Cyclic plasticity, creep and fracture in a Nimonic alloy

Abstract

Deformation and fracture are examined for a cyclic loading condition in which a 24 h period of forward creep in IN 597 at 850°C is reversed (a) by a similar period of creep and (b) by fast monotonic strain-limited plasticity with and without a subsequent 1 h period of relaxation. When ratchetting is evident, it is shown that a life fraction rule based upon the Monkman—Grant law describes the observed behaviour and that, for a given creep stress, the accumulated creep strain—time plot correlates with a simple creep curve. At lower-magnitude creep dwell stresses a cyclic equilibrium state is achieved. The absence of ratchetting during equilibrium results in a net increase in rupture life and fracture strain when compared with those for simple creep under the same stress. Time-limited cyclic behaviour is compared with further cyclic tests in which the reversal occurs at fixed limits of total strain. These did not show an equilibrium condition but a progressive cyclic softening in which the creep time and strain successively diminished. Predictions to individual creep curves are made from the mechanical equation of state and the time hardening law. However, empiricism is preferred to describe the softening behaviour observed for the alloy more precisely. A theoretical assessment is made of the effect of imposing a period of relaxation at peak reversed strain upon cyclic creep deformation and life.