EUTECTICS. 



i;y J. L. W. RHODES, Assoc. Ixst. M.E. 



It has long been known that most alloys melt cufccfic jioint and the allo\- the eutectic allo\-. 



at lower temperatures than their constituent The question now arises as to why this alio}- 



metals. should possess such peculiar properties. Is it a 



Thus solder containing 32 per cent, of lead chemical compound, since it possesses definite com- 



and 68 per cent, of tin melts at a lower position and melting-point ? To this we can say no 



temperature than either tin or lead ; and fusible at once, for every chemical and electrical test shews 



allo}-s consisting of lead. tin. bismuth, and that the eutectic contains free lead, free tin, and 



sometimes cadmium, melt readih' in hot water. nothing else. To soh'e the difficult\- the microscope 



FiGUKE 1. 

 Micropegmatite in dioritic granophyre. 



Figure 2. 

 Perthite parallel to basal plane. 



Figure 3. 

 Perthite parallel to clinopinacoid. 



The same princijile has been found to apply 

 to salts: thus fusion mixture consisting of sodium 

 and potassium carbonates in molecular proportions 

 melts much more readilv than either of them. The 

 use of fluxes in metallurg\- frequently furnishes 

 another instance, so that we ma\' regard the fusion 

 of alumina in presence of crxiilite as due to a similar 

 lowering of melting-point. 



Let us now take up the study of the tin-lead 

 alloys and plot out the melting points against the 

 percentage of tin. We find our curve consists of 

 two branches with a cusp shown at A in Figure 4. 

 The increase in the percentage of tin continuously 

 lowers the melting-point until A is reached at 180" C 

 and 68 per cent, of tin after which the melting-point 

 rises continuously. This point is called the 



is brought into use. It is at once seen that of all 

 the allo\-s the eutectic is the onl}- one that is 

 homogeneous: all the others consist of more or less 

 perfect crystals imbedded in a fine grained matrix 

 identical with the eutectic. 



It is now easier to understand the phenomenon. 



Taking an a\\o\ fairly rich in lead, X in Figure 4, 

 we see that as the temperature falls no crystallisa- 

 tion occurs till we reach a point Y on the curve 

 when pure lead separates out and the temperature 

 falls along the line YA. When .\ is reached all the 

 lead o\-er and above eutectic proportions has 

 crystallised out and the tin and lead remaining 

 crystallise simultaneoush'. .A eutectic point is there- 

 fore the point at which two substances crystallise 

 simultaneoush'. Now if this occurs, obviously each 



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