The influence of imposed strain on the development of microstructure and transformation characteristics of Ni-Ti shape memory alloys

Presented paper describes the influence of imposed strain on the development of microstructure (substructure) in Ni-Ti materials during the application of ECAE (equal channel angular extrusion). The interrelationship between imposed strain and resulting transformation characteristics is in the main point of interest while the chemical composition, namely the occurrence of secondary phases such as TiC and Ti₄Ni₂O resulting from the technology used for the sample alloy preparation is also investigated. The alloy Ni50.6-Ti (at.%) was prepared by melting in the HF vacuum induction furnace using a graphite crucible. The deformation was carried out via ECAE combined with prior rotary forging/swaging. ECAE was carried out at 280 °C. The imposed strain had the value ∼2. The alloy was processed by deformation route Bc. It was established the high dislocation density generated by the imposed strain stabilized the B2 and R phases and shifted the transformation R ↔ B19′ towards the lower temperature region. Direct effect of the deformation stress was demonstrated in samples after the second pass when the occurrence of the deformation induced transformation B2 → B19′ was confirmed. Deformation induced transformation also represents contribution to the overall deformation of the samples during the process.     

Passivation and oxygen ion implantation double surface treatment on porous NiTi shape memory alloys and its Ni suppression performance

The surfaces of porous NiTi shape memory alloys (SMAs) were modified by double treatment of passivation (PA) in HNO₃ solution and oxygen plasma immersion ion implantation (O-PIII) methods. SEM and XPS were used to characterize the modified surface. It has been found that a protective film consists of three layers formed on the surface after modification. From the outmost surface towards inside, they are a 10 nm thick layer consisting of rutile and anatase TiO₂ with pure rutile TiO₂ on the surface and their ratio varying in gradient, a 45 nm thick layer containing anatase TiO₂ and NiTi with constant ratios and a 40 nm thick layer of TiO₂, TiO and NiTi with their ratio varying in gradient. The immersion tests in simulated human body fluid demonstrated that the modified porous NiTi samples exhibit a good Ni suppression performance, approaching to that of the untreated dense NiTi samples. The Ni ion content released from the porous NiTi SMAs double treated by PA and O-PIII is one magnitude lower than that from the untreated porous samples. Moreover, the released Ni ion content after 8 weeks can be reduced into the safe range acceptable to the human body.     

Selective surface oxidation and nitridation of NiTi shape memory alloys by reduction annealing

The current investigation aims in understanding the effect of short-term (300 s) annealing of NiTi shape memory alloys (SMAs) in a reducing atmosphere of N₂-10% H₂. The influence of temperature on the resulting surface morphology and chemistry is elucidated. On annealing at 600 °C, the surface is covered with a thin layer of titanium oxide, which is 7.5 nm thick, while at 800 °C, the surface is covered with a golden-yellow layer of TiN of thickness more than 100 nm. The surface analysis carried out by XPS on the specimen annealed at 800 °C confirms the formation of TiN and more notably, the surface is devoid of Ni.     

Compressive training of the shape memory alloy washer

This paper evaluates the performance and engineering aspects of 44Ti-47Ni-9Nb alloy in a pretensioning washer application. Previously the behavior of this alloy has mainly been studied for tensile predeformation with wires and strips up to 1 mm thickness. The present work investigated the effect of compression deformation and different homogenization temperatures on ring-shaped washers with larger dimensions. The best results with the compression-trained washer were achieved after homogenization at 1073 K where the compression training deformation was -10.4%, the lowest applied. The deformation training was carried out at 213 K near the martensitic reaction start temperature of the material. The expansion method, applied to the commercial reference ring, seemed to be a fairly efficient training method for the studied application, even though the parallel course of the radial surfaces was poor.     

Chemical and mechanical treatments to improve the surface properties of shape memory NiTi wires

In this paper the results of an experimental study concerning the effect of different surface treatments on NiTi shape memory alloy wires are presented. These treatments were conducted in order to improve the adhesion properties between the NiTi wires and an epoxy resin, acting as the matrix of a composite material.Mechanical and chemical surface treatments (immersion in acid and alkaline solutions), and different combinations of the above surface preparation procedures were studied.For the characterisation of the resulting alloy surface conditions electrochemical impedance spectroscopy, polarisation curves and potential versus time measurements were carried out.The alloy wire/epoxy matrix adhesion was characterised through pull out tests. The results proved that all adopted treatments can remarkably influence the electrochemical properties of the wires. The acid treatments favour the formation of a surface passivation layer, while the alkaline treatments are effective in producing a rougher surface morphology. Moreover, these basic treatments significantly reduce corrosion resistance of the alloys, another material property that has been incidentally investigated in the present context. The main effect of the mechanical surface treatment, consisting in abrading the alloy wires using an emery paper, was to increase the homogeneity of surface roughness.From the experimental results clear indications on the most promising surface treatments can be inferred.     

合金化对NiTi合金相变点影响机制的电子结构分析①

根据固体与分子的经验电子理论,就合金化元素对NiTi形状记忆合金马氏体晶胞的价电子结构的影响进行了计算分析,为揭示其电子结构与合金的成分及组织之间的内在联系做了初步的探索。     

铸态FeMnSiCrNi合金的形状记忆效应

分别研究了预变形量对超低碳与含0.12%碳的铸态FeMnSiCrNi形状记忆合金的形状记忆效应的影响,并将以上结果与冷拉FeMnSiCrNi形状记忆合金的结果进行了比较。结果表明:预变形量小于5%时,两种铸态合金的形状记忆效应基本不变;而预变形量大于5%时,随预变形量的增加形状记忆效应降低。当预变形量相同时,含0.12%碳的合金的形状记忆效应低于超低碳合金。预变形量为5%时,超低碳与含0.12%碳的合金的形状回复率分别达到73%、62%,具有与冷拉FeMnSiCrNi丝相当的形状记忆效应。     

First-principles study of shear deformation in TiAl alloys

The stress-induced phase-transition γ-TiAl ↔ ₂-Ti₃Al in TiAl alloys was systemically studied by density functional theory (DFT) calculations. We found that the key role in such phase-transition is the alloy composition. Firstly through studying the relationship between the formation energy, structure (fct and hcp) and alloy composition (Ti:Al = 1–3), we acquired that the stability of fct or hcp phases depends on the Ti:Al ratios. When the ratio meets about 1.5, the formation energies of fct and hcp phases are very close, which means both structures have the similar stability. Further shear deformation calculations on such alloys with different structures and compositions show that phases with Ti:Al value close to 1.5 have smaller dislocation nucleation energies and lower maximum shear strengths than those of the stoichiometric TiAl and Ti₃Al phases. Such results not only reveal the composition's effect on the reversible phase transition, but also exhibit the intrinsic controlling role on the alloying properties.     

热爆反应合成多孔NiTi形状记忆合金的性能

利用热爆方法来制备了多孔NiTi形状记忆合金。 研究了在不同热爆温度下制备出的样品与其机械性能之间的关系。结果表明： 在1223K下热爆反应制备的NiTi合金, 具有大的孔隙度, 高开孔率和基本各向同性,同时表现出较好的超弹性。 对断口分析发现, 断裂为脆性断裂和韧性断裂的复合。 这表明改善孔洞分布和形态, 可以极大地提高多孔NiTi形状记忆合金的机械性能和超弹性。     

Twinning stress in shape memory alloys: Theory and experiments

Utilizing first-principles atomistic simulations we present a twin nucleation model based on the Peierls–Nabarro formulation. We investigated twinning in several important shape memory alloys, starting with Ni₂FeGa (14M modulated monoclinic and L1₀ crystals) to illustrate the methodology, and predicted the twin stress in Ni₂MnGa, NiTi, Co₂NiGa, and Co₂NiAl martensites, all of which were in excellent agreement with the experimental results. Minimization of the total energy led to determination of the twinning stress accounting for the twinning energy landscape in the presence of interacting multiple twin dislocations and disregistry profiles at the dislocation core. The validity of the model was confirmed by determining the twinning stress from experiments on Ni₂FeGa (14M and L1₀), NiTi, and Ni₂MnGa and utilizing results from the literature for Co₂NiGa and Co₂NiAl martensites. This paper demonstrates that the predicted twinning stress can vary from 3.5 MPa in 10M Ni₂MnGa to 129 MPa for the B19′ NiTi case, consistent with the experimental results.     

高温形状记忆合金

马氏体逆相变开始温度As点稳定在120℃以上的形状记忆合金称之为高温形状记忆合金,有多种合金系可供选择。本文综述了现有的各种高温形状记忆合金系的研究现状。     

可加工变形的Au-Cu-Zn-Al形状记忆合金

报道了一种新的金基形状记忆合金(Au-Cu-Zn-Al)的初步研究,结果表明,成分配比合适的合金可以用常规的方法加工变形,有典型的热弹性马氏体相变及良好的单程和双程形状记忆效应。合金的金含量可达50wt％。     

Ni-Mn-Ga-V高温形状记忆合金研究

本文采用光学显微镜、X射线衍射仪、扫描电子显微镜、透射电子显微镜以及压缩力学试验机系统的研究了钒元素对(Ni56Mn25Ga19)100-xVx (x = 0, 1, 3) (at.%)高温形状记忆合金性能的影响。结果表明,由于钒取代方法的不同,我们所研究合金的组织结构和某些性能与已报到的Ni56Mn25Ga19-xVx合金差异较大。未掺杂钒元素时,试验合金由单相非调制四方结构马氏体构成,当钒含量为1at.%和3at.%时,合金由四方结构马氏体和竹叶状γ相构成。随着钒含量的增加,γ相的数量增多,但其尺寸和形状保持不变。钒掺杂改善合金的力学性能和形状记忆效应。 (Ni56Mn25Ga19)99V1合金变形10%后加热,可以得到6.7%的可逆应变。     

Microstructure,mechanical properties and shape memory effect of Ni–Mn–Ga–B high-temperature shape memory alloy

The effects of boron addition on the microstructure, transformation temperature, mechanical properties and shape memory effect of (Ni₅₄Mn₂₅Ga₂₁)_100−xB_x alloys were investigated. The results showed that the martensitic transformation start temperatures M_s decreased monotonically from 465 K for x = 0–278 K for x = 3. Boron addition refined the grain and significantly enhanced the mechanical properties. The compressive fracture strain of 22.3% and reversible strain of 6.8% were obtained in (Ni₅₄Mn₂₅Ga₂₁)_99.5B_0.5 alloy.     

Damping capacity of TiNi-based shape memory alloys

The damping capacity mainly comes from the motion of martensite boundaries, which is larger than that from different phase interfaces in TiNi-based shape memory alloys. The fine and disperse precipitation in the TiNi matrix will increase the damping capacity of the TiNiNbMo alloy in the martensitic state but little affect the damping capacity during the martensite transformation. The dispersed Nb-rich particles and the internal stress fields around them influence the change tendency of damping capacity with the increasing of strain amplitude.     

退火处理对NiTi记忆合金丝力学性能的影响

以德国Memory-Metalle有限公司生产的两种型号NiTi合金丝为研究对象,其一是具有中温激励特性的M型,其二是具有超弹性的N型,通过拉伸实验,研究了退火处理对其力学性能的影响.结果表明:不同的退火温度和退火时间对试样的形状记忆性能以及超弹性具有不同程度的影响.     

Influence of pre-strain on recovery stress of annealed NiTi thin wire during isothermal holding

The present research aims to understand the recovery stress of NiTi wires generated during heating and isothermal holding under constrained condition. Both pre-strain and isothermal holding temperature have significant effects on the evolution of recovery stress during heating and isothermal holding. In general, increasing isothermal holding temperature and/or pre-strain causes an increase of recovery stress reduction or relaxation. This effect can be attributed to different dislocation densities and mobility of dislocations generated in those conditions.