20 BELL SYSTEM TECHNICAL JOURNAL 



arrangement may be clearer to some if it is considered a "compound," 

 although strictly speaking, the arrangement does not correspond to a 

 compound in the chemical sense. The regular arrangement is now 

 ordinarily called a "superlattice" or a "superstructure," and as such, 

 will be designated in this paper. If it is clearer to the reader, he may 

 substitute for the term "superlattice" or "superstructure," the term 

 "compound" without great sacrifice in the sense of the discussion. 



With this picture in mind, the effect of heat treatment is immediately 

 evident. Rapid cooling prevents the formation of the "superlattice" 

 arrangement, which occurs somewhere in the region of 500° C, and a 

 disordered distribution of atoms is retained. This is the distribution 

 possessing the higher magnetic permeabilities. Slow cooling or baking 

 promotes the transformation to a superlattice of lower magnetic 

 quality. 



Heat treatment effects are much less in the alloys in the region of 50 

 per cent nickel, and in fact would not be expected, since a face centered 

 cubic binary alloy with the atomic ratio of 1 : 1 would not be expected to 

 have a special structure (superstructure). 



The hypothesis of superlattice formation in the 75 per cent nickel 

 region is supported further by the effect of heat treatment on other 

 physical characteristics, for example the electrical resistivity and the 

 tensile strength. That such special structures are formed in the solid 

 states is well established from detailed studies of the copper-gold 

 system.^'* Unfortunately, in the nickel-iron system where the nickel 

 and iron atoms are so near in atomic number, the detection of super- 

 structure by x-ray methods appears, at the present time, a difficult task. 



The effect of heat treatment and the general magnetic softness of 

 iron-nickel alloys in the region of 75 per cent nickel may be explained 

 by another hypothesis which returns for its basis to the considerations 

 set forth in the section on high purity iron. The improvement in 

 magnetic softness may be attributed, first, to the effect of nickel in 

 decreasing the residual quantities of interstitial elements originally 

 present in iron. Whether or not such an effect is present awaits a care- 

 ful investigation of the interstitial element content of iron-nickel 

 alloys. Secondly, assuming the interstitial elements the responsible 

 factors, the effect ^^ of heat treatment on magnetic quality can be ex- 

 plained in that the quench (rapid cooling) retains the residual elements 

 in solution, while slow cooling permits precipitation in a form more 

 deleterious to magnetic quality. 



The two hypotheses possess interesting possibilities, but it appears 

 that further investigation will be required before the structural rela- 

 tionships are definitely established. 



