THE MICROSCOPE IN METALLURGICAL RESEARCH. 

 By E. F. Law. 



It would be difficult to over-estimate the importance of the 

 part played by the microscope in metallurgical research during the 

 last 15 years. Its introduction threw a flood of light on problems 

 hitherto unsolved, and it was not surprising that the early work 

 of the pioneers — Sorby, Osmond, Roberts- Austen and others — was 

 followed by a rush of eager recruits anxious to take part in the 

 campaign. Nor was it surprising that this display of zeal should 

 be followed by a lull, if not an actual reaction; such periods in- 

 variably follow a period of exceptionally rapid progress, and when 

 they occur it is wise to take stock of the existing position and 

 endeavour to prepare the way for the next advance. With that 

 object in view we may briefly consider the metallurgical problems 

 which have already been solved by means of the microscope, and 

 then turn to some of those which are awaiting solution and which 

 require either more knowledge or more perfect instruments. 



Frohlems Solved. — Before the introduction of the microscope we 

 knew from the chemical analysis of an alloy its ultimate chemical 

 components, but we did not know in what form those components 

 occurred. Probably the only exception to this rule was to be found 

 in the case of carbon in cast iron, which was invariably divided into 

 free or graphitic carbon and combined carbon. The microscope 

 altered all this, and explained not only the relationship between 

 the structure of the alloy and its mechanical properties, but the 

 structural alterations and consequent changes in mechanical proper- 

 ties produced by heat treatment. 



Problems to he Solved. — With very few exceptions, it may be 

 «aid that the finer or smaller the structure of an alloy, the more 

 useful it is from a commercial standpoint ; and it frequently happens 

 that the best of our commercial steels possess so fine a structure 

 that they are imperfectly resolved by the highest powers of the 

 microscope now available. How often do we read in descriptions 

 of microstructures such expressions as "a confused groundmass *' 

 or "a matrix whose structure is not resolved by the microscope " ? 

 Another problem awaiting solution is to be found in the inter- 

 crystalline weakness of metal. During the last few years a wonderful 

 edifice of hypotheses has been erected on the foundation of a so- 

 called amorphous phase which is said to exist between the crystals 

 of a metal, and this amorphous material is made to serve as an 

 explanation either of its strength or weakness. 



Unfortunately, there is very little direct evidence in support of 

 these theories ; but with a higher degree of magnification it is possible 

 we may learn more of the intercrystalline structure of metals. Even 

 of the crystalline structure we know very little, and there is scope 

 for much research on the " dendritic " structure which is shown 

 on heat-tinting, and which has been so beautifully developed by 

 Humfrey. 



For these and other investigations we require instruments which 

 will give us a higher degree of magnification, and we look to the 

 manufacturers for their assistance. But, if we are provided with 

 such instruments, we, on our part, must be prepared to supply a 

 much higher degree of skill in the preparation of samples for exam- 

 ination than is commonly met -with. 



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