Cyclic Deformation Behavior And Fatigue Damage Mechanism Of Copper Bicrystals

The cyclic deformation behaviors of single- and poly- crystals were well documented in the last three decades. It has been recognized that there existed great difference in fatigue mechanisms between single- and poly- crystals. In order to reveal the effects of grain boundaries (GBs) and component crystals, a series of work on the cyclic deformation behaviors of the copper bicrystals with large-angle GBs and the copper multicrystals containing low-angle GBs were carried out and reported in this paper. Some bicrystals with the size of 200×50×10mm~3 were grown from OFHC copper of 99.999% purity by Bridgman method in a horizontal furnace. Different kinds of copper bicrystal specimens with parallel, perpendicular, inclined and surrounding GBs and some copper multicrystal specimens containing low-angle GBs were prepared by electro-spark method, respectively. The fatigue tests were performed on a Shimadzu servo-hydraulic testing machine. The surface slip morphology, saturation dislocation patterns and fatigue cracks were observed by using optical microscopy, scanning electron microscopy (SEM) and electron channeling contrast (ECC) technique in SEM. Finally, the cyclic stress-strain responses and fatigue damage mechanisms of these copper crystals were discussed.The cyclic deformation behaviors of the really grown [ 679]//[ 145] copper bicrystal (RB) with a parallel GB, the crystals G1[ 679], G2[ 145] and the bicrystal (CB) combining with G1 and G2 were compared. All the cyclic stress-strain curves (CSSCs) of the crystals G1, G2 and the bicrystal CB showed the plateau feature, however, the CSSC of the bicrystals RB did not exhibited any plateau region and its saturation stress was always higher than that of the bicrystal CB. By optical microscopy, a GB affected zone (GBAZ) containing secondary slip and deformation bands (DBs) was observed in the vicinity of GB in the bicrystal RB. With increasing strain amplitude, the width of the GBAZ was also increased. By introducing an orientation factor Ω_B of the bicrystal and in combining with the effect of GBAZ, the resistance of GB to slip bands and the mean higher stress in the GBAZ were calculated and a GB strengthening model was proposed. Meanwhile, the CSSC of the copper multicrystals containing low-angle GBs also showed...