Comparison of stress relaxation and creep strain rates for the superalloy IN738LC

The strain rates derived from stress relaxation tests are compared to those from creep tests at 760 degrees C for the superalloy IN738LC for stresses between 500 and 600 MPa. The strain rates agree well from 500 to 575 MPa, but a difference exists at 600 MPa. The results are interpreted in terms of the deformation mechanism controlling the strain rate. It is suggested that dislocation mechanisms control the deformation, but at 600 MPa the mechanisms controlling the creep strain rate differs from that controlling recovery during stress relaxation. The time efficiency associated with stress relaxation testing can be used to advantage within a limited stress range, however, extrapolation of stress-relaxation-derived strain rates may lead to the same erroneous results associated with extrapolation of creep strain rates.