Amelioration of weld-crack resistance of the M951 superalloy by engineering grain boundaries

•Microstructures of GBs can be engineered by altering cooling rate of heat-treatments.•Lowering cooling rate, more M23X6 phase precipitates on GBs with segregation of B.•Lower cooling rate, GBs become more serrated in morphology.•Correlation between the microstructures of GBs and the weldability has been established.•Weld-crack resistance of Ni-base superalloys can be ameliorated via engineering GBs.