Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

To clarify the strengthening effect of grain boundaries (GB), cyclic deform ation behaviour of really grown [(4) over bar 79]parallel to[(1) over bar 2 5] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, R B-6, and RB-8) of component crystals and a combined copper bicrystal (denot ed CB-6), obtained by sticking component single crystals G1 [(4) over bar 7 9] and G2 [(1) over bar 25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused diffe rent degrees of strengthening, which increased with the decreasing width of the RE bicrystals, By surface observation, it was found that only the prim ary slip system was activated in the combined bicrystal during cyclic defor mation. However, an additional slip system appeared near the GB within the crystal G2 [(1) over bar 25] in the RB bicrystals (except in the primary sl ip system), and formed a GB affected zone (GBAZ), The width of the GBAZ was about 400 and 600 mu m at plastic strain amplitudes of 0.1% and 0.2% respe ctively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the (GB and within the component crystals were observed. It was found that a two phase structure of persiste nt slip bands (PSBs) and matrix (or veins) can form in these bicrystals, si milar to that in copper single crystals. But these PSBs cannot transfer thr ough the GB during cyclic deformation. Based on the results above, the effe ct of grain size on GB strengthening of copper bicrystals was discussed. MS T/4267.