Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase.     

Order-disorder phase transition in CoPd thin films

The thin films of a CoPd alloy in the equiatomic composition region are prepared by condensation at different substrate temperatures. The substrate temperature is varied from the liquid nitrogen temperature to +280°C. At low substrate temperatures, the crystal structure of the condensed films is the single-crystal blocks of the hexagonal close-packed (hcp) phase. As the substrate temperature is further increased, the domains characterized in the initial state by the microdiffraction patterns in the form of a diffuse halo appear in the films, and these domains have a clear-cut boundary with the regions indicated by point reflections in the electron diffraction patterns. At substrate temperatures from +150 to 160°C, the CoPd alloy films in the equiatomic composition region are fully amorphous. The given state is a polymorphic transformation of the martensitic type. It arises in the martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase. Original Russian Text ? E.M. Artem’ev, M.E. Artem’ev, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 838–840.     

Ferromagnetic resonance in Ni-Mn-Ga thin films and thin-film tubes

We report on FMR experiments performed for the first time on thin Ni-Mn-Ga films clamped to the mica substrates and then fully released from them. The aim is to evaluate the role of magnetoelastic coupling in stressed Ni-Mn-Ga Heusler alloy films that undergo martensitic transformation. The experimental results show that the difference in the effective magnetization 4π(M_eff ^tubes-M_eff ^films) is negligible in the austenite phase and it increases to about 1–1.5 kG at temperatures well below the martensitic transformation. The data suggests that magnetoelastic coupling in the martensite phase of Ni-Mn-Ga thin films is typical of normal thin magnetic films with magnetostriction of about 50 ppm.     

Effect of preliminary deformation on the fine structure of a TiNi-based alloy in the premartensitic region

Results are given of an in situ X-ray diffraction study of the crystal-lattice state of a TiNi-based alloy during deformation under the effect of an external stress in the pretransition region preceding the B2-R-B19′ martensitic transformations. The pretransition state preceding martensitic transformations in the alloy under study was found to manifest itself in a specific behavior of the lattice parameter and thermal expansion coefficient in the B2 phase. The magnitude of the thermal expansion coefficient in the B2 phase nonmonotonically depends on the amount of deformation and the applied stress.     

Martensitic transformation in a copper-aluminum-nickel alloy single crystal after short-term neutron irradiation

The influence of neutron irradiation on the temperature kinetics of thermoelastic martensitic transformation in a Cu-Al(13.4%)-Ni(5%) alloy single crystal is investigated by measuring the electrical resistivity directly under irradiation of the sample in a nuclear reactor channel. It is revealed that, after irradiation of the crystal in a martensitic or two-phase state, the temperature of the phase transition upon heating becomes 25–30 K higher than that prior to irradiation. This shift in the transition temperature is observed only upon the first heating, and the kinetics of martensitic transformation is restored in subsequent thermocycles. The shift in the transformation temperatures after irradiation increases with an increase in the fluence. The experimental results are explained by a disturbance of coherence at the interfaces in the irradiated crystals.     

18 MeV质子辐照对TiNi形状记忆合金R相变的影响

 研究了用HZ-B串列加速器的18MeV质子辐照对TiNi形状记忆合金R相变的影响，辐照在奥氏体母相状态下进行。示差扫描量热法（DSC）表明，辐照后R相变开始温度T^s_R和逆马氏体相变结束温度T^f_A随辐照注量的增加而降低。当注量为1.53×10¹⁴/cm²时，T^sR和T^f_A分别下降6K和13K，辐照未引起R相变结束温度T^s_R和逆马氏体相变开始温度T^f_A的变化。表明辐照后母相(奥氏体相)稳定。透射电镜（TEM）分析表明辐照后没有引起合金可观察的微观组织变化。辐照对R相变开始温度T^s_{R和逆马氏体相变结束温度Af的影响可能是由于质子辐照后产生了孤立的缺陷团，形成了局部应力场，引起晶格有序度的下降所造成的。}     

Mechanism of the shape memory effect in martensitic alloys: an assessment

The (one-way) shape memory effect is a phenomenon that when a martensitic alloy is deformed in a martensitic state it recovers its original shape upon heating to the parent phase. This is a universal effect for certain martensitic alloys. We will assess the mechanism of the effect critically and select the essential factors which govern the effect. We try to understand it from a unified view, invoking the group–subgroup symmetry relation between the parent and martensite phase, along with analysis of reversible twinning modes in martensite. By such an assessment, we will show why typical shape memory alloys, such as Ti–Ni, Cu–Al–Ni etc., exhibit good shape memory characteristics, while others, such as ferrous alloys, do not. Thus, we will show that most of the shape memory characteristics of various martensitic alloys can be understood consistently from such an approach.     

Ni4Ti3沉淀相对NiTi形状记忆合金相变行为的影响

采用第二近邻修正型嵌入原子势的分子动力学方法,建立了共格沉淀相与半共格沉淀相块状/柱状模型,模拟了温度诱发相变和应力诱发相变,分析了Ni₄Ti₃沉淀相对Ni Ti形状记忆合金相变行为的影响．结果表明,Ni₄Ti₃沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时,共格沉淀相促进部分马氏体变体的形核生长,能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时,Ni₄Ti₃沉淀相使马氏体早于无沉淀相区域形核,导致了相变应力降低、抑制了马氏体解孪,减小了应力-应变曲线的滞回环.     

Pressure effect of magnetic and electronic properties of Mn2PtGa Heusler alloy

First-principles calculations are performed to investigate pressure effects on structure, magnetism, martensitic phase transition and Curie temperatures of Mn₂PtGa Heusler alloy in framework of the density functional theory. It is shown that Mn₂PtGa prefer to crystallize in the inverse Heusler type structure. Besides, we predict an extraordinary occurrence of pressure induced metallic ferrimagnetism to half-metallic ferromagnetism transition in cubic phase of Mn₂PtGa alloy under hydrostatic pressure up to 43 GPa and the half-metallic ferromagnetism is found to be robust even the lattice further compression to 90 GPa. However, with the pressure up to 100 GPa, the spin-down gap starts to close and the half metallicity begin to disappear, while with the pressure increasing from 100 GPa to 300 GPa, the alloy returns to metallic characteristic. In addition, the energy difference between the austenitic and martensitic phases is found to increase with increasing pressure followed by a decrease when pressure reaches to 43 GPa, which implies a variation trend of martensitic phase transition temperature. Furthermore, Curie temperatures in both austenitic and martensitic phases are estimated under pressure by using the standard mean-field approximation which agrees well with the theoretical results in literature. The robustness of the half metallicity, magnetic transition and the high Curie temperature under pressure make Mn₂PtGa alloy a promising candidate for applications in spintronic devices.     

Ni53.2Mn22.6Ga24.2单晶的两步热弹性马氏体相变及其应力应变特性

对具有两步完全热弹性的Ni_53.2Mn_22.6Ga_24.2单晶的物性采用多种测量手段进行了表征,特别研究了不同温度下的应力-应变特性.研究表明,热诱发的中间马氏体相变应变远大于马氏体相变应变.在较低的形变温度下,沿单晶母相［001］方向的压应力诱发的是两步马氏体相变,材料表现出赝弹性;在较高的形变温度下,只能观察到一步马氏体相变,材料展现出完全超弹性特性.此外,利用热力学理论分别计算了诱发马氏体相变和中间马氏体相变的临界应力与形变温度的关系,与实验测量得到的结果相符. 关键词： 马氏体相变 形状记忆效应 应变 超弹性     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

NiTi形状记忆合金马氏体相变的第一性原理研究

为了理解NiTi形状记忆合金马氏体相变的机理，基于密度泛函的第一性原理研究了温度和应力对电子结构的稳定性的影响.通过对态密度的分析，发现随着温度的降低和变形的增加，B2相的态密度升高，能级兼并性提高，从而导致结构的稳定性降低，促进相变的发生. 关键词： NiTi 电子结构 第一性原理计算 相变     

Corrosion of stainless steels in chloride solution: An XPS investigation of passive films

Five commercial steels ranging from the martensitic stainless steel containing 12% chromium to the superferrite containing 29% chromium, 4% molybdenum, and 2% nickel have been studied by XPS. In addition, a pure iron-chromium alloy containing 7% chromium has been investigated. Armco iron and pure chromium (99.99%) were included as references. The formation of the passive films (or corrosion) occurred in deoxygenated 0.1 M NaCl solution (pH=5.6), from which the samples were transferred directly to the XPS chamber under controlled atmosphere (Ar). Concentration profiles (at.-%) of the alloy constituents in their oxidized and metallic states have been determined separately from the measured XPS depth profiles. Forc= 12% chromium the passive films have the following structure: there is a depletion of Cr in the inner region, followed by an enrichment (concentration maximum) in the central region of the films. The height of this maximum increases, and its position shifts towards the surface with increasing chromium content in the alloy. The outermost monolayers are rich in water and hydroxyl groups. Various significant properties of the films change drastically at the critical chromium concentration of about 12%. This behaviour is rather independent of the other components (Mo, Ni, Cu) present in the alloys and is discussed in terms of a phase transition in the films which is controlled by the chromium concentration.     

Characterization of defects in a martensitic CuAlNi shape-memory alloy

A water-quenched martensitic CuAlNi shape-memory alloy was investigated by a combination of coincidence Doppler broadening and positron-lifetime spectroscopy, supported by positron-lifetime calculations. We find a high defect concentration in the as-quenched samples. The positron-lifetime calculations suggest that the defects are not only single vacancies but also vacancies associated with dislocations and stacking faults. Annealing in the martensitic phase has no significant influence on the vacancy concentration but results in a different chemical environment around the vacancies. After aging in the austenitic phase the vacancy concentration decreases significantly. PACS 61.72.Ji; 78.70.Bj     

Phase transitions in films prepared from Alnico-type alloys

Phase transitions taking place in films of an alloy based on Fe-Ni-Co-Al-Cu (YuDNK-24) as a result of a thermomagnetic treatment and which lead to a high coercivity state have been investigated. In this state, three phases are detected in the films: a phase with a bcc lattice, and a phase with a fcc lattice, and a phase based on the compound NiAl (or Ni₃Al).     

Structure of heterogeneous states and magnetic properties of CoPd nanocrystalline films

Thin films of a CoPd equiatomic alloy have been obtained by condensation at a high substrate temperature. In their initial states, they are characterized by a microdiffraction pattern in the form of a diffuse halo. This state is a polymorphic martensite-type transformation, which occurs for the martensite transition from the low-temperature hexagonal close-packed phase to the high-temperature face-centered cubic one. Crystalline metastable structures formed in films of this alloy have been identified, and their magnetic characteristics have been determined.     

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.     

内应力对Mn2NiGa铁磁形状记忆合金的结构、相变和磁性能的影响

通过施加压应力的方法,在铁磁形状记忆合金Mn₂NiGa中引入残留内应力,研究了内应力对 Mn₂NiGa材料的结构、相变和磁性能的影响.研究发现,加压过程使材料发生了塑性形变,在材料内部引入了大量的位错缺陷.卸载后保留的位错缺陷在材料中造成了残留的内应力,导致了马氏体相变温度大幅度提高, 使原本室温下的母相转变成了马氏体相.测量到导致样品转变成马氏体的阈值压应力为1.0 GPa.加压形成的马氏体中的残留内应力将矫顽力从低于50 Oe提高到350 Oe.残留内应力在730 K的热处理中由于位错缺陷的消失而得以消除,样品实现了马氏体逆相变.如此高的逆相变温度使得 Mn₂NiGa马氏体的居里温度测量成为可能,获得了530K的数值.     

18 MeV质子辐照对TiNi形状记忆合金R相变的影响

研究了用HZ B串列加速器的18MeV质子辐照对TiNi形状记忆合金R相变的影响,辐照在奥氏体母相状态下进行。示差扫描量热法(DSC)表明,辐照后R相变开始温度TsR和逆马氏体相变结束温度TfA随辐照注量的增加而降低。当注量为1.53×1014/cm2时,TsR和TfA分别下降6K和13K,辐照未引起R相变结束温度TfR和逆马氏体相变开始温度TsA的变化。表明辐照后母相(奥氏体相)稳定。透射电镜(TEM)分析表明辐照后没有引起合金可观察的微观组织变化。辐照对R相变开始温度TsR和逆马氏体相变结束温度Af的影响可能是由于质子辐照后产生了孤立的缺陷团,形成了局部应力场,引起晶格有序度的下降所造成的。     

低温时效处理对铁磁形状记忆合金Mn2NiGa的结构、相变和磁性能的影响

对在较低温度范围的时效处理铁磁形状记忆合金Mn₂NiGa的结构、相变和磁性进行了研究.研究发现,母相基体析出了细小的析出相,引起了晶格扭曲和畸变,导致了系统内产生了很大的内应力.在其浓度超过晶格的容忍度之后,提升了体系的马氏体相变温度,使母相在时效温度下转变成马氏体相,并在其中测量到高达900 Oe的矫顽力.由于这种马氏体相的逆相变温度大幅提高,外推获得其居里温度在530 K附近.细小析出相的粗化使内应力消失,样品又回到母相状态.观察到细小析出相粗化的两个阈值温度,分别为423 K和 关键词： 铁磁形状记忆合金 2NiGa')" href="#">Mn₂NiGa 时效处理 内应力