Fatigue crack growth and fracture behavior of bismuth-doped copper bicrystals

Abstract

Crack growth rate measurements in Cu-Bi bicrystals have shown that for a given cyclic stress intensity factor ΔK, the crack growth rateda/dN increases with increasing Bi concentration. The increase inda/dN is due to the surface energy reduction that occurs because of the presence of bismuth in the copper. For a given bicrystal orientation, the Paris exponentm was found to increase with increasing bismuth content. Them value was found to be between about 0.5 and 3. This finding points to the need for fatigue crack growth theories that incorporate a variation inm in their crack growth laws. The grain boundary fracture surfaces of weak bicrystals showed steps whose formation is thought to arise from the need for bismuth to segregate to low energy surfaces. The results also indicate that the fracture surface energy of weak bicrystals can be lower than that of pure bismuth. This result might help explain the finding that when present, bismuth particles at the boundary tended to pull out of the matrix of one of the single crystals instead of cleaving flush with the boundary. Bismuth coverage at the grain boundary fracture surfaces of weak bicrystals was found to be between about 0.5 and 2 monolayers.