125 



The Microstructure of Metals and Alloys. 



By Sydney W. Smith, A.R.S.M. 



Communicated by D. J. Scourfield^ F.R.M.S. 



{Read June 21s#, 1901.) 



Plate 7. 



In ojQfering these brief notes to those whose interest in micro- 

 scopical work centres largely in biological science, the writer hopes 

 that some slight assistance may be afforded to any who may wish 

 to extend their work to the field of physical science, of which the 

 subject of these remarks is one of the most striking branches, 

 both in its novelty and in the practical utility of its results. It 

 is impossible, however, to give here more than a general outline 

 of the nature of the work. 



The interest and importance attaching to the microscopical 

 examination of metals and alloys depend on the fact that when 

 a highly polished surface is etched by attack with suitable 

 reagents and viewed by reflected light a certain "structure" is 

 seen which will depend upon (1) the composition of the metal 

 under examination, (2) the conditions under which it solidified, 

 and (3) the treatment, either thermal or mechanical, which it 

 has received subsequent to solidification. For example, it is well 

 known that steel which has been slowly cooled from a bright 

 red heat, or in other words has been annealed, has the properties 

 of ductility and malleability to some considerable extent. If the 

 same steel, however, instead of being allowed to cool slowly, is 

 suddenly plunged into water, or " quenched," its properties of 

 ductility and malleability entirely disappear and the material 

 becomes quite brittle. Tempered steel is the compromise between 

 these two states. Each, however, has a distinct and unmistakable 

 microstructure (see Figs. 1, 2, and 3). This will be readily 

 realised by those familiar with the somewhat analogous pheno- 

 mena revealed by the study of rocks. For instance, the ultimate 

 components of granite may give rise to bodies differing widely in 



JouRN. Q. M. C, Series II.— No. 49. 9 



