The body-centered tetragonal ordered phase Ni₃Nb formed by Nb in Ni-based alloys such as 718. It precipitates coherently and strengthens the alloy, but with long use above about 650 °C it changes to δ phase.
Japanese name: γ''相
γ'' phase appears in 1 practice question across 1 topic, including Nonferrous metals and alloys.
Field: Nonferrous metals and alloys
Answer: C. It is strengthened mainly by body-centered tetragonal γ'' (Ni₃Nb), which precipitates slowly so it resists post-weld cracking, but in long use above about 650 °C it turns to δ phase and softens
In alloy 718, Nb precipitates coherently with the matrix as the body-centered tetragonal ordered phase γ'' (Ni₃Nb), which provides strengthening together with a small amount of γ'. Because γ'' precipitates more slowly than γ', strain-age cracking, which occurs when shrinkage from precipitation combines with residual stress during the aging heat after welding, is unlikely, so the alloy suits parts assembled by welding. γ'', however, is metastable, and with long use above about 650 °C it coarsens and transforms to the stable orthorhombic δ phase (Ni₃Nb), so the strength falls. The alloys used above 1000 °C are those rich in γ', such as single-crystal blade alloys. Alloy 718 is a high-strength precipitation-strengthened alloy, so the option calling it solid-solution strengthened is also wrong.
In alloy 718, Nb precipitates coherently with the matrix as the body-centered tetragonal ordered phase γ'' (Ni₃Nb), which provides strengthening together with a small amount of γ'.
The Ni-based alloy 718 (a Ni-Cr-Fe alloy containing about 5…Al-Li alloy Alclad Beryllium copper GP zone Temper designation Tough-pitch copper