1911 Encyclopædia Britannica/Metallography

From Wikisource
Jump to navigation Jump to search

METALLOGRAPHY.-The examination of metals and alloys by the aid of the microscope has assumed much importance in comparatively recent years, and it might at first be considered to be a natural development of the use of the microscope in determining the constitution of rocks, a study to which the name petrography has been given. It would appear, however, that it is an extension of the study of the structure of meteoric irons. There can be no question that in the main it was originated by Dr H. C. Sorby, who in 1864 gave the British Association an account of his work. Following the work of Sorby came that of Professor A. Martens of Charlottenburg, presenting many features of originality. F. Osmond has obtained results in Connexion with iron and steel which are of the highest interest. A list of the more important papers by these and other workers will be found in the appended bibliography.

Preparation of the Specimen.—Experience alone can enable the operator to determine what portion of a mass of metal or alloy will afford a trustworthy sample of the whole. In studying a series of binary alloys it has been found advantageous in certain cases to obtain one section which will show in a general way the variation in structure from one end of the series to the other. This has been effected by pouring the lighter constituent carefully on the surface of the heavier constituent, and allowing solidification to take place. A section through the culot so obtained will show a gradation in structure from pure metal on one side to pure metal on the other. A thin slice of metal is usually cut by means of a hack-saw driven by mechanism. The thickness of the piece should not be less than 1/4 in. and in order that it may be firmly held between the fingers it should not be less than 1 in. square. The preliminary stages of polishing are effected by emery paper placed preferably on wooden disks capable of being revolved at a high rate of speed. The finest grade of emery paper that can be obtained is used towards the end of the operation. Before use the finer papers should be rubbed with a hard steel surface to remove any coarse particles. The completion of the operation of polishing is generally effected on wet cloth or parchment covered with a small amount of carefully washed jeweller’s rouge. Various mechanical appliances are employed to minimize the labour and time required for the polishing. These usually consist of a series of interchangeable revolving disks, each of which is covered with emery paper, cloth or parchment, according to the particular stage of polishing for which it is required. In the case of brittle alloys and of alloys having a very soft constituent, which during polishing tends to spread over and obliterate the harder constituents, polishing is in many cases altogether avoided by casting the alloy on the surface of glass or mica. In this way, with a little care, a perfect surface is obtained, and it is only necessary to develop the structure by suitable etching. In adopting this method, however, instances have occurred in which the removal of the cast surface has shown a structure differing considerably from the original.

Polishing in Bas-Relief.—If the polishing be completed with fine rouge on a sheet of wet parchment, placed upon a comparatively soft base such as apiece of deal, certain soft constituents of an alloy may often be eroded in such a manner as to leave the hardest portions in relief. For the later stages of polishing H. L. Le Chatelier recommends the use of alumina obtained by the calcination of ammonium alum; and for the final polish of soft metals, chromium oxide.

Although in some cases a pattern becomes visible after polishing, yet more frequently a mirror-like surface is produced in which no pattern can be detected, or if there is a pattern it is blurred, as if seen through a veil or mist. This is due to a thin layer of metal which.has been dragged, or smeared, uniformly over the whole surface by the friction of the polishing process. Such a surface layer is formed in all cases of polishing, and the peculiar lustre of burnished silver or steel is probably due to this layer. But to the metal log rap hist it is an inconvenience, as it conceals scratches left by imperfect polishing, and also hides the pattern. It is therefore desirable to conduct the polishing so as to make this layer as thin as possible: it is claimed for alumina that it can be so used as to produce a much thinner surface layer than that due to the employment of rouge. The surface layer is very readily removed by appropriate liquid reagents, and, the true surface of the metal having been laid bare, the etching reagent acts differently on the individual substances in the alloy and the pattern can thus be emphasized to any required extent. Osmond divides etching reagents into three classes-acids, halogens and salts. As regards acids, water containing from 2 to 10% of hydrochromic acid is useful. It is made by mixing 10 grams of potassium bichromate with 10 grams of sulphuric acid in 100 grams of water. The use of nitric acid-requires much experience. It is frequently employed in the examination of steels, Sir W. C. Roberts-Austen preferred a 1% solution in alcohol, but many workers use concentrated acid, and effect the etching by allowing a stream of water to dilute the film of acid left on the surface of the specimen after dipping it. Of the halogens, iodine is the most useful. A solution in alcohol is applied, so that a single drop covers half a square inch of surface. The specimen is then washed with alcohol, and dried with a piece of fine linen or chamois leather. Tincture of iodine also affords a means of identifying lead in certain alloys by the formation of a yellow iodide of lead, while the vapour of iodine has in certain cases been used to tint the constituents. Thin coloured films may often be produced by the oxidation of the specimen when heated in air. This, as a means of developing the structure, in the case of the copper alloys is specially useful. Tinted crystals may thus be distinguished from the investing layer caused by the presence of a minute quantity of another constituent. The temper colours produced by heating iron or steel in air are well known. Carbide of iron is less oxidizable than the iron with which it is intimately associated, and it assumes a brown tint, while the iron has reached the blue stage. These coloured films may be fixed by covering with thin films of gelatine.

In some cases the alloy may be attacked electrolytically by exposing it for a few minutes to a weak electric current in a bath of very dilute sulphuric acid. Certain organic bodies give very satisfactory results. The Japanese, for instance, produce most remarkable effects by simple reagents of which an infusion of certain forms of grass is a not unimportant constituent. In the case of iron and steel a freshly prepared infusion of liquorice root has been found to be most useful for colouring certain constituents of steel. Osmond, who was the first to use this reagent, insisted that it should be freshly prepared and always used under identical conditions as regards age and concentration. His method of applying it was to rub the specimen on parchment moistened with it, but he has subsequently modified this “polish attack” by substituting a 2% solution of ammonium nitrate for the liquorice infusion. In each case a small quantity of freshly precipitated calcium sulphate is used on the parchment to assist the polishing.

Micrographic Apparatus

Appliances used in Micrography.—The method of using the microscope in connexion with a camera for photographic purposes will now be considered. Every micrographer has his own views as to the form of an installation to be adopted, and it will therefore be well to give an illustration of a definite apparatus which has been found to give satisfactory results. It consists of a microscope A with a firm base placed in a horizontal position. The microscope can be connected by a tube; B with the expanded camera CC, at the end of which is the usual frame to receive the photographic plate. A practised observer can focus on a plate of clear glass by the aid of a subsidiary low-power microscope lens. If a semi-transparent plate is employed it should be as fine as possible. The surface of the table is cut in such a way near H that the observer who is seated may conveniently examine the object on the stage of the microscope, the portion B turning aside for this purpose. The subsequent focusing is effected by a rod, FFF, and gearing attached to the fine adjustment of the microscope, GA; flap J when raised forms the support of the lamp used for illumination. As an illuminant an arc light has many advantages, as the exposure of the plate used will seldom exceed 10 seconds. The filament of a Nernst lamp can be used as the source of light; though not so brilliant as the arc it possesses the great advantage of perfect immobility. For the best results, especially with high powers, the source of light must be small, so that its image can be focussed on to the surface of the object; this advantage is possessed by both of these illuminant's. Next in value comes the acetylene iiame, and an incandescent lamp or a gas lamp with a mantle will give good results, but with much longer exposure. Actual illumination is best effected by a Beck vertical illuminator or a Zeiss prism. It is necessary that the lens used for concentrating the light on the illuminator should be an achromatic one, as colour effects cause trouble in photographing the objects. For lower powers the Lieberkuhn parabolic illuminator is useful. Certain groups of alloys show better under oblique illumination, which may be effected by the aid of a good condensing lens, the angle of incidence being limited by the distance of the object from the objective in the case of high magnification. As regards objectives, the most useful are the Zeiss 2 mm., 4 mm. and 24 mm.; two other useful objectives for low powers being 35 mm. and 70 mm., both of which are projecting objectives. A projecting eye-piece, preferably of low power, should be employed with all but the two latter objecti ves. The immersion lens, the Zeiss 2 mm., is used with specially thickened cedar oil, and if the distance from the objective to the plate is 7 feet, magnifications of over 2000 diameters can easily be obtained. As regards sensitized plates, excellent results have been obtained with Lumiére plates sensitive to yellow and green. The various brands of “ process ” plates are very serviceable where the contrasts on the specimen are not great. Some reproductions of photo-micrographs of metals and alloys will be found in the plate accompanying the article Alloys.

Authorities.—H. C. Sorby, “ On Microscopical Photographs of Various Kinds of Iron and Steel," Brit. Assoc. Report (1864), pt. ii. p. 189; “ Microscopical Structure of Iron and Steel," Journ. Iron and Steel Inst. (1887), p. 255; A. Martens, “ Die mikroskopische Untersuchung der Metalle," Glaser's Annalen (1892), xxx. 201; H. Wedding, “Das Gefüge der Schienenköpfe," Stahl und Eisen (May 15, 1892), xii. 478; F. Osmond, “ Sur la metallographie microscopique,” Rapport présenté à la commission des méthodes d’essai des matériaux de construction le 10 février 1892; et ii. 7–17 (Paris, 1895); “ Microscopic Metallography," Trans. Amer. Inst. Mining Eng. xxii. 243; J. E. Stead, “Methods of preparing Specimens for Microscopic Examination," Journ. Iron and Steel Inst. (1894), pt. i. p.292 W. C. Roberts-Austen and F. Osmond, “ On the Structure of Metals, its Origin and Causes, ” Phil. Trans. Roy. Soc. clxxxvii. 417-432; and Bull. de la Soc. d’encouragement pour l’industrie nationale, 5° série, i. 1136 (Aout 1896); G. Charpy, “Microscopic Study of Metallic Alloys,” Bull. de la soc. encouragement pour l’industrie nationale (March, 1897); A. Sauveur, “Constitution of Steel,” Technology Quarterly (June, 1898); Metallographist, vol. i. No. 3; “ Metallography applied to Foundry Work, ” The Iron and Steel Magazine, vol. ix. No. 6, and vol. x. No. 1; J. E. Stead, “Crystalline Structure of Iron and Steel,” Journ. Iron and Steel Inst. (1898), i. 145; “Practical Metallography,” Proc. Cleveland Inst. of Engineers (Feb. 26, 1900); Ewing and Rosenhain, “ Crystalline Structure of Metals,” Phil. Trans. Roy. Soc. cxciii. 353 and cxcv. 279; F. Osmond, “ Crystallography of Iron,” Annales des Mines (January 1900); Le Chatelier, “ Technology of Metallography," Metallographist, vol. iv. No. 1; Contribution à l'étude des alliages. Société d'encouragement pour l’industrie nationale (1901); Smeaton, “Notes on the Etching of Steel Sections,” Iron and Steel Magazine, vol. ix. No. 3.  (W. C. R.-A.; F. H. Ne.)