| 其他摘要 | Equal channel angular pressing (ECAP) is one of important techniques to produce ultrafine-grained (UFG) materials. In this paper, commercially pure Al and Cu-Ni alloy were chosen as the materials to be conducted by ECAP. After ECAP, a series of investigations were conducted to reveal their mechanical properties, deformation mechanism, tension/compression (T/C) asymmetry and plastic deformation under different strain rates. Based on those studies, the main goal is to comprehensively understand the deformation and fracture mechanisms of the ECAPed materials and give some instructions on how to design advanced metallic materials through ECAP.
Although the strength of commercially pure Al has been improved profoundly after ECAP, the elongation is reduced remarkably. Compared with the 90° die, the tensile properties of the specimens produced by ECAP with the 120° die are more scatter, which should be closely associated with the non-uniform shear deformation during pressing. And the non-uniform deformation has been proved in the following hardness test. With increasing the number of ECAP pass, the amount of small dimples on the fracture surface increases, and the dimples become shallow and more uniform. The distribution and size of dimples are strongly dependent on the matrix strength, impurity and grain size. In addition, the microstructure is inhomogeneous at few pass of ECAP, inducing no sensitivity to the strain rate.
When compressing along different directions, the specimens displayed different compressive stress–strain curves and surface deformation morphologies. The ND specimens show practically no strain hardening after yielding when the compressive strain is low and some shear bands formed on the specimen surface. However, there is obvious strain softening after yielding in the compressive stress–strain curves of ED specimens and shear bands arise on the specimen surface. After 2-passes ECAP, dense shear bands along two directions arise to form a cross-weaved structure. With increasing the compressive strain, the specimens along two directions show continual strain softening. In addition, the tensile yield strength is higher than that along the ED under compression, but lower than that along the ND under compression. Moreover, the strength asymmetry under tension and compression along both of directions decreases with increasing the ECAP pass, which is caused not only by crystallographic texture and grain size, but also grain morphology and grain boundaries. On the other hand, the shear plane induced by ECAP also plays an important role under tensile and compressive loadings. The piling-up of dislocations on the grain boundaries is one of the reasons causing no strain hardening or even strain softening under compression.
The grains of ECAPed Cu-Ni alloy were elongated along the ECAP shear direction. In specimens ECAPed under high temperature, some recrystallized grains were formed. Similar to most of the ECAPed materials, the strength of the Cu-Ni alloy is improved and the ductility is reduced. Besides, the strain hardening exponent n is reduced rapidly, and the lower strain hardening exponent demonstrates that the resistance to the plastic deformation and necking is low.
The deformation and fracture of Cu-Ni alloy are also affected by the strain rate. With the increase of strain rate, the yield strength of specimen E0 was not improved but the ultimate tensile strength slightly increased. Therefore, both strength and shear fracture angle increase with increasing the strain rate in the EH specimens. In the fracture zone of specimen E0, there are some shear lips and the equiaxial dimples appear under all the strain rates, whereas it turns into shear dimples under high strain rate in the EH specimens. Based on the tensile tests, the strain rate sensitivity m was calculated. It is shown that both specimens have a low m value, indicating a slight strain rate hardening, and the value m of the specimen EH is slightly higher than that of specimen E0. The lower strain hardening index n and strain rate sensitivity m will induce the lower resistance to the plastic deformation, necking and shear deformation. Under lower strain rate, dimples have enough time to propagate along the cross direction and then form equiaxial dimples. However, aggregation of dimples is rapidly shear dehiscence under high strain rate, which will induce the dimples shearing and elongating along the shear stress direction, resulting in the shear dimples eventually. |
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