Abstract
The isothermal section of the 
system at 1000°C has been defined by determining the compositions and structures of the equilibrated solid phases. Wustite can coexist with 
alloys containing up to 79.6 a/o (atomic percent) Ni. The three‐phase field wustite‐spinel‐alloy consists of wustite containing 0.51 a/o Ni, spinel containing 0.6 a/o Ni, and an alloy containing 79.6 a/o Ni. The three‐phase field, spinel‐nickel oxide‐alloy, consists of nickel ferrite, nickel oxide containing 6.5 a/o Fe, and an alloy containing 99.5 a/o Ni. Tie lines in the spinel‐alloy field were investigated in detail. Wustite‐nickel oxide solid solutions exhibited nonideal behavior.