INTERFACE CONTROL FOR RESISTANCE TO INTERGRANULAR CRACKING

Theoretical and experimental results are presented, with the primary o bjective of improving the resistance of conventional polycrystalline a lloys to intergranular degradation phenomena, through the application of grain boundary design and control. Geometric considerations are dis cussed, which show that, as a consequence of both energetic and crysta llographic constraints associated with twinning, a grain boundary char acter distribution (GBCD), consisting entirely of low Sigma grain boun daries, is attainable. A geometric model of crack propagation through active intergranular paths is used to evaluate the potential effects o f Sigma grain boundary fraction and grain size on intergranular cracki ng. The effect of the GBCD on intergranular stress corrosion cracking and intergranular corrosion in a nickel-based alloy 600 (Ni-16Cr-9Fe) is determined. Important factors in achieving microstructural optimiza tion of alloy 600 are presented. These results provide direct experime ntal support for the model of intergranular crack propagation, and dem onstrate the importance of grain boundary structure control for enhanc ing the resistance of a material to inter,oranular degradation.