The γ/γ′ mismatch in Ni based superalloys:: In situ measurements during a creep test

The lattice mismatch between the fcc gamma matrix and the ordered gamma' cuboidal precipitates in superalloys induces large internal stresses within the material. These stresses have a major effect on its mechanical behaviour and on the anisotropic evolution of the microstructure (rafting...) during its lifetime. The evolution of the effective lattice mismatch of the AM1 and MCNG superalloys was measured continuously during high temperature creep tests (I 100 degreesC, 150 MPa) at the ID 15 (high energy) beamline of the ESRF. The bulk profiles of the 200 reflection (parallel to the tensile axis) were recorded using the triple crystal diffractometer. Both materials have a negative mismatch, and exhibit a transition between the initial wide gamma' peak to a two peaks profile during stage I of the creep curve, as rafting takes place. During stage II, the 200 mismatch decreases in magnitude. During stage 111, as a microstructural transition (coalescence) occurs and the strain rate increases, the mismatch changes in the same direction, but at a faster rate, while the thickness of both peaks increases. Evolution of the MCNG specimen was the same as AM1 ones, but rafting and transition to stage II take place at a different rate. (C) 2002 Elsevier Science B.V. All rights reserved.