Precipitation of γ′ phase in δ-precipitated Alloy 718 during deformation at elevated temperatures

Alloy 718 samples aged to precipitate only δ particles (with maximum volume fraction) when tensile deformed to fracture at elevated temperatures revealed precipitation of γ′ and γ″ phases. The γ′ precipitation was found to precede the γ″ phase precipitation unlike in the case of specimens subjected to standard ageing treatment where both the γ′ and the γ″ phases precipitate simultaneously. This sequence is explained on the basis of the relative concentration of Al, Ti and Nb in the matrix of δ precipitated Alloy 718 microstructure. The precipitation sequence was consistent with the Cozar and Pineau's model that predicts such sequences on the basis of (Al + Ti) to Nb atom ratios.