A comparative study of strain relaxation and stress relaxation in nickel-base superalloy in-738LC

Short-term closed-loop controlled "constant-strain" stress relaxation tests (SRT) and "constant-stress" strain relaxation and recovery tests (SRRT) were carried out successively on the same specimen of a nickel-base superalloy, IN-738LC, at 1000degreesC. Both techniques have been found to be suitable for evaluating the initial microstructure dependent flow properties of a material, However, the SRT tends to give a higher strain rate at a given stress level than that given by the SRRT. This is attributed to the contribution of the delayed elastic strain rate (due to the grain-boundary shearing mechanisms and hence grain size dependent) to the total strain rate during the SRT. The delayed elastic strain is not negligible and its role in the process of relaxation cannot be ignored. SRRT, in contrast, is capable of separating delayed elastic strain and viscous strain from the total strain. SRRT is therefore more powerful than SRT and is easier to perform.