Browsing by Author "Kennedy, A. J."

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    Dislocations and twinning in graphite
    (College of Aeronautics, 1960-04) Kennedy, A. J.
    The twin composition plane in graphite is a 20o tilt boundary between lattices which are rotated, relatively, about an axis In the basal plane. Previous work has led to the proposition that some special type of structure must necessarily exist in the neighbourhood of the boundary which violates normal hexagon arrangement of the carbon atoms. It is demonstrated that tilt boundary of the required form can be explained as an array of partial dislocations, such a boundary being possible in either the hexagonal or the rhombohedral form. A boundary of this type is mobile, and can, by its movement, introduce or eliminate stacking faults and thus change the volume rhombohedral graphite present in the normal hexagonal lattice. Such effects have been reported previously. The true twinning plane in this model is not the composition plane, which is the plane {1101} referred to the structural (not the morphological) axes, but the plane {1121}
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    Graphite as a structural material in conditions of high thermal flux: a survey of existing knowledge and an assessment of current research and development
    (College of Aeronautics, 1959-11) Kennedy, A. J.
    The state of fundamental knowledge on the subject of graphite and the graphitisation process is reviewed. The principle methods of manufacture may be considered to include (1) conventional graphitisation of a coke filler-binder mix, (2) the compaction at high pressure and temperatures of natural or artificial graphite particles without a binder, (3) pyrolytic graphites derived from gaseous deposition, and (4) conventional graphites impregnated by liquid or gas and re-graphitised. The present state of development of these processes is examined. The erosion of graphite by high velocity gases at high temperatures is due primarily to oxidation effects which occur preferentially at crystallite boundaries. Coatings of carbides and nitrides improve the resistance at temperatures below about 1700 degrees C, but above this, pyrolytic coatings are more successful. The addition of vapourising compounds, iodides and fluorides, or the addition of carbides and nitrides to the graphite mix, are both beneficial, but of little value at very high temperatures. The development of new graphites, either the impregnated type, or those produced by pressure baking, may offer a margin of improvement, as the best surface structure at temperatures of 3000 degrees C and above appears to be simply graphite. Additions may do little to improve the mechanism of erosion, but they may usefully lower the surface temperature. Considerations relating to thermal shock, creep and fabrication are surveyed. Some of the conclusions are: that graphite is of singular importance to high temperature technology; that commercial issues cannot be allowed to impede vigorous development towards more resistant forms; that much is to be gained by viewing graphite from a metals standpoint; that the fundamental theory of the basic crystal mechanics is undeveloped; that the present wide variability in properties should not be regarded overseriously; that non-destructive assessment by damping measurements needs development, that coatings and impregnants are of high priority, and that, of all factors, oxidation is the most serious limitation to use at the present time.
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    Temperature effects on material characteristics
    (College of Aeronautics, 1960-08) Murphy, A. J.; Kennedy, A. J.
    Some of the physical properties of the main elements of interest in high temperature technology are reviewed. Some general trends emerge when these properties are viewed as a function of melting point, but there are a few notable exceptions. Titanium, zirconium, niobium and tantalum all have disappointingly low moduli; chromium is excellent in many ways, but has a limited ductility at lower temperatures; molybdenum oxidises catastrophically above about 700° C, and niobium suffers from severe oxygen embrittlement. Beryllium and carbon (in the graphitic form) both stand out as exceptional materials, both have very low densities, beryllium a very high modulus but an unfortunately low ductility, while graphite has a relatively low strength at the lower temperatures, although at temperatures of 2000° C and above it emerges as a quite exceptional (and probably as the ultimate) high temperature material. Some of the fundamental factors involved in high temperature material development are examined, in the light, particularly, of past progress with the nickel alloys. If a similar progress can be achieved with other base elements then a considerable margin still remains to be exploited. Protection from oxidation at high temperatures is evidently a factor of major concern, not only with metals, but with graphite also. Successful coatings are therefore of high importance and the questions they raise, such as bonding, differential thermal expansion, and so on, represent aspects of an even wider class covered by the term “composite structures". Such structures appear to offer the only serious solution to many high temperature requirements, and their design, construction and utilization has created a whole series of new exercises in materials assessment. Matters have become so complex, that a very radical and fundamental reassessment is required if we are to change, in any very significant way, the wasteful and ad hoc methods which characterise so much of present-day materials engineering.