The sintering process of a high permeability Ni-Fe-Cu-Mo alloy made by powder metallurgy

The magnetic permeability of a 77 Ni-14 Fe-5 Cu-4 Mo wt % alloy made by powder metallurgy is known to be improved by extending the sintering time considerably beyond that normally used. The room temperature measurement of resistivity during the sintering cycle of such an alloy clearly shows the overlapping stages of change that occur during the sintering process. The variation in resistivity and its relationship with the changes in density, in weight and in bend strength of compacts shows that de-oxidation of the constituent element powders occurs initially. De-oxidation is followed by sintering and alloying of the nickel and iron which is followed in turn by alloying of the molybdenum. The final stage involves the alloying of the copper and the elimination of pores.Electron microprobe analysis has shown that the copper does not alloy substantially until the copper particles melt, and that alloying is hindered if copper powder of large particle size is used. Sintering occurs more rapidly than alloying, but the rate of alloying is the most important factor in determining the electrical and magnetic properties of the alloy.