Shape memory heat engines

The mechanical shape memory effect associated with a thermoelastic martensitic transformation can be used to convert heat directly into mechanical work. Laboratory simulation of two types of heat engine cycles (Stirling and Ericsson) has been performed to measure the amount of work available per cycle in a Ni-45 at% Ti alloy. Tensile deformations at ambient temperature induced martensite, while a subsequent increase in temperature, caused a reversion to the parent phase during which a load was carried through the strain recovery, i.e. work was accomplished. The amount of heat necessary to carry the engines through a cycle was estimated from calorimeter measurements and the work performed per cycle. The measured efficiency of the system tested reached a maximum of 1.4% which was well below the theoretical (Carnot) maximum efficiency of 35.6%.     

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Study of the Thermal Properties of a Ni<Subscript>3</Subscript>Ta Shape Memory Alloy

Dilatation characteristics, thermal diffusivity, and thermal conductivity of a Ni₃Ta shape memory alloy were studied over the temperature range from room temperature up to 950 °C. The Ni₃Ta alloy was investigated in both polycrystalline and single crystal forms. The shape memory effect was positive for the polycrystalline samples and negative for the single crystal. While the transformation temperature of the M (martensite) → A (austenite) phase transformation was the same for both types of alloys and all measurements, the transformation temperature for the reverse phase transformation A → M was dependent on the maximum cycle temperature. Higher maximum temperatures of the thermal cycle yielded lower transformation temperatures for the A → M transformation. The thermal diffusivity and thermal conductivity of the austenite were higher than those of the martensite. No latent heat was found for the phase transformations.     

材料状态对NiTi形状记忆合金相变行为的影响

用金相显微镜观察了冷加工和固溶状态的显微组织形貌,用示差热量扫描法(DSC)系统研究了冷加工、固溶和时效处理对近等原子比的NiTi形状记忆合金的相变温度的影响。试验结果表明,冷加工态NiTi合金组织形态呈纤维状,固溶处理后组织形态呈等轴状。冷加工带来的大量变形缺陷抑制了热弹性马氏体的相变;冷加工态NiTi合金直接进行时效发生了P→M相变;经固溶处理后再进行时效则发生了P→R→M相变。NiTi合金在不同的热处理条件下发生了不同类型的热弹性马氏体相变。分析认为,应力、位错密度及析出相对NiTi合金热弹性马氏体的相变行为有重要的影响。     

Effect of variable material properties and environmental conditions on thermomechanical phase transformations in shape memory alloy wires

This paper reports a computational study of the impact of variable material properties and environmental conditions (thermal boundary conditions and convection coefficients) on shape memory alloy wires undergoing (i) zero-stress, thermally-induced phase transformations, and (ii) stress-induced phase transformations at constant stress rates. A finite difference numerical approach has been employed, and has been validated by comparing with two analytical solutions. The results have been all given in non-dimensional form, and within the context of the range of parameters that have been studied, the following recommendations can be made for shape memory alloys (SMA) actuator design: (i) an uncertainty in the thermal boundary condition is not as important as long as the design process allows for a full transformation back to martensite at the end of a cycle of martensite–austenite–martensite thermal transformation, (ii) uncertainties in the thermal boundary condition, convection coefficient and thermal material properties are not as important when the phase transformation in a SMA is induced by stress.     

形变时效对FeMnSiCrNi合金形状回复率和相变温度的影响

为了进一步提高Fe-14Mn-6Si-8Cr-5Ni合金的形状记忆效应,对固溶态合金采用了形变时效的方法处理,并利用光学显微分析、X射线衍射分析和透射电子显微分析的测试手段分析了时效温度和时效时间对合金形状回复率和相变温度的影响.结果表明,固溶态合金经10%拉伸和600℃时效10 min时,形状回复率提高幅度最大,由固溶态的48%提高到84.7%,并且合金γ→ε马氏体转变的起始温度Ms由固溶态的34℃降低到13.2℃.合金的形状回复率得到提高的主要原因是合金中热诱发ε马氏体已经消失,组织为奥氏体和大量定向α’马氏体,这样的组织特征有利于应力诱发γ→ε马氏体相变以及它们的逆相变.     

Ni50.1Mn24.1Ga20.3Fe5.5形状记忆合金多晶纤维的双程形状记忆效应

通过熔体抽拉技术制备Ni_50.1Mn_24.1Ga_20.3Fe_5.5多晶纤维,采用步进式热处理释放因快速凝固引起的内应力和缺陷,利用场发射扫描电子显微镜、透射电子显微镜、XRD衍射仪对其微结构和相结构进行表征,采用动态机械拉伸仪测试其相变行为和双程形状记忆性能。结果表明:热处理后原子有序度显著提高,孪晶界平直,在恒应力作用下一个热循环中母相和马氏体相的形状得到完全恢复。双程形状记忆曲线显示了热弹性马氏体相变的两个基本特征:可逆性和热滞性。在热循环实验中,纤维被加载到198 MPa时,其马氏体态总应变达到1.32%。根据热机械拉伸测量,发现相变温度遵循Clausius-Clapeyron关系式。与诸如Ti-Ni和Cu-Al-Ni的其他合金相比,Fe掺杂的纤维显示出较小的应变-应力依赖性,在恒应变输出的驱动中是有益的。     

CuZnAl合金在应力下的马氏体相变

采用附加有加载装置及电视系统的光学金相显微镜,对CuZnAl形状记忆合金的应力诱发马氏体进行了动态观察,发现该合金的应力诱发马氏体具有热弹性马氏体的相变特征,马氏体的形貌及转变量随应力的变化而变化;此外,还发现该合金有些晶粒的马氏体在应力诱发过程中消隐,代之出现取向、大小不同的新的马氏体。     

Microstructure, Compression Property and Shape Memory Effect of Equiatomic TaRu High Temperature Shape Memory Alloy

The microstructure, phase transformation, compression property and strain recovery characteristics of equiatomic TaRu super high temperature shape memory alloy have been studied by optical microscope, XRD, DTA, compression tests and TEM observations. When cooling the alloy specimen from high temperature to the room temperature,β(parent phase)→β′(interphase) →β"(martensite) two-step phase transformations occur. The microstructure at room temperature show regularly arranged band morphology, with the monoclinic crystal structure. The twinning relationship between the martensite bands is determined to be (101) of Type I. Reorientation and coalescence of the martensite bands inside the variant happened during compression at room temperature. The β′→β reversible transformation contributes mainly the shape memory effect, with the maximum completely recovery strain of 2%.     

Microstructures of 18R martensite induced by deformation and thermomechanical cycles in CuZnAl shape memory alloy

Microstructures of martensite in a Cu-26.4Zn-4.8Al shape memory alloy after deformation and thermomechanical cycles are reported. By detailed transmission electron microscopy investigation, it is clearly established that the microstructure induced by deformation is different from that induced by thermomechanical cycles, and the effects on the thermoelastic martensite transformation are also not the same at all. The microstructure of martensite induced by deformation is mainly deformation twins when the deformation exceeds the range of recoverable strain of the shape memory alloy. The main microstructure characteristic of martensite which has undergone thermomechanical cycles is regularly distributed dislocations, which are mainly aligned but sometimes tangled. Micro-twins similar to those found in deformed martensite and crossed bands also exist inside the martensite of samples after thermomechanical cycles, but the amounts are too small to be regarded as important microstructure features.     

Temperature memory effect in two-way shape memory TiNi and TiNiCu springs

An incomplete thermal cycle upon heating in a shape memory alloy (arrested at a temperature between A_s and A_f) induced a kinetic stop in the next complete thermal cycle. The kinetic stop temperature is closely related to the previous arrested temperature. This phenomenon is named temperature memory effect (TME). In this article, the TME in two-way shape memory TiNiCu and TiNi springs was investigated by performing either a single incomplete cycle, or a sequence of incomplete cycles. N points of temperatures could be memorized if N times of incomplete cycles on heating were performed with different arrested temperatures in a decreasing order. The capability is enhanced by performing repetitive incomplete cycles with the same temperature, and the TME can be eliminated by appropriate complete transformation cycle. The TME is originated from the relaxation of both the strain energy between martensite and coherent strain between parent phase and martensite.     

Thermomechanical behavior of Fe–Mn–Si–Cr–Ni shape memory alloys modified with samarium

The deformation and training behavior of Fe–14Mn–3Si–10Cr–5Ni (wt.%) shape memory alloys containing samarium addition has been studied in the iron-based shape memory alloys. It is noticed that thermomechanical treatment (training) has significant influence on proof stress, critical stress and shape memory behavior of the alloys. The improvement in shape memory behavior can be attributed to the decrease in the proof stress and critical stress which facilitates the formation of ? (hcp martensite). It is also observed that alloy 2 containing samarium undergoes less softening as compared to alloy 1 with training which inhibits the formation of ? (bcc martensite) and thus enhances the shape memory behavior. The excessive thermomechanical treatment with increase in the training cycle has led to the formation of ? (bcc martensite) along with ? (hcp martensite) in the alloy 1 which appeared to have decline in the shape memory effect. This has been demonstrated by the examination of microstructure and identification of ? (bcc martensite) martensite in the alloy 1 as compared to alloy 2.     

Effect of Incomplete Thermal Cycle on Transformation Behavior of Deformed TiNi Thin Film

Compared with the undeformed TiNi film,the martensite-austenite transformation(M-A) of the deformed one is elevated to a higher temperature on the first heating,but it nearly returns back to the original temperature on the second heating.An incomplete M-A transformation of the deformed TiNi film on the first heating divides the total martensite population into the self-accommodating martensite M2 and the oriented martensite M1.Thus,two transformations corresponding to M1-A and M2-A transition occur on the second heating.However,the forward transformation is not affected by the incomplete thermal cycle.     

The effect of thermal and stress cycling on thermoelastic martensite formation in Cu-Al-Mn alloys

The effect of thermal cycling on the thermoelastic martensitic reaction and stress cycling on stress induced martensite formation and pseudoelasticity have been studied in alloys in the Cu-Al-Mn system. The martensite to high temperature phase start and finish transformation temperatures,A _s andA _f, of alloys quenched from 850 C were considerably higher than after repeated thermal cycling. The excess vacancy concentration present after quenching is postulated to enhance low temperature ordering reactions in these alloys through defect assisted diffusional mechanisms and so stabilize the martensite leading to higherA _s andA _f temperatures. Stress cycling of a phase alloy increased pseudoelastic recovery and decreased the stress necessary to form stress induced martensite. Ageing specimens at room temperature after stress cycling was found to increase considerably the stress required to form stress induced martensite on subsequent stress cycling. Both pseudoelastic and shape memory recovery were observed in a martensitic alloy.     

CuAlBeCr形状记忆合金组织结构与形状记忆效应的研究

本文研究了热处理制度对含微量Cr的CuAlBe合金的马氏体逆转变及形状记忆效应的影响,并通过正电子湮没Doppler增宽线性参数的测量及金相组织和相结构分析,探讨了组织结构与记忆效应间的内在联系。结果表明,分级淬火态的CuAlBeCr合金具有相对最好的记忆性能(逆变起始温度低,恢复率大),而直接淬火的记忆性能最差。合金中空位型晶体缺陷的浓度显著影响马氏体逆转变开始的温度,空位浓度愈低,则马氏体逆转变开始的温度愈低,所研究的CuAlBeCr合金逆转变完成的区间宽(逆转变终了温度A′_f≈200℃),但该类合金在较高的温度下(≈400℃)具有比CuZnAl合金更好的记忆效应(逆转变动作快,恢复率大)。     

On role of atomic migration in amnesia occurrence during complex thermal cycling of Cu–Zn–Al shape memory alloy

Abstract

The reproducibility of thermally induced reversible martensitic transformation, occurring in a Cu–15Zn–6Al (mass-%) shape memory alloy during heating, was studied on three different fragments of martensitic alloy subjected to complex thermal cycling comprising four series of five heating–cooling cycles, applied on a differential scanning calorimetry (DSC) device, between room temperature (RT) and three maximum temperatures: 453, 463 and 473 K respectively. After each series of heating–cooling cycles the fragments were naturally aged at RT for 37 days (～3·2 Ms). Thermograms of DSC revealed a gradual diminution in the magnitude of martensite reversion to parent phase, accompanied by decreasing tendencies of both critical temperatures and transformation rates, which reflect a gradual loss of thermal memory, associated with ‘amnesia’ occurrence. Scanning electron microscopy observations coupled with energy dispersive X-ray spectroscopy performed at the end of complex thermal cycling revealed that ‘amnesia’ occurrence was associated with changes in the morphology of martensite plates, from parallel plate-like to interblocking needle-like, and with an increase of chemical fluctuations, as an effect of the intensification of atomic migration.     

Microstructure and phase transformations in a Ni<Subscript>50</Subscript>Mn<Subscript>29</Subscript>Ga<Subscript>16</Subscript>Gd<Subscript>5</Subscript> alloy with a high transformation temperature

A Heusler Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy with a high transformation temperature has been obtained by substituting 5 at% Gd for Ga in a ternary Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloy. The microstructure and phase transformations in the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy have been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. It is shown that the microstructure of the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy consists of matrix and hexagonal Gd (Ni,Mn)₄Ga phase, which indicates a eutectic structure composed of these two phases. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy exhibits a martensite transformation start temperature up to 524 K, approximately 200 K higher than that of Ni₅₀Mn₂₉Ga₂₁ alloy. At room temperature, non-modulated martensite with twin substructure is observed in Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy.     

Thermal expansion of composites with shape memory alloy inclusions and elastic matrix

Incorporation of shape memory alloy (SMA) inclusions into a continue matrix can make composites with various thermal dilatation behavior, and this depends sensitively on the microstructural parameters of the composite. A micro-mechanical method is proposed to relate quantitatively the overall thermal dilatation with the microstructures and the transformation characteristics of SMA materials. Composites with aligned and with two populations of perpendicularly oriented SMA inclusions are analyzed in detail in this paper. It is found that the composite with SMA fibers can have a large transformation temperature range during the heating process, and a linear shrinkage may take place during the cooling because of the large difference between the austenite finish and martensite start temperatures of the composite. Design aspects to minimize the overall thermal dilatation during a full thermal cycle are also discussed.     

多孔Ni-Mn-Ga-Co磁性形状记忆合金的制备及磁学特性研究

首先采用球磨法制备了不同粒度的Ni-Mn-Ga-Co合金粉末,然后通过3D打印技术成功制备了泡沫结构的多孔Ni-Mn-Ga-Co磁性形状记忆合金。利用SEM、DSC和XRD等研究了合金的微观组织特征、物相结构、相变特性和相关的磁性行为。结果表明,球磨后经过分筛得到的不同粒径尺寸的合金粉末均为不规则形状。Ni-Mn-Ga-Co合金粉末在室温下为非调制四方马氏体结构,其特征峰十分明显。Ni-Mn-Ga-Co合金的DSC曲线上出现宽峰相变,添加Co元素对马氏体转变温度开始值(Ms)基本没有影响,但其居里温度(Tc)有显著的提高。采用粒径为50~100μm的合金粉末烧结制备的磁性合金,饱和磁化强度最大可达68 Am^2/kg。合金粉末粒径越小,烧结制备的多孔Ni-Mn-Ga-Co磁性形状记忆合金致密度越高。当合金粉末粒径<50μm时,致密度可达90%;当合金粉末粒径为50~100μm时,致密度仅为75%。相较于粒径较小的合金粉末,粒径较大的合金粉末制备的磁性合金磁感生应变能力更高,这是由于泡沫结构能够有效减少内部和外部的约束,从而有利于提高磁场诱导应变。     

Deformation behaviour and shape memory effect of near equi-atomic NiTi alloy

The mechanical shape memory effect associated with the martensitic-type transformation which occurs in polycrystalline Ti-50.3 at. % Ni alloy has been investigated using the techniques of transmission and optical microscopy. Deformation of initially partially transformed material within the recoverable strain range was found to occur by: (1) stress-induced transformation of the most favourably oriented existing martensite variants at the expense of adjacent unfavourably oriented variants and retained high temperature phase (2) stress-induced re-orientation of favourably oriented martensite by utilizing the most favourably oriented twin system, and (3) stress-induced twin-boundary migration within the martensite. The reverse transformation during heating restores the original grain structure of the high-temperature phase in a highly coherent manner. It was concluded that deformation modes limited to those involved in the transformation process and the reversibility of the transformation give rise to the memory effect.