Grain boundary-induced shielding of Ni-base single crystals: Experimental and analytical study

Fracture behavior was studied for Ni-base single crystals and Ni-base bicrystals. The single crystals were machined from the bicrystal specimens to ensure that the single crystal samples had the same properties as those of the grains in the bicrystals. Using the three-point bend specimens, the fracture toughness K_c of single crystals and grain boundaries was measured, respectively. The two toughness values were little dependent on the ratio of the crack length to the height of the specimens. The grain boundary-induced shielding effect of fracture toughness was found in the grain-cracked bicrystals with normal front grain boundaries. The toughness varied with the distance between the crack tip and grain boundary. With the help of SEM analyses and numerical calculations, the mechanisms for the grain boundary-induced shielding are revealed. It is the grain boundary-induced redistribution of stresses and strains near the crack tip that induces the grain boundary-induced shielding. And the strain energy density criterion can be applied to the fracture of bicrystals. Agreement with experiments is good.