CREEP AND RELAXATION MECHANISMS IN A NICKEL-BASE SUPERALLOY AT 650-DEGREES-C

An analysis of the creep and relaxation behavior at 650 degrees C of the nickel base alloy N-18 is made in order to determine the physical mechanisms, as observed by TEM investigations, which are likely to govern the macroscopic mechanical response of the material. In relaxation tests, the evolution of the plastic strain rate with decreasing applied stress indicates that several viscosity domains can be identified and each of them is characterized by an apparent activation volume. Thin foils cut out of samples submitted to relaxation tests interrupted after each domain are examined: they reveal various dislocation- arrangements and stacking fault configurations typical of each domain of viscosity. The first domain, following the initial loading at a rate of 10(-5) s(-1) under stresses larger than 1000 MPa is characterized by the multiplication and movement of perfect dislocations often moving by pairs, thus shearing both phases gamma + gamma' and finally dissociating into Shockley partials. The second stage extending below 1000 MPa exhibits a higher activation volume (approximate to 0.5 nm(3)) and seems to be controlled essentially by restoration processes. The third stage sets in below 850 MPa when strain rates of 10(-9) s(-1) are reached and only strongly coupled Shockley partials can move by viscous glide through both phases: a mechanism characterized by a small activation volume (approximate to 0.2 nm(3)). The creep tests are analyzed by use of log epsilon(p) = f(epsilon(p)) diagrams. Dislocation mechanisms governing creep in the stress domain 850 to 1000 MPa are analogous to those identified during the last stage of relaxation and the apparent activation volumes measured experimentally are similar.