METALLOGRAPHY

metallography is the study of ihe internal structure of metals and alloys, and of its relation to their composition and their physical and mechanical properties. beginning with the use of the microscope for the examination of polished and etched surfaces of metals by sorby in 1864, the scope of metallography has been widened to include other meth- ods for the examination of the internal constitution of solids, such as the measurement of changes of volume and of heat con- tent, and of electrical resistance, magnetic quality or behaviour towards x-rays. the properties of metals and alloys, like those of other solids, depend partly on their chemical composition and partly on the internal arrangement of their constituents. it is possible for two specimens of metal, taken from the same homogencous mass and identical in chemical composition, to have widely differing properties if they have been subjected to different thermal or mechanical treatments. since this important fact has become generally recognised, other methods of examination must supple- ment chemical analysis in the control of materials for engineering and similar purposes, and in recent years there has been a great development of metallographic methods, together with a greatly increased application of them jn industry, so that the microscope has become as indispensable as the balance in metallurgical works, whilst dilatometers, magnetic and electrical measuring instruments, and accurate pyrometers are becoming more and more frequent items of the equipment of a laboratory for the control of materials by manufacturers or users. preparation of specitmens.—the improved microscopical tech- nique of the present day, and the use of high magnifications, make it necessary to prepare the surfaces of specimens with a very perfect polish. polishing by hand on fine emery papers is preferred to the use of a machine, except in the last stage, when a rotating disc covered with cloth is used, the finest polish being given by levigated magnesia, although for low magnifications alumina is quite satisfactory. steels are best etched by a 4% solution of picric acid in alcohol to which a small quantity of nitric acid may be added to bring out the boundaries of the crystal grains. stainless steel and other highly alloyed steels require special reagents. many tables are now published giving the most suitable reagents for the ctching of the commonly occurring metals and alloys. much light has been thrown on questions of metallurgical importance by the use of high magni- fications, and progress is being made in this direction. it is im- portant to observe the conditions of accurate microscopy by making a sultable choice of objective and eyepicce for the work. mere magnification without higher resolution gives no informa- tion of value. the internal structure of martensite in hardened steel is a severe test of the resolving power of a microscope, and has been studied in detail with some success. the inverted type of microscope, in which the specimen is placed with its polished face downwards on the stage, has many advantages in conven- ience and rapid working, and has become popular in metallo- graphic laboratories, in spite of certain optical disadvantages due to the larger number of reflections. macroscopic kitching —in another direction, the examination of etched specimens with little or no magnification has proved of great practical value. for this macroscopic etching the sur- face to be examined is roughly polished and then deeply etched, acid solutions of copper salts being mostly used for steels, the copper which is deposited at first being completely removed by washing. segregation is revealed by differences of etching, a very distinct pattern being produced, which may be recorded by 878 photography or by the method employed long ago by sorby in making “‘ nature prints,” and temporarily forgotten, of using the etched surface as a printing block, rubbing printer’s ink into the depressions, and transferring to paper by pressure. macroscopic etching has proved of value in the detection of defects in ingots and castings, and also in following out the treat- ment of a metal during such processes as forging, the direction of the flow during working bcing shown by the curving of the lines representing small differences in composition in the original mass. this method is employed in some works as a regular means of control in the forge. strain etching —in 1921 a method of etching was described by a. fry, having as its object the detection of plastic strain in mild steel. the steel to be examined is polished, care being taken to avoid any distortion, and is then annealed for a short time at about 200°c., after which it is etched in an acid solution of cupric chloride. clearly defined dark bands make their appearance where the crystals have been deformed, and much information as to the distribution of plastic movement is obtained from a study of the figures. rather strangely, whilst some mild steels give well defined patterns with ease, others of almost identical composition give no result, and it has not been possible to trace any connection between the composition and the degree of suc- cess of the etching. the method has therefore so far a limited value, but it is almost certainly capable of further development. on a microscopic scale, the mechanism of deformation has been studied by many investigators. metals and alloys vary greatly in the degree of apparent confusion in the crystalline structure produced by deformation. cupro-nickel is remarkable for its property of undergoing severe cold-working and continuing to give perfectly sharp and definite structures when etched, and this alloy, together with pure aluminium and an alloy consisting of tin to which 1-5°%% of antimony has been added, have been largely used in the study of the cold-working of metals and of the microscopic changes which occur on subsequent annealing. plastic deformations.—the nature of plastic deformation be- ing of such great importance to the engineer, this part of the subject has attracted much attention. a mass of metal, such as a iorging, consists of an immense number of crystalline grains, the axes of which are directed at random, so that for practical purposes the metal is commonly regarded as isotropic, the indi- vidual differences being lost in the aggregate. at the bounding surfaces of the grains there is a discontinuity of properties, the origin of which is a subject of controversy. the failure of metals by fatigue (see fatigue of metals) under rapidly alternating stresses, cannot be explained by any assumption of isotropic character, and it becomes necessary to take into account the stresses in the individual grains. the problem is simplified by the comparatively recent discovery of methods for the prepara- tion of single metallic crystals of such large size that they may be tested in an ordinary testing machine, the microscopic and other changes being observed. aluminium has been studied in detail in this way by carpenter and elam; tungsten (in the form of wire) by goucher and zinc, tin and bismuth by polanyi and weissenberg. in these instances the direction of slip and its relation to the crystalline structure have been determined. the influence of the crystal boundary is studied by using test pieces composed of two crystal grains, a boundary crossing the specimen. beyond the fact that the boundary offers an obstacle to slip, little has yet been established in regard to this factor, although the hypothesis of a layer of amorphous material be- tween the grains has been applied with great ingenuity and suc- cess by rosenhain to explain many of the facts of deformation and also of chemical attack on metals. the low tensile strength of metals, as compared with the theoretical cohesion calculated by indirect means, has led to the suggestion by a. a. griffith that all ordinary solids contain innumcrable minute flaws, and this hypothesis has attracted the attention of mathematical physicists, who have applied it with some success. by special devices, vitreous silica and glass have been prepared in an un- stable condition in which the strength is of the order of the theo- retical cohesion, but this has not been attained with metals. metallography x-ray analysis —some departments of metallography have been transformed by the introduction of the methods of x-ray (see x-rays) analysis. the use of x-rays for the penctration of large masses of metal for the detection of blowholes and other defects 1s obvious, and whilst useful in practice has no theoretical significance; but the refined methods of crystal analysis due to laue and to bragg are far-reaching in their effects. the arrange- ments of the atoms in a space lattice (see crystallography) and the absolute dimensions of that lattice have now been deter- mined for most of the important metals, and many interesting results have been obtained, amongst others the fact, established by westgren, that a and @ iron below 900°c. and 6 iron above 1,410°c. have the same space lattice, a body-centred cube, whilst ¥y iron between goo° c. and 1,410° c. has a face-centred cubic lat- tice, and this conclusion is in harmony with the physical proper- ties of these modifications of iron. the same method provides an accurate determination of the direction of slip in single crys- tals during straining, whilst the examination of drawn wires and of rolled sheets has shown that there is a tilting of the space lattices of the individual crystal grains, leading to a more or less parallel direction of one of the crystal axes throughout, this “tibre structure ”’ being similar to that which the x-rays also show to be present in natural fibres. studics of this kind are throwing much light on the question of the mechanical strength of materials. thermal analysis ——-the methods of thermal analysis have undergone comparatively little change. to ensure a uniform rate of cooling, the specimen of metal may be suspended in a furnace in which a steady temperature gradient has been estab- lished by winding a metal tube with resistance wire suitably insu- lated, and making the windings closer at the upper part. the specimen is then lowered through the furnace at a uniform rate by a mechanical clevice. it has been shown that the specimen should have a spherical or pear-shaped form in order to obtain sharply defined points on the heating and cooling curves. ther- mo-couples are almost invariably used for the measurement of temperature, although the platinum resistance thermometer, used in the early and extremely accurate work of heycock and neville, has great advantages. japanese workers, investigating the alloys of iron, have made much use of magnetic determina- tions as a means of fixing temperatures of transformation, the curves having much the same form as those derived from ther- mal observations; whilst other investigators have used the changes in electrical resistance with temperature for the same purpose. several types of dilatometer have also been devised, in which the changes of length of a rod of the metal or alloy dur- ing heating and cooling are measured directly or by comparison with a standard or with a rod of vitreous silica with negligible expansion. where the change of length is indicated by the tilt- ing of a mirror, this method is susceptible of very great accuracy. it has proved of value in the study of hardened steels, and is now making its way from the research laboratory into technical practice. practical a pplications—the higher standards of strength and endurance demanded of metallurgical products by the modern motor and aeroplane industries (see aero-engines) and by such other branches of engineering as the manufacture of steam turbines (q¢.v.), have been responsible for the interest now being taken by engineers as well as by metallurgists in the study of metallography. alloy steels, and many of the higher qualities of plain carbon stecls and of non-ferrous alloys, have to be “ heat- treated ’’ to develop their maximum toughness or other desirable properties. to determine whether the treatment has been a correct one the methods of the metallographic laboratory must be applied. moreover, the engincer is concerned with such prob- lems as that of fatigue (see fatigue of metaatls), and with the general nature of mechanical deformation beyond the limits to which the theory of elasticity 1s applicable. ; the methods devised for the study of metallic alloys have proved to be of value in the scientific examination of other solids. whilst the original idea of the examination of metals by means of the microscope occurred to sorby in consequence of his earlier metallurgy introduction of microscopical petrography, the technique of metallography has been applied to opaque minerals, and has made possible the detailed study of mineral veins and ore de- posits. the work of the geophysical laboratory at washington on the equilibrium in rock magmas (see petrology), with its methods of accurate measurement at high temperatures, is also based to a great extent on experience gained in the study of metallic systems. binliography.—the most comprehensive treatise is that of w. guertler, metallographie, berlin, in course of publication since 1909, and extending to many volumes... other text-books are: l. guillet and a. portevin, an introduction to the study of metallography and macrography (eng. trans., 1922); e. heym, physical metallography (eng. trans., 1925); g. tammann, 4 text- book of metallography (eng. trans., new york, 1925); c. h. desch, metallography (1922). on plastic deformation, z. jeffrics and r. s. archer, the sctence of afetals (new york, 1924); g. sachs, grund- begriffe der mechanischen technologie der metalle (leipzig, 1925); g. t. beilby, aggregation and flow of solids (1921). on x-ray analysis: w. h. and w. l. bragg, x-rays and crystal structure (1924); r. w. g. wyckoff, the structure of crystals (new york, 1924); whilst papers on metallography are to be found in many journals. the principal sources are the journal of the iron and steel instiiuie, journal of the institute of metals, zeitschrift fiir metall- kunde, and revue de metallurgie, all of which include abstracts and bibliographies as well as original communications. the afitteilungen aus dem kaiser-wilhelm institut fiir eisenforschung contain many important papers. for the practical applications see also the transactions of the american society for sieet treating. (c.h.d.)