Apkil 29, 1909] 



NA TURE 



structure are not so definitely known as could be 

 desired. Fortunately, the list of such materials is 

 rapidly diminishing, but, owing to the wide range 

 and complexity of industrial alloys, cases of this kind 

 will continue to occur occasionally. 



Although the microscopic evidence is, as a rule, 

 quite conclusive, it is eminently desirable to supple- 



FlG. I.— Section through a fracture of mild steel, after embedding in an 

 electro-deposit of copper. The path of the fracture among the con- 

 stituents of the steel can be clearly traced. Magnification, 100 diameters. 



ment it wherever possible by the data of a careful 

 chemical analysis and complete mechanical tests, 

 dynamic as well as "static." Even where these 

 additional data are not needed to confirm the conclu- 

 sions drawn from the microscopic examination, they 

 are valuable as throwing a light upon the indications 

 of the various forms of test relied upon by engineers 

 in drafting their specifications. 



This consideration raises the question how far it 

 would be possible or desirable to include an examina- 

 tion of the micro-structure in the regular tests carried 

 out on engineering materials. Some time ago metal 

 manufacturers, and more particularly steel makers, 

 would have met such a proposal with every means 

 of opposition in their power, but greater familiarity 

 with questions of micro-structure has, it may be 

 supposed, diminished this feeling. If it were simply 

 a question of imposing an additional test, or of 

 placing an additional difficulty in the way of the 

 manufacturer who has to comply with specifications, 

 a hostile attitude would, of course, be readily under- 

 stood, but the effect of the inclusion of microscopic 

 examination in regular testing would not be at all 

 likely to increase the stringency of the specifications 

 in question. Thus, as regards chemical compositions, 

 specifications are so drafted that, even with un- 

 favourable structure, the material may be strong 

 enough to meet the mechanical requirements of the 

 engineer. Were it possible to rely upon obtaining a 

 favourable structure in the material as used, the 

 necessity for stringency in regard to composition 

 would be materially reduced. 



Further, the use of the microscope in this connec- 

 tion should enable the engineer to rely more securely 

 upon both the uniformity of his materials and on 

 their conformity with the test specimens. The reason 

 is that, by the microscopic examination of a number 

 of very small pieces chosen from a variety of different 

 pieces or parts of the material, it would be at once 

 ascertained whether the material was uniform, and 

 whether the test-pieces chosen for mechanical testing 

 or for chemical analysis fairly represented the bulk 



NO. 2061, VOL. 80] 



of the material. This again is an application of 

 the microscope in engineering practice which could 

 not be regarded as operating against the interests of 

 the makers of the material ; the rejection of metal 

 on the results of tests carried out on samples which 

 happen to be below the average of the batch would 

 be prevented quite as often as the acceptance of a 

 batch on results obtained from an unduly favourable 

 sample. 



In the case of large pieces of metal also, the appli- 

 cation of the microscope would prevent the occurrence 

 of failures which sometimes arise as a consequence 

 of want of uniformity in the materials forming 

 different parts of the same forging. Such differences 

 may arise either from segregation, i.e. from a non- 

 uniform distribution of the constituents or the 

 impurities in the metal as originally produced, or it 

 may be the result of insufficient or of wrongly applied 

 working. Thus, if rods of ductile metal, such as ' 

 brass, are drawn down cold too rapidly, or if the 

 reduction at each pass through the dies is incorrectly 

 adjusted, the result is the production of a surface 

 layer of material which has been much more heavily 

 deformed than the core of the rod, and this results 

 in a condition of serious internal stress which may 

 even produce subsequent spontaneous fracture. 



In large forgings also, an external layer of fine- 

 grained material is sometimes found superposed on 

 a coarse-grained core as the result of inadequate 

 working. This also is liable to lead to failure in 

 use, while the indications of test-pieces cut from the 

 fine-grained layer are entirely falsified by the real 

 behaviour of the piece as a w'hole. An example of 

 the diversity of structure to be met with in different 

 parts of the same piece of metal is shown in Fig. 2, 

 the two halves representing, to the same magnifica- 

 tion, the structure as seen in the outer and central 





tm^ 



w 



the 



Fig. 2.— Sections from two parts of the same large 



half of the figure represents the fine structure of the extern: 

 while the left-hand half represents the coarse structure of the 

 The magnification of both is 50 diameters. The dark and ligh 

 both portions represent pearlite and ferrite respectively. 



ight-hand 

 il layers. 



layers of a large forging. This example is, of course, 

 abnormal, but the intelligent use of the microscope in 

 ordinary testing practice would prevent such a piece 

 from passing into use. 



Examples of other uses of the microscope in con- 

 nection with the materials of engineering could be 

 given in great numbers. Perhaps one of these which 



