Fatigue Crack Propagation in Copper Bicrystals Having the Grain Boundaries of Σ3 Vicinal Domain

Abstract

The criterion for occurrence of intergranular fatigue cracking in copper has been investigated from the view point of both the grain boundary (GB) character and the cyclic deformation property of constituent grains. The copper bicrystals were prepared to have several orientation relationships close to Σ3(1 1 1) coherent twin (Σ3 vicinal domain) so as to change the GB character rapidly with increasing deviation angles |Δθ| from the Σ3 relation. These bicrystals were shaped to single-edge-notched specimens in which a GB plane was perpendicular to the tensile axis. The fatigue crack propagation tests were carried out in air at room temperature. The specimens having deviation angles |Δθ| less than 3° involved no intergranular fatigue cracking. When the |Δθ| values were ranged from 3° to 5°, the ratio of the intergranular cracking increased. In the specimens having the |Δθ| values more than 9°, the intragranular cracking became predominate again. The increase in the intergranular cracking with increasing deviation angle at the |Δθ| values less than 5° could be understood in terms of the increasing GB susceptibility to the GB damage due to air environment. On the other hand, the intragranular cracking at the |Δθ| values more than 9° could be attributed to the formation of the persistent slip bands in the constituent grains and subsequent crack propagation preferentially along them.