CONSTITUTION OF THE ALLOYS OF ALUMINIUM AND ZINC. 339 



freezing-point of pure aluminium !H reached. The phases which commence to 

 i-r\ stallize along the four branches of the liquidus curve just described are 



(1) The a phase, which is zinc, or zinc carrying in solid solution less than 1 per 

 cent, of aluminium. 



(2) The ft phase, which has been shown to be a definite compound of aluminium 

 and zinc, probably of the formula AljZnj. This compound is characterized by a well- 

 marked tendency to crystallize in six-rayed dendrites ; it has only a limited range of 

 stable existence, being decomposed on heating above 443 C. into y and liquid and 

 on cooling below 256 C., breaking up into tin- two phases a and y. The product 

 of the decomposition of this Compound, or of its solid solutions with the y phase, are 

 found to assume a typical laminated structure closely resembling that of the 

 " pearlite " of carbon steels. The ft compound is not capable of forming solid solutions 

 \\itli ;m excess of zinc, but with aluminium it forms a continuous series of solid 

 solutions cxtfinling up to pure aluminium itself. 



(8) The y phase, which is a solid solution of aluminium containing up to approxi- 

 mately 40 per cent, of zinc. Between concentrations of from 40 to 85 per cent, of 

 y.mc this solid solution reacts with zinc which may be present in either the liquid or 

 solid form, to produce the compound Al a Zn s ; from 78'3 to 85 per cent, of zinc the 

 product of this reaction is the free ft compound, with a residue of liquid, but between 

 40 and 78'3 per cent, of zinc the resulting compound dissolves in the residual solid 

 y phase, forming a series of solid solutions which have been termed the 3 phase. 

 Between the concentrations named, this solid solution undergoes decomposition on 

 cooling to the temperature of 256 C., separating into the a and y phases. It is 

 probable, although it has not been- definitely proved, that both these reactions are 

 continued in the alloys lying between the concentrations of 40 per cent, of zinc and 

 pure aluminium. 



The researches described in the present paper have been carried out in the 

 Department of Metallurgy and Metallurgical Chemistry at the National Physical 

 Laboratory. The authors are indebted to members of the chemical staff", particularly 

 Messrs. G. BARR and L. L. BIRCUMSHAW for carrying out the numerous analyses 

 required in the research. As has already been indicated, they constitute a portion of 

 a larger scheme of investigation of the alloys of aluminium which is being carried 

 out at the Laboratory under the auspices of the Alloys Research Committee of the 

 Institution of Mechanical Engineers. The authors desire to express their appreciation 

 of the interest taken in this research by Dr. R. T. GLAZEBROOK, C.B., F.R.S., Director 

 of the Laboratory. 



x 'J 



