An experimental study of high-permeability nickel-iron alloys

Because the properties of commercial alloys are inconsistent, designers of electrical equipment normally assume properties greatly inferior to those usually quoted. Further, very thin strips or sheets have losses greater than those predicted by simple theory. Factors which might influence either the ultimate performance or the consistency of the metals have therefore been studied, with particular reference to the 77/14/5/4 nickel-iron-copper-molybdenum alloy, using samples made by powder-metallurgy. If high permeabilities are to be attained the permissible tolerance on composition is very small, the limit for iron and nickel being only ±0.2%; further, any introduction of silicon makes the alloy susceptible to damage by water vapour in the annealing atmosphere. Using conventional melting techniques it is difficult to hold such tolerances and to avoid contamination by silicon. The types of low-permeability layer which have been previously reported are not intrinsic and are probably due to impure metal which has been unsatisfactorily or incompletely annealed. Although metal having a permeability of 30000 can be consistently made by strict control of composition and annealing, there is an unexpected component of a.c. loss which becomes serious at thicknesses less than about 30 microns. Spiral cores, if free from strain, can have properties almost as good as those of the strip they are made from.