Effect Of Cyclic Pre-deformation On The Uniaxial Mechanical Properties Of Copper Single Crystals With Different Orientations

In practical engineering applications, the mechanical properties of materials would often become weakened with the accumulation of fatigue damage during the process of their service. Therefore, studies on the general rule and mechanism for the influence of pre-fatigue damage on the mechanical properties of materials are of particular significance. However, there is much less information about such investigations. In view of this, in the present work, copper single crystals, which possess typical face centered cubic (FCC) structure, are selected as the model materials to study the effect of previous cyclic deformation on their uniaxial static mechanical properties. Owing to the fact that during the deformation of the actual polycrystalline metals the double or multiple slip is a general phenomenon, three kinds of double-slip orientations are chosen in the present work at three different sides of (001) standard triangle, namely [017] critical double-slip,[233] coplanar double-slip and [112] conjugate double-slip orientations, for investigations, with the purpose of revealing the micro-mechanisms for the effect of fatigue damage on the static mechanical properties of metallic crystals.For [017] critical double-slip-oriented copper single crystals, there exists an obvious effect of cyclic pre-deformation at different plastic strain amplitudes on their uniaxial mechanical properties. The results show that after cyclic pre-deformation at a plastic strain amplitude (γp1=7.0×10-4), which is located within the quasi-plateau in the cyclic stress-strain (CSS) curve, the tensile strength of the [017] crystal is obviously enhanced, but the plasticity is slightly decreased; this is probably because cyclic strain hardening during cyclic pre-deformation causes an remarkably increased dislocation density but does not induce much permanent damage in crystals in this case. In contrast, as the plastic strain amplitude increases to be higher than the quasi-plateau region, i.e., γp1=3.0×10"3, the plasticity of the pre-fatigued [017] crystal further decreases, meanwhile, its tensile strength also drops to be less than that of the unfatigued crystal. The dislocation structures induced by pre-fatigue deformation at γp1=7.0×10-4have nearly disappeared after subsequent uniaxial tensile deformation. However, the dislocation structures induced by the previous cyclic deformation at γp1=3.0×10-3are partially retained after uniaxial deformation. Similar conclusive results are also obtained with uniaxial compression experiments.[233] coplanar double-slip-oriented copper single crystals are cyclically pre-deformed to saturation stage and then uniaxially tensioned to fracture. The results show that the samples cyclically pre-deformed at two plastic strain amplitudes within the quasi-plateau of the CSS curve exhibit evidently different tensile behavior from others. The uniaxial tensile plasticity of these two samples keeps almost unchanged, but their strengths are improved markedly, as compared with those of the directionally tensioned sample, indicating that the fatigue damage induced by cyclic pre-deformation is not so serious in these cases. However, the tensile strength and plasticity of the crystal samples, which are pre-fatigue deformed at plastic strain amplitudes slightly lower or higher than the quasi-plateau region, are somewhat decreased or unchanged. TEM observations indicate that the dislocation structures of [233] copper single crystal induced by cyclic pre-deformation are mainly composed of dislocation cells, and that the dislocation structures of the samples pre-fatigued and then uniaxially deformed consist also of dislocation cells. Therefore, it is comparatively hard to distinguish whether these dislocation cells formed in the pre-fatigued and then tensioned samples are inherited from those formed in the course of cyclic pre-deformation.Cyclic pre-deformation tests of [112] conjugated double-slip-oriented copper single crystals are performed, respectively, at plastic strain amplitudes, which are lower than, higher than, or within the plateau region. Afterwards, the pre-fatigued samples are uniaxially deformed by tension or compression. The experimental results show that the plasticity of the sample pre-deformed at a lower plastic strain amplitude than the plateau region is not obviously declined, while its strength is clearly improved. However, both of the strength and plasticity of the samples pre-deformed at two plastic strain amplitudes within the plateau region are noticeably decreased. Compared with the cases of former two samples, the tensile strength and plasticity of the sample pre-fatigued at a higher strain amplitude than the plateau region are obviously improved. The above experimental phenomena are believed to be closely related to the fact that various volume fractions of PSB ladders induced by cyclic pre-deformation at different plastic strain amplitudes would yield a determinative influence on the subsequent uniaxial deformation behavior.