| 其他摘要 | It is well known that cyclic deformation behaviors of fcc single crystals have been widely studied since “persistent slip band” (PSBs) was defined as the common presence in fatigued crystals by Thompson et al. in 1956. In 1950’s – 1970’s, a large number of researchers (Broom and Ham (1959), Helgeland (1965), Laufer and Roberts (1966), Woods (1973), Winter (1974), Finney and Laird (1975)) focused on various PSBs’ mechanical and structural characteristics, including hardness, distribution, volume fraction, two-phase model and so on. Based on these early works, Mughrabi (1978) established the famous cyclic stress-strain (CSS) curve of single-slip-oriented Cu single crystal, which is the first quantitative determination of the relationship between the macro-scale deformation behaviors and micro-scale dislocation structures. Since 1980’s，Prof. Bretschneider and Prof. Holste have devoted to the cyclic deformation behaviors of Ni single crystals based on the temperature and orientation effects all along. During the same period, various other fcc single crystals, including Al, Cu-αAl, Cu-αZn and so on, were paid much more attentions on the fatigue behaviors. Combined with plenty of previous research results, we found that the influencing factors of cyclic deformation behaviors can be listed as follows: temperature, frequency, orientations and stacking fault energy (SFE). Among them, the orientation and SFE are the core factors. Then we proposed a new criterion to judge the cyclic deformation behaviors of different fcc crystals and explained the physical nature of formation mechanism of PSBs in various fcc crystals according to the criterion.
Temperature and frequency can be regarded as the extrinsic factors on cyclic deformation behaviors of fcc single crystals. As testing temperature increases, the plateau of CSS curves gradually disappears and the saturation resolved shear stress decreases distinctly at low strain amplitudes. At the same time, the space between PSB ladders in Cu single crystals is gradually enlarged, which causes the decrease in the volume fraction of the rungs in PSBs and thus leads to a lower of the saturation stress. On the other hand, the increase of frequency does not affect the appearance of the plateau behavior, but the corresponding plateau stress slightly increases. This is because a higher frequency increases the probability of interaction between edge dislocations, which further leads to the increase in the local flow stress in the rungs, and eventually causes the increment of saturation stress.
The CSS curves of silver single crystals of different orientations show a clear plateau region over a strain range of γpl=8.0×10-5-6.0×10-3 with two saturation shear stresses of about 18~21MPa or 25-26MPa, respectively. After that, the dislocation configurations from differently oriented silver single crystals are well summarized as follows: 1) PSB ladders or walls appear in [-239],[011]and [-459]silver single crystals; 2) labyrinth is the main dislocation structure in the [-1 8 18]silver single crystal at high strain amplitudes; 3) vein and cell structures form in the [-233] silver single crystal at low and high strain amplitudes, respectively; 4) the interaction between the primary and secondary PSBs arises in the [-1 4 14] silver single crystal.
Combined with the results of copper, nickel and silver single crystals, it can be concluded that the effects of the orientations on the cyclic deformation behaviors follow a general principle: the orientation-dependent dislocation configurations can be divided into three regions in the stereographic triangle, including the central region,[011] and [-111] regions. Based on further research about the three multiple-slip orientations, it can be found that the formation of more complicated labyrinth, cell or wall structures depends on which slip system is priority to actuate. And the choice of slip system shows that the physical nature of the orientation effect is strongly associated with fcc crystal structure.
The saturation stress of cyclic saturated Al single crystal is far lower than that of Cu single crystal. In fatigued Al single crystal, the cell structure is the most classical dislocation configuration. These cells are mainly composed of loose clusters of dislocations, so dislocations in the cells move much freely. The CSS curves of Cu-Al alloys have a shorter plateau with Al content increase. Meanwhile, the dislocation configurations change from PSB ladders in Cu-Al crystal with low Al content (≤5at%Al) to persistent Lüder’s bands (PLBs) in Cu-Al crystal with medium, even high Al content (≥8at%Al). Studies show that PLBs mainly consist of the dipole array or stacking faults (SFs) by dislocation reactions and interactions with other secondary dislocations.
The cyclic deformation behaviors, especially dislocation configurations from different fcc single crystals show obvious regularity with the change of SFE. Combined our understanding on the formation of dislocation configurations with previous theoretical models, it is summarized that characterizing the width of extended dislocation and the trap distance of dipole, respectively, G/γsf and τs/G are regarded as two important parameters to influence the formation of PSB ladders or not. In view of the formation of PSB ladders, the evolution processes of the macroscopic dislocation configurations can be shown as below: dipole segment → vein → PSB ladder; the corresponding microscopic dislocation configurations go through the course: primary edge dipole → primary edge dipole + faulted dipole → faulted dipole. Further researches show that in this process the ratio dextend/dtrap plays a vital role. Based on this criterion, the physical fundamental on the cyclic deformation behaviors of fcc crystals is established.
According to the width of extended dislocation, the relationships between slip and twinning, dislocation and stacking fault are shown. With the increment of the width, the slip mode of fcc crystals experiences the wavy-to-planar-slip transition and the formation of the SFs becomes easier, which leads to the transition of deformation mode from slip to twinning. |
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