Magnetic and electrical properties of iron-nickel alloys

Part 1. This investigation was undertaken to determine whether any iron-nickel alloys could be found having a higher saturation value than pure iron. Alloys were prepared containing 0–100 per cent Ni. Pure Fe-Ni alloys do not forge readily, and to make them forgeable it is necessary to add alloying elements like Mn or Ti. The results show that the saturation value decreases slowly with increase in Ni content up to 20 per cent Ni; then rapidly to 30 per cent; again rises rapidly to 50 per cent and falls off gradually toward 100 per cent Ni. At no point does it exceed that of pure iron. For values of II between 100 and 400 the permeability is about 5 per cent higher for 6 to 8 per cent Ni than for pure iron, but this advantage is offset by the large increases in hysteresis loss. Alloys containing 35 to 70 per cent Ni have high permeability at low and medium densities and low hysteresis loss, the highest permeability occurring for 50 per cent. 30 to 50 per cent alloys are characterized by a nearly straight line B-H curve from the origin to B = 2000 to 4000 gausses and also by low retentivity and coercive force, properties which are of value in connection with certain electromagnetic meters. Part II. Previous investigations on commercial iron-nickel alloys have shown that 25 to 35 per cent alloys have irreversible magnetic and electrical transformation points occurring below ordinary temperatures. The present investigation confirms these results for pure alloys. A 30 per cent alloy, annealed and cooled to room temperature, had its saturation value, 4πIs increased from 2500 to 17,800 gausses and its electrical resistance decreased from 81 to 32 microhms per cu. cm. after being cooled to liquid air temperature and reheated to room temperature. Alloys containing 15, 35 and 50 per cent nickel showed practically no change after the above treatment. After allowing all transformations from the austenitic state to the α state to take place the curves for 4πIs and for electrical resistances both have definite cusps for 34.5 per cent nickel, corresponding to the compound Fe2Ni, thus giving evidence of the existence of this compound. It is pointed out that the irreversible transformation causes an enormous increase in the hysteresis loss.