Solidification cracking of superalloy single- and bi-crystals

Superalloy single crystals with cellular-dendritic microstructures and different crystal orientations were laser-welded under otherwise identical conditions. In this way, single crystals and bi-crystals with pure tilt grain-boundaries could be produced with well-defined misorientation angles and the effect of grain boundary misorientation on hot cracking tendency could be studied. Under certain conditions, cracks formed in the last stage of solidification. Measurement of the crack length showed that solidification cracks were absent in single crystals and low angle bi-crystals, but formed beyond a critical grain boundary angle, which in our experiments was 13°. The experimental findings are discussed in the light of recent theory of solidification cracking and of crystal coalescence in the final stage of transformation. The reason for the increased sensitivity of high angle grain boundaries to solidification cracking is their low coalescence temperature. This leads to an increase of the length of the vulnerable region of the mushy zone and localises the strain at the boundaries. The results indicate that, even in alloys of high cracking sensitivity, solidification cracks may be avoided when only sub-boundaries are present, i.e., when the grain boundary angle is below a critical value, which corresponds to a critical grain boundary energy.