FUSION AND SOLIDIFICATION 7J 



page 72, represents a microscopic photograph of 

 the eutectic of gold and aluminium; while in Fig. 5 

 is shown the structure of an alloy with a com- 

 position not quite that of the eutectic. Here 

 large primary crystals have appeared, the intervals 

 being filled with the same eutectic which is seen 

 in Fig. 4. The metal of Fig. 4 has been cooled 

 more slowly than that of Fig. 5, and therefore 

 the eutectic in Fig. 4 has larger crystals and a 

 coarser structure. 



The eutectic alloy has a constant melting or 

 freezing-point ; but, during the process of fusion 

 or solidification of other alloys, the temperature 

 will generally change. As the primary crystals 

 of one or other pure metal form, they leave the 

 residual liquid richer in the other constituent, and 

 thus with a lower freezing-point. This process 

 continues till the liquid has the composition of 

 the eutectic alloy, when any further loss of heat 

 will precipitate crystals of both metals side by 

 side. A thermometer immersed in the mixture 

 will show the temperature at which primary 

 crystals begin to form, and the temperature at 

 which the composition of the residual liquid 

 reaches that of the eutectic, for the rate at which 

 it falls becomes suddenly much slower when solid 

 first appears, and the fall stops altogether while 

 the eutectic is freezing out. Thus, in such a 

 simple case as that of silver and copper, useful 

 information can be obtained by merely drawing 

 the curve giving the observed relation between 

 the time and the temperature for the heated alloy. 

 Such curves have forms more or less resembling 

 that shown in Fig. 7. 



With silver and copper no chemical compounds 



