Multiaxial creep deformation of single crystal superalloys: Modelling and validation.

The creep behaviour of the single crystal nickel-base superalloy CMSX-4 has been studied under multiaxial loading, using double Bridgman-notch specimens with <001> and <111> nominal axial orientations, at 850 degreesC and net section stresses of 600 to 850 MPa. The creep lives measured were an order of magnitude greater than those obtained with the same load in uniaxial loading. The disposition of the triaxial stress state with respect to crystallographic orientation varies around the perimeter of the notch. The creep-deformed material has been examined by electron back-scatter diffraction (EBSD) to characterise local changes in crystal orientation resulting from creep strain. It is shown that there is a spatial distribution of crystal rotation across the specimen diameter that is consistent with the activation of <111>{1 (1) over bar0} and <001>{110} slip for <001> and <111> specimens respectively. A model of anisotropic creep, developed at Imperial College over a number of years that assumes viscous slip on such a restricted set of slip systems, has been implemented in a commercial finite element code to simulate the notch-creep behaviour. There is good agreement between the computer simulation and the experimental results.