The precipitation of Ni3Nb phases in a Ni−Fe−Cr−Nb alloy

The age hardening of a Ni?Fe?Cr?Nb alloy containing 4.85 wt pct Nb has been studied using transmission electron microscopy. The major hardening phase in this alloy isγ*, DO₂₂-ordered Ni₃Nb, which precipitates as a fine dispersion of square plates. It is shown that nucleation ofγ* plates may be dependent upon matrix excess vacancy concentration, but nucleation ofγ* plates is also observed at dislocations and extrinsic stacking faults. Theγ* phase is metastable with respect to the orthorhombic Ni₃Nb phase, β, which precipitates by either a cellular or an intragranular reaction, depending upon the aging temperature. It is proposed that the intragranular nucleation of β laths proceeds by the growth of stacking faults from withinγ* plates; theγ* plates subsequently dissolve in favor of the β laths. Room temperature deformation of theγ* dispersion is shown to produce faults within theγ* plates; possible dislocation reactions occurring during this deformation are discussed.