DETERMINATION OF THE LOCAL TENSILE AXIS DIRECTION IN A TEM INSITU STRAINED GAMMA-' SINGLE-CRYSTAL - A FINITE-ELEMENT APPROACH

An in situ study of the plastic deformation mechanisms occurring in the ordered γ′ phase of the CMSX2 superalloy has been performed inside a Jeol 200 CX electron microscope. This type of study necessitated the development of a single crystal tensile test specimen with a special curved profile which allowed us to concentrate the stresses at the edge of a hole generated by a polishing treatment. We then deformed this highly resistant material in a wide temperature range within the microscope. Since the mechanical properties of this material are strongly orientation dependent, we were interested in determining the orientation of the local tensile axis in the specimen produced during the experiment. This was obtained by performing a finite element analysis using the code MSC/NASTRAN® and extracting the state of principal stresses near the edge of the hole. By recalculating the Schmid factors in the vicinity of the hole, we were able to explain the development of the slip systems observed during the experiments. We will show that the orientation of the stresses occurring in the in situ experiments can be accurately predicted, and that there exists a zone, the extent depending on the diameter of the hole, in which the major principal stress axis is parallel to the global tensile direction. These results from the particular study of the CMSX2 γ′ phase can be of interest to all in situ straining experiments. © 1990.