Band 99, Heft 1

Dense bulk Ta 2 AlC ceramic was fabricated by in-situ reaction/hot pressing of Ta, Al and C powders. The reaction path and effects of initial composition on the purity were investigated. It was found that Ta 2 AlC formed through the reactions between AlTa 2 and graphite, or between Ta 5 Al 3 C, TaC and graphite at 1500–1550°C. By modifying the molar ratio of the initial Ta, Al, and C powders, single-phase Ta 2 AlC was prepared at 1550°C under an Ar atmosphere with an optimized composition of Ta: Al: C = 2: 1.2: 0.9. The lattice parameter and a new set of X-ray diffraction data of Ta 2 AlC were obtained. In addition, Ta 2 AlC was reported unstable above 1600°C and decomposed to Ta 4 AlC 3 , and then to TaC x .

A unified equation on the viscosity of pure liquid metals (published recently by the author) is combined with the well-known Sutherland – Einstein equation to obtain a new equation for the temperature dependence of the self-diffusion coefficients of pure liquid metals. The equation does not contain new adjustable parameters. It reproduces perfectly the experimental values, measured under micro-gravity conditions for liquid Sn, Pb, In and Sb. The experimental data, obtained under normal gravity conditions appear to be equal or somewhat higher than those calculated from the present model. This is explained by the effect of gravity induced convection in the liquid.

The system Bi – Ga shows a pronounced miscibility gap in the liquid state ranging from 38.5 to 91.5 at.% Ga at 495 K with a maximum at 535 K at about 70 at.% Ga. Heat capacities of liquid Bi – Ga alloys were determined by heat flow differential scanning calorimetry over the entire composition range at temperatures between 520 and 600 K. In order to optimize the differential scanning calorimetry measurements in the liquid state, the sample masses were increased to 250–400 mg and the heating and cooling rates were reduced to 10 or 5 K min −1 , respectively. The experimental results show a positive deviation from the Neumann-Kopp rule in the composition range of the miscibility gap. The results are in good agreement with theoretical considerations. Additionally, enthalpies of mixing were determined by drop calorimetry in the composition range from 30 to 100 at.% Ga at three different temperatures: 513, 573 and 673 K.

The objective of the present work was to study the effects of forced flow on solute transport and associated phase composition in an Al – Mg – Si alloy. Possible solidification paths were calculated using Thermo-Calc from macrosegregation patterns obtained for the experimental alloy solidified in the melt velocity range between 0.14 m s −1 and 0.44 m s −1 . The comparison of the calculated and experimentally observed phase compositions showed good agreement. In order to reveal the microstructural features and to identify the phases present, metallographic analysis in combination with electron-probe analysis and X-ray measurements was extensively used.

It is well known that a high pressure ω-phase is formed in Ti at high pressures in the range between 2 and 8 GPa. This martensitic-type transformation demonstrates very large hysteresis, and hence the ω-phase can be retained in the material after release of pressure. Additionally, applied shear stresses are known to facilitate the α → ω transformation. This paper describes an investigation on the ω-phase formation after high pressure torsion under a wide range of pressures and shear strains by means of X-ray diffraction and transmission electron microscopy. We show that the ω-phase forms in Ti upon high pressure torsion deformation after 300 s under a pressure of 3 GPa. This suggests that the transformation kinetics are notably increased as compared with the kinetics of pressure-induced transformation.

The thermal properties and elastic constants of Nd – Mg ordered intermetallic alloys are studied using molecular dynamics simulations. The calculated results are in good agreement with the available experimental data and the first-principles data at various temperatures. The calculated elastic constants decrease as the temperature increases, which indicates that thermal softening occurs. Furthermore, the thermal volume expansion values of NdMg, NdMg 2 , NdMg 3 , Nd 5 Mg 41 and NdMg 12 are 7.26 × 10 −5 , 7.96 × 10 −5 , 7.54 × 10 −5 , 8.29 × 10 −5 , and 8.02 × 10 −5 K −1 , respectively, at 298 K. The results indicate that among those intermediate phases, the interatomic force of NdMg is the strongest, and the melting point of the phase is the highest, which is also proved by bulk modulus. In addition, the heat capacities of NdMg, NdMg 2 , NdMg 3 , Nd 5 Mg 41 and NdMg 12 are 23.46, 23.32, 23.48, 23.38 and 23.40 J mol −1 K −1 , respectively, at 298 K.

To investigate the influence of equal channel angular pressing on the microstructure and texture of the magnesium alloy AZ31, electron backscattering diffraction and well as neutron diffraction experiments were carried out. Through these experiments it was possible to trace the microstructure and texture evolution with strain accumulated with the increasing number of equal channel angular pressing passes. It was further demonstrated by subsequent compression tests that the microstructural changes produced by equal channel angular pressing have a beneficial effect on both the compressive strength and ductility of AZ31.

The compressive creep behaviour of AZ91 processed by Thixomolding with varying solid phase content (2–45%) was investigated. It can be shown that with increasing solid phase content the creep rate at 150°C is reduced by a factor of 3–10. The results can be rationalised in terms of a grain size effect. The solid phase (primary α-Mg phase solidified prior to mould filling) is relatively coarse (40 μm), i. e. the average grain size increases significantly when the material is processed in the semi-solid regime. Grain boundary sliding which is known to contribute significantly to creep deformation depends on the grain size. The creep rate of high pressure die cast material is found to be similar to thixomolded material. The high pressure die cast materials although nominally processed in the fully liquid regime, contains pre-solidified coarse primary α-Mg phase as well. The coarse solid phase particles are formed early in the process when the melt cools rapidly in the shot sleeve. AZ91 with Ca-additions up to 5 wt.% were prepared using a newly developed second feeding system that allows the addition of a second sort of granules to the screw in parallel to the main feed. As expected, Ca increases the creep strength very significantly. The reasons for this increase will be discussed.

The effect of C addition on texture, phase formation, and magnetic properties of PrCo 5 – x C x ( x = 0–0.5) ribbons has been examined. C addition does not favor the formation of (200) texture perpendicular to the ribbon plane for the ribbons spun at 5 m s −1 . As x increases, the fraction of PrC 2 nonmagnetic phase increases, which enhances the resistance of domain wall movement and thus leads to an obvious coercivity increase in the specimens. The increase in the wheel speed further strengthens the beneficial effect of C addition on the magnetic properties of the PrCo 5 -based ribbons. When the wheel speed is increased to 25 m s −1 , the coercivity and the energy product of the ribbons increase from 1.6 kOe and 1.6 MGOe to 6.7 kOe and 4.3 MGOe with the increase of x from 0 to 0.5. Simultaneously, the coercivity mechanism changes from the reverse domain nucleation type to the domain wall pinning type.

The exchange coupling between Nd 2 Fe 14 B/α-Fe phases is investigated for Nd 10 Fe 85 B 5 and Nd 11 Fe 80 B 9 melt spun ribbons. The exchange coupling is present in Nd 10 Fe 85 B 5 ribbons when the grain size of the α-Fe phase is as high as 30 nm due to better inter-grain interaction. The exchange coupling is enhanced when the exchange length increases due to the reduction in the value of magnetocrystalline anisotropy constant which is verified by δM measurements at various temperatures. The as-spun nanocomposite ribbons show both exchange and dipolar couplings, whereas annealing results in an increase in the strength of the exchange coupling due to the removal of grain boundary anisotropy as demonstrated from δM studies.

The microstructural characteristics and elevated temperature wear behavior of a Ti-11Si-16Al alloy have been investigated, compared with the as-cast Al-free Ti – Si eutectic alloy. Experimental results show that addition of 16 at.% Al greatly modifies the microstructure by forming near-equiaxed Ti 5 Si 3 , rod-like Ti 3 Al and rosette-like eutectic cells distributing on a continuous α-Ti (Al, Si) solid solution phase. Therefore, mechanical properties such as Vickers' hardness, elastic modulus and strength are significantly improved. In-situ hot pressing and sintering of the hybrid oxide particles form a compact-and-hard glaze layer on the worn surface of the alloy. It has protective effects on the Ti-11Si-16Al specimen by relieving oxygen diffusion, alleviating rubbing of the counterpart and providing a solid lubricant effect with the hybrid oxide particles. This effectively improves the elevated temperature wear resistance and friction stability.

A new method was developed to produce nickel coated fibres, which could be used as a raw material for making lightweight electrodes in nickel based secondary batteries. Hydrazine was employed as the reducing agent, ammonia solution as a complexing agent, nickel sulphate as the source of nickel ions and ammonia – ammonium sulphate as the buffer system. While the experiments confirmed that the ammonia – ammonium sulphate system could be applied for plating nickel on viscose, polyester, polyamide and carbon fibres, viscose fibre was found to be the more suitable material. The suitable operating conditions were reported.

Plasma nitrocarburizing was applied to AISI 316L stainless steel substrates using an electrolytic plasma technique, based on a dielectric barrier discharge created during cathodic saturation in an aqueous electrolyte. The samples were biased cathodically with a direct current high voltage. The wear resistance of the treated samples was evaluated using a pin-on-disk wear testing method. Results indicate that the resultant hardened nitrocarburized layers exhibited excellent abrasive wear resistance. Investigation of the wear characteristics showed that the magnitude of weight loss and the wear mechanisms strongly followed from the treatment experimental parameters. Additionally, in order to evaluate the corrosion properties of the layers a potentiodynamic polarization test was performed on the hardened samples. Results indicated that short time as well as high treatment temperature provided a slight increase in corrosion resistance while in the case of long treatment time the corrosion resistance decreased. These changes in the properties could be attributed to the different nanocrystalline morphologies of the nitrocarburized layers.

Microstructural changes of rapidly solidified Al alloy ribbons of various thicknesses and containing different concentrations of iron were analysed. The kinetics and sequence of microstructural changes occurring at a constant heating rate were analysed through the measurement of electrical resistance. This method allowed detection of the transition between metastable and stable phases of rapidly solidified Al – Fe alloy ribbons with various concentrations of iron and various thicknesses. After determining the temperature regions throughout the heating cycle, the quenched microstructure was analysed at transition points using both optical and transmission electron microscopy. Precipitation is the most common reaction for these types of alloys, and the most important alloy formed is Al 13 Fe 4 .

Liquid metal quality has been an important topic for aluminum alloys in both industry and academic circles. Pressure filtration, or pre-concentration techniques have been among the principal methods of measuring the contents of solid inclusions. In this work, the mutual relationships between the main variables related to liquid metal quality, filter medium, filtration process, and the formed filter cake are effectively established, based on the classic filtration theories of cake mode combined with the quantitative metallurgy of the filter cake. For the first time the specific cake resistance, formation and growth dynamics of filter cakes can be quantitatively calculated. This progress will significantly help to quantitatively understand the filtration behavior during the detection of liquid metal quality. A typical example is illustrated in this work.

Ni/YSZ (Y 2 O 3 -stabilized ZrO 2 ) composite cathode is fabricated by high-nergy ball milling of Ni and YSZ powders for high temperature electrolysis of water vapor. Composite powder composition, time and rotation speed of ball milling were optimized by trial and error for suitable performance with maximum efficiency of fabricated cathode in high temperature electrolysis of water vapor (steam) at 800°C. The composite powders thus synthesized were characterized using various analytical tools such as X-ray diffraction, scanning electron microscopy and particle size analysis, and a self-supporting planar type unit cell was prepared with Ni/YSZ composite cathode and Pt paste anode screen-printed on each side of YSZ electrolyte disk (30 mm dia.) and then sintered at 1450°C for high temperature electrolysis. X-ray diffraction and particle size analysis results showed that the Ni/YSZ composite effectively consisted of crystalline Ni and YSZ distributed on a sub-micron scale and evenly distributed without aggregation of particles. To study the effect of anode and cathode thickness on the efficiency of the hydrogen production rate, screen-printing of anode and cathode was carried out on each side of YSZ electrolyte to three different thickness values during the fabrication of a self-supporting planar type unit cell which operated quite satisfactorily during high temperature electrolysis at 800°C: a significantly higher hydrogen production rate of 13 sccm (standard cubic centimeters per minute) was reproducibly observed for triple screen-printing of anode and cathode over each side of the YSZ electrolyte disk compared to 0.5 sccm for the single screen-printed cathode under an applied direct current of 0.7 mA cm −2 . Additionally, linear increase in hydrogen production was observed with applied current for triple screen-printed electrodes in contrast to single and double screen-printed electrodes.

Glass-ceramics, prepared from the crystallization of interface-free, homogeneous glass, offer a unique perspective to study the influence of interfaces owing to the controlled manner in which interfaces (grain boundaries) can be introduced into the material. This review begins by considering the kinetics of interface development, itself strongly influenced by surface energetics. We then take up mechanical properties, the increase of which over that of their glassy precursors remains a cornerstone of what makes glass-ceramics desirable. Here, the role of the interface, both between crystals and, in some cases, within crystals (e. g., twin planes), serve to provide multiple ways in which to produce tough and strong materials. Hermetic crystal – glass interfaces are probably the rule in most glass-ceramics, rather than the exception, and are well demonstrated by several systems. Crystal clamping – the process by which crystals surrounded by residual glass in a glass-ceramic undergo stress due to coefficient of thermal expansion mismatches or by the inability of the crystal to go through a polymorphic transition upon cooling – has now been demonstrated in a number of studies with calculated stresses reaching 1 GPa. Electrical property studies of multi-phase glass-ceramics have shown the wide variety of phenomena possible with these materials, although more detailed studies will be necessary to fully understand the complex interplay between crystals, grain boundaries, space-charge regions, and residual glass. Light scattering is one of the more important implications of interfaces in glass-ceramics and quantitative approaches are more and more used to characterize this often unwanted feature. Concluding this review, we attempt to address four key questions that seek to extract those features of interfaces in glass-ceramics that have been application-enabling, require additional understanding and, finally, which might point a way towards new applications in the future.