EVALUATION OF CASTING DEFECTS IN POLYCRYSTALLINE, DIRECTIONALLY SOLIDIFIED AND SINGLE CRYSTAL SUPERALLOYS USING SEM-EDS-EBSD METHODS

摘要

Aero engine turbine blades are flight safety critical components and first of all they are characterized by high mechanical properties. Increase of mechanical properties is achieved by replacing equi-axed (EQ) polycrystalline blades by directionally solidified (DS) and single crystal ones (SX). During the investment casting, the formation of many defects is possible. Typical casting defects are: non-metallic inclusions, high angle boundaries and freckles. Comprehensive evaluation of these defects can be performed using the combined techniques of scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDS), and electron backscatter diffraction (EBSD). Using EDS and EBSD techniques, phase identification in non-metallic inclusions was carried out. The identification was often ambiguous or even impossible due to overlap of Kikuchi patterns originated from several compounds. High angle boundaries (HAB) were detected in experimental DS turbine blade with columnar grains. Application of EBSD technique allows to determine the angle between [001] direction of grains and blade axis and the misorientation between neighbouring grains. Obtained results show that in this blade columnar grains are rotated relative to each other with simultaneous deviation from the [001] orientation. Freckles are casting defects appearing on DS and SX castings surface in the form of long chains of equi-axed grains aligned parallel to the direction of gravity. The main reasons for freckles forming are thermosolutal convection and buoyancy forces in the mushy zone caused by density inversion in the interdendritic liquid. Some investigations suggest that freckling tendency strongly depends on the orientation of the SX: casting with [001] orientation should be much more prone to freckles than casting with [001] direction highly deviated from the solidification direction. In the investigated casting, freckles appear although the angle between the [001] direction and the solidification direction was about 40°. EBSD technique was applied to determine crystallographic orientation of freckle grains in relation to SX casting on their longitudinal section. These results reveal some new details of freckles formation mechanisms. Orientation of some freckle grains is very close to the SX, but in many cases it is quite different. It indicates on different rotation facility of growing nuclei in plume of interdendritic liquid.