Fig. 1. GB microstructures of the pre-weld heat-treated M951 superalloy with three different cooling ways: the (a) AC, (b) IFC, and (c) FC samples. More detailed GB microstructures under a higher magnification of the (d) AC, (e) IFC, and (f) FC samples.

Fig. 2. SEM micrographs of the GB regions of the heat-treated (a) AC, (b) IFC, and (c) FC samples with the corresponding elemental mapping of B, Cr and Nb by SIMS.

Fig. 3. TEM micrograph of the IFC sample. The diffraction spots in the SAED pattern (inset) from the circle region can be indexed as the M23X6 phase and the γ phase.

Fig. 4. Schematic diagram on the division of regions for calculating the crack index.

Fig. 5. Statistical results of the average crack length and the crack index of the welded three M951 samples.

Fig. 6. SEM micrographs of the welded M951 superalloy: (a) AC, (b) IFC, and (c) FC samples. More detailed GB microstructures under a higher magnification of the welded (d) AC, (e) IFC, and (f) FC samples.

Fig. 7. The statistical result of the mutual area-fraction of the MC and M23X6 phases in the three pre-weld heat-treated M951 samples.

Fig. 8. Equilibrium phase diagrams of the M951 superalloy calculated by Thermal-Calc.

Fig. 9. Element mapping of Cr, C, B, Nb, and Mo in the HAZ of the welded FC sample.

Fig. 10. Schematic illustration of the microstructural evolution of GBs during welding. The (a) pre-weld, (b) welding, and (c) post-weld microstructures of the AC sample. And the (d) pre-weld, (e) welding, and (f) post-weld microstructures of the FC sample. The formation of continuous liquid films on GBs contributes to the amelioration of weld-crack resistance.