Strengthening behavior in non-isothermal monotonic and cyclic loading in a Ni-based single crystal superalloy

Dwell-fatigue tests and variable strain rate tensile tests followed by cycling tests were performed using 〈0 0 1〉-oriented specimens made of a first-generation Ni-based single crystal superalloy. A short thermal jump from the nominal temperature of 1050 to 1200 °C was introduced along the lifetime of dwell-fatigue experiments and at the beginning of tensile tests. A fine γ′ precipitation occurred in the γ matrix upon such event which induced a large transient strengthening effect on the mechanical properties. In fact, a transient decrease of the plastic strain rate, corresponding to a reduced magnitude in the hysteresis loops, was measured after the temperature peak during the dwell-fatigue experiments. In addition, a temperature peak produced a large hardening effect and a mechanical behavior change during the tensile tests since a hardening of 160 MPa was recorded and a perfectly plastic behavior was obtained. These transient phenomena were due to a temporary additional strengthening provided by fine γ′ precipitates lasting for the time necessary to not hinder dislocation motions anymore. Furthermore, the experimental results were also used to determine how fine γ′ precipitates modified the magnitude and the saturation speed of the isotropic and kinematic hardenings.