Repeatable temperature memory effect of TiNi shape memory alloys

An incomplete transformation cycle induces a kinetic stop in the following complete transformation cycle in TiNi shape memory alloys. The kinetic stop can be regarded as a memory of the previous interruption temperature. This memory was generally believed to be a one-time phenomenon. Herein, we show that the temperature memory effect is actually a long-lasting phenomenon. Experimental results show that a repeatable temperature memory effect can be introduced into a TiNi alloy by a deformation lager than 12%. Deformation induced dislocations are considered a main factor to the persistence of the memory. The memory becomes more distinct with increasing numbers of the incomplete thermal cycle, but the memory becomes less distinct with increasing numbers of subsequent complete transformation cycling.     

TiNi基形状记忆合金的辐照效应

材料辐照效应是入射粒子与物质交互作用造成的物质微观组织结构与宏观性能的变化。辐照效应不仅是改善材料表面性能的重要手段,而且也是特殊环境应用材料可靠性评价的重要组成部分。TiNi基形状记忆合金是一种重要的金属智能材料,具有独特的形状记忆效应和超弹性,已在卫星、空间站等航天器以及生物医学中广泛应用。本文阐述了Ti-Ni基形状记忆合金在空间粒子(质子、电子)以及离子辐照改性的研究进展,辐照效应会对TiNi合金的微观组织结构产生影响,进而改变合金的相变行为和力学行为。然而目前关于TiNi基合金的辐照效应的研究仍处于起步阶段,组织结构和相变行为的变化规律和机理还未研究清楚,有关形状记忆效应的研究较少,仍需深入研究辐照参数、组织结构、相变行为和功能特性之间的内在联系。     

The electro-discharge machining characteristics of TiNi shape memory alloys

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current I _P and pulse duration _P. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration _P and a lower pulse current I _P should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO₂, TiNiO₃ and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration _P. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti₄₉Ni₅₁, Ti₅₀Ni₅₀ and Ti₅₀Ni₄₀Cu₁₀ SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.     

Amorphization and crystallization characteristics of TiNi shape memory alloys by severe plastic deformation

Differential scanning calorimetry (DSC) was used to determine the crystallization fraction and rate in TiNi alloys by severe plastic deformation. Results showed that the reverse martensitic transformation peak was not observed during the first heating at the rate of 40 K/min in the as-rolled samples, but one exothermic peak was observed at 620 K, which was associated with the amorphous crystallization process. During the second heating, reverse martensitic transformation was recovered. The onset crystallization temperature was low in the initial stage of crystallization with lower heating rates, but the crystallization fraction was found to increase with increasing temperature. However, the crystallization fraction was almost constant in the initial stage of crystallization with a relatively high heating rate. In all heating rates, the amorphous crystallization rates almost always reached maximum as the volumetric fraction of amorphous crystallization rose to 50%.     

Study of incomplete transformations of near equiatomic TiNi shape memory alloys by DSC methods

The incomplete transformations of near equiatomic TiNi shape memory alloys were investigated by differential scanning calorimeter (DSC). The results showed that the incomplete transformation can induce multistage phase transformation in a sample showing R-phase transformation, when the turn back temperature located in a value between R_f and M_s, the heat flow detected upon following heating only shows one endothermic peak. With decreasing the turn back temperature to a temperature between M_s and M_f, two endothermic peaks (multistage phase transformation) can be observed upon the following heating. There were no incomplete transformation induced multistage phase transformations in a sample without R-phase transformation.     

热等静压法制备多孔NiTi形状记忆合金

运用热等静压法(hot isostatic pressing,HIP),制备出多孔的NiTi形状记忆合金.制备出的多孔NiTi合金具有接近球状的孔,孔径在50～200μm,孔的分布均匀且各向同性.本文研究了多孔NiTi形状记忆合金在不同时效条件下的微观结构和马氏体相变行为,发现富Ni的多孔NiTi形状记忆合金的相变机制与富Ni致密NiTi合金相类似,时效后的冷却曲线出现两个峰,表示母相B2→R的转变和R→B19'的转变过程.     

Multi-strengthening effects on the martensitic transformation temperatures of TiNi shape memory alloys

The multi-strengthening effects, introduced by combining cold rolling, aging and thermal cycling, on the martensitic transformation temperatures of Ti₅₀Ni₅₀ and Ti₄₉Ni₅₁ shape memory alloys have been studied by using DSC and microhardness measurements. Experimental results show that both pre-cold-rolling of Ti₅₀Ni₅₀ alloy and aging treatments of Ti₄₉N₅₁ alloy can impede the further introduction of dislocations during the thermal cycling and hence effectively reduce the effect of thermal cycling on transformation temperatures. For Ti₄₉Ni₅₁ alloy, whether the cold rolling is conducted before or after the aging treatment, the strengthening of cold rolling can significantly increase the specimen hardness but only slightly affect the following thermal cycling effect. The multi-strengthening effects on the martensitic transformation temperatures are also found to follow the equation of Ms = T₀–K_y.     

Effect of thermomechanical training temperature on the two-way shape memory effect of TiNi and TiNiCu shape memory alloys springs

In this article, the effect of thermomechanical training temperature on the two-way shape memory effect (TWSME) of TiNiCu and TiNi alloys springs were investigated. The results showed that when the springs were thermomechanical-trained at pure martensite, there is an increase of the recovery rate to a saturated value, the maximum recovery rate was about 55% and 45% for TiNiCu and TiNi alloys, respectively. As the springs were thermomechanical-trained at pure austensite and martensite+austensite, there is an increase of the recovery rate to a maximum value and decreased with ongoing training after having passed the maximum value and the maximum TWSME recovery rate is less than that of the shape memory alloys spring-trained at pure martensite. Dislocations generated by martensite reorientation were effective in developing two-way memory effect. Since the amount of the stress-induced martensite variants is less than that of thermal-induced martensite variants, thus, the recovery rate showed a different rule with increasing thermomechanical training cycles at different training temperature.     

Optimization of spark plasma sintering parameters for NiTiCu shape memory alloys

Nanostructured nickel titanium copper-shape memory alloys (NiTiCu-SMAs) were fabricated using spark plasma sintering (SPS) by varying the significant process parameters. The NiTiCu elements with different particle size were consolidated in a temperature range of 700–900°C and pressure from 20 to 40 MPa with 5 min of soaking time. The sintered products were subjected to mechanical analysis such as density and microhardness. Genetic algorithm (GA) and particle swarm optimization (PSO) techniques were used with integrated artificial neural network (ANN) to optimize the SPS process parameters to obtain better mechanical characteristics. The results indicate that the density and microhardness can be enhanced by the reduction of particle size and increase in pressure and temperature. A maximum density of 6.21 g/cc and Vickers hardness of 766 Hv were obtained the optimal for process parameters of temperature, pressure, and particle size of ~ 800°C, ~ 26 MPa and ~ 6 µm, respectively, in case of NiTiCu nanostructured SMAs.     

单辊旋淬法制备含Ti的Fe基形状记忆合金的组织与性能

采用单辊旋淬法制备了3种不同组成的Fe基形状记忆合金(shape memory alloy,SMA):Fe-30Mn-6Si、Fe-30Mn-6Si-1Ni、Fe-30Mn-6Si-1Ni-1Ti。采用X射线衍射仪(XRD)、金相显微镜(OM)、扫描电子显微镜(SEM)分析了合金的成分及结构,通过拉伸实验测量合金的形状恢复率。结果表明,在Fe-30Mn-6Si合金中,除奥氏体晶粒外出现少量枝晶偏析,其二次枝晶臂间距为0.2μm。Fe-30Mn-6Si-1Ni、Fe-30Mn-6Si-1Ni-1Ti中等轴奥氏体晶粒均匀分布,且后者晶粒较大。预变形量为1%,1.5%时,Fe-30Mn-6Si-1Ni的形状恢复率高于Fe-30Mn-6Si-1Ni-1Ti;预变形量为2%时,Fe-30Mn-6Si-1Ni-1Ti的形状恢复率高于Fe-30Mn-6Si-1Ni;预变形量为3%时,Fe-30Mn-6Si-1Ni-1Ti的形状恢复率出现最大值2%。     

Improved shape memory composites combined with TiNi wire and shape memory epoxy

In order to develop high functionality of shape memory materials, the shape memory composites combined with TiNi wire and shape memory epoxy were fabricated, and the mechanical and thermomechanical properties were studied. The results showed that TiNi wire can compensate for the stiffness decrease of SMPs at elevated temperature, and the strength of interface and strength of interface matrix were important to further increase elevated temperature mechanical properties. The recovery stress of composites could be adjusted by changing the pre-strain, and the maximum recovery stress was obtained at 8% which was TiNi wire maximum recoverable strain. The addition of 1 vol% TiNi wire could increase the maximum recovery stress from 1.36 MPa to 4.04 MPa, which was almost 3 times of the matrix and at the same time maintained the rates of shape fixity and shape recovery close to 100%.     

形状记忆合金驱动的微执行器

以TiNi基为代表的形状记忆合金（SMA）具有做功密度大、可恢复应变应力大、驱动电压低、良好生物相容性等优点，在微执行器领域具有极为广阔应用前景。本文主要总结了TiNi基SMA丝、薄带、薄膜在微执行器方面的国内外研究应用现状，并对目前SMA在微驱动领域应用所存在的问题及其未来的发展趋势进行了分析讨论。     

机械合金化和粉末冶金法制备Fe-Mn-Si基形状记忆合金的研究进展

铁基形状记忆合金由于价格低廉、强度高、加工性能好、可焊接等优点引起广泛重视。机械合金化(MA)和粉末冶金(PM)作为制备材料的新工艺,可以用来制备性能优越的形状记忆合金。本文详述了机械合金化和粉末冶金工艺在制备Fe-Mn-Si基形状记忆合金过程中对合金相变、组织与性能的影响,以及此类合金在新领域的应用。最后提出了现阶段在研究MA/PM工艺制备Fe-Mn-Si基SMA中有关工艺参数、相变机制以及回复应力和低温应力松弛所存在的问题。     

Influence of size factor on martensite transformations and shape memory effects in TiNi based alloys

The results of the studies of electrical and mechanical properties of fine and hyperfine wires prepared from titanium nickelide based alloys (TN-10 grade) are presented. It is established that the temperature range of the martensite transformations shifts to lower temperatures with a decrease in the specimen diameter. Oxide films formed on the surface of thin specimens deteriorate the parameters responsible for shape changes in the specimens possessing the shape memory effect.     

NiTiHf-based shape memory alloys

Abstract

NiTiHf-based shape memory alloys have been receiving considerable attention for high temperature, high strength and two-way shape memory applications since they could have transformation temperatures above 100°C, shape memory effect under high stress (above 500 MPa) and superelasticity above 100°C. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf-based alloys have some drawbacks such as low ductility and high slope in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing. It has been revealed that microstructural control is crucial to govern the shape memory properties (e.g. transformation temperatures, matrix strength, shape recovery strain, twinning type, etc.) of NiTiHf-based alloys. A summary of the most recent improvements on selected NiTiHf-based systems is presented to point out their significant shape memory properties, effects of alloying, aging and microstructure of transforming phases and precipitates.     

Characteristics of two-way shape memory TiNi springs driven by electrical current

The electrothermal driving characteristics of the two-way shape memory effect (TWSME) extension TiNi springs, which can extend upon heating and contract upon cooling, were investigated with alternating and direct current. It was found that the response time (the time interval between the start and the end of the spring's shape change) and the maximum elongation greatly depend on the magnitude of the electrical current. The response time was large for a small electrical current and small for a high one. The maximum elongation increased to a maximum value with increasing current first and then decreased. For practical applications, electrothermal driving working cycles cannot be avoided in repeated use. So the degradation of the TWSME was also investigated by repeated application of the effect. There was a rather strong decay during the first electrothermal cycling. In the latter stage the degradation rate was small.     

TiNi shape memory clamps with optimized structure parameters

The different treatments of TiNi shape memory alloys applied as a material for so-called TiNi clamps and their influence on substructure parameters and deformation behaviour are discussed. The forces generated in fine-grained materials with higher dislocation density are stable after the cooling of the clamp to body temperature, which is a necessary condition for stable fixation of bone fragments. The forces generated and reversible strains depend on the substructure state and system stiffness parameter. The yield point and the elastic moduli are modified by controling the B2/B19' interface mobility. The mechanical parameters obtained after annealing at 400 and 500 °C, respectively, following the 15% cold-working reduction are optimal for the TiNi clamp design. The TiNi clamps with optimal properties have been used in a total of 68 patients. The new TiNi clamps enhance the fixation of the bone fragments involved, helping to prevent redislocation and provide the ability for earlier active rehabilitation.