M.J. Whiting

An experimental study of bonding and crystal structure modifications in MoSi2 and MoSi2+xAl (x=10 to 40 at% Al) via Auger parameter shifts and charge transfer calculations

Abstract The alloying behaviours of as-cast MoSi2 and MoSi2+xAl alloys have been studied using high-energy XPS with Cr Kβ radiation. The charge transfer occurring upon alloying was calculated using the variations in the Auger parameters of Mo, Si and Al between alloyed and unalloyed conditions and the linear potential model of Thomas and Weightman and the non-linear potential model of Cole, Gregory and Weightman. In MoSi2 there was a significant increase in the Auger parameter of Si, while the shift in the Auger parameter of Mo was negligible. The charge transfer towards the Si atoms was close to zero and is smaller compared to theoretical calculations. It is concluded that the atomic bonding between Mo and Si is of a covalent p–d character. In MoSi2+xAl alloys, similar observations were made for Mo and Si, while the Auger parameter of Al was reduced. Donation of electronic charge by Al atoms is possible; covalent bonds of Al with Mo are formed. The plasmon loss structures of the Si 1s and Al 1s peaks showed reduced intensity in the alloys relative to the pure metals. This was attributed to more strongly bound valence electrons. The opposite was the case for the Mo 2p3/2 peak. The substitution of Si by Al atoms is confirmed, in agreement with previous studies.

Preliminary results on the chemical characterisation of the cathode nickel: emissive layer interface in oxide cathodes

Abstract In cathode ray tube (CRT) thermionic oxide cathodes, the nickel-oxide interface properties are key to understanding the mechanisms of operation. At the elevated operational temperatures, free barium is formed at the interface by the reaction of reducing activators, from the nickel alloy, with barium oxide. The free barium diffuses to the outer surface of the oxide providing a low work function electron-emitting surface. However, during cathode life an interface layer grows between the nickel alloy and oxide, comprised of reaction products. The interfacial layer sets limits on the cathode performance and useful operational lifetime by inhibiting the barium reducing reaction. This paper discusses sample preparation procedures for exposure of the interface and the use of several surface and bulk analytical techniques to study interface layer formation. SEM, AES and SIMS data are presented, which provide preliminary insight into the mechanisms operating during the cathode’s lifetime. There is evidence that the activator elements in the nickel alloy base, Al and Mg, are able to diffuse to the surface of the oxide during activation and ageing and that these elements are enriched at the interface after accelerated life.

Morphological evolution of lamellar structures: The CuAl eutectoid

Abstract The growth and morphological evolution of the CuAl eutectoid were investigated. CuAl pearlite is shown to exhibit direction steps at the interlamellar heterophase boundary which maintain a (111) x |(110) γ ) habit plane. The CuAl pearlite is also demonstrated to possess a complex three-dimensional morphology which explains the variety of structures observed in two dimensional sections. Termination migration and conventional coarsening are the dominant mechanisms of morphological evolution in the temperature range 788–828 K. A new termination generation mechanism and a facetting phenomenon make minor contributions to shape change. Choice of the operative mechanism(s) in a given grown lamellar structure (e.g. eutectoid or eutectic) is affected by both crystallography and interfacial structure.

The ledge mechanism of pearlite growth: Some thoughts on the solution to the kinetic problem

The temperature dependence of the spacing of pearlite lamellae has been of interest for a long time. The linear relationship between the reciprocal of interlamellar spacing and the undercooling during growth is known to hold for the whole spectrum of ferrous pearlites and in those non-ferrous systems investigated. Many workers have attempted to rationalize this relationship over more than fifty years, while also predicting the growth velocity. The problem is to find a unique description of both pearlite spacing and growth rate for a given undercooling. For the purpose of this discussion, the approaches have been broadly divided into three groups. The authors shall discuss why each of these approaches is phenomenologically inaccurate, in the light of recent work on pearlite growth. A possible route to a more accurate description which involves ledge migration will be outline. Support for the ledge mechanism will be given by the agreement of available growth velocity data for the Cu-Al eutectoid with the predictions of approaches based on ledge migration.

A study of the combustion synthesis of MoSi2 and MoSi2-matrix composites

The combustion synthesis of MoSi2 from elemental powders was investigated. Higher completion of the reaction was achieved by the use of prior compaction of powders and higher heating rates. The reaction mechanism was studied by performing experiments at a heating rate of 20 K/min. It was established that reaction occurred via capillary motion accompanying the flow of molten silicon to the molybdenum powder particles. During the synthesis of MoSi2/TiC composites, two ternary (Mo,Ti) Si2 phases were observed, in contradiction to the thermodynamic predictions of the Mo-Si-Ti-C system. Combustion synthesis of (Mo, W) Si2 yielded an inhomogeneous product where the composition of the solid solution varied throughout the sample. These observations were discussed in terms of the characteristics of the reaction process.

ON THE LEDGE MECHANISM OF PEARLITE GROWTH - THE CU-AL LAMELLAR EUTECTOID

Recently the classical views of pearlite growth were challenged by a series of studies of the three heterophase boundaries present in growing lamellar eutectoids. The ferrite/cementite boundary was found to contain linear defects (direction steps), which maintained a micro-habit plane between the pearlitic phases, in both Fe-C and Fe-C-Mn steels. Observations of the ferrite/austenite and cementite/austenite growth interfaces revealed the presence of facets and growth ledges. From these observations Hackney and Shiflet proposed that pearlite grows from the parent austenite by a co-operative ledge mechanism. The motion of ledges on the growth interface determines the position of the triple points and hence the introduction of direction steps into the inter-lamellar boundary. Further observations have indirectly supported this ledge mechanism of pearlite growth. Direction steps have also been observed at the inter-lamellar boundaries in Cu-Be and Fe-Ti pearlites. This work aims to show that features of the lamellar eutectoid ([beta] [r arrow] [alpha] + [gamma]) in a Cu-11.95 wt%Al alloy can be readily rationalized in terms of a co-operative ledge mechanism.

THE LEDGE MECHANISM OF PEARLITE GROWTH

The nature of the pearlite phase transformation has received much attention over the last decade. Most notably a body of work has presented observations that have been rationalized in terms of the operation of a ledge mechanism during pearlite growth and of a greater role for crystallography than previously accepted. Arguments have been presented against this interpretation of the observations reported recently and presentations of well known data have been made in defense of the classic view of pearlite growth. Other studies have ignored the new ideas. This short not attempts to clarify some points about the ledge mechanisms of pearlite growth.

FINITE ELEMENT ANALYSIS AS A TOOL FOR CALCULATING THE THERMAL CONTACT CONDUCTANCE FOR A MACHINED SURFACE

Heat transfer between contacting surfaces is a key factor in the thermal behaviour of engineering components in turbomachinery and various other areas of technology. Thermal contact conductance (TCC) is a parameter that quantifies this heat flow. An ongoing challenge in measuring and modelling TCC is the different length scales of surface topography exhibited on real components. Manufacturing techniques such as turning and fly cutting introduce repeatable surface deviations of medium wavelength which are the focus of this study.An instrumented split tube with in-line washers, loaded under carefully controlled conditions, was used to measure the TCC of washers made of PE16. Fly cutting was used in this study to introduce a repeatable lay typifying a range of manufacturing techniques. Experimental determination of TCC for (i) flat contacting washers and (ii) contact between flat washers and fly cut washers with repeatable lay is reported. As expected the latter case gives rise to significantly lower TCC than the former. A 2D finite element model is described which models the elastic-plastic deformation of a representative machining induced contact line. Using the TCC data for flat contacting surfaces, FEA is used to calculate the reduced TCC for the machined case. Predicted values of TCC assuming plane stress and plane strain are compared with experimental data.Copyright

FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings

The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W2C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape.

An Improved Silicon Carbide Monofilament for the Reinforcement of Metal Matrix Composites

As part of ongoing research in the UK, TISICS have developed an improved 140 µm carbon coated silicon carbide monofilament for the reinforcement of metal matrix composites. The monofilament is fabricated in a single reactor using a high speed chemical vapor deposition process at a rate of 8 m/min (26 ft/min). Statistical analysis of monofilament properties over two years of production has demonstrated excellent reproducibility of the process. The monofilaments have an average tensile strength of 4.0 ± 0.2 GPa with a Weibull modulus of 50 ± 10. Composites incorporating the monofilaments show similar low variability in yield and tensile strength with the latter exhibiting a mean value above 90% of the maximum theoretical strength predicted by the rule of mixtures. By varying the volume fraction and orientation of the monofilament reinforcement, composite properties can be tailored to fit design requirements. Examples are given of demonstrator components made for the European aerospace sector.