Creep Damage Evaluation of DS CM247 Nickel Base Superalloy Using Alternate Current Potential Drop Technique

The evolution of creep damage in a directionally solidified DS CM247 alloy during creep at 950 A degrees C/250 MPa has been evaluated using alternate current potential drop (ACPD) technique in which a constant alternating current was passed through the specimen and corresponding potential drop was measured. Creep tests were interrupted at different strain levels in order to examine the microstructural degradation of the alloy during creep. The creep curve of the alloy exhibited limited primary and secondary creep regimes, but extended tertiary creep regime. Microstructural examination revealed the progressive directional coarsening of gamma' precipitates i.e., rafting and widening of gamma matrix channel with accumulation of creep strain. The specimen exhibited a considerable amount of creep strain prior to fracture. Further, creep cavitation was observed only close to the fractured end of the specimens. Various factors responsible for the extended tertiary creep regime of the alloy are discussed. The potential of ACPD technique to be used as a NDE tool to monitor in service/online creep damage evaluation, has been established/demonstrated.