A Study of the Order-Disorder Transformation in Iron-Nickel Alloys in the Region FeNi₃

The order-disorder transformation in iron-nickel alloys of high purity has been studied by measurements of electrical resistivity, by x-ray analysis and by an examination of the magnetic properties in both low and high fields. It is shown that the transformation appears to take place over a wide range of composition, from about 50 to 80 atomic% nickel, with a maximum change in structure sensitive properties at 75 atomic% nickel. The formation of order is extremely sluggish and takes place in the temperature range 494°C to 500°C. The decrease in lattice parameter of the 75% alloy which occurs on the formation of the ordered structure is 2 × 10 -3 Å. The existence of the superlattice was confirmed and, using the counter-spectrometer technique, an estimate of the degree of order was obtained. Low field magnetic measurements show that the maximum and initial permeabilities of FeNi 3 are decreased considerably by the annealing process known to produce the ordered alloy. The coercive force is increased by the same process. The effects of the addition of small percentages of aluminium, copper, molybdenum and manganese to pure FeNi 3 have been examined. It is shown that the experimental results support the assumption that the `spare' electrons of the added elements enter the 3d shell of the nickel atoms only, the contribution of the iron atoms to the total magnetic moment being unaltered Thus the iron and nickel atoms may be considered to act independently.