Effect of fine γ′ precipitation on non-isothermal creep and creep-fatigue behaviour of nickel base superalloy MC2

A novel technique, combining on the one hand creep-fatigue tests with an overheating and creep tests with thermal cycling in the other hand, performed on γ/γ′ nickel base single crystal superalloy MC2 have led to an increased understanding of fine γ′ precipitation and its strengthening effect.Both creep and creep-fatigue tests were conducted at 1050 °C with 1200 °C overheating for creep-fatigue experiments and with repeated overheatings at 1100 °C and 1150 °C for creep.The resulting microstructures of these experiments were examined using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It appears, both on creep or creep-fatigue, once an overheating is experienced a fine γ′ precipitation occurs in γ matrix. These precipitates seem to have a transient strengthening effect on the mechanical properties. For the creep-fatigue experiments a decrease of the plastic strain rate was measured at once after the temperature peak. In the case of the creep tests under thermal cycling, no extra deformation induced by the overheating at 1100 °C was recorded. However, overheatings at 1150 °C lead to a plastic strain jump which progressively decreases upon thermal cycling, due to the formation of fine γ′ precipitates. Furthermore, the γ′ fine particles seem to have a hardening effect that vanishes once they dissolve.