Volume 96, Issue 3

The relationship between substrate materials and the deposition of diamond or graphitic soot was studied in the low-pressure synthesis of diamond. Electrocatalytic metals such as Fe, Ni, Rh, Pd, Pt, and Ir result in soot deposition while insulating materials such as Al 2 O 3 and SiO 2 result in diamond deposition. There exists a strong correlation between the electrocatalytic substrate materials and soot deposition. The substrate dependence of the deposition behavior is approached by the theory of charged clusters. The substrate dependence is suggested to be related to the charge-transfer rate at the substrate surface for charged clusters, which had formed in the gas phase. The evolution of soot and diamond is explained in terms of the floccula-tion and deflocculation behavior of colloid particles, respectively.

Dual-Phase steels are excellent candidates for applications in the automotive industry. However, because of their high content in alloying elements and the formation of the corresponding surface oxides during recrystallisation annealing, the Dual-Phase steels exhibit major problems in hot-dip galvanising. In the present work, the influence of short-time gas nitriding at recrystallisation temperature in an atmosphere containing reactive nitrogen on the galvaniseability of the Dual-Phase steel DP500 is investigated. The experiments are performed in an annealing and hot-dip simulator. Galvanising is performed in a standard zinc bath containing 0.2 wt.% Al. After nitriding, the surface-near regions of the steel exhibit a nitrogen diffusion zone with a much finer-grained microstructure with higher hardness than the unnitrided reference samples. The density of surface oxides is substantially reduced, and the corresponding samples show a distinct improvement in their galvanising behaviour. In the microstructural investigations with transmission electron microscopy, particular attention is paid to the formation of the Fe 2 Al 5 intermediate layer. After pre-nitriding, the layer becomes denser, and larger plate-like crystals develop at the interface. Fe – Zn phases were found to nucleate on the faceted Fe 2 Al 5 crystals, which also contain a finite amount of Zn in solution. The results will be discussed, and a model for the evolution of the zinc coating will be presented.

Magnetoelectronic devices contain nanoscaled layer stacks. Their structural and chemical characterization is necessary for the control of the deposition process as well as for the understanding of structure formation at different temperatures. The analytical transmission electron microscopy including scanning transmission electron microscopy with high angle annular dark field imaging, energy-dispersive X-ray analysis and electron energy-loss spectrometry meets this challenge. This is demonstrated by results of investigation of multilayers for Giant Magnetoresistance devices and of layer stacks for Tunnel Magnetoresistance units. The results are confirmed by other, more integral methods like X-ray diffraction and reflectometry, Auger electron, and photoelectron spectroscopy.

Cu films were grown on (0001) α -Al 2 O 3 single-crystals by magnetron sputtering. The growth behavior was manipulated by Ar + -ion sputter cleaning of the substrates at kinetic energies between 100 and 500 eV, changing the sputter rate from 0.75 to 1.1 nm/s, and using nominal substrate temperatures of 100 and 200 °C, respectively. Polycrystalline Cu films formed on α -Al 2 O 3 substrates after an Ar + -ion bombardment at 500 eV, while epitaxial Cu films evolved when Ar + -ion energies of 100 and 200 eV were used. The epitaxial Cu films always consisted of two twin-related growth variants. However, two different orientation relationships emerged which differ by a 30° in-plane rotation of the (111) oriented Cu films when the deposition rate is changed from 0.75 to 1.1 nm/s. The results will be discussed on the basis of differences in the growth process.

W- 15wt.% Cu nanocomposite powder samples were fabricated by ball-milling and subsequent hydrogen-reduction. The powder samples were heated to 1200 °C at 5 and 20 °C min –1 and sintered for 1 h to obtain different sintered microstructures. The homogeneity of the sintered microstructures was evaluated by the standard deviation of the Vickers hardness. The W–W contiguity was calculated by using Voronoi diagrams. The sintered microstructure in case of the 20 °C min –1 sample was more homogeneous and had lower W–W contiguity than the 5 °C min –1 sample. The microstructural homogeneity was directly related to the W–W contiguity. Thermal conductivity of the 20 °C min –1 sample was about 5% higher than the thermal conductivity of the 5 °C min –1 sample. This indicates that the thermal conductivity is affected by the W–W contiguity resulting from the homogeneity of the sintered microstructure.

When deposits obtained by physical and chemical vapor deposition, electrodeposition, and electroless-deposition are annealed, they undergo recrystallization or abnormal grain growth to reduce their energy stored during deposition. The driving force for recrystallization is mainly due to dislocations, whereas that for abnormal grain growth is due to grain boundary energy, surface energy, interface energy or strain energy, or their combinations. During recrystallization and abnormal grain growth, a texture change can take place. Models for the evolution of recrystallization and abnormal grain growth textures are discussed.

The corrosion of NiAl alloys containing zirconium in molten carbonate salt at 650 °C was studied as a potential coating material on a stainless steel separator. A mixed layer of NiO, ZrO 2 , and Al 2 O 3 was formed on the surface of the NiAl alloy during preoxidation at 1000 °C in air. The Ni 2 AlZr phase, precipitated along the grain boundaries of NiAl alloys containing zirconium, was the source of the formation of the outermost layer of Al 2 O 3 only on the mixed oxide layer during the oxidation. The Al 2 O 3 layer inhibited the dissolution of NiO on the NiAl – Zr alloy in molten carbonate salt. Therefore, the oxide layer formed on the NiAl – Zr alloy was more corrosion-resistant than the oxide layer on the NiAl alloy. As the amount of zirconium in NiAl – Zr alloy was increased, the corrosion rate greatly decreased due to an increasing amount of Al 2 O 3 at the outermost surface layer oxide. The mixed oxide layer on the NiAl alloy was spalled off the alloy by air cooling after high temperature isothermal oxidation, but the spalling phenomenon of the oxide layer on the NiAl – Zr alloy with more than 0.84 at.% zirconium was greatly reduced by the improvement of the oxide adhesion due to the formation of oxide pegs of Ni 2 AlZr phase along grain boundaries, which were mechanically anchored to the surface oxide layer.

The effects of annealing temperature and time in 5083 Al alloy deformed at cryogenic temperature were investigated and compared with alloys deformed at room temperature. It was found that mechanical properties of deformed and annealed alloys were mainly influenced by the volume fraction of recrystallized grains. The formation of a bimodal microstructure, consisting of equiaxed grains and elongated subgrains, seems to be responsible for the good combination of uniform elongation and tensile strength. Additionally, the small variation of hardness for annealing temperatures of 300 – 400 °C indicates that mechanical properties of 5083 Al alloy deformed and annealed are significantly influenced by the volume fraction of recrystallized grains rather than by the coarsening of recrystallized grains.

The microstructures and formation processes of MoSi 2 – Si 3 N 4 or MoSi 2 – SiC nanocomposite coatings on Mo substrates by ammonia nitridation or carburizing process followed by chemical vapor deposition of Si was investigated using optical microscopy, field-emission scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and X-ray diffraction (XRD). The present study demonstrated that the crack density formed in the MoSi 2 -base nanocomposite coatings due to mismatch in the coefficient of thermal expansion (CTE) between the nanocomposite coatings and the Mo substrate could be reduced by the control of volume percentage of the Si 3 N 4 or SiC particles with the low CTE values. Especially, no cracks were observed in the MoSi 2 -(30– 33) vol.% Si 3 N 4 nanocomposite coating, indicating that its CTE value was close to that of the Mo substrate. The microstructure of MoSi 2 -base nanocomposite coatings showed the oblate-spheroidal-type Si 3 N 4 or SiC particles dispersed in the equiaxed MoSi 2 grains. The average size of equiaxed MoSi 2 grains ranged from 300 to 100 nm depending on the processes, and that of Si 3 N 4 or SiC particles ranged from 150 to 90 nm. The growth of MoSi 2 grains was inhibited by the nanosize Si 3 N 4 or SiC particles located mostly at the grain boundaries of MoSi 2 .

Single-phase bulk Cr 2 AlC, a non-Ti-containing carbide with superior expected properties, was successfully fabricated by in-situ hot pressing/solid-liquid reaction process. The reaction process was investigated using differential scanning calorimetry. The products were characterized using X-ray diffraction and scanning electron microscopy. Four distinguished stages of the reactions from Cr, Al, and C elemental powders were discussed. The obtained X-ray diffraction data are in good agreement with calculated ones and those from JCPDS card No. 29-0017. A new set of X-ray diffraction data comprising reflections, 2 θ , and intensities of Cr 2 AlC is presented. Lattice parameters obtained by Rietveld XRD refinement of Cr 2 AlC are a = 2.858 Å and c = 12.818 Å, respectively. The measured Vickers hardness of the as-synthesized Cr 2 AlC is 5.5 ± 0.4 GPa, which is twice that of Ti 2 AlC. Cr 2 AlC displays excellent oxidation resistance. The parabolic rate constant of Cr 2 AlC was decreased by 3 –4 orders of magnitude compared with those of Ti 3 SiC 2 at 1200 °C.

Two compositions of aluminium alloys Al-1.2 wt.% Cu-0.9 wt.% Ni-0.7 wt.% Si and Al-1.1 wt.% Cu were chosen to produce conventional bearing alloys with about 6 & 20 wt.% of tin respectively and other two alloys with same basic compositions were produced using lead as dispersoid in same amount instead of tin. The alloys were prepared by impeller mixing and bottom discharge chill casting technique. Their density, microstructure, mechanical properties were evaluated at room temperature. Sliding wear characteristics of these alloys under lubrication were studied in detail. Debris and worn surface analysis was also done. Tin and lead decreased tensile and compression strength of aluminium matrix alloys, percentage elongation was increased in both the cases. Strength properties of aluminium-tin alloys are slightly more than the equivalent experimental aluminium-lead alloys. Wear rate of leaded alloys is lower than the aluminium-tin alloys. Leaded aluminium bearing alloys exhibit superior wear resistance with slightly lower mechanical properties, to that of aluminium-tin alloys.

The thermodynamics and kinetics associated with the peri-tectic transition have become greatly discussed topics. A high-temperature confocal scanning laser microscope has been used to observe the growth of γ-austenite along the pro-peritectic δ-ferrite/liquid-Fe boundary and to measure the rate at which this occurs. It was found, for Fe-18 wt.% Co, Fe-19 wt.% Co, and Fe-20 wt.% Co alloys, that the reaction rate increased with increasing undercooling over the temperature range observed and that there was no discernible difference in rate between the three alloys. The growth rate dependence on undercooling was found to be ν = (62.82 Δ T 2 –70.90 Δ T + 149.04) ± 15.61 μm s –1 . This rate compared well with the growth model proposed by Bosze and Trivedi when the uncertainties in available phase diagram information were considered.