FACTORS AFFECTING MAGNETIC QUALITY 



271 



den changes in the Curie point curve are associated with a, 7 phase boun- 

 daries, as mentioned earlier in this chapter. The peak of the permeability 

 curve (Fig. 22) occurs at the composition for which atomic ordering is stable 

 at the highest temperature (see also Fig. 2). The sharp decline near 95 per 

 cent cobalt coincides with the phase change y,e at this composition. Addi- 

 tions of vanadium, chromium and other elements are used in making com- 

 mercial ternary alloys. 



Some useful alloys based on the binary iron-sihcon, iron-nickel and iron- 

 cobalt alloys are described in Table III. 



The hardening of material resulting from the precipitation of one phase in 

 another is often used to advantage when magnetic hardness (as in per- 

 manent magnets), or mechanical hardness, is desired. To illustrate this 



10^x2.0 



ftr 1.2 



-jUJ 



= 



5 "0.8 



0.4 



600 



20 40 60 80 



PER CENT COBALT IN IRON 



Fig. 22 — Variation of permeability at H = 10 oersteds, and of the critical temperature 

 of ordering, with the composition of iron-cobalt alloys. 



process consider the binary iron molybdenum alloys, a partial phase dia- 

 gram of which is given in Fig. 23. The effect of the boundary between the 

 a and a -j- e fields is shown by the variation of the properties with composi- 

 tion (Fig. 24a). Saturation magnetization and Curie point are affected but 

 little, the principle change in the former being a slight change in the slope 

 of the curve at the composition at which the phase boundary crosses 5(X)°C, 

 the temperature below which diffusion is very slow. The Curie point curve 

 has an almost imperceptible break at the composition at which the phase 

 boundary lies at the Curie temperature. The changes of maximum per- 

 meability and coercive force are more drastic ; Hm drops rapidly as the amount 

 of the second phase, e, increases and produces more and more internal strain 

 (Fig. 24b), and He increases at the same time. The experimental points 

 correspond to a moderate rate of cooling of the alloy after annealing. 



