Comparison of cyclic deformation behavior between copper bicrystals and their component crystals
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ODUCTION
It is well known that copper, single crystal oriented for single slip, exhibits a plateau region over a resolved shear plastic strain range from 6.0 3 1025 to 7.5 3 1023 in cyclic stress-strain curve (CSSC).[1] Most investigators[2,3] have found that the plateau saturation resolved shear stress at room temperature lies in the range from 28 to 30 MPa, nearly independent of the crystal orientations when the crystal orientation orients for single slip. Meanwhile, the cyclic deformation behavior of copper polycrystals was also investigated extensively. It was found that the CSSCs of some copper polycrystals also exhibited a plateau feature over certain strain ranges.[4–7] However, Mughrabi[8] and Lukas and Kunz[9] suggested that there should not be any plateau region in the CSSCs of copper polycrystals. In order to compare the deformation features between monocrystals and polycrystals, the effects of both grain boundaries (GBs) and crystal orientations should be considered. Each grain in a polycrystal has its own stress-strain characteristic, whereas the stress-strain response of the whole polycrystal is not a simple ‘‘average’’ over all the grains.[10] The reason is that the grains in a polycrystal do not deform independently. The strain compatibility requirement leads to the operation of secondary slip in the vicinity of GBs.[11,12] To clarify the difference in plastic deformation between monocrystals and polycrystals, the bicrystal is often regarded as an important model material for revealing the effects of GBs and component crystals. However, most of the studies on bicrystals were performed under monotonic loading conditions.[12–17] Only very limited work on Z.F. ZHANG, Postdoctoral Student, and Z.G. WANG, Professor, are with the Institute of Metal Research, Chinese Academy of Science, Shenyang 110015, People’s Republic of China. Manuscript submitted August 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
the fatigue behavior of bicrystals such as a-b brass,[18] aluminum,[19] a-Ti,[20] an Fe-Si alloy[21] and copper bicrystals,[22–26] can be found. Recent studies[23,24] on copper bicrystals with a parallel GB revealed that the saturation stresses can be increased by the GB. However, the component crystal orientations have a decisive effect on the CSSC of the bicrystal with a perpendicular GB.[22,25,26] In order to further study the cyclic deformation behavior of copper bicrystals, the effects of both component crystals and GB on the fatigue behavior of copper bicrystals will be investigated separately in this article. II.
EXPERIMENTAL PROCEDURE
A bicrystal plate measuring 200 3 50 3 10 mm3 was grown from oxygen-free high-conductivity copper of 99.999 pct purity, by the Bridgman method, in a horizontal furnace, and the GB was parallel to the growth direction. In order to compare the cyclic deformation behavior of bicrystals with their component crystals, three kinds of fatigue specimens were made from the grown bicrystal plate. They are (1) the fatigue specimen of a naturally grown copper bicrystal (R
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