Origins of misorientation defects in single crystal castings: A time resolved in situ synchrotron X-ray radiography study

The presence of grain boundaries in single crystal castings is intolerable owing to the detrimental impact on creep and fatigue behaviour. Whilst the origins of many defects such as freckles have been understood since the 1970s, other defects such as slivers, or indeed the small mosaicity observed in many castings have eluded comprehensive treatments. In the present work, in situ X-ray imaging has been used to examine the origin of misorientation defects that arise during solidification. Dendrite deformation was observed, which impacted growth characteristics and led to a permanent misoriention. Digital image correlation analysis showed the dendrites becoming increasingly bent as solidification progressed. In order to probe the deformation modes further, a method was devised to convert standard EBSD data into measurements of bending and torsion angle. It was demonstrated that such defects form as the result of bending moments arising from differential thermal contraction and gravity.