Local order and mechanical properties of the γ matrix of nickel-base superalloys

For the first time gamma single crystals, closely matching with the chemical composition of the matrices of the industrial single crystal superalloys AM3 and MC2 have been made. Macroscopic tensile tests as well as "in situ" deformation tests show that in both alloys a strong localization is the main characteristic of the deformation at different scales of observation. Collective movements of dense planar arrays of dislocations are observed related to the presence of order which appears to be different depending on the alloy. These ordering phenomena introduce high friction forces larger than the applied stress (tau(f) approximate to 2.4 tau(el)) opposing dislocation movement. The MC2 matrix has a tensile strength 15 % higher than that of the AM3 matrix. This improvement in strength is partially due to the higher content in W. From deformation and neutron diffraction experiments, it appears that the hardening mechanism is not only a solid solution one, but also associated with a local order hardening which depends on the alloy. The superior mechanical properties of the MC2 gamma phase are attributed presumably to the presence of a DO22 short range order due to W.