R.A. Varin

Particle size, grain size and γ-MgH2 effects on the desorption properties of nanocrystalline commercial magnesium hydride processed by controlled mechanical milling

The hydrogen desorption properties of commercial nanocrystalline magnesium hydride (Tego Magnan® from Degussa–Goldschmidt) processed by controlled mechanical milling (CMM) are investigated. A profound effect of the powder particle size on the hydrogen desorption characteristics has been observed. The onset (TON) and peak hydrogen desorption temperatures measured by differential scanning calorimetry (DSC) decrease initially slowly with decreasing mean particle size of hydride, and when the particle size reaches a certain critical threshold value, the desorption temperatures start decreasing more rapidly with further decrease of particle size. The total drop of desorption temperature from its initial value for the as-received MgH2 to the value attained for the milled MgH2 having a particle size of ~500–600 nm is within the range 40–60 °C. The metastable γ-MgH2 hydride coexists with the stable nanocrystalline β-MgH2 in the microstructure of the MgH2 powders ball milled for 10 h and longer. Quantitative evidence shows that two factors, namely the refined powder particle size and the γ-MgH2 phase residing within the powder particles, acting additively, are responsible for a substantial reduction of the hydrogen desorption temperature of MgH2 hydride.

Photocatalytic Activity of Hydrogenated TiO2

Photocatalysis is a promising advanced water treatment technology, and recently the possibility of using hydrogenation to improve the photocatalytic efficiency of titanium dioxide has generated much research interest. Herein we report that the use of high-temperature hydrogenation to prepare black TiO2 primarily results in the formation of bulk defects in the material without affecting its electronic band structure. The hydrogenated TiO2 exhibited significantly worse photocatalytic activity under simulated sunlight compared to the unhydrogenated control, and thus we propose that high-temperature hydrogenation can be counterproductive to improving the photocatalytic activity of TiO2, because of its propensity to form bulk vacancy defects.

Magnesium silicide intermetallic alloys

Methods of induction melting an ultra-low-density magnesium silicide (Mg2Si) intermetallic and its alloys and the resulting microstructure and microhardness were studied. The highest quality ingots of Mg2Si alloys were obtained by triple melting in a graphite crucible coated with boron nitride to eliminate reactivity, under overpressure of high-purity argon (1.3 X 105 Pa), at a temperature close to but not exceeding 1105 °C ± 5 °C to avoid excessive evaporation of Mg. After establishing the proper induction-melting conditions, the Mg-Si binary alloys and several Mg2Si alloys macroalloyed with 1 at. pct of Al, Ni, Co, Cu, Ag, Zn, Mn, Cr, and Fe were induction melted and, after solidification, investigated by optical microscopy and quantitative X-ray energy dispersive spectroscopy (EDS). Both the Mg-rich and Si-rich eutectic in the binary alloys exhibited a small but systematic increase in the Si content as the overall composition of the binary alloy moved closer toward the Mg2Si line compound. The Vickers microhardness (VHN) of the as-solidified Mg-rich and Si-rich eutectics in the Mg-Si binary alloys decreased with increasing Mg (decreasing Si) content in the eutectic. This behavior persisted even after annealing for 75 hours at 0.89 pct of the respective eutectic temperature. The Mg-rich eutectic in the Mg2Si + Al, Ni, Co, Cu, Ag, and Zn alloys contained sections exhibiting a different optical contrast and chemical composition than the rest of the eutectic. Some particles dispersed in the Mg2Si matrix were found in the Mg2Si + Cr, Mn, and Fe alloys. The EDS results are presented and discussed and compared with the VHN data.

IN SITU INVESTIGATION OF THE EARLY STAGES OF PLASTIC DEFORMATION IN AN AUSTENITIC STAINLESS STEEL

The early stages of plastic deformation in stainless steel with a small grain size have been studied by in situ straining of a thin foil inside the electron microscope. TEM observations indicate that the deformation process has a sequential character: Stage 1—activation of lattice dislocations; Stage 2—activation of triple point sources and formation of dislocation pile-ups; Stage 3—propagation of slip across some of the grain boundaries by dislocation generation induced by stress concentration at the pile-up. The shear stresses acting in each deformation stage have also been calculated using data based on the geometry of the pile-ups observed in thin foil. It is shown that the stress necessary to generate a dislocation from special grain boundary close to Σ = 9 CS1 misorientation is about 40 times higher than the stress necessary to emit a dislocation from a triple point. The calculated values of the shear stresses are in agreement with those registered in bulk specimens of stainless steel as well as they justify the sequence of dislocation events observed during in situ straining.

Carbon fibres coated with titanium carbide using the liquid metal transfer agent technique

Protective coatings of titanium carbide were applied to PAN type carbon fibres by a liquid metal transfer agent (LMTA) technique using tin as a transfer agent. The effect of the coating on the strength of the fibres was evaluated by performing single fibre tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. Carbide coating thicknesses obtained ranged from approximately 0.05 to 0.5 μm and the coatings were found to be uniform and adherent to the fibres. It was found that wetting of the fibres by the tin alloy is associated with the spontaneous formation of a carbide layer with a thickness dependent upon the melt temperature, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 187 kJ mol−1. The strength of the carbon fibres decreased with increasing coating thickness according to a Griffith relation.

Coating of graphite fibers with tungsten carbide using solid and liquid copper as a transfer medium

An attempt was made to coat graphite fibers with tungsten carbide. Solid and liquid copper were used as a diffusion medium for tungsten to migrate to the surface of the graphite fibers at elevated temperature to form carbides. It was found that it is possible to electroplate Cu-1 pct W alloy on graphite fibers. It was established that this alloy contained tungsten in atomic dispersion. It was fur-ther discovered that tungsten was not in an equilibrium condition in copper and upon heat treatment formed fine precipitates instead of diffusing to the surface of the fiber to form tungsten carbide. However, it was also found that tungsten has a certain solubility in liquid copper and thus it was possible to obtain carbide coatings on graphite fibers by using liquid copper as a transfer agent for tungsten.

Grain boundary diffusion and free energy during the recrystallization of type 316 stainless steel

Abstract Experimentally obtained pairs of values of the temperature and time at which extrinsic grain boundary dislocations spread, i.e. the contrast of the extrinsic grain boundary dislocations disappeared, have been used to calculate the activation enthalpy ΔHb and grain boundary free energy γGB in type 316 stainless steel cold rolled to 82% reduction in thickness and then recrystallized at 757°C. The calculated ΔHb value of 177.52 ± 2.08 kJ mol−1 is in excellent agreement with the value reported very recently by Patil and Sharma (1982) for type 316 stainless steel annealed at higher temperatures (900–1200 °C). Such agreement indicates that grain boundaries as well as recrystallization fronts in partially recrystallized type 316 steel are fully equilibrated in spite of the fact that they have incorporated a large number of dislocations from the heavily deformed matrix during recrystallization. This fact is accounted for on the basis of the molybdenum-enhanced lattice diffusivity which in turn causes the time necessary to attain complete equilibrium of a grain boundary structure to be significantly shorter.

Fracture toughness of intermetallic compacts consolidated from nanocrystalline powders

Abstract Ball-milled and fully disordered intermetallic powders of Fe–45at.%Al (iron aluminide) and titanium trialuminide (Al 3 Ti) stabilized to cubic (L1 2 ) structure by alloying with 9 at.% Mn, with nanocrystalline (nanophase) grain size in the range of ∼10 and ∼3 nm (from X-ray diffraction, XRD), respectively, were successfully consolidated into nearly pore-free bulk compacts. Fe–45at.%Al powders were consolidated only by explosive shock wave compaction and titanium trialuminide powders were consolidated by hot pressing and explosive shock wave compaction. After shock consolidation a microcrystalline structure appeared in larger powder grains of the Fe–45Al compacts. Compacts were re-ordered after hot or shock consolidation. Vickers indentation fracture toughness of compacts was investigated. Fe–45Al compacts did not develop any corner cracks up to 2000 g indentation load, indicating some intrinsic fracture resistance. Cubic titanium trialuminide compacts developed corner cracks under the indentation load and their average measured fracture toughness was barely ∼2 MPa m 0.5 , i.e. even lower than the fracture toughness of bulk specimens of coarse-grained cubic titanium trialuminides (∼4–5 MPa m 0.5 ). The results demonstrate that refining the grain size towards the nanolevel is not sufficient to beneficially modify toughness of brittle intermetallics.

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising ∼30 vol% Mg2Ni and ∼61 vol% MgNi2 intermetallic compounds (remaining ∼9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg2Ni and MgNi2. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi2 particles and the ‘mantel’ is a thoroughly milled mixture of Mg2Ni, MgNi2 and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg2Ni phase. The remnant Mg2Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi2 phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.

Microstructure and ordering of L12 titanium trialuminides

The present article reports and discusses the results of the microstructural characterization of various modifications of Ll2 trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll2 trialuminide alloys Al3Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al3Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al3Ti + Cu alloy. The homogenized Al3Ti + Cr and Mn alloys have greater lattice parameters than the Al3Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll2 Al3Ti + X alloys dramatically decreases with increasing titanium content.