New type of γ′ phase in Ni based single crystal superalloys: Its formation mechanism and strengthening effect

New type of γ′-precipitate in Ni-(6-8)Al-(7-12)Mo-4.5Ta-1.5Re (weight%) single-crystal superalloys and its effect on mechanical properties have been investigated. The phase exhibits a rattan form from cross-sectional observation while a curved thin film in stereoscopic structure. Its density is strongly linked to Al and Mo contents and holding time of solid solution. Its formation arises as a result of i) the overlap of γ′-precipitate and ii) the subsequent interface migration; the former is caused by the mismatch of γ-γ′ periodicity, which is inherited from the congruent ordering and spinodal decomposition independently occurring in different regions; the latter is driven from the chemical potential difference between γ-ribbon and γ′-precipitate induced by interface curvature and misfit stress. The γ′-rattan improves room temperature tensile properties through changing the slip lines and inhibiting the crack growth, which is ascribed to the change in the stress field of γ-γ′ structure.