274 BELL SYSTEM TECHNICAL JOURNAL 



When the amount of the second phase is considerable (as in the 15% Mo 

 alloy) it is common practice to quench the alloy from a temperature at which 

 it is a single phase (e.g. 1100 or 1200°C) and so maintain it temporarily as 

 such, and then to heat it to a temperature (e.g., 600°C) at which diffusion 

 proceeds at a more practical rate. During the latter step the second phase 

 separates slowly enough so that it can easily be stopped at the optimum 

 point, after a sufficient amount has been precipitated but before diffusion 

 has been permitted to relieve the strains caused by the precipitation. A 

 conventional heat treatment for precipitation-hardening of this kind, used 

 on many permanent magnet materials, has already been given in Fig. 16. 



In some respects the development of atomic order in a structure is like 

 the precipitation of a second phase. When small portions of the material 

 become ordered and neighboring regions are still disordered, severe local 

 strains may be set up in the same way that they are during the precipitation 

 hardening described above. The treatment used to estabhsh high strains is 

 the same as in the more conventional precipitation hardening. The decom- 

 position of an ordered structure in the iron-nickel-aluminum system has 

 been held responsible, by Bradley and Taylor,^" for the good permanent 

 magnet qualities of these alloys. 



Some of the common permanent magnets, heat treated to develop in- 

 ternal strains by precipitation of a second phase, or by the development of 

 atomic ordering, are described in Table IV. 



The changes in properties to be expected when the composition varies 

 across a phase boundary of a binary system are shown schematically by the 

 curves of Fig. 25. 



Impurities 



The principle of precipitation hardening, as just described, apphes also 

 to the lowering of permeability by the presence of accidental impurities. 

 For example, the solubilities of carbon, oxygen and nitrogen in iron, de- 

 scribed by the curves of Fig. 26, are quite similar in form to the curve sep- 

 arating the a and a -\- e areas of the iron-molybdenum system of Fig. 23 ; 

 the chief difference is that the scale of composition now corresponds to con- 

 centrations usually described as impurities. One expects, then, that the 

 presence of more than 0.04 per cent of carbon in iron will cause the perme- 

 ability of an annealed specimen to be considerably below that of pure iron. 

 The amount of carbon present in solid solution will also affect the magnetic 

 properties. 



Because the amounts of material involved are small, it is difficult to carry 

 out well defined experiments on the effects of each impurity, especially in 



"> A. J. Bradley and A. Taylor, Proc. Roy. Soc. (London) 166, 353-75 (1938). 



