| 其他摘要 | The stacking fault energy (SFE) is an important factor by which the micro
structure and mechanical properties of materials are deeply affected. During the processes of fatigue and the severe plastic deformation (SPD), metals with different SFEs exhibit different deformation characteristics and mechanical behaviors. In this paper, three Cu-Al polycrystalline alloys with different Al contents (Cu-5at%Al, Cu-8at%Al, Cu-16at%Al) were chosen as the starting materials and their cyclicstress-strain(CSF) responses, fatigue cracking along twin boundaries, microstructural evolution and tensile behaviors at RT during SPD (ECAP) were systematically investigated. Besides, the effects of annealing on the microstructures and mechanical properties of the ECAPed Cu-Al alloys for 4 passes were investigated.
During the fatigue of constant plastic strain amplitude, the slip mode changed from wave slip to planar slip with decreasing the SFE of the three Cu-Al polycrystalline alloys; the cyclic non-hardening stage appeared and became longer. The cyclic hardening rate becomes smaller, and the cyclic saturation stress in creases. The plateau in the CSS curves for the three Cu-Al alloys. The fatigue cracking observations showed that the common large-angle grain boundaries (GBs) often become the preferential sites for the fatigue crack initiation in the three Cu-Al polycrystalline alloys. With in creasingthe plastic strain amplitude, fatigue cracks can be formed along the cross-slip bands when the Al content is low (Cu-5at%Al) as well. However, fatigue cracks were also found along the duplex slip bands like a sawtooth in the Cu-16at%Al also.
With the decrease in the SFE of Cu-Al alloys, the slip mode near TBs also changed from wavy slip feature to planar slip feature during fatigue. The severely deformed persistent slip bands （PSBs） in Cu-5at%Al beside the TBs distributed symmetrically along the TBs and can go through TBs. While, in Cu-16at%Al, the slip deformation is different at both sides of TBs. The severely deformed SBs are interlaced along TBs. Accordingly, the fatigue cracking behaviors along TBs are different in the three Cu-Al polycrystalline alloys. In the high-SFE Cu-Al alloy (Cu-5at%Al), fatigue cracks were found to nucleate along those severely plastic deformed SBs and these fatigue cracks along SBs distributed symmetrically along the TBs and can pass through the TBs into the neighboring twin grains. However, for the Cu-16at%Al alloy with low SFE, the fatigue cracks mainly nucleated along the TBs and both SB and TB cracking modes can be found in the Cu-8at%Al alloy with moderate SFE. Meanwhile, the resistance of to fatigue cracking TBsdecreases with decreasing the SFE.
During the processes of ECAP, a transition of the grain refinement mechanism from dislocation subdivision to twin fragmentation was analyzed with the decrease in SFE. The homogeneous microstructures of materials with high or low SFE, are much more readily gained than that of medium-SFE metals. And several microscale shear bands can be easily formed resulting in the shear fracture along the main shear plane in the materials with extremely low SFE at large strain. The grain size of Cu-Al alloys with low SFE can be refined into nanometer scale, hence the minimum grain size decreased with lowering SFE. Meanwhile, dmin/b may roughly fit a linear relationship with the normalized SFE, /Gb and the dependence of dmin/b on /Gb, qualitatively reflected by the K value, is determined by the severity degree of external deformation condition. Compared with the other deformation modes, lowering SFE can also enhance the value lnZ by changing the intrinsic factors. This can be indirectly substantiated by the relationship between minimum grain size and activation energy of diffusion. And the factors influencing the microstructure after large plastic deformation were carefully examined. Differing from those of high-/low- SFE materials which are nearly insensitive to the external deformation conditions, the post-deformation microstructures of medium-SFE materials are highly dependent on them. Moreover, the strength and ductility can be simultaneously improved with decreasing the SFE in Cu-Al alloys, which can be attributed to the formation of deformed twins, SFs, microscale shear bands and their interplays.
After annealing, the thermal stability of ECAPed ultrafine grain (ufg) Cu-5atAl is better than that of ECAPed ufg Cu-8atAl . Annealing induces a static recrystallization accompanied with a strength drop and an enhanced ductility in deformed Cu-Al samples. The strength-ductility combination of the annealed ECAPed ufg Cu-8atAl is superior to that of ECAPed ufg Cu-5atAl. With the same uniform elongation, the yield strength of annealed ECAPed ufg Cu-8atAl is about 100 MPa higher than that of ECAPed ufg Cu-5atAl. |
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