126 S. W. SMITH OX THE MICROSTRUCTURE OF METALS AND ALLOYS. 



physical properties, according to the conditions and rate of cooling 

 of the original molten mass. Thus, complete crystallisation has 

 occurred in a true granite, partial crystallisation in rhyolites 

 and andesites which show crystals imbedded in a matrix of 

 crystallites ; while in the glassy lavas, such as obsidian, the rate 

 of cooling has not been sufficiently slow to allow the formation of 

 crystals to take place. 



It may be presumed that the idea of crystallisation of minerals, 

 either from a solution or from a molten mass, is familiar to all. 

 In extending this to the case of metals, and in explanation of what 

 follows, it may be well to consider briefly what happens during 

 the solidification of a mass consisting of two metals, as being the 

 simplest case of an alloy. Assuming that no segregation of 

 the constituents into distinct layers of different specific gravity 

 occurs, but that, as a whole, the mass remains homogeneous, then 

 what happens is, in general, somewhat as follows : — Either one 

 of the constituents first solidifies in the form of practically pure 

 crystals or grains, or else the first portions to solidify consist of 

 crystalline grains of a definite compound of the two constituents. 

 These grains which are first to solidify remain, as a rule, dis- 

 tributed evenly through the mass. This may be followed by the 

 solidification of a second compound of the two constituents, but, 

 of course having a lower temperature of solidification. In this 

 way solidification proceeds until the still molten material reaches 

 a composition which for the particular pair of metals under 

 consideration possesses the lowest " freezing " point, and then this, 

 the so-called " eutectic " alloy, solidifies as a whole. This eutectic 

 mixture or alloy usually possesses a characteristic banded structure 

 common to alloys of all metals. Thus, if the composition chosen 

 is that of the eutectic, then the whole mass will solidify at one 

 temperature and will be made up entirely of eutectic, showing 

 the characteristic banded or lamellar structure already referred to. 

 The structure, then, of an alloy of two metals will, in general, 

 show crystalline grains of a pure metal, or a compound of the two, 

 set in a matrix of eutectic which has solidified round the grains 

 (see Figs. 4, 5, and 6). The structure of the eutectic has been 

 called "pearlitic," from the pearly appearance of the first observed 

 eutectic — that of the iron-carbon alloy which is a characteristic 

 constituent of annealed steel. 



From the foregoing remarks it is easy to imagine how Sorby (to 



