0.091 Am~2/kg,磁化率为-1.82×10~(-5).当合金部分晶化后,合金的磁性仍保持抗磁性,但是比磁化强度的绝对值均显著增加.当磁场强度为0.5 T时,Al_(90)Ni_2Y_8合金对应的比磁化强度的绝对值从急冷态的0.083 Am~2/kg增大到部分晶化后的0.231 Am~2/kg,Al_(84)Ni_8Y_8对应的比磁化强度的绝对值从急冷态的0.091 Am~2/kg增大到部分晶化后的0.163 Am~2...     

U-Rh体系非晶合金的成分设计与形成特性

为了进一步认识U合金体系的非晶形成规律,本文探究了U-Rh体系的非晶形成特性。在富U端共晶区65～80 at.% U范围内设计了系列U-Rh合金,通过真空电弧熔炼制备母锭样品与甩带技术制备条带样品,采用X射线衍射和热分析手段研究了两种样品的相组成与热行为。发现U₇₀Rh₃₀合金在急冷凝固条件下形成了部分非晶相,晶化温度为642K,晶化激活能接近200kJ/mol,与之共生的是g-U高温固溶体相,这是U基非晶合金通过急冷冻结高温U固溶体的首次证据,反映出U-Rh合金非平衡凝固相变行为的独特性。结果表明,U-Rh是一个新的铀基非晶合金体系,其非晶形成能力不强,这很可能与g-U高温固溶体能被急冷直接捕获有关。     

Tuning the Planar-Flow Melt-Spinning Process Subject to Operability Conditions

An amorphous structure improves the physical properties of metal and enhances material performance. Planar-flow melt-spinning (PFMS) is a rapid solidification process for producing microcrystalline and amorphous metal ribbons. In PFMS, molten metal is fed through a nozzle onto a rotating wheel where the melt freezes and a continuous ribbon is spun. This study proposes a rapid method for process tuning. Examples were used to determine the applied pressure and wheel speed for designing the ribbon thickness and a wheel-nozzle gap with a preset nozzle size. The determined operating variables are suited to operation conditions, enabling the successful production of continuous ribbon. The proposed method was tested using computational fluid dynamics by treating the liquid metal and ambient air as a two-phase flow using the volume of fluid method. This study used the model to predict the puddle shape and ribbon thickness by fixing the dimensions of the wheel-nozzle gap and nozzle slot and varying the pressure and wheel speed. The results from the simulation confirmed the viability of the method and showed a concerted trend for the ribbon thickness compared with previous studies. In addition, the simulation revealed a fluctuation in the ribbon thickness, which was attributed to the first vibration mode of the puddle that was related to the natural frequency of the liquid inertia balanced by surface tension.     

Generation,development and degradation of the intrinsic two-way shape memory effect in different alloy systems

A uniaxial two-way shape memory effect is induced in wire specimens by a thermomechancial treatment. In order to enable a systematic variation of the microstructure with respect to dislocation density, second phase particles and grain size, the investigations are carried out on three different alloy systems. 4000 thermal cycles are performed on the trained shape memory elements, continuously observing the changes in the deformation behavior. The influence of work hardening, grain size and high internal stress-fields on the development and the stability of the intrinsic two-way shape memory effect is discussed, supported by microstructural investigations.The degradation due to cyclic application is related to instabilities in the high- and low-temperature modification. The decrease of the effect size is found to consist of two characteristic stages, where the decay is distinguished by different origin and rate. Definite conclusions about the degradation mechanisms are drawn by analyzing the influence of the preliminary heat treatment on the fatigue behavior. It is shown that the degradation can be minimized and hence the stability of the two-way shape memory effect can be optimized by establishing an appropriate microstructure.