FACTORS AFFECTING MAGNETIC QUALITY 



255 



(d) the a phase becomes ordered on cooling, i.e., the iron and cobalt atoms 

 tend to distribute themselves regularly among the various atom positions 

 so that each atom is surrounded by atoms of the other kind. This phenome- 

 non is especially important in connection with the properties of iron-alumi- 

 num and manganese-nickel alloys. 



The transition at (e) is entirely in the non-magnetic region but it has 

 its influence on the properties of iron at room temperature. If iron is cooled 

 very slowly through (e), the internal strains caused by the change in struc- 

 ture will be relieved by diffusion of the metal atoms, but if the cooling is too 

 rapid there will not be sufficient time for strain relief. Practically this means 

 that to obtain high permeability in iron it must be annealed for some time 

 below 900°C, or cooled slowly through this temperature so that diffusion 

 will have time to occur. In most ferromagnetic materials diffusion occurs 

 at a reasonably rapid rate only at temperatures above about 500 to 600°C. 



10^x16 



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FIELD STRENGTH, H, IN OERSTEDS 



Fig. 6 — Effect of tension on the magnetization curve of 68 permalloy. 



The effect of a homogeneous strain on the magnetization curve can be 

 observed in a simple way, as by applying tension to an annealed wire and 

 then measuring B and H. The efifect of tension on some materials is to 

 increase the permeability and on other materials to decrease it, as shown 

 in Fig. 6. Compression usually causes a change in the opposite sense. 



The internal strains resulting from plastic deformation of the material, 

 brought about by stressing beyond the elastic hmit, as by pulling, rolling 

 or drawing, almost always reduce the permeability. The material is then 

 under rather severe local strains similar to those present after phase change, 

 and these strains are different in magnitude and direction in different places 

 in the material and have quite different values at points close together. 

 Strains of this kind can usually be relieved by annealing; therefore, metal 

 that has been fabricated by plastic deformation is customarily annealed to 

 raise its permeability. Figure 1 shows the effect of annealing a permalloy 

 strip that has been cold-rolled to 15 per cent of its original thickness. 



