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Aviation History
1941
1941 - 2436.PDF
2f>0 FLIGHT OCTOBER I6TH, 1941. SPECIMENS Preparing Aluminium Alloys for Microscopical Examination WHILST the internal crystallinestructure of various alumin-ium alloys has come in for con- siderable scrutiny of recent years, the best results are very largely depend- ent upon the manner in which the specimens are subjected to preliminary preparation In earlier years, micro- scopical examination of practically all metals was undertaken by the one in- vestigator. Thus iron and steel, brass and copper alloys, anti-friction alloys. and aluminium alloys were more or less in the hands of one metallographist, who examined the metal, took photo- micrographs and proffered opinions on the structure and condition which was presented. Probably the greatest fillip was given to this subject at the last war period when the need for special- ists for each particular metal was recognised. Complete staffs were en- gaged on gun-metals, or alloy steels for different armament purposes, who with long and continuous experience of the same subject could detect any variation in the structure at a glance, and what treatment would be required to rectify it. This resulted in all preliminary pre- paration of the specimens being brought up to a high standard, and the same remarks applied to anti- friction alloy investigators, at a later date. Many of the earlier samples of white bearing alloys had their struc- ture impaired by imperfect grinding and polishing, and the soft structure was distorted by too heavy treatment. To-day all specimens of soft metals are ground and polished by hand, and By A. G. AREND even slow-working grinding wheels are not utilised, with the result that no distorted surfaces are produced, and the microscopical examination pro- ceeds without delay. In a similar manner, all etching solutions have been more or less standardised to bring up the structure with the greatest uni- formity and consistency, and saves time in undue experimentation. With the expansion of aluminium alloys, a similar state of affairs has arisen, but it is only of quite recent years that the work of preparation and etching has been more or less stan- dardised. The reason for this is easy to understand, since aluminium can alloy with almost any other metal, and it would be difficult to envisage how many fresh aluminium alloys will be utilised industrially in the future Most aluminium alloys are soft, although not obtaining to the great softness of white bearing metals, but possess the additional possibility of oxidising. Hence the initial grinding requires greater care, whilst the use of water in polishing and cleaning is to be deprecated. Preparing the Specimens Best results are secured by initially working down the specimen on differ- ent grades of emery paper by hand, taking care that perfect dryness is en- sured. This kind of grinding is most satisfactory when the specimen is slung along the surface of the papers (which latter are fixed on wooden blocks) rather than held firmly by the hand and pressed tightly against the polishing surfaces. This might at first sight appear a circuitous system, since direct rubbing will remove scratches, and slight inequalities more rapidly, but it saves time in the long run, as the surface will remain unimpaired. The specimen is slung back and forwards over one paper, and followed on the next paper in a line at right angles to the action on the former, and this method is continued until an un- blemished surface is obtained. Moist- ening the papers with pure petroleum is to be recommended, but too much petroleum can cause the surface to be- come unduly rough. Where the aluminium alloys are not too soft, the usual polishing disc, covered with felt containing '' diamantine '' powder made into an emulsion with methyl alcohol, can be taken advantage of. Where there is any suspicion that this action may be too severe, an ex- amination under the highest power magnification available will reveal if any slight distortion has been intro- duced. In other words, although a magnification of some 200 diameters will suffice for taking photo-micro- graphs to show the structure to best advantage, a glance at the specimen under, say, 2,000 diameters will reveal if the crystal boundaries have been moved from the position which they would occupy with lighter treatment. Only sufficient powder should be used to simply render the felt white, and it must remain thoroughly wet with alcohol throughout. The more polish- ing that is done the finer does the polish become, so that, where time per- Fig. 1. Aluminium alloy etched with sodium hydrate. The outline lacks sharpness. Fig. 2. Aluminium alloy containing silicon, etched with hydrofluoric acid, showing dendritic structure. Fig. 3. Aluminium alloy containing tin and copper, etched with hydrofluoric acid, showing eutectic network. Fig. 4. Magnesium die casting alloy, etched in sulphuric acid, showing mixed crystals in the eutectic.
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