Nonferrous metals and alloys
The Ni-based alloy 718 (a Ni-Cr-Fe alloy containing about 5% Nb) is widely used for jet engine disks and similar parts. Which statement about its strengthening and characteristics is correct?
- AIt is strengthened mainly by L1₂ γ' (Ni₃Al), which precipitates quickly so it is prone to strain-age cracking in post-weld heat treatment, but it keeps its strength above 1000 °C
- BIt is a solid-solution-strengthened alloy without precipitates, low in strength but excellent in corrosion resistance, and used mainly for chemical plant piping
- CIt 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
Show answer and explanation
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.