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xucz

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[交流] 高手帮忙润色一下这段英文

*      This change is also reflected in the hardness. The variation of their microhardness vs. the number of ECAP passes are shown in Fig. 3. As can be seen, the improvement of hardness with the number of ECAP passes is evident. Further observations of Fig.3, it can be seen that the microhardness of samples increases rapidly with the number of ECAP passes in the initial several passes, and then the increase gradually tends to saturate. For example, in pure Cu, after 4 passes, the hardness value is the 205% of that of the pre-ECAP one; but after 12 passes, its value just increases to 218% of pre-ECAP one. Obviously, after several passes, the rate of the increase in microhardness is not as significant as that on initial stage. But the number of ECAP passes is visibly different between pure Cu and Cu-Cr alloy when the increase of hardness reached their saturation. After 4 passes, the increase in hardness of pure Cu is prone to be stable. But as for Cu-Cr alloy, there is still slowly increase in hardness even after 6 passes. These behave accords with the microstructure characteristic discussed in Section 3.1. The state of the structure influences the hardness in materials. While the stable microstructure being established, as the deformation continue, the metal matrix can no longer accommodate more deformation strains by dislocations and grain boundaries propagation, and then the subgrain microstructure begins to rotate independently so as to accommodate more deformation. Thus, the increase of hardness is slow down and prone to saturation. In Cu-Cr alloy, there need more ECAP pass to establish a stable microstructure, so, its hardness still slowly increases after more passes.


*      The much more increment in microhardness of Cu-Cr alloy with ECAP passes might be due to following reasons. First, the mean grain size in Cu-Cr alloy are smaller than that in pure Cu due to the presence of Cr. the effect of the grain size on strength follows the classical Hall-Petch type equation in general, i.e. the yield strength increases with grain size decreasing. This is due to the number of dislocations in the pile-ups is decreased with the grain size decreasing, which should produce a much smaller stress concentration in the next grain [29, 30]. Therefore, a much larger applied stress is needed to cause slip to pass through the boundary for UFG materials as reported  [17, 20]. Second, the accumulated plastic strain increases with the number of ECAP passes for materials, which creates dislocations and dislocation strengthening gradually during ECAP [31]. The presence of Cr precipitations reduces the dislocation mobility, so, the dislocations are not easily to annihilate, which leads to much more stable and denser dislocations distribution clustering near the boundaries and precipitations. The large quantity of dislocations and boundaries favourably become effective barriers to crack propagation. Third, the Cu-Cr alloy is a kind of precipitation hardened alloy. The aging treatment results in the formation of the fine precipitate particles dispersed within the matrix, which would effectively strengthen Cu-Cr alloy. Overlap effect mentioned above three strengthening mechanism results in the hardness furthest improvement. Thus, there are much higher hardness in Cu-Cr alloy than that in pure Cu.


*     Conclusions：   Ultra-fine grain pure Cu and Cu-Cr alloy were prepared by ECAP, the effect of Cr addition on their microstructure was investigated and discussed in present work. The results have shown that the trace addition of Cr has an apparent effect on their microstructure. As a low SFE and precipitate strengthening metal, although the effect of Cr addition in Cu on microstructure is not as sharp as that of Mg addition in Al, the effect is visible yet. The microstructures of Cu-Cr alloy are finer and the boundaries are more poorly-defined than those of pure Cu. The mean grain size of pure Cu and Cu-Cr alloy under stable structure are about ~305 nm and ~230 nm, respectively; meantime, more ECAP passes are required in order to attain a reasonably stable microstructure in Cu-Cr alloy. Hardness of Cu-Cr alloy is significantly higher than those of pure Cu counterparts. The overlap effects of grain refinement, cold deformation and precipitation strengthening mechanisms result in hardness further improvement in Cu-Cr alloy. All of those significant changes are due to the trace addition of Cr.
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1楼 2009-09-02 14:28:13
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xucz

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没人应答啊
再增加金币

主要是改改语法就可以了

[ Last edited by xucz on 2009-9-3 at 08:29 ]
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2楼2009-09-03 08:28:23
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rain841230

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那么多，看得头晕。。。
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3楼2009-09-04 22:03:36
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xucz(金币+66,VIP+0):thaks a lot 9-5 12:07
This change is also reflected in the hardness. The variation of microhardness vs the number of ECAP passes are shown in Fig. 3. As can be seen, the improvement of hardness with the number of ECAP passes is evident. Further observations showed that the microhardness of samples increased rapidly with the number of ECAP passes in the initial several passes, and then the increase gradually tended to saturate. For example, in pure Cu, after 4 passes the hardness value was 205% that of the pre-ECAP material; but after 12 passes the hardness only increased to 218% of pre-ECAP material. Obviously, after several passes, the rate of the increase in microhardness is not as significant as that on initial stage. However, the number of ECAP passes is visibly different between pure Cu and Cu-Cr alloy when the increase in hardness reached saturation. After 4 passes, the increase in hardness of pure Cu is observed to be stable. In contrast, for Cu-Cr alloy, there was a slow increase in hardness even after 6 passes. This behaviour corresponded with the microstructure characteristics discussed in Section 3.1. The state of the structure influences the hardness in materials. While the stable microstructure is being established, as the deformation continues the metal matrix can no longer accommodate more deformation strains by dislocations and grain boundaries propagation. Further, the subgrain microstructure begins to rotate independently, so as to accommodate more deformation. Thus, the increase in hardness is reduced and saturation occurs. In Cu-Cr alloy, more ECAP passes were needed to establish a stable microstructure, in order for the hardness to slowly increase.


*      The increased increments in microhardness of Cu-Cr alloy with the number if ECAP passes could be due to a number of reasons as described as follows. First, the mean grain size in Cu-Cr alloy were smaller than that in pure Cu due to the presence of Cr. The effect of the grain size on strength follows the classical Hall-Petch type equation, i.e. the yield strength increases with a decrease in grain size. This is due to the number of dislocations pile-ups which is decreased with decreasing grain size, consequently producing a much smaller stress concentration in the next grain [29, 30]. Therefore, a much larger applied stress is needed to cause slip to pass through the boundary for UFG materials as reported  [17, 20]. Second, the accumulated plastic strain increases with the number of ECAP passes for materials, which creates dislocations and dislocation strengthening gradually occurs during ECAP [31]. The presence of Cr precipitations reduces the dislocation mobility, therefore the dislocations are not easily to annihilate, which leads to much more stable and denser dislocation distribution clustering near the boundaries and precipitations. The large quantity of dislocations and boundaries favourably then become effective barriers to crack propagation. Third, the Cu-Cr alloy is a type of precipitation hardened alloy. The aging treatment results in the formation of the fine precipitate particles dispersed within the matrix, which would effectively strengthen Cu-Cr alloy. The overlap effect of these three strengthening mechanism results in the further improvement of hardness. Thus, in the hardness of Cu-Cr alloy is greater than that of pure Cu.


*     Conclusions：   Ultra-fine grain pure Cu and Cu-Cr alloy were prepared by ECAP, the effect of Cr addition on their microstructure was investigated and discussed in the present work. The results have shown that the trace addition of Cr has an apparent effect on the materials’ microstructure. Due to the low SFE and precipitate strengthening of the metal, the effect of Cr addition in Cu on the microstructure is not as sharp as that of Mg addition in Al, however the effect is still visible. The microstructures of Cu-Cr alloy are finer and the grain boundaries are less defined than those of pure Cu. The mean grain size of pure Cu and Cu-Cr alloy under a stable structure were about ~305 nm and ~230 nm respectively; In addition, more ECAP passes are required in order to attain a reasonably stable microstructure in Cu-Cr alloy. Hardness of Cu-Cr alloy is significantly higher than those of pure Cu. The overlap effects of grain refinement, cold deformation and precipitation strengthening mechanisms, result in further hardness improvement in Cu-Cr alloy. All of those significant changes are due to the trace addition of Cr.
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