Smart Control Systems for Smart Materials

Shape memory alloys (SMAs) are thermally activated smart materials. Due to their ability to change into a previously imprinted shape by the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations for macroscopic systems. Most commonly used SMAs for actuators are binary nickel-titanium alloys (NiTi). The shape memory effect relies on the martensitic phase transformation. On heating the material from the low temperature phase (martensite) the material starts to transform into the high temperature phase (austenite) at the austenite start temperature (A _s). The reverse transformation starts at the martensite start temperature after passing a hysteresis cycle. To apply these materials to a wide range of industrial applications, a simple method for controlling the actuator effect is required. Today??s control concepts for shape memory actuators, in applications as well as in test stands, are time-based. This often leads to overheating after transformation into the high temperature phase which results in early fatigue. Besides, the dynamic behavior of such systems is influenced by unnecessary heating, resulting in a poor time performance. To minimize these effects, a controller system with resistance feedback is required to hold the energy input on specific keypoints. These two key points are directly before transformation (A _s) and shortly before retransformation (M _s). This allows triggering of fast and energy-efficient transformation cycles. Both experimental results and a mechatronical demonstrator system, exhibit the advantages of systems concerning efficiency, dynamics, and reliability.     

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.     

Factors Influencing the Reversion of Stress-induced Martensite to Austenite in a Fe-Mn-Si-Cr-Ni Shape Memory Alloy

It is well known that one way shape memory effect (SME) in Fe-Mn-Si-based shape memory alloys (SMAs) is related to the thermally induced reversion of ε (hexagonal close packed, hcp) stress-induced martensite (SIM) to γ (face centered cubic, fcc) austenite. In the case of a Fe-Mn-Si-Cr-Ni SMA, this reverse martensitic transformation was analyzed in regard to the critical temperature for the beginning of austenite formation (A _s) in different states characterized by quenching temperature and permanent tensile strain. For this purpose, dynamic mechanical analysis (DMA), dilatometry (DIL), differential thermal analysis (DSC), and optical microscopy (OM) were employed to determine the influence of quenching temperature and permanent tensile straining on SIM reversion to austenite during heating.     

真空感应重熔NiTi形状记忆合金室温拉伸力学性能

采用真空感应熔炼炉制备NiTi合金,并对重熔后的合金进行不同制度热处理。研究NiTi形状记忆合金边角料重熔合金不同温度时效后的室温拉伸力学性能。结果表明,NiTi形状记忆合金边角料重熔后,其室温屈服强度和抗拉强度均有不同程度的提高,断后延伸率略有增加,应力诱发马氏体相变对应的形状回复率降低;合金塑性变形能力随时效温度的升高而变差;合金中R相重新取向所提供的延伸率从总体上看比由单质原材料一次熔炼而成的合金要高。     

A micromechanical analysis of the coupled thermomechanical superelastic response of textured and untextured polycrystalline NiTi shape memory alloys

In this paper a micromechanical model that incorporates single crystal constitutive relationships is used for studying the pseudoelastic response of polycrystalline shape memory alloys (SMAs). In the micromechanical framework, the stress-free transformation strains of the possible martensite twinned structures, correspondence variant pairs (CVPs), obtained from the crystallographic data of NiTi are used, and the overall transformation strain is obtained by defining a set of martensitic volume fractions corresponding to active CVPs during phase transformation. The local form of the first law of thermodynamics is used and the energy balance relation for the polycrystalline SMAs is obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled based on Voronoi tessellations. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples, the effects of texture and the tension–compression asymmetry in polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), the number of grains and the texture on the mechanical response of polycrystalline SMAs is studied. It is found that the number of grains (or size) affects both the stress–strain response and the phase transformation propagation in the material. In addition to tensile and compressive loadings, textured and untextured NiTi micropillars with different sizes are also studied in bending. The coupled thermomechanical framework is used for analyzing the effect of loading rate and the phase transformation latent heat on the response of both textured and untextured samples. It is shown that the temperature changes due to the heat generation during phase transformation can affect the propagation of martensite in samples subjected to high strain rates.     

Research and Development in NiTi Shape Memory Alloys Fabricated by Selective Laser MeltingEI北大核心CSCD

由于NiTi形状记忆合金(SMAs)具有高反应敏感性和低热导率等物性,导致其初步成形件的后续加工十分困难,作为一种典型的金属增材制造技术,选区激光熔化(SLM)在近净成形复杂几何形状的金属构件方面具有显著优越性,能够有效解决NiTiSMAs冷加工难、加工成本高的问题。为实现SLMNiTiSMAs的工程应用,需厘清其工艺参数-微观结构-功能特性的内在联系,揭示其相转变行为与功能特性变化的机理,建立坚实的理论基础。基于此,本文重点对SLMNiTi SMAs的成形性、相转变行为、微观结构、力学性能和热机械性能的相关研究结果进行了分析与总结。同时,对近来SLM多孔NiTiSMAs的设计及其生物相容性的探索研究进行了阐述。最后,本文展望了SLMNiTiSMAs研究过程中需要重点突破的问题。     

Shape-Memory Intermetallics Development in Taiwan

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形状记忆合金非常规加工综述(英文)

形状记忆合金(SMAs)由于具有多种特殊性能,如伪弹性、形状记忆效应、生物相容性、高的比强度、高耐蚀性、高耐磨性、良好的抗疲劳性能,成为不断发展的先进材料。因此,形状记忆合金被广泛应用于航空航天、医疗和汽车等方面。然而,由于严重的加工硬化和伪弹性,形状记忆合金的传统加工会造成严重的刀具磨损、费时以及低维畸变。这些材料可以使用非传统的方法,如激光加工、水射流加工(WJM)和电化学加工(ECM)进行机械加工,但这些方法受限于该材料的复杂性和力学性能。而电火花加工(EDM)和线切割(WEDM)能够很好的加工具有复杂形状和精密尺寸的形状记忆合金。介绍大量关于使用电火花和线切割加工形状记忆合金的研究,分析不同研究的差异,并展望未来的研究趋势。     

Prediction of martensite and austenite start temperatures of the Fe-based shape memory alloys by artificial neural networks

In this study, martensite start (Ms) and austenite start (As) temperatures of Fe-based shape memory alloys (SMAs) were predicted by using a back-propagation artificial neural network (ANN) that uses gradient descent learning algorithm. An ANN model is built, trained and tested using the test data of 85 Fe-based SMAs available in literature. The input parameters of the ANN model are weight percentages of seven elements (Fe, Mn, Si, Ni, Cr, Cu and Al) and three different treatment conditions (hot rolling, homogenizing temperature and quenching). The ANN model was found to predict the Ms and As temperature well in the range of input parameters considered. A computer program was devised in MATLAB and different ANN models were constructed with this program for prediction of As and Ms temperatures of iron-based SMAs. A comprehensive analysis of the prediction errors of Ms and As temperatures made by the ANN is presented. This study demonstrate that ANN is very efficient for predicting the Ms and As temperatures of iron-based SMAs.     

Influence of aging on microstructure,martensitic transformation and mechanical properties of NiTiRe shape memory alloy

Aging is an effective way to adapt the microstructure, phase transformation and consequently the mechanical properties of NiTi shape memory alloys. In the present study, Ni₅₂Ti_47.7Re_0.3 shape memory alloy was solution treated at 1000 °C for 24 h then aged at various temperatures of 300, 400, 500 and 600 °C for 3 h. The influence of aging treatment on microstructure, martensitic transformation and mechanical properties of Ni₅₂Ti_47.7Re_0.3 was investigated. The microstructure of the solution treated alloy was martensite as a matrix phase and precipitates of Ti₂Ni phase. The aged alloys had a microstructure as same as that of solution treated alloy in addition to the existence of other types of precipitates like Ni₄Ti₃ and Ni₃Ti. The martensitic — austenitic transformation during heating and cooling was going through one stage of transformation. The martensitic phase transformation temperature increased by the increase of aging temperature but still lower than that of solution treated alloy.     

Wide Range Temperature Memory Effect of an In Situ NiTi Alloys Composite

Incomplete martensitic transformation characteristics of NiTi shape memory alloys after cold deformation were studied. By designing the shape of the original surface curve, macroscopic domains with different dislocation density were introduced into the NiTi alloys and materials possessing the characteristics of composites were obtained. Due to the interactions between the dislocation texture and martensite variants, the reverse transformation temperatures were found to expand over a large temperature window, which gives temperature memory effect a higher potential for application.     

(NiTi)_(50-0．5x)Nb_x形状记忆合金的阻尼性能及力学性能

通过加入Nb制备了具有双相组织的(NiTi)50-0.5xNbx(x=5,10,15,20)形状记忆合金,合金兼具高阻尼性能和高屈服强度.随着Nb含量x的增大,合金中(NiTi+β-Nb)共晶组织比例含量增加,合金轧制样品在马氏体状态自协作屈服强度随之升高,在x=15时达到最高(289 MPa);同时,合金轧制样品保持高阻尼性能,本征阻尼性能tan δ＞0.01,并随x增大而升高.根据形状记忆合金阻尼理论以及NiTiNb形状记忆合金的阻尼性能随温度的变化规律,探讨了β-Nb和NiTi相界面阻尼对合金阻尼性能的影响.     

Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips – Berkovich and spherical – are employed to examine the SMAs’ indentation responses as a function of the representative strain, ε_R. In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, ε_R is systematically varied and its effect on the depth recovery ratio, η_d, is examined. Results indicate that RTA has higher η_d than RTM, but the difference decreases with increasing ε_R such that η_d values for both the alloys would be similar in the fully plastic regime. The experimental trends in η_d vs. ε_R for both the alloys could be described well with a η_d ∝ (ε_R)^?1 type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum ε_R, below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found.     

CuAlPd alloys for sensor and actuator applications

Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115–370°C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180°C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modified Bridgeman technique.     

Reproducibility Study of NiTi Parts Made by Metal Injection Molding

Powder metallurgy (P/M) is an attractive manufacturing process for net-shaped NiTi parts considering the limited machinability of NiTi alloys. Nevertheless, the industrial implementation of P/M processing for NiTi alloys is not trivial. To become competitive to manufacturing of NiTi alloys based on established ingot metallurgy, combination of fully pronounced shape memory behavior with sufficient mechanical properties is required. Successful use of P/M technology is strongly influenced by high affinity of NiTi alloys for uptake of oxygen and carbon, which leads to the formation of oxygen-containing Ti₂Ni and TiC phases coupled with increase of Ni content in the matrix. In the case of Ni-rich NiTi alloys, this increase leads to a shift of phase transformation temperatures to lower values. Furthermore, precipitation of Ni₄Ti₃ during cooling from sintering temperature is difficult to avoid. Even if these precipitates might be used to decrease the Ni:Ti ratio of the matrix balancing oxygen and carbon uptake, significant loss of ductility arises, especially in the case of finely dispersed Ni₄Ti₃ precipitates. In the present work, each step of P/M manufacturing is discussed regarding its influence on the specific properties of NiTi alloys. The work is based on the application of prealloyed, gas atomized NiTi powders. Metal injection molding was used for net-shaped manufacturing of tensile samples, which enabled detailed study of sintering behavior combined with investigation of shape memory and mechanical properties depending on particle size, oxygen and carbon content as well as precipitation of Ni₄Ti₃ phase.     

Investigation of the structural stability of Co2NiGa shape memory alloys via ab initio methods

CoNiGa alloys have received considerable interest due to their high-temperature shape memory properties. Although there have been many investigations on the mechanical and magnetic behavior of these materials, little is known about the microscopic basis for the observed macroscopic behavior. In this work, we discuss the stability of Co₂NiGa-based structures. The austenite phase is modeled as fully ordered L2₁ and as partially ordered B2. The stability of the austenite phase with respect to tetragonal distortions is examined. Lattice dynamics calculations suggest that the mechanism for the transformation is different from that of the much more studied and chemically similar Ni₂MnGa shape memory alloy (SMA). The electronic basis for the observed metastability of the cubic Co₂NiGa austenite is found to be qualitatively different from that observed in other ferromagnetic SMAs, especially Ni₂MnGa.     

Influence of Fe addition on phase transformation behavior of NiTi shape memory alloy

Three different NiTi-based alloys, whose nominal compositions were Ni₅₀Ti₅₀, Ni₄₉Ti₄₉Fe₂, Ni₄₅Ti_51.8Fe_3.2 (mole fraction, %), respectively, were used in the current research to understand the influence of Fe addition on phase transformation behavior in NiTi shape memory alloy (SMA). The microstructure and phase transformation behavior of the alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results show that the matrix of the Ni₅₀Ti₅₀ alloy consists of both B19′ (martensite) phase and B2 (austenite) phase. Moreover, the substructures of twins could be observed in the B19′ phase. However, the ternary alloys of NiTiFe exhibit B2 phase in the microstructures. Such microstructures were also characterized by large presence of Ti₂Ni precipitates dispersed homogenously in the matrix of the two kinds of alloys. The addition of Fe to the NiTi SMA results in the decrease in phase transformation temperatures in the ternary alloys. Based on mechanism analysis, it can be concluded that this phenomenon is primarily attributed to atom relaxation of the distorted lattice induced by Ni-antisite defects and Fe substitutions during phase transformation, which enables stabilization of B2 phase during phase transformation.     

Atomistic characterization of pseudoelasticity and shape memory in NiTi nanopillars

Molecular dynamics simulations are performed to study the atomistic mechanisms governing the pseudoelasticity and shape memory in nickel–titanium (NiTi) nanostructures. For a 〈1 1 0〉 – oriented nanopillar subjected to compressive loading–unloading, we observe either a pseudoelastic or shape memory response, depending on the applied strain and temperature that control the reversibility of phase transformation and deformation twinning. We show that irreversible twinning arises owing to the dislocation pinning of twin boundaries, while hierarchically twinned microstructures facilitate the reversible twinning. The nanoscale size effects are manifested as the load serration, stress plateau and large hysteresis loop in stress–strain curves that result from the high stresses required to drive the nucleation-controlled phase transformation and deformation twinning in nanosized volumes. Our results underscore the importance of atomistically resolved modeling for understanding the phase and deformation reversibilities that dictate the pseudoelasticity and shape memory behavior in nanostructured shape memory alloys.     

A new ferromagnetic shape memory alloy system

It is shown that the alloys Co₂Ni_1−xGa_1+x, x = 0.06, 0.09, 0.12, 0.15, are ferromagnetic shape memory alloys. In the as-solidified state their martensite start temperatures vary in the range 20 °C<T<60 °C as the concentration parameter x decreases. The high and low temperature phases are body centered cubic and orthorhombic and/or monoclinic, respectively, The transformation hysteresis, i.e. the difference between the martensite and austenite start temperatures, equals approximately 30 degrees. The saturation magnetization of the alloys resembles that of nickel while their coercive force is of the order of 100mT.     

Phase Transformation Behavior of Hot Isostatically Pressed NiTi-X (X?=?Ag, Nb, W) Alloys for Functional Engineering Applications

Owing to their unique properties, NiTi-based shape memory alloys (SMAs) are highly attractive candidates for a lot of functional engineering applications like biomedical implants (stents), actuators, or coupling elements. Adding a third element is an effective measure to adjust or stabilize the phase transformation behavior to a certain extent. In this context, addition of alloying elements, which are low soluble or almost insoluble in the NiTi matrix is a promising approach and??with the exception of adding Nb??has rarely been reported in the literature so far, especially if the manufacturing of the net-shaped parts of these alloys is aspired. In the case of addition of elemental Nb, broadening of hysteresis between austenitic and martensitic phase transformation temperatures after plastic deformation of the Nb phase is a well-known effect, which is the key of function of coupling elements already established on the market. In the present study, we replaced Nb with additions of elemental Ag and W, both of which are almost insoluble in the NiTi matrix. Compared with Nb, Ag is characterized by higher ductility in combination with lower melting point, enabling liquid phase sintering already at moderate temperatures. Vice versa, addition of W might act in opposite manner considering its inherent brittleness combined with high melting temperature. In the present study, hot isostatic pressing was used for manufacturing such alloys starting from prealloyed NiTi powder and with the additions of Nb, Ag, and W as elemental powders. Microstructures, interdiffusion phenomena, phase transformation behaviors, and impurity contents were investigated aiming to better understand the influence of insoluble phases on bulk properties of NiTi SMAs.