METALLOGRAPHY. A polished piece of metal presents an appearance of complete homogeneity which does not suggest the existence of a complex internal structure. When the metal is broken, however, the fracture indicates a structure or texture capable of microscopic examination. This fruitful method, which forms the foundation of the science of Metallography, was first applied to metals by H. C. Sorby of Sheffield in 1861. It was subsequently developed in Germany by Martens, in France by Osmond and in England by Roberts-Austen and many others.
The microscopic examination of metals cannot well be made by the use of sections thin enough to be transparent. It is therefore made by looking at a prepared surface of the specimen, and not through it. Careful preparation of the surface eliminates all accidental markings and scratches. An ordinary "polished" metal surface is much too rough for the purpose. Metallographic surfaces are prepared by rubbing a flat specimen on successively finer grades of special emery papers, the scratches made by one grade being replaced by finer ones made by the next grade. The finest scratches are removed by a revolving disk covered with soft cloth or leather and fed with water and extremely fine polishing material, such as specially washed alumina or magnesia powder.
The featureless mirror, so produced, is covered with a very thin surface layer of "flowed" metal which has been smeared or dragged over the surface during the polishing process. This surface is lightly attacked by an "etching reagent" For many metals a weak solution of nitric acid in alcohol, or of picric acid in alcohol, serves as a useful etching reagent, but a great variety of solutions has been developed. Lists of these are to be found in the text books, etc.
actual "colouring" of the surface by the formation of a coloured film or from a roughening of the surface by partial solution—i.e., the partial "unbuilding" of the structure at certain points.
First we have the simple aggregate of crystals which we find in pure metals and in those alloys which consist of a single solid solution. A typical example is shown in fig. 1, Plate I. The structure, seen under "normal" lighting, presents the appearance of an irregular tesselated pavement ; each of the roughly polygonal fields into which the surface is divided is the section of a crystal of metal, the mutual boundaries of these crystals being deter mined, however, not by their geometrical shapes or properties but by the manner in which adjacent crystals have met. Within their boundaries, however, these crystals possess the typical geometrical regularity of true crystals. Another structure often found in alloys is the finely laminated duplex structure, illus trated in fig. 2, Plate 1., which is typical of what are known as "eutectic" alloys. These are the alloys of lowest melting point which occur in certain series of the alloys of two metals. Many alloys consist of simple crystals of the type illustrated in fig. i, Plate 1, embedded in a matrix of the laminated type. An example is shown in fig. 3, Plate i. There are many other types of struc ture, such as those which occur when a well-defined chemical compound of two metals occurs in the alloy. An example is given in fig. 4, Plate 1. Further types of micro-structure are found in alloys which undergo changes— as many alloys do—after they have become completely solid. An example of the complex structures which are sometimes produced in this way is shown in fig. 5, Plate I. Other examples will be found in reference to ALLOYS and in connection with the articles dealing with particular metals and alloys, such as STEEL, COPPER, BRASS, BRONZE, ALUMINIUM and others.