Training, constraints, and high-cycle magneto-mechanical properties of Ni-Mn-Ga magnetic shape-memory alloys

Magneto-mechanical experiments with a rotating magnetic field of 0.97 T were performed with a Ni-Mn-Ga single crystal. Periodic strains exceeding 1% were recorded over a hundred million magneto-mechanical cycles. The twin microstructure of the cycled crystal was characterized using atomic force microscopy (AFM) and magnetic force microscopy (MFM). In the center of the sample, no twin boundaries were found. At the sample edges, the microstructure shows a dense twin pattern. The results are compared with previous experiments of differently trained crystals. It is useful to distinguish between “ineffective training”, which results in a nearly self-accommodated martensite, and “effective training”, which results in a nearly single-variant crystal. The evolution of twin structure is discussed in terms of training, magneto-mechanical cycling, and extrinsic constraints imposed by the experimental setting. It is concluded that the response of a magnetic shape memory alloy to an alternating excitation depends strongly on the initial twin microstructure established through training. In particular, ineffective training results in a twin microstructure which can adapt to extrinsic constraints resulting in continued large periodic magnetic-field-induced deformation. In contrast, the twin microstructure of an effectively trained crystal can not adapt to extrinsic constraints resulting in early failure by fracture.     

A Two-Dimensional Relaxation Study of the Evolving Microstructure in a Mixed Biopolymer Gel

It is shown that two-dimensional (2-D) T ₁–T ₂ relaxation spectra are sensitive to the changing microstructure in a model food system comprising water-in-water gelatin–agarose gelled emulsions. This 2-D relaxation–microstructure relationship suggests that T ₁–T ₂ spectra have the potential of acting as “fingerprint” molecular biomarkers of microstructure in complex water-rich biological systems and this has important future implications for in vivo magnetic resonance imaging studies of food digestion and of the changing structure of cancerous tissue.     

Solidification microstructure of AZ91D Mg alloy after laser surface melting

The solidification microstructure plays a critical role in determining the surface properties of laser-treated magnesium alloys. The purpose of this paper is to study the solidification microstructures of AZ91D Mg alloy following millisecond- and nanosecond-pulse Nd:YAG laser irradiation. The solidification microstructural evolution of laser-melt AZ91D Mg alloy was investigated using X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectrometer and transmission electron microscopy. Much refined α-Mg phase and β-Mg₁₇Al₁₂ intermetallics were observed in the microstructure after laser surface melting. Periodic and successive structure was observed in the millisecond irradiated surface and the melt depth was more than 100 μm. The solidification microstructure was mainly cellular/dendrite structures together with a large number of β-Mg₁₇Al₁₂ nano-particles. Micron holes were found in the nanosecond irradiated surface and the melt depth was shallow at 50 μm. Millisecond-pulse Nd:YAG laser was found to be more suitable for Mg alloy surface treatment due to sufficient melt depth.     

Heat and mass transfer characteristics during rapid solidification of Fe-Cu peritectic alloys

The viscose flow and microstructure formation of Fe-Cu peritectic alloy melts are investigated by analyzing the velocity and temperature fields during rapid solidification, which is verified by rapid quenching experiments. It is found that a large temperature gradient exists along the vertical direction of melt puddle, whereas there is no obvious temperature variation in the tangent direction of roller surface. After being sprayed from a nozzle, the alloy melt changes the magnitude and direction of its flow and velocity rapidly at a height of about 180 μm. The horizontal flow velocity increases rapidly, but the vertical flow velocity decreases sharply. A thermal boundary layer with 160–300 μm in height and a momentum boundary layer with 160–240 μm in thickness are formed at the bottom of melt puddle, and the Reynolds number Re is in the range of 870 to 1070 in the boundary layer. With the increase of Re number, the cooling rate increases linearly and the thickness of thermal boundary layer increases monotonically. The thickness of momentum boundary layer decreases slowly at first, then rises slightly and decreases sharply. If Re < 1024, the liquid flow has remarkable effects on the microstructure formation due to dominant momentum transfer. The separated liquid phase is likely to form a fiber-like microstructure. If Re>1024, the heat transfer becomes dominating and the liquid phase flow is suppressed, which results in the formation of fine and uniform equiaxed microstructures. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

Rapid solidification of Al-Cu-Ag ternary alloy under the free fall condition

The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al₂Cu), α(Al) and ξ(Ag₂Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔT _Max=171 K (0.20T _L). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ(Al₂Cu) phase of the alloy grows into coarse dendrite. When 78 K⩽ΔT⩽171 K, its refined θ(Al₂Cu) phase grows alternatively with α(Al) phase. Once ΔT⩾171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

Morphologies of Fe-66.7 at.%Si alloy solidified in a drop tube

Solidification of 0.1–1.0 mm diameter droplets of Fe-66.7 at.%Si alloy was achieved in a 3 m drop tube. The XRD, EDS, and SEM measurements reveal that all the droplets are composed of the primary phase α and the α+ε eutectic. With decreasing droplet diameter, the growth mode of the primary phase α changes from faceted to nonfaceted growth and the eutectic changes from needle-like to anomalous eutectic. In addition, the width of the primary phase α decreases with decreasing droplet size. The different cooling rates and undercooling levels corresponding to the samples with different sizes are responsible for the morphology changes. The cooling rates of the samples with different diameters during free fall were calculated and their effects on the microstructure formation were discussed. This kind of transition is also found inside the same sample, which is due to the larger cooling rate on the surface than at the center.     

Theoretical study of relative width of photonic band gap for the 3-D dielectric structure

Calculations for the relative width (Δω/ω₀) as a function of refractive index and relative radius of the photonic band gap for the fcc closed packed 3-D dielectric microstructure are reported and comparison of experimental observations and theoretical predictions are given. This work is useful for the understanding of photonic crystals and occurrence of the photonic band gap.     

Growth of large microcones in steel under multipulsed Nd:YAG laser irradiation

We report the growth of conical microstructure arrays on a stainless steel substrate under multi-pulsed Nd:YAG laser irradiation (wavelength of 1.064 μm, pulse duration of 300 ns, repetition rate of 5 kHz) at atmospheric air pressure. The average period of microcones is 70 μm, and they protrude 50–60 μm above the substrate. At an air pressure of 1 Pa, the well-defined conical shape is lost and the resulting microstructure shows a smaller period and height. At 10^-3 Pa, only small protrusions separated by about 5 μm are observed. The different mechanisms involved in the growth of conical microstructures are discussed. PACS 42.62.-b; 81.65.-b     

Water sorption characteristics of carbon nanostructures calculated on the basis of the Ornstein–Zernike equation

Molecular systems in contact with the bounding surfaces are examined. New apparatus describing adequately quasi–two-dimensional structures whose cross sectional dimensions are comparable with the correlation length is suggested. It is based on the modified method of the Wigner distribution functions and the system of Ornstein–Zernike equations generalized for one- and two-particle distribution functions. Modeling of the physical gas sorption process in carbon nanofibers and nanotubes allows local microstructure, sorption characteristics, effective sizes, and optimal technological filling parameters to be calculated.     

Spreading of diblock copolymer droplets: A probe of polymer micro-rheology

We present an experimental study of the spreading dynamics of symmetric diblock copolymer droplets above and below the order-disorder transition. Disordered diblock droplets are found to spread as a homopolymer and follow Tanner’s law (the radius grows as R ∼ t ^m , where t is time and m = 1/10 . However, droplets that are in the ordered phase are found to be frustrated by the imposed lamellar microstructure. This frustration is likely at the root of the observed deviation from Tanner’s law: droplet spreading has a much slower power law ( m ∼ 0.05±0.01 . We show that the different spreading dynamics can be reconciled with conventional theory if a strain-rate-dependent viscosity is taken into account.     

Microstructure and gas sensitive properties of α(M: Sn and Ti) materials prepared by ball milling

Metastable α-Fe₂O₃-MO₂(M: Sn and Ti) solid solutions can be synthesized by mechanical alloying. The alloy formation, microstructure, and gas sensitive properties of mechanically milled α-Fe₂O₃-SnO₂ materials are discussed. Tin ions in α-Fe₂O₃ are found to occupy the empty octahedral holes in the α-Fe₂O₃ lattice. This interstitial model can also describe the structure of α-Fe₂O₃-TiO₂solid solutions. Finally, a correlation of gas sensitive properties with microstructure of α-Fe₂O₃-SnO₂ materials is presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Numerical simulation of the dynamics of current channel microstructure formation in atmospheric nanosecond discharges in a uniform electric field

The results of simulation of the current channel microstructure formation in atmospheric nano- second discharges in a uniform electric field due to the development of instability of the ionization process in the avalanche stage followed by cycling breakdowns of the avalanche are considered. It is shown that the enhancement of the electric field at the ionization front due to the intrinsic field of the avalanche leads to the contraction of the path length between consecutive avalanche breakups; after several breakups, the ionized gas passes to the plasma state. The effect of small electric field perturbations on the dynamics of microstructure formation is investigated; as a result, the possibility of “induced” avalanche breakup at the instant of action of perturbations is established.     

Microstructural characterization of ultrafine-grain interstitial-free steel by X-ray diffraction line profile analysis

This paper highlights the microstructural features of commercially available interstitial free (IF) steel specimens deformed by equal channel angular pressing (ECAP) up to four passes following the route A. The microstructure of the samples was studied by different techniques of X-ray diffraction peak profile analysis as a function of strain (ε). It was found that the crystallite size is reduced substantially already at ε=2.3 and it does not change significantly during further deformation. At the same time, the dislocation density increases gradually up to ε=4.6. The dislocation densities estimated from X-ray diffraction study are found to correlate very well with the experimentally obtained yield strength of the samples.     

Laser cleaning of copper roofing sheets subjected to long-lasting environmental corrosion

The study was concerned with the effect of the laser fluence and pulse duration on the microstructure of copper roofing of the Wilanów Palace in Warsaw (Poland) subjected to environmental degradation since the turn 19th century. The techniques used for the microstructure examinations included: SEM+EDS, X-ray diffraction analysis, surface profiling and colour analysis. The measurements of colour of the laser-cleaned surfaces showed that series of 100-μs pulses offer the most effective cleaning. It was also found that by controlling the number of laser pulses it is possible to control the roughness of the cleaned surface.     

Electrochemical cell with two layers cathode for NO decomposition

An electrochemical cell composed of an yttria-stabilized zirconia disk and two layers cathode was used for nitrogen monoxide decomposition. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the performance of the electrochemical cell for NO_x decomposition in the presence of excess oxygen. The decomposition activity was measured for the one-compartment cell oxide|(cathode)|YSZ|(anode) by applying a DC voltage lower than 3.7 V in the temperature range 550–700 °C. The microstructure of the YSZ-NiO mixed oxide electrodes was investigated in dependence of the cell operating condition and the working electrode sintering temperature. The correlation between the microstructure of the mixed oxide electrode and conversion rate of NO was studied. The phenomenon of self-optimization of the microstructure of the NiO-YSZ working electrode during the cell operation was observed and investigated. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Phase Composition of Iron Compounds in the Products of Annealing of the Tertiary Basaltic Rock from Góra Obłoga (the Lower Silesia, Poland)

The work presents results of the research on thermal transformations of iron minerals which are present in basalt rock from Góra Obłoga in Pogórze Złotoryjskie District. The rock under study shows microporphyric texture and compact microstructure and contained: olivine, augite, diopside, plagioclases, serpentines, zeolites, apatite and also opaque minerals (oxides). The basalt was annealing within the temperature range between 573–1473 K, in air, and then, microscopic observations, X-ray diffraction as well as Mössbauer spectroscopy measurements were performed. During the heat treatment of the rock it was found that the content of olivine and augite was gradually decreasing due to progressive oxidation of iron compounds, resulting in formation of magnetite and hematite. The comparison of the results of thermal Mössbauer spectral analysis showed that transformation of iron minerals occurred already under conditions when the basic rock mass underwent only insignificant changes.     

Determination of the density of states in high-Tc thin films using FET-type microstructures

A simple electronic experiment using a field-effect-transistor–type microstructure is suggested. The thin superconductor layer forms the source-drain channel of a layered structure across which an AC current is applied. It is found necessary to measure the second harmonic of the source-gate voltage, and the third harmonic of the source-drain voltage; these electronic measurements then give the logarithmic derivative of the density of states, which is an important consideration when fitting parameters of the band structure.     

Effects of composition and cooling rate on the microstructure of?Sn–3.7Ag–0.9Zn–Bi solders

The effects of Bi addition, of less than 3 wt.%, and applied cooling rate on the solidified microstructure of the eutectic Sn–3.7Ag–0.9Zn (weight percent, hereafter) solder were investigated. As observed by microstructural analysis, the increase of Bi content favors the separation of the β-Sn and AgZn intermetallic compounds (IMCs) in the eutectic Sn–Ag–Zn solder. And there are some Bi precipitates formed along with the primary β-Sn dendrites as the concentration of Bi exceeds 2%. As the applied cooling rate increases, the microstructure of the Sn–3.7Ag–0.9Zn–Bi solder is refined, and the segregation of Bi is restrained. By increasing the amount of Bi, the microhardness of the solder increases.     

Influence of the short-range order parameter on the optical properties of amorphous nickel-phosphorus alloy

The influence of the short-range order of amorphous nickel-phosphorus alloy on the IR absorption is studied. The relation between the optical properties and the microstructure of amorphous nickel-phosphorus alloys obtained by various technological methods is considered. It is shown that a coating of amorphous Ni−P alloy obtained by electrolytic deposition has the most homogeneous structure; this permits the use of such coatings as protective layers for optical and magnetic memory disks. Optical methods permitting the estimation of the dimensions of the microcrystalline regions in an amorphous matrix are outlined. T. Shevchenko Kiev University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–40. May, 1997.     

Microstructure control of low-resistivity tin-doped indium oxide films grown by photoreaction of nanoparticles using a KrF excimer laser at room temperature

A tin-doped indium oxide (ITO) film on a SiO₂ substrate was prepared by photo-irradiation of spin-coated nanoparticles using a Xe excimer lamp and a KrF excimer laser. The effects of the excimer lamp and the excimer laser on the resistivity, mobility, and carrier concentration of the film were investigated. To better understand how to control the microstructure of the film, we investigated the effect of thickness on the resistivity of a film prepared by the two-step process, and found that the resistivity was higher in a thicker film. Using two-step irradiation plus one-step KrF irradiation in N₂ at room temperature, we produced an ITO film with lowest resistivity of any in this study. The electrical resistivity of this film was 5.94×10⁻⁴ Ω cm. On the other hand, when using a simple thermal process, the resistivity of a film sintered at 500°C in N₂ was 4.10×10⁻³ Ω cm. The differences in resistivity are discussed on the basis of the microstructure of the films using atomic force microscopy and Hall measurements.