28 BELL SYSTEM TECHNICAL JOURNAL 



ranges, the saturation value is approached by that of the less expensive 

 material, magnetic iron. In connection with high flux density applica- 

 tions, the permalloys, the perminvars, and modified alloys of this 

 character are of.little interest since they are saturated at comparatively 

 low flux densities. 



The data given here are not sufficient to select a material for a spe- 

 cific application since many other detailed properties must be con- 

 sidered in connection with each individual problem. The materials 

 which have been described, however, cover, in a general manner, the 

 entire range of magnetic fields. 



Magnetic Alloys in Dust Form 



For certain purposes where a substantially constant low permeability 

 is desired, and in particular, for high frequency applications where eddy 

 current losses are of consequence, it is desirable to produce the magnetic 

 material in a fine powder, which is subsequently insulated and pressed 

 into the desired core shape. One alloy prepared in this form is the 

 permalloy ^^ containing approximately 80 per cent of nickel. This is a 

 material suited for use at audio frequencies, for example in the cores 

 of loading coils. 



The alloy may be prepared in powdered form by a number of 

 methods. In one method, which is essentially metallurgical in nature, 

 advantage is taken of the effect of small amounts of added elements.^^ 

 It has been found that the addition of a few thousandths of a per cent 

 of sulfur to the iron-nickel alloy containing approximately 80 per cent 

 of nickel produces a structure which can be hot-rolled to a small section, 

 but which when cold, is exceedingly brittle and can be pulverized to a 

 fine dust. The presence of other elements in small amounts also 

 affects the properties of the alloy. One element, manganese, has an 

 effect opposite to that of sulfur, and if present in sufficiently large 

 amounts, nullifies the action of sulfur by producing a tough and 

 malleable casting. 



The structural behavior of sulfur and manganese in permalloy is 

 interesting in explaining the embrittling action. Sulfur exists in the 

 structure as microscopic films of complex sulfides at the crystallite 

 boundaries as illustrated in Fig. 13. These films are brittle, and when 

 they extend over the greater portion of the crystallite surface, produce 

 an interface of weakness permitting easy pulverization. The satis- 

 factory hot-working properties can be explained in that at the high 

 temperature, either the sulfide film is malleable, or dissolves in the 

 iron-nickel solid solution. If manganese is added to an alloy contain- 

 ing sulfur, the sulfide constituent is blackened, loses its continuous 



