Co-x%Mn合金中的fcc→hcp相变和相关的形状记忆效应

Co-Mn合金是磁诱发形状记忆效应候选材料之一，本文制备了不同Mn含量的Co-Mn合金，研究了合金成分对Co-Mn二元合金中应力诱发γ(fcc)→ε(hcp)马氏体相变及形状记忆效应的影响作用，研究结果发现，对于能够发生γ→ε马氏体相变的Co-Mn合金，具有部分形状记忆效应；随着Mn含量的增加，马氏体量减小，基体的强度降低，导致合金的形状回复率和可回复应变均有不同程度的下降。     

NiTi合金形状记忆效应的微观机制研究进展

NiTi合金具有优异的形状记忆功能和良好的生物体兼容性,近年来对它的应用研究受到工程界和医学界的重视,同时对NiTi合金形状记忆效应的微观机制的研究也在逐步深入.介绍了NiTi合金的主要特性及影响其形状记忆功能的主要因素,总结了NiTi合金的形状记忆效应和超弹性的微观机制研究现状,并指出了需对该合金进一步研究的一些问题.     

约束态相变中TiNi形状记忆合金的马氏体自拉伸过程

研究了约束态加热过程中产生的回复力对TiNi形状记忆合金剩余马氏体的影响．结果表明，产生的回复力使剩余马氏体发生了塑性变形，使逆相变温度升高，剩余马氏体分数和其逆相变温度之间存在特定的函数关系．但是，外部约束条件的变化对马氏体的自拉伸过程所造成的剩余马氏体分数与其逆相变温度之间的关系影响很小．     

TiNi形状记忆合金约束态相变及驱动特性研究进展

TiNi形状记忆合金作为一种驱动材料具有广阔的应用前景,同时TiNi合金丝增强复合材料的研究和应用也得到人们的重视。TiNi合金的驱动能力与其约束条件下的相变行为有密切的关系。综述了不同约束态条件下TiNi形状记忆合金丝的马氏体相变特征及其驱动特性的研究进展。     

Sb对Mn50Ni40In10合金相变及磁性的影响

研究了Sb替代部分In对铁磁性形状记忆合金材料Mn50Ni40In10相变和磁性的影响。研究表明,在掺杂后形成的Mn50Ni40In10-x Sbx系列合金中,随着Sb含量的增加,马氏体的逆相变温度逐渐升高,居里温度逐渐降低。其相变温度的升高主要归结于电子浓度和晶格体积的变化以及电子轨道的杂化作用。Sb掺杂会导致样品马氏体相和奥氏体相之间的饱和磁化强度差ΔM的变化,x<0.6的微量掺杂会使ΔM增大,x>0.6时,ΔM随着掺杂量的增加而逐渐降低甚至减为零。x=0.6的样品(Mn50Ni40In9.4Sb0.6)在20kOe的外场作用下ΔM达到74emu/g,同时在该材料中观察到了磁场驱动的马氏体到奥氏体的转变,显示出该材料作为磁驱动形状记忆材料的潜在应用前景。     

铁基形状记忆合金马氏体相变研究进展

铁基形状记忆合金的形状记忆效应和超弹性取决于合金的马氏体相变特征,掌握铁基合金的马氏体相变规律是开发和优化铁基形状记忆合金的前提。根据马氏体相变类型将目前发现的铁基形状记忆合金分成3类：Fe-Mn-Si系,Fe-Ni-Co系和Fe-Pt／Fe-Pd系,分别阐述了3类铁基形状记忆合金马氏体相变的研究进展,总结了铁基合金形状记忆效应的不同机理和影响马氏体相变特征的各种因素,探讨了开发新型铁基形状记忆合金的需要关注的方向。     

Magnetic structure of the modulated martensite phase in Ni-Mn-Ga ferromagnetic shape memory alloys

The magnetic structure of the seven layered (7 M) modulated martensite phases in Mn-rich Ni-Mn-Ga alloys was studied using Mössbauer spectroscopy. The Mössbauer results clearly demonstrate that in contrast to the non-modulated tetragonal structure two new magnetic sublattices exists for the 7 M orthorhombic martensite phase. Based on the unit cell symmetry and atomic coordination, the additional magnetic sublattices have been assigned to the Ni site. The variation in the magnetic properties of the martensite phases has been related to the underlying magnetic structure.     

Selective laser melting of NiTi alloy with superior tensile property and shape memory effect

It is a challenge to develop complex-shaped NiTi shape memory alloy parts by traditional processing methods, due to the poor machinability of NiTi alloy. It is reported that selective laser melting (SLM) of additive manufacturing could overcome this problem. However, the reported SLM-produced NiTi exhibits poor tensile ductility due to the inner defects and adverse unidirectional columnar grains from SLM process. In this work, the defect-less SLM-NiTi with nondirective columnar grains was fabricated by optimizing the intraformational laser scanning length and interformational laser scanning direction. The obtained lath-shaped SLM-NiTi sample exhibits tensile strain of 15.6%, more than twice of the reported maximum result (˜7%). Besides, the SLM-NiTi part with complex geometry displays a shape memory recovery of 99% under compressive deformation of 50%.     

陶瓷形状记忆效应的研究进展

本文综合评述了（１）二氧化锆陶瓷中马氏体相变和形状记忆机理，（２）钙钛矿石类氧化物陶瓷的位移相变及形状记忆特征，并与合金的形状记忆效应进行对比。     

The effect of grain orientation on the tensile‐compressive asymmetry of polycrystalline NiTi shape memory alloy

The purpose of the present study is to thoroughly understand the influence of crystallographic texture on the stress‐strain asymmetric behavior of polycrystalline NiTi shape memory alloy under tension and compression. To do this, a 3D thermo‐mechanical model has been implemented in a finite element program and textured and untextured polycrystalline NiTi have been considered. In our polycrystalline finite element model, each element represents one grain and a set of crystal orientations which approximate the initial crystallographic texture of the NiTi are assigned to the elements. From the calculated results, it is found that the crystallographic texture is the important reason for the tension‐compression asymmetry. For the textured polycrystal, the tension‐compression asymmetry can be observed clearly, but for the polycrystal containing randomly oriented grains, the stress‐strain curves show low levers of asymmetry between tensile and compressive loading, and the evolutions of martensite volume fractions are similar under two stress states.     

On the robustness and efficiency of integration algorithms for a 3D finite strain phenomenological SMA constitutive model

Most devices based on shape memory alloys experience large rotations and moderate or finite strains. This motivates the development of finite‐strain constitutive models together with the appropriate computational counterparts. To this end, in the present paper a three‐dimensional finite‐strain phenomenological constitutive model is investigated and a robust and efficient integration algorithm is proposed. Properly defining the variables, extensively used regularization schemes are avoided and a nucleation–completion criterion is defined. Moreover, introducing a logarithmic mapping, a new form of time‐discrete equations is proposed. The solution algorithm as well as a suitable initial guess for the resultant nonlinear equations are also deeply discussed. Extensive numerical tests are performed to show robustness as well as efficiency of the proposed integration algorithm. Implementation of the integration algorithm within a user‐defined subroutine UMAT in the commercial nonlinear finite element software ABAQUS/Standard makes also possible the solution of a variety of boundary value problems. The obtained results show the efficiency and robustness of the proposed approach and confirm the improved efficiency (in terms of solution CPU time) when a nucleation–completion criterion is used instead of regularization schemes, as well as when a logarithmic mapping is used for the time‐discrete evolution equation instead of an exponential mapping. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of applying external stress during aging on martensitic transformation and the superelastic behavior of a Ni-rich NiTi alloy

Superelastic properties and martensitic transition temperatures of Ni_50.9Ti shape memory wires were studied after aging under different applied stresses at 450 °C for 45 min. The results indicate that applying external stresses during aging of Ni_50.9Ti shape memory wires changes the strain value and stress level of plateaus related to stress induced martensitic transformation. Furthermore, martensitic transformation sequences change after applying external stresses but transformation temperatures measured with DSC remain constant in samples aged under stresses up to 120 MPa. Applying stress of 60 MPa during aging enhances the superelastic properties by increasing the strain of plateaus during loading and unloading cycles. Applying stresses more than 60 MPa, decreases the strain of the plateau during unloading.     

On nucleation and propagation of PLC bands in an Al–3Mg alloy

The processes of nucleation and propagation of Portevin–LeChatelier deformation bands during tensile deformation of the alloy AA5754 have been investigated by a special optical technique for local strain measurements, combining both a line-scan and a CMOS camera to record the changes of surface strain, applying digital image correlation. Examples for bands of types A and B are observed and discussed with respect to the micro-mechanisms of their nucleation and propagation, emphasizing local stress concentrations in addition to the well-known effects of dynamic strain aging and dislocation interactions for explaining the PLC behavior. Finally the nucleation and propagation of PLC bands are simulated by computations based on a simple constitutive model.     

Degeneration and rejuvenation of shape memory effect associated with the precipitation of coherent nano-particles in a Co-Ni-Si shape memory alloy

In a solution treated Co-20Ni-6Si shape memory alloy,coherent nano-particles were precipitated after annealing at 873 K for 1 min,but the shape memory effect almost vanished.It is attributed to that the coherent nano-particles not only suppressed the stress-induced face-centered cubic to close-packed hexagonal martensite transformation but also damaged the crystallographic reversibility of reverse martensite transformation.After further annealing at 1073 K for 1 min,the shape memory effect was reju-venated owing to the dissolution of nano-particles.Besides,the recovery strain significantly increased to 5.1％ from the solution treatment of 3.1％ after annealing at 1073 K for 1 min.     

Impact of B alloying on ductility and phase transition in the Ni-Mn-based magnetic shape memory alloys: Insights from first-principles calculation

Brittleness is a bottleneck hindering the applications of fruitful functional properties of Ni-Mn-based multiferroic alloys.Recently,experimental studies on B alloying shed new light on this issue.However,the knowledge related to B alloying is limited until now.More importantly,the mechanism of the improved ductility,which is intrinsically related to the chemical bond that is difficult to reveal by routine experiments,is still unclear.In this context,by first-principles calculations,the impact and the correlated mechanism of B alloying were systemically studied by investigating four alloying systems,i.e.,(Ni2-xBx)MnGa,Ni2(Mn1-xBx)Ga,Ni2Mn(Ga1-xBx) and (Ni2MnGa)1-xBx.Results show that B prefers the direct occupation manner when it replaces Ni,Mn and Ga.For interstitial doping,B tends to locate at octahedral rather than tetrahedral interstice.Calculations show that the replacement of B for Ga can effectively improve (reduce) the inherent ductility (inherent strength) due to the weaker covalent strength of Ni(Mn)-B compared with Ni(Mn)-Ga.In contrast,B staying at octahedral interstice will lead to the formation of new chemical bonds between Ni(Mn) and B,bringing about a significantly improved strength and a greatly reduced ductility.Upon the substitutions for Ni and Mn,they affect both the inherent ductility and strength insignificantly.For phase transition,the replacement of B for Ga tends to destabilize the austenite,which can be understood in the picture of the band Jahn-Teller effect.Besides,the substitution for Ga would not lead to an obvious reduction of magnetization.     

On the electropolishing of NiTi braided stents – challenges and solutions

Nickel Titanium (NiTi) alloys possess special mechanical properties and good biocompatibility hence used as base material for the production of vascular stents. Normally, vascular stents are machined from NiTi tubes, using laser cutting processes. Braiding is a promising alternative for the machining of certain NiTi stents. However, a surface finish treatment, such as electropolishing of the braided stents, is still required in order to achieve a medical‐grade surface finish. The thermally‐grown oxide resulting from the shape‐setting heat treatment, following the braiding must be removed. Moreover, electropolishing is required to achieve optimum corrosion resistance. Therefore, the aim of this study is to find suitable parameters for the effective electropolishing of NiTi textile stents. Electropolishing of a device with such a complex geometry is challenging, hence a custom‐designed electrolytic cell was constructed and used in this study. We examined the stent surfaces before and after electropolishing, using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Potentiodynamic tests were performed in NaCl 0.9% solution for as‐received and electropolished samples. The results from the present study indicate an improvement in surface quality of the braided stents after electropolishing. Potentiodynamic tests revealed that electropolishing improves the corrosion resistance of the NiTi stents.     

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part II: Size and lay-up effects

The in-plane size, thickness and lay-up effects on the measured fracture toughness associated with fibre tensile failure were investigated for a T300/920 laminated carbon/epoxy material system. Compact tension specimens were tested with scaled in-plane size, increased thickness, and having various proportions of plies orientated at 0° and 90° to the loading direction. No in-plane size effects were discovered; however, testing revealed a thickness dependence. It was found that the ply toughness is significantly dependent on the thickness of the 0° layers. Propagation values of toughness were measured to be 132 kJ/m² for specimens made up of [90/0₂] sub-laminates and between 57 and 69 kJ/m² for all other configurations. Investigation of the fracture surfaces using SEM revealed that the increase in measured toughness for specimens with thicker 0° plies was due to an increase in the amount of pulled-out 0° fibres.     

Modelling Residual Strains During Cycling of Ti–Ni and Ti–Ni–Cu Shape Memory Alloys in a Pseudoelastic Range of Behaviour Conditions

Abstract: Pseudoelastic behaviour of three types of Ti–Ni shape memory alloys in a pseudoelastic state has been studied under conditions of maximum strain‐ and maximum stress‐controlled cycling. Experimental results proved that residual deformation after unloading increases with the number of cycles; however, critical stress for the induction of martensite and the energy dissipated in one cycle decline during cycling. A higher critical stress for slip, and more intense cyclic dislocation hardening promoted by greater maximum deformation and greater maximum applied stresses, generally reduce the rate at which residual elongation grows with the number of cycles, and tend to stabilise the cyclic stress‐elongation diagrams. The small magnitude of critical stress for slip in low‐nickel alloys, and also cyclic strain hardening, induce greater internal stresses and a more marked decrease in critical stress for the induction of martensite as cycling progresses. Detailed analysis of plastic deformation propagation in cyclically loaded specimen helped develop a model of dependence of residual elongation on the number of cycles. This model enables identification of three main factors that govern the magnitude of residual elongation: one residual plastic elongation caused by dislocation hardening after the alloy is heat treated, and two cyclic strain hardening parameters describing how residual elongation grows with number of cycles, and how this residual elongation is reduced, as cycles increase, by the rising critical stress level for slip. The model has proved to yield very close agreement with experimental findings.