Structure of martensite in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures

After annealing at different temperatures, there are different types of precipitates in Ti-rich Ti–Ni–Cu thin films: plate-like Guinier–Preston (GP) zones, Ti₂Cu precipitates and spherical Ti₂Ni precipitates. The (0 1 1) compound twins and (1 1 1) type I twins are dominant in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures, which suggests that the precipitates do not change the twinning modes of the B19 martensite. However, the amount of the (0 1 1) compound twin increases with increasing annealing temperature due to its small twinning shear. In thin films with GP zones or Ti₂Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti₂Cu + Ti₂Ni precipitates, hardly any martensite with a single-pair morphology is observed. For the different types of precipitates at the different annealing temperatures, the obstacle of the precipitates to the growth of the B19 martensite plate also varies. The GP zones slightly hinder the growth in the width of martensite, resulting in wavy twin boundaries at the martensite variant tip. The Ti₂Cu precipitates can change both the width and the direction of the martensite plates. Ti₂Ni precipitates also significantly disturb or impede the growth of the martensite variants. These effects lead to a decrease in the maximum shape recoverable strain with increasing annealing temperature.     

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

The martensite structure in a Ti₃₆Ni₄₄Hf₁₅Cu₅ ribbon annealed at different temperatures is investigated. When the annealing temperature is <873 K, spherical (Ti,Hf)₂Ni particles 20–40 nm in diameter precipitate in the grain interior. Transmission electron microscopy analysis shows that (0 0 1) compound twins are dominant in the ribbon containing homogeneously distributed (Ti,Hf)₂Ni precipitates. When the annealing temperature is 773 K, the boundaries between the martensite domains with the (0 0 1) twins are blurry and vague. When the annealing temperature is 873 K, four types of boundaries among the martensite domains are found: {1 1 1}, (0 0 1)//{1 1 1}, {1 1 3} and (1 1 0)//{1 1 3} types. When the annealing temperature is 973 K, the (0 1 1) twins become dominant, and the martensite variants show mainly spear-like and mosaic-like morphologies. However, martensite domains with (0 0 1) twins also exist around the coarse (Ti,Hf)₂Ni precipitates. Fine (Ti,Hf)₂Ni precipitates should be responsible for the improvement in shape memory effect and the superelasticity of Ti–Ni–Hf–Cu ribbons.     

Porous hierarchical TiO2 nanostructures: Processing and microstructure relationships

A dual porous hierarchical coating of TiO₂ nanotubes (～50 nm diameter) on the nanoscale and large (～1 to 20 μm) pores on the micro-scale can be fabricated on the surface of Ti by anodic oxidation. This unique coating may have potential applications as bioactive coatings for Ti bone implants. This paper details several important aspects of the coating microstructure. TiO₂ coatings were fabricated by anodic oxidation in 1 M H₂SO₄ + 0.1 M NaF solution. Microstructure characterization was carried out using scanning electron microscopy. We also report on the observation of precipitates which form as both a continuous surface layer and of a conical geometry. The mechanism for nanotube formation, precipitate layer formation, and microscopic pitting was discussed. The effect of processing variables (i.e. time, temperature, pH) on the TiO₂ microstructure was studied. Anodization time was found to affect nanotube length and also pit size and density. Lowering the electrolyte pH decreased the nanotube length and microscopic pit density. Increasing electrolyte temperature decreased nanotube length and increased pit/pore and precipitate density. Microscopic pitting, in the nanotube coating was found to occur above grain boundaries in the Ti substrate and above Ti grains with (0 0 0 1) orientation.     

Probing structure and microstructure of epitaxial Ni–Mn–Ga films by reciprocal space mapping and pole figure measurements

The crystal structure and complex twinning microstructure of epitaxial Ni–Mn–Ga films on (1 0 0) MgO substrates was studied by X-ray diffraction using 2θ scans, pole figure measurements and reciprocal space mapping (RSM). The orientation distribution of all variants is visualized by RSM, which forms the basis for a better understanding of the crystallographic relation between variants and substrate. Above the martensitic transformation temperature the film consists of single austenite phase with lattice constant a = 5.81 Å at 419 K. At room temperature some epitaxially grown residual austenite with a = 5.79 Å remains at the interface with the substrate, followed by an intermediate layer exhibiting orthorhombic distortion, a_trans = 6.05 Å, b_trans = 5.87 Å, c_trans = 5.73 Å and a major fraction of 14M (7M) martensite, a = 6.16 Å b = 5.79 Å c = 5.48 Å. The seven-layered modulation of this metastable martensite structure is directly observed by RSM. The intermediate phase observed close to interface indicates the existence of an instable, pre-adaptive martensite phase with a short stacking period.     

Inhibition performance of a gemini surfactant and its co-adsorption effect with halides on mild steel in 0.25 M H2SO4 solution

The inhibition performance of a cationic gemini-surfactant (CBB) and its co-adsorption behavior with halides on mild steel in 0.25 M H₂SO₄ solution was studied by weight loss and electrochemical techniques. Results showed that the compound could effectively inhibit the mild steel corrosion and acted as a mixed-type inhibitor by suppressing simultaneously anodic and cathodic reactions. Addition of the halides improve the inhibition efficiency of CBB and the synergistic effect increase in the order of I^? > Br^? > Cl^?, revealing that halides radii and their electronegativity may play significant roles in co-adsorption with the cationic inhibitor.     

Size effects in the superelastic response of Ni54Fe19Ga27 shape memory alloy pillars with a two stage martensitic transformation

The superelastic behavior of Ni₅₄Fe₁₉Ga₂₇ shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10 μm and 200 nm were cut on a single crystalline bulk sample oriented along the [1 1 0] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L2₁ austenite to 10M/14M modulated martensite and then to L1_o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500 nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1 μm and below, which were shown to exhibit a direct austenite to L1_o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars.     

Deformation twinning in Ni2MnGa

Deformation twinning is investigated in the martensitic phase of a Ni_46.75Mn₃₄Ga_19.25 (at.%) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a “monoclinic” unit cell, designated 2M: then K₁, η₁, K₂, and η₂ are (0 0 1), [1 0 0], (1 0 0), and [0 0 1] respectively. The Burgers vector of the active twinning disconnections is close to 1/6[1 0 0] and the disconnections are associated with steps of height d₍₀₀₂₎. These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M.     

A comparative investigation between poly(1-ethynylpyrene) and poly(1,6-(3-ethynylpyrenylene)): Influence of the structure on the thermal,optical, electrochemical properties and conductivity

A comparative investigation between trans-poly(1-ethynylpyrene) (trans-PEP) obtained chemically and poly(1,6-(3-ethynylpyrenylene) (E-PEP) prepared electrochemically was carried out. Thermal and optical properties of the polymers were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and absorption spectroscopy. Electrochemical properties were evaluated by cyclic voltamperometry, in a 0.1 M Et₄NClO₄/THF solution at 10 mV/s, using a Pt disc as working electrode and Ag/AgCl as reference electrode. On the other hand, the conductivity of both polymers was measured in pressed pellet. Trans-PEP (T₁₀ = 369 °C) showed a higher thermal stability than its homologue E-PEP (T₁₀ = 256 °C). DSC of the polymers showed that trans-PEP exhibits a softening point at 330 °C, whereas E-PEP does it at 117 °C. Absorption spectra of the polymers revealed that trans-PEP exhibits two absorption bands at λ = 336 nm and λ = 580 nm due to the pyrene moieties and the highly conjugated polyacetylene main chain, respectively. By contrast, E-PEP showed only an absorption band at λ = 358 nm followed by a tail, which reveals that this polymer possesses a lower degree of conjugation. Molecular modelling performed in short segments of these polymers confirmed this hypothesis. Regarding the electrochemical properties, trans-PEP showed an anodic peak at 1500 mV and a conductivity value σ = 2.7 × 10^?2 S/cm, whereas E-PEP exhibited an anodic oxidation peak at 1670 mV and σ = 8.4 × 10^?2 S/cm.     

Structure of martensite in sputter-deposited (Ni,Cu)-rich Ti–Ni–Cu thin films containing Ti(Ni,Cu)2 precipitates

The martensite structure in sputter-deposited thin films of Ti_48.6Ni_35.9Cu_15.5 was studied. The Ti(Ni,Cu)₂ phase precipitates during the annealing process. Fine Ti(Ni,Cu)₂ precipitates can be deformed by the shear deformation of martensitic transformation, but they obstruct the movement of the twin boundaries to some extent. Coarse Ti(Ni,Cu)₂ precipitates seriously impede the growth of martensite plates and lead to a rectangular-cell-like structure of martensite in the film annealed at 873 K. The resistance of Ti(Ni,Cu)₂ precipitates to the growth of the martensite plates enhances with the coarsening of Ti(Ni,Cu)₂ precipitates, which is one of the reasons for the decrease in the maximum recoverable strain with increasing annealing temperature. B19′ martensite with (0 0 1) compound twinning is frequently observed near coarse Ti(Ni,Cu)₂ precipitates and grain boundaries in films annealed at 873 and 973 K. The local stress concentration should be responsible for the presence of B19′ martensite.     

Inhibition by Ginkgo leaves extract of the corrosion of steel in HCl and H2SO4 solutions

The inhibition effect of Ginkgo leaves extract (GLE) on the corrosion of cold rolled steel (CRS) in 1.0–5.0 M HCl and 0.5–2.5 M H₂SO₄ solutions was investigated for the first time by weight loss, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) methods. The results show that GLE is a good inhibitor, and exhibits more efficient in 1.0 M HCl than 0.5 M H₂SO₄. The adsorption of GLE on CRS surface obeys Langmuir adsorption isotherm. GLE acts as a mixed-type inhibitor in 1.0 M HCl, while a cathodic inhibitor in 0.5 M H₂SO₄.     

The effect of Co doping on the magnetic entropy changes in Ni44−xCoxMn45Sn11 alloys

A series of Ni_44?xCo_xMn₄₅Sn₁₁ (x = 0, 1, and 2) ferromagnetic shape memory alloys (FSMAs) were prepared by arc melting method. The martensitic transition (MT) and Curie temperatures vary obviously with Co addition. With the increasing temperature, the magnetization increases from a weak-magnetic martensite to a ferromagnetic austenite, for x = 0 and 1. But in the case of x = 2, the magnetization increases from a ferromagnetic martensite to another ferromagnetic austenite. Under an applied magnetic field of 10 kOe, the peak values of magnetic entropy changes are 10.1, 14.1, and 6.2 J/(kg K), for x = 0, 1, and 2, respectively. The magnetic phase transition near the martensitic transition temperatures and the field-induced metamagnetism should account for the large magnetic entropy changes.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

Electrosynthesis of polyaniline films on titanium by pulse potentiostatic method

Polyaniline (PANI) was synthesized on titanium electrode from aqueous solution containing 0.3 mol L⁻¹ aniline and 1 mol L⁻¹ HNO₃ by pulse potentiostatic method. The chronoamperogram during polymerization process of aniline was recorded. The effects of the synthesis parameters, such as anodic pulse duration (t_a), cathodic pulse duration (t_c), lower limit potential (E_c) and upper limit potential (E_a), on the morphology and electroactivity of the PANI films were investigated by scanning electron microscopy (SEM) and cyclic voltammetry (CV). SEM results present that flake, mica-like, quasi-fibrous and nano-fibrous PANI film could be synthesized with various polymerization parameters. Under the following conditions, t_a = 0.8 s, t_c = 0.1 s, E_c = 0 V and E_a = 1.0 V, high quality nano-fibrous PANI film with the best electroactivity was obtained. The CV results show that the PANI films with different morphologies, which were prepared under the same anodic polymerization charge, have obvious different characteristics. This means that the PANI films with different morphologies have different electrochemical activity.     

Study of Ni50+xMn25Ga25−x (x = 2–11) as high-temperature shape-memory alloys

Ni_50+xMn₂₅Ga_25−x (x = 2–11) alloys were studied as high-temperature shape-memory alloys, with regard to their microstructure, martensitic transformation behavior and high-temperature shape-memory effect. Single phase of martensite with tetragonal structure was present for x < 7, and dual phases containing martensite and γ were observed for x ⩾ 7. The martensitic transformation peak temperatures M_p increase monotonically from 39.1 °C for x = 2 to 443.8 °C for x = 7, then remain almost constant at 440 °C for x ⩾ 7. The shape-memory strains of the alloys decreased gradually from 6.1% for x = 4 to 2.8% for x = 8 and 0% for x = 11 under the same pre-strain. The variations of the martensitic transformation temperatures and the shape-memory effects with Ni contents correlate with changes in size factor, electron concentration and precipitation of γ phase.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.     

Preparation of self-organized porous anodic niobium oxide microcones and their surface wettability

Porous anodic niobium oxide with a pore size of ～10 nm was formed at 10 V in glycerol electrolyte containing 0.6 mol dm^?3 K₂HPO₄ and 0.2 mol dm^?3 K₃PO₄ at 433 K. After prolonged anodizing for 5.4 ks, niobium oxide microcones develop on the surface. X-ray diffraction patterns of the anodized specimens revealed that the initially formed anodic oxide is amorphous, but an amorphous-to-crystalline transition occurs during anodizing. As a consequence of the preferential chemical dissolution of the initially formed amorphous oxide, due to different solubility of the amorphous and crystalline oxides, crystalline oxide microcones appear on the film surface after prolonged anodizing. The surface is superhydrophilic. After coating with fluorinated alkylsilane, the surface becomes superhydrophobic with a contact angle of 158° for water. The surface is also oil repellent, with a contact angle as high as 140° for salad oil.     

SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions

Surface-enhanced Raman scattering (SERS) and cyclic voltammetry (CV) were used to investigate oxidation–reduction processes of single-wall carbon nanotube (SWNT) films deposited on Au supports in 0.5 M H₂SO₄ solutions. In the potential range (0; +1000) and (0; +1500) mV versus saturated calomel electrode (SCE), the oxidation–reduction reactions of SWNT films are quasi-reversible and irreversible, respectively. Anodic polarization of SWNT films until +1000 mV versus SCE produced compounds similar to the bisulfate intercalated graphite. Regardless of excitation wavelength, i.e. 1064 or 676.4 nm, variation in the Raman spectra exhibited a decrease in the intensity of the bands associated with the radial breathing mode (RBM) situated in the 120–240 cm⁻¹ spectral range. Also an increase in the intensity of the D band is accompanied an up-shift of this band. A gradual decrease of the Breit–Wigner–Fano component was observed at λ_exc=676.4 nm. Partial restoration of the Raman spectra was achieved by a subsequent alkaline solution treatment. Potentials higher than +1000 mV versus SCE resulted in SWNTs breakage and fragments of different length were formed such as closed-shell fullerene. This was observed in the SERS spectrum by: (i) the disappearance of the RBM band, (ii) the increased D-band shifted to ca. 1330 cm⁻¹ and (iii) the appearance of a new band at 1494 cm⁻¹, frequently observed also in the Raman spectrum of fullerenes on the type C₇₀, C₈₄, C₁₁₉, as well as in its derivative compounds (e.g. C₆₀O, clathrates, etc.). Appearance and increase in the intensity of the Raman band at 1494 cm⁻¹ as result of an anodic polarization of the SWNT film in solution of H₂SO₄ 0.5 M in 1-butanol is a further evidence of the nanotubes breakage.     

Martensitic transformation and thermal cycling effect in Cu–Co–Zr alloys

Cu_50?xCo_x Zr₅₀ (x = 0, 0.5, 1, 2, 3, 4 and 5 at.%) alloys were cast in a cylindrical copper mold by a suction casting device. In order to investigate the thermal behavior of the as-cast rods, the samples were heated from 313 to 573 K and then cooled down to about 253 K. The structure of the samples was studied by X-ray diffraction and optical microscopy. Thermal cycling measurements were also done for alloys with 2, 3, 4 and 5 at.% cobalt. It was found that increasing the cobalt content decreases martensite (M_s) and austenite (A_s) start temperatures, while it increases the temperature region in which austenite is stable. Thermal cycling measurements revealed that by increasing the number of cycles, the austenite start temperature increases while martensite start temperature shifts to lower temperatures.     

Polypyrrole nanowire modified graphite (PPy/G) electrode used in capacitive deionization

With high specific capacitance and good conductivity, polypyrrole nanowire modified graphite (PPy/G) electrode has great promising applications in capacitive deionization (CDI). Preparation parameters of modified electrode such as concentration of supporting electrolyte solution (LiClO₄), concentration of monomer (pyrrole, Py), pH of polymerization medium, polymerization potential and time have significant effects on the electrode adsorption capacity of NaCl. The experimental results indicate that the optimal preparation condition of the PPy/G electrode used for CDI is 0.10 M LiClO₄, 0.19 M Py and pH 5.91 which was controlled by phosphates buffer solution (PBS, 0.10 M), polymerized at 0.85 V vs saturated calomel electrode (SCE) with polymerization time of 150 s. The obtained electrode has an area specific capacitance of 0.188 F/cm² determined by cyclic voltammetry (CV) method in 1.0 M HClO₄ at a scanning rate of 0.05 V/s. In addition, the desalination experiments of the electrode were carried out in 500 ppm NaCl solution at a working voltage of 1.4 V. The experimental results indicate that the NaCl can be removed from the feed solution by electroadsorbing of the electrode with good desalination stability and the electrode can be regenerated efficiently by its electrodesorbing.     

Effect of precipitation on the shape memory effect of Ti50Ni25Cu25 melt-spun ribbon

The present research aims to provide accurate understanding of the relation between precipitation (volume fraction, morphology, type) and shape memory effect of Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon. Rapid thermal annealing was used to control the microstructural development while the shape memory effect of the ribbon was determined under constraint thermal cycling. The results show that the precipitation process takes the following sequence: B11 TiCu → B11 TiCu + Ti₂(Ni, Cu) → Ti₂(Ni, Cu) with increasing annealing temperature or duration. The shape memory effect is found to depend on both the volume fraction and the distribution of the precipitates. The former affects the shape recovery strain through reduction of the transformation volume participating the shape recovery. The latter affects the shape recovery strain through strengthening the matrix thus reducing the martensite strain which is more predominant under low constraint stresses. Precipitation strengthening, on the other hand, reduces the tendency of dislocation generation/movement, thus reducing the irreversible strain and improving shape recovery strain. This understanding provides guidelines on the optimization of the shape memory properties of the Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon via post-processing annealing.