20 



Messrs, C. T. Heycock and F. H. Neville. 



"Gold- Aluminium Alloys." By C. T. Heycock, F.E.S., and F. H. 

 Neville, F.E.S. Eeceived October 31, — Read December 7, 

 1899. 



(Abstract.) 



The first part of this paper gives the equilibrium curve for the 

 liquid alloys and the various solid bodies that can form in them. The 

 curve is based on the determination of the freezing points of mixtures 

 varying in composition from pure gold to pure aluminium. 



The freezing points were determined by means of platinum resist- 

 ance j)yrometers of the Callendar-Griffiths type, and the composition 

 of each alloy was found by extracting a sample from the crucible and 

 analysing it. 



The ordinate in the curve is the freezing point on the air-centigrade 

 scale, and the abscissa is the composition of the alloy expressed in 

 atomic percentages of aluminium. 



The curve was found to consist of seven branches, each branch cor- 

 responding to a state in which a particular solid crystallises first. In 

 harmony with this, seven substances can be detected in the solid alloys. 

 The bodies are : — 



Gold ; AU4AI ; AU5AI2 or perhaps AusAls ; AU2AI ; a body which is 

 probably AuAl ; AuAL, Roberts-Austen's purple alloy ; aluminium. 



The bodies AuoAl and AuAl2'^ are indicated by well-marked summits 

 in the curve, at 33*4 and at 66*6 atomic per cents, of aluminium re- 

 spectively. 



The melting or freezing point of AU2AI is at 625° C., that of AuAU 

 is at 1062° C, apparently identical with the melting point of gold 

 itself. The body whose formula we give as either AU5AI2 or AugAlg 

 has its melting point at 575° C. The body AU4AI has its melting- 

 point near 550° 0. That of the hypothetical AuAl is not given by the- 

 curve. 



The curve has three well-marked euteetic angles. One of these is at 

 527° C. and 3'6 per cent, by weight of aluminium, the alloy here being- 

 a mixture of AU4AI and AU5AI2. The next is at 569° C. and 8'36 per 

 cent, by weight of aluminium ; this alloy has a composition corre- 

 sponding to the formula AU3AI2, but it is a mixture of AuoAl and 

 « AuAl. The third euteetic is at 648° C., and the alloy contains 1-87 

 per cent, by weight of gold ; it is a mixture of AUAI2 and aluminium. 

 We see from the ixhove that the mixture with the lowest melting point 

 of all is that containing 3'6 per cent, by weight of aluminium, this- 

 small percentage depressing the melting point of gold from 1062° C. ta 

 527°, that is mc.'e than 500°. A liquid of this composition, though 

 * See footnote, p. 21. 



