Experiments in Micro-metallurgy. 



85 



"Experiments in Micro-metallurgy: — Effects of Strain. Pre- 

 liminary Notice." By James A. Ewing-, F.E.S., and Walter 

 Bosenhain, 1851 Exhibition Besearch Scholar, Melbourne 

 University. Eeceived and Bead March 16, 1899. 



[Plates 1 — 5.] 



Much information has been obtained regarding the structure of 

 metals by the methods of microscopic examination initiated by Sorby and 

 successfully pursued by Andrews, Arnold, Charpy, Martens, Osmond, 

 Boberts- Austen, Stead, and others. When a highly polished surface of 

 metal is lightly etched and examined under the microscope, it reveals 

 a structure which shows that the metal is made up in general of irre- 

 gularly shaped grains with well defined bounding surfaces. The 

 exposed face of each grain has been found to consist of a multitude of 

 crystal facets with a definite orientation. Seen under oblique illumi- 

 nation, these facets exhibit themselves by reflecting the light in a 

 uniform manner over each single grain, but in very various manners 

 over different grains, and, by changing the angle of incidence of the 

 light, one or another grain is made to flash out comparatively brightly 

 over its whole exposed surface, while others become dark. 



It is also well known that the grains are deformed when the metal 

 is subjected to such processes as cold hammering, or cold rolling, or 

 wire-drawing. On polishing and etching a piece strained in any such 

 way, the grains are found to be on the whole longer in the direction in 

 which the metal is extended than in other directions. But on heating 

 the metal sufficiently a re-formation of structure occurs, and the grains 

 are found to have again assumed forms in which there is no direction 

 of predominating length. In iron this recrystallisation occurs at a red 

 heat. It is also known that prolonged exposure of iron to a tempera- 

 ture of about 700° C. tends to produce a larger granular structure than 

 is found if the metal is somewhat quickly cooled from a higher tem- 

 perature. 



The grains appear to be produced by crystallisation proceeding, more 

 or less simultaneously, from as many centres or nuclei as there are 

 grains, and the irregular more or less polygonal boundaries which are 

 seen on a polished and etched surface result from the meeting of these 

 crystal growths. The grains are, in fact, crystals, except that each of 

 their bounding surfaces is casually determined by the meeting of one 

 growth with another. This is, we believe, the view usually accepted 

 by metallurgists •* but there is considerable difference of opinion as to 

 the part played by foreign matter in possibly contributing to form a 

 cement at the intergranular junctions. 



* See especially two papers by Mr. J". E. Stead ('Jour. Iron and Steel Inst.,' 

 1898). 



