458 BELL SYSTEM TECHNICAL JOURNAL 



Tests on other compositions showed that those alloys in which the 

 magnetic properties depended on the rate of cooling below 600° C. 

 gave results similar to those for the 78.5 permalloy. The temperature 

 range was also found to be substantially the same as for the 78.5 

 permalloy, although in some cases there were indications that small 

 changes occurred above 600° and below 400° C. 



This connection between heat treatment and magnetic properties 

 led me to conclude that the differences in these properties in the alloys 

 which had been heat treated differently below 600° C. were caused by 

 constitutional changes in the alloys. Such changes are common in 

 alloys in the solid state, often at low temperatures. The progressive 

 change in the permeability as the temperature of the alloy decreases 

 slowly below 600° C, the gradual increase in the time required for a 

 change to complete itself as the temperature drops, and the prevention 

 of the change by rapid cooling through this temperature range support 

 this conclusion. 



It is well known that some alloys are in the state of homogeneous 

 solid solutions at high temperatures, but segregate into two or more 

 phases as the temperature drops. Such segregation ordinarily takes 

 place in a definite temperature range, and is progressive in nature. 

 It is a change of this type which I picture as taking place in these 

 alloys during slow cooling. At the upper end of the critical tempera- 

 ture range for each alloy, the homogeneous solid solution begins to 

 segregate into constituents of different composition. This segregation 

 continues until the temperature has dropped to a point where no 

 further changes take place. Rapid cooling prevents this segregation 

 and the alloys remain after cooling in a metastable condition. 



We would suppose that if such a change takes place, confirmatory 

 evidences might be found. I shall refer, briefly, to some of our 

 attempts to obtain evidence to confirm our speculations as to the 

 nature of these magnetic changes. 



The resistivity was measured for rapidly cooled and for baked 

 alloys both of permalloy and perminvar compositions. These measure- 

 ments showed that for both types of alloys, baking reduced the 

 resistivity. For example, the resistivity of a 78.5 permalloy sample 

 was measured after baking and after air quenching. The resis- 

 tivities for the two conditions were 14.2 and 15.8 microhm-cm., 

 respectively. Upon rebaking the resistivity again dropped to 14.5, 

 about the same as before it was air quenched. This change is in 

 accordance with what would be expected if a segregation took place 

 with annealing. A homogeneous solid solution has the highest 

 resistivity and segregation tends to lower it. 



