Grain boundary cracking during directional solidification of columnar grained nickel base superalloys has been investigated. A quantitative test has been developed that allows a classification of the castability of directionally solidified nickel base superalloys: CM247L C showed excellent castability, while IN792 showed severe cracking. T he cracking was found to originate at a temperature near the solidus temperature (hot tearing). A number of experimental alloys based on the composition of IN792 were tested, with the eVects of variations in hafnium, aluminium/titanium, carbon, boron and zirconium contents being examined. A combination of 0%B and 0•01%Zr shows significant promise. A number of approaches were applied to explain the cracking susceptibility. A relationship was found between a small nil ductility region, obtained by Gleeble testing, and good alloy castability for IN792 and CM247L C. A correlation between the eutectic content of the alloy and its cracking tendency was also found. At low (<5%) and high (>22%) amounts of eutectic, a significant reduction in cracking was apparent, while with intermediate amounts (15%), severe cracking was observed.MST /3958

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Before looking at solutions, it is necessary to first look at the features which impose special requirements.The aircraft operating environment is unique, since for much of its time the aircraft structure operates high above the Earth's surface.A typical inter-continental airliner travels at altitudes of 35,000ft (10,700m), and at an air speed of 564mph (900kph). Normal outside temperatures in these conditions is -72ºF (-58ºC). Temperature and air pressureThere are vast temperature variations on both the aircraft structure and on the coating. Also at high altitude the air pressure is considerably reduced which can make any residual liquid in the coating much more volatile, as well as changing the dimensions of the aircraft structure. Flexing and adhesionThe effect of temperature in itself is probably nothing to worry about. However, in combination with the movement of the metal or composite substrate that makes up the airframe, the paint needs to be both flexible and adhesive. It needs to retain this flexibility throughout its life, even when it is likely to be changed by the effect of UV degradation or loss of volatile coating components at lowpressures. It must also cope with a host of general ageing effects.Adhesion in combination with flexibility is all important. It is only possible for the paint coating to function, either for protection or for decoration, if it can continue to adhere to the substrate. Special effort goes into formulating primers with very good adhesion and topcoats with very good flexibility. The adhesion of a primer is not only a combination of the intrinsic adhesive properties of the coating but the pre-treatment of the substrate as well. CondensationAnother aspect related to temperature is that of condensation. When an aircraft travels subsonic at altitude the structure becomes very cold. When it lands, it enters relatively humid air resulting in extreme condensation which can collect inside the unpressurised or unheated areas of the aircraft structure. This condensation, being initially pure water, 451

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