CONSTITUTION OF Till! ALLOYS OF ALUMINIUM AND ZINC. 337 



crystals of impurities are often scattered about the specimens. This " dirty " 

 appearance of the sections may be due partly to the great difficulty of polishing 

 them perfectly, but it certainly makes it difficult if not quite impossible to state 

 definitely at which concentration the appearance of a given phase ceases it becomes 

 difficult to decide whether certain markings are due to the causes mentioned above, 

 or whether they indicate the separation of the a phase along the line KL. For 

 these reasons the lines in question have been drawn as dotted lines only, and it is 

 prot>able that for the higher concentrations of aluminium they merely represent 

 theoretical possibilities. 



As has already been indicated, the position of the solidus curve of these alloys has 

 been determined by a series of quenching experiments. These were preferred to the 

 in ploy merit of heating-curves because of the greater certainty of results from 

 quenching, although a number of heating-curves were also observed. The quenching 

 i-\pri imcuts in question consist, as is well known, in quenching a series of specimens 

 from diUI-iviit t rmperatures, and subsequently examining the micro-structure of the 

 quenched specimens in order to ascertain whether the presence of liquid at the 

 moment of quenching is indicated or not. In the present series of alloys it was, 

 however, essential that the specimens of alloy should be brought to a condition of 

 equilibrium before being quenched, and for that reason a prolonged heating at a 

 definite temperature was often required before the actual quenching operation itself 

 could be carried out. 



The quenching apparatus devised by one of the authors* was employed for this 

 purpose throughout the research. The special advantages of this apparatus made 

 themselves felt in a very marked degree in carrying out this work. The specimens 

 are heated in vacua, and this made it possible to employ very small specimens 

 without fear of oxidation ; these small specimens, generally measuring about 6 mm. 

 cube, remained perfectly bright and were cooled with very great rapidity. The 

 temperatures of the specimens, up to the moment of quenching, was measured by 

 means of a thermo-couple placed in a small hole drilled to the centre of the specimen 

 of metal, and since in this apparatus the specimen is quenched by a powerful stream 

 of cold water which strikes the specimen in the furnace itself, and carries it out of 

 the furnace with the water, the quenching-temperatures were known as accurately as 

 those of the arrest-points on the cooling-curves. 



The first series of quenching experiments were undertaken in order to prove that 

 the line GH really constitutes part of the solidus curve ; this fact is important not 

 only for fixing the position of the solidus curve itself, but also as supporting the 

 evidence for the existence of a definite compound at the concentration where the 

 step-up in the solidus is found. The result of one of these quenching experiments 

 has already been (It-scribed and illustrated in connection with alloy No. 75 (74'5 per 



* W. K .si MI UN, " The Metallurgical Laboratories at the National Physical Laboratory," ' Journ. Iron 

 and Steel Inst.,' 1908, I. 



VOL. OCX I. A. 2 X 



