Formation of stray grains during directional solidification of a nickel-based superalloy

Superalloy CMSX-4 is directionally solidified and initiated by bicrystal seeds. It has been found that diverging boundaries are the most favorable location for stray grain formation. The phenomenon cannot be attributed to nucleation of crystals. A reasonable mechanism is the bending or detachment of side arms during extension of secondary arms and the development of tertiary branches at the diverging boundaries. Solute interaction of the neighboring dendrites increases the likelihood of bending or detachment and thus leads to an enhanced frequency of stray grains.

► Bicrystal castings were produced to study the factors that promote stray grains during directional solidification.
► Occurrence of stray grains is not due to random nucleation, but due to bending or detachment of side arms on dendrites.
► Solute interaction of the neighboring dendrites promotes likelihood of bending or detachment of side arms.