Calorimetric measurements of transformation thermodynamics and thermal efficiencies of NiTi helices

The latent heats and thermal efficiency of a NiTi helix as the driving element of a Shape Memory Effect (SME) heat engine was measured by an adiabatic calorimeter capable of simulating a heat engine cycle. The latent heat measurements were found to be highly variable. Thermal efficiencies between 2.6 and 4.5% were measured in the limit of low external stresses needed for reproducible cycling behaviour.     

热处理对NiTi形状记忆合金的相变的影响

对含Ti 50.8%的NiTi形状记忆合金在300℃、400℃、500℃保温不同时间进行时效处理,然后通过差示扫描量热仪DSC对其相变温度点进行测定,发现经时效处理调节后,Af最大可下降20~30℃左右,而Ms变化不大。通过不同时效温度和保温时间的DSC曲线比较,得出热处理对其相变温度点的影响规律,即在300~500℃时效处理时,保温时间越短,Af降低越明显,在相同保温时间时,保温温度越低,Af降低越明显。     

Effect of micro-arc oxidation surface modification on the properties of the NiTi shape memory alloy

In this paper, the effects of micro-arc oxidation (MAO) surface modification (alumina coatings) on the phase transformation behavior, shape memory characteristics, in vitro haemocopatibility and cytocompatibility of the biomedical NiTi alloy were investigated respectively by differential scanning calorimetry, bending test, hemolysis ratio test, dynamic blood clotting test, platelet adhesion test and cytotoxicity testing by human osteoblasts (Hobs). The results showed that there were no obvious changes of the phase transformation temperatures and shape memory characteristics of the NiTi alloy after the MAO surface modification and the coating could withstand the thermal shock and volume change caused by martensite-austenite phase transformation. Compared to the uncoated NiTi alloys, the MAO surface modification could effectively improve the haemocopatibility of the coated NiTi alloys by the reduced hemolysis ratio, the prolonged dynamic clotting time and the decreased number of platelet adhesion; and the rough and porous alumina coatings could obviously promote the adherence, spread and proliferation of the Hobs with the significant increase of proliferation number of Hobs adhered on the surface of the coated NiTi alloys (P?<?0.05).     

A shape memory effect swashplate heat engine

A novel heat engine based on the shape memory effect (SME) of helical NiTi wire and utilising the swashplate geometry is described. Measurements of engine power cycled through 12°C and 80°C water baths were as high as 3 W/kg, which compare favourably with more complex SME heat engines designs.     

Effects of Mold Pressure on Mechanical, Microstructures, Oxidation Behavior, and Thermodynamic Properties of an Al-Based Alloy

In this study, the effects of mold pressure variation on the microstructure, mechanical, oxidation behavior, and thermodynamics properties of an AlSi12CuNi alloy have been investigated by means of optical microcopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, differential thermal analysis, and thermo-gravimetric analysis techniques. It is observed that hardness and tensile strength of the alloys increase as die-casting mold pressure increases due to an improvement in the distribution pattern of secondary phases and reduction in porosity. Oxidation behavior parameters and thermodynamic parameters such as formation enthalpy, heat capacity, and entropy are determined from TGA and DTA. It is found that mold pressure also has a significant effect on the oxidation behavior and thermodynamic parameters.     

Relevant aspects in the clinical applications of NiTi shape memory alloys

NiTi shape memory alloys showing pseudoelastic behaviour have great potential in dental and orthopaedic applications where constant correcting loads may be required. In most of the clinical applications the device may have been heat treated and during its life in service it will be cyclically deformed. It is therefore important to investigate the effect of cyclic straining and heat treatments upon the transformation stresses and temperatures of the material. The aim of this work is to study the thermal and mechanical ageing of a pseudoelastic NiTi shape memory alloy, as well as the environmental in vitro degradation of the alloy due to the effect of artificial saliva.This paper was accepted for publication after the 1995 Conference of the European Society of Biomaterials, Oporto, Portugal, 10–13 September.     

Influence of Rolling on the Thermal Diffusivity of Metal Alloys by Photothermal Infrared Radiometry

The thermal diffusivities of metallic foils subjected to cold-rolling processes have been studied by photothermal radiometry in a thermal transmission and reflection configuration. In this work, measurements were conducted on foils of Al, Cu, and stainless steel (V2A) that were subjected stepwise to cold-rolling process, reducing the sample of around 1 mm, as prepared, to approximately 0.1 mm. It was found that the effect of the cold-rolling is manifested as a relationship between the relative diffusivity and the relative thickness, both with respect to their corresponding initial values. This empirical relationship consists of a linear decrease of the relative diffusivity with the negative of the logarithm of the relative thickness of the sample. Within the approximation of small deformations, this behavior is consistent with linear diminishing of the thermal diffusivity with the degree of rolling. An influence of rolling on both the thermal diffusivity and effusivity was previously observed for a polycrystalline NiTi shape memory alloy with a nearly equi-atomic composition. Due to the thermal-diffusivity behavior of metal alloys due to rolling, a simple microscopic model is proposed to explain this influence upon the effective thermal parameters within the framework of a one-dimensional heat flow perpendicular to the foil surface. The model assumes the reduction of grain sizes and the consequent increase of grain interfaces during rolling as responsible for a larger effective thermal resistance. Numerical results are shown using the available polycrystalline NiTi for both the thermal-diffusivity and thermal-effusivity values.     

Mg/NiTi复合材料的制备及阻尼性能

采用Mg粉的无压熔渗法制备Mg/NiTi复合材料以提高多孔NiTi合金的强度和阻尼性能。通过OM、SEM、EDS和XRD分析Mg/NiTi复合材料的显微组织结构,采用压缩实验分析其抗压强度、吸能能力,采用热机械分析仪分析其内耗和存储模量。结果表明:经Mg粉无压熔渗后,多孔NiTi合金的孔隙被Mg填充,其孔隙率由原来的50.38%下降至5.6%,且Mg与NiTi合金的界面结合良好。多孔NiTi合金主要由B2奥氏体相和B19'马氏体相及少量Ni₃Ti相和NiTi₂相组成;Mg/NiTi复合材料除增加了熔渗的Mg相外,还新生成了Mg₂Ni相。Mg的渗入未改变多孔NiTi合金相变行为,但提高了相变温度。Mg/NiTi复合材料的抗压强度可达554 MPa,较多孔NiTi合金提高了61%,压缩断裂方式也由多孔NiTi合金的孔壁崩塌断裂转变为Mg/NiTi复合材料的剪切断裂。Mg/NiTi复合材料的吸能较多孔NiTi合金有大幅提高。同时,Mg/NiTi复合材料的内耗值有所增加,而存储模量大幅提高,整体呈现出更佳的阻尼性能。      

NiTiCu合金相变过程的研究

本文用差示扫描量热法研究了 Cu 对 NiTi 记忆合金马氏体相变点及相变潜热的影响。结果表明,以 Cu 代替 Ni 至30 at.%,合金仍具有良好的记忆性能;合金相变温度及相变潜热值略有变化,相变滞后明显减小;Cu 含量对合金相变类型没有影响。相变潜热值△H 与 T_0=1/2(A_s+M_s)并不存在严格的直线关系。Cu 含量超过7.5at.—%时,合金的热加工性能变差。     

造孔剂法制备孔隙率可控的多孔镍钛合金

用常规粉末冶金烧结法结合造孔剂技术制备了多孔镍钛形状记忆合金.研究结果表明,孔隙特征主要取决于所用造孔剂的尺寸和形貌,而孔隙率可以通过加入造孔剂的含量来调控;所制备合金的主要成分是B2相和B19'相的NiTi,在经高温固溶处理后仍有极少量的Ti2Ni,Ni3Ti和Ni4Ti3等杂质相存在,而且合金杂质相的含量随造孔剂加入量的增加而相应提高.对合金的相变热分析表明,在从高温降至低温过程中多孔镍钛发生特殊的三阶段相变,这主要是由合金成分和相组成在晶界的不均匀造成的.最后,研究表明相变对多孔镍钛合金的阻尼性能(内耗)有独特的影响,而孔隙率对阻尼的影响则不明显.     

Combustion synthesis of TiNi intermetallic compounds

The microstructural characteristics of NiTi intermetallic shape memory compounds produced by combustion synthesis have been investigated. It was found that the microstructure consists mainly of NiTi parent phase and NiTi₂ second phase. The faceted or coarse dendritic NiTi₂ phase, which is produced using a low cooling rate of the synthesized liquid product, can be modified to provide an evenly distributed, fine dendritic structure by increasing the cooling rate. This fine dendritic product can be readily hot-rolled into plate exhibiting the shape memory effect (SME). The initial nickel particle size has an important influence on the microstructure and also on the SME transition temperature. The morphology of the microstructure can also be modified to that of conventionally produced alloy by solution treatment. It is proposed that the synthesis reaction mechanism occurs by two combustion stages, i.e. precombustion and combustion. The particle size plays a key role in the precombustion stage.     

多孔NiTi合金的化学氧化处理表面改性研究

在H2O2+HCl溶液中对多孔NiTi合金进行了化学氧化处理表面改性。利用EPMA、SEM、原子吸收光谱对合金表面氧化膜的组成、形貌及化学氧化处理前后的Ni离子释出进行了研究。结果表明,经过H2O2+HCl溶液化学氧化表面处理后合金表面Ni含量显著降低,在生理盐水中的Ni释放速率明显下降;H2O2+HCl溶液浓度和氧化时间均对合金表面氧化膜的生长有较大影响。     

铝合金高温储热材料及储热系统设计

对Al-7％Si合金,Al-7％Si-4％Cu合金和Al-33％Cu合金进行了差热扫描分析,设计了高温储热系统,并建立了相应的测试与评价方法。结果表明,3种合金的熔化潜热分别为305．5J／g,245．1J／g和202．7J／g,且Al-Si合金的相变潜热与含铝量成正比关系。该测试与评价方法可对储热材料的储热性能和储热系统的换热效率,热交换动力学等指标进行综合评价。     

NiTi基形状记忆合金弹热效应及其应用研究进展

NiTi合金作为性能最优异的形状记忆合金之一,已经广泛应用于航空航天、电子、建筑、生物医学等领域。近年来,NiTi基合金极佳的力学性能、巨大的弹热效应和良好的机械加工性使其在弹热制冷领域引起了广泛关注。然而,传统NiTi二元合金超弹性应力滞后大,超弹性和弹热效应循环稳定性差,达不到实际应用所需的长期服役要求。本文介绍了NiTi基合金的弹热效应研究进展,从掺杂合金元素、热机械处理、改变制备方法等角度综述了近几年NiTi基合金弹热效应改进优化的研究进展,同时本文也简要介绍了已经开发的基于NiTi基合金的弹热装置或原型机。但是目前NiTi基合金弹热材料的研究和原型机的开发仍处于实验阶段,实现其商业化应用需要进一步深入研究和优化,未来前者研究重点将集中在材料小型化、合金化或特殊处理及改变循环方式等方面,后者也将从提高热量传输效率、加强热量交换、减小摩擦等损耗、改进机械负载和循环模式等方面不断优化和完善。     

基于物质熵增纳米铜-赤藻糖醇循环稳定性分析

本文介绍了纳米铜-赤藻糖醇的配制方法,通过材料热物性变化的内部机理研究了循环过程中此相变材料(PCM)的衰减过程。根据差示扫描量热仪(DSC)以及导热系数测试仪(Hotdisk)的测试数据,对导热系数、过冷度、相变潜热在100次热循环过程中的0次、20次、40次、60次、80次、100次的变化原因进行了理论分析。采用热力学熵的热力学原理引出物质熵,从物质熵增的角度,总结了相变材料的衰减原因。结果表明:相变材料经过100次循环后,各项性能参数都出现了不同幅度的下降。导热系数随着纳米铜的添加量增加下降幅度减小;赤藻糖醇添加纳米铜后的过冷度整体上小于没有添加的过冷度;随着循环次数的增加,潜热值后期整体下降幅度加大。     

Effect of microstructure on the fatigue of hot-rolled and cold-drawn NiTi shape memory alloys

We present results from a systematic study linking material microstructure to monotonic and fatigue properties of NiTi shape memory alloys. We consider Ni-rich materials that are either (1) hot rolled or (2) hot rolled and cold drawn. In addition to the two material processing routes, heat treatments are used to systematically alter material microstructure giving rise to a broad range of thermal, monotonic and cyclic properties. The strength and hardness of the austenite and martensite phases initially increase with mild heat treatment (300 °C), and subsequently decrease with increased aging temperature above 300 °C. This trend is consistent with transmission electron microscopy observed precipitation hardening in the hot-rolled material and precipitation hardening plus recovery and recrystallization in the cold-drawn materials. The low-cycle pseudoelastic fatigue properties of the NiTi materials generally improve with increasing material strength, although comparison across the two product forms demonstrates that higher measured flow strength does not assure superior resistance to pseudoelastic cyclic degradation. Fatigue crack growth rates in the hot-rolled material are relatively independent of heat treatment and demonstrate similar fatigue crack growth rates to other NiTi product forms; however, the cold-drawn material demonstrates fatigue threshold values some 5 times smaller than the hot-rolled material. The difference in the fatigue performance of hot-rolled and cold-drawn NiTi bars is attributed to significant residual stresses in the cold-drawn material, which amplify fatigue susceptibility despite superior measured monotonic properties.     

Thermal conductivity of inhomogeneous materials

The effective thermal conductivity is calculated from the rate of entropy production per unit volume. Thermal conductivity and the temperature field are expressed in terms of Fourier components and these are related. The rate of entropy production is then obtained in terms of the volume-averaged thermal conductivity and the Fourier components of thermal conductivity. A simple expression for the effective thermal conductivity is found. In the case of striations it leads to well-known results. The formalism is applied to solids with inhomogeneously distributed solutes. It is shown that the thermal conductivity is less than the volume-averaged thermal conductivity and that homogenization by diffusion increases the thermal conductivity. Similar results would apply to the electrical conductivity of inhomogeneous alloys.     

Grain size effect on the R-phase transformation of nanocrystalline NiTi shape memory alloys

Development of nanoscale actuators and sensors in recent years calls for functional materials with small dimensions and high strengths. High strength nanocrystalline NiTi alloys which experience the R-phase transformation with a small thermal hysteresis are ideal candidates for these applications. To facilitate the application of the R-phase transformation in nanocrystalline NiTi alloys, this study investigated the effect of grain size on the R-phase transformation of a nanocrystalline Ti-50.2at.%Ni alloy. The nanometric grain size was created by severe cold deformation and low temperature anneal. It was found that in the recrystallized state, achieving nanoscale grain sizes (<100 nm) was effective in suppressing the B2→B19’ martensitic transformation and revealing the B2?R transformation. The B2?R transformation temperature was found to increase with the decreasing grain size within the range of 22–155 nm. The suppression of the B19’ martensite in nanograins is attributed to the limited space within the grains to allow the formation of self-accommodation structures to contain the large lattice distortion of the martensite.     

Dilatometric study of martensitic transformation in NiTiCu and NiTi shape memory alloys

Dilatometric measurements have been carried out for the study of nature of martensitic transformation in the NiTiCu and NiTi shape memory alloy wire samples. Investigation has been done in the heat-treat temperature range 300–800°C. NiTiCu exhibits only single stage A M martensitic transformation in the entire heat-treat temperature range indicating the suppression of R-phase by Cu substitution. NiTi shows the two-stage A R M martensitic transformation in the heat-treat temperature range 340–410°C and the single-stage A M martensitic transformation above heat-treat temperature 410°C. The extent of dilation during phase transformation decreases with increasing heat-treat temperature in both the alloys. Effect of first 15 thermal cycles on transformation temperatures in both the alloys has been studied. It is found that transformation temperatures are unaffected with thermal cycles in NiTiCu whereas considerable decrease in transformation temperatures has been observed in the case of NiTi. The stability of transformation temperatures in NiTiCu during M A transformation against thermal cycling may be attributed to the associated smaller thermal hysteresis compared to NiTi.     

Using a modified Knoop Indentation Technique to estimate the cavitation erosion resistance of NiTi

An Ni-rich (50.8 at.% Ni) NiTi alloy was heat treated with different aging temperatures to obtain specimens possessing different cavitation erosion resistance. A modified Knoop Indentation Technique that combines indentation measurements and thermal recovery has been used to determine the total-recovery/deformation ratio for the heat-treated NiTi specimens. This ratio, which reflects both superelasticity and pseudoplasticity, has been found to correlate well with the cavitation erosion resistance for NiTi. The technique thus serves as a simple method for estimating the performance of NiTi against cavitation erosion.