| Bulk ultrafine-grained metals (grain size 0.1~1μm) with unique physical and mechanical properties is one of most interesting topic in the field of materials research. Equal channel angular pressing (ECAP) is an effective route to obtaining bulk ultrafine-grained metals through severe plastic deformation (SPD). Deformation parameters of ECAP and die design are the key factors for ultrafine-grained metals. Although the outer arc angle of the ECAP die has important effects on materials flow, microstructure and mechanical properties, it has not been well investigated to this day. On the other hand, mechanical properties of electronic deposited copper with ultrafine twins have been greatly enhanced. The present work tries to find some possibilities to strengthen ultrafine-grained metals by deformation twins via changing SPD routes. Furthermore, mechanical behavior of ultrafine-grained materials is distinctly different to that of coarse-grained materials. To model mechanical behavior of ultrafine-grained materials produced by SPD is very important to deeply understand strengthening theories and the relationship of microstructure and mechanical properties.Investigations on fabrication of bulk ultrafine-grained copper (99.98%), the effects of outer arc angle of ECAP die, the number of passes of ECAP and cold rolling on microstructure and mechanical properties as well as the factors for the formation of deformation twins were conducted in detailed. A composite model of twin strengthening was proposed to describe the mechanical behavior of ultrafine-grained materials. The main research work for the dissertation focused on eight aspects of ultrafine-grained materials. (1) Based on the detailed analysis of ECAP principle and mainly technical parameters including die angles, pressing routes, the number of ECAP passes, pressing speed and temperature, the equivalent strain and the upper-bound solution of ECAP is compared and deduced respectively. The influence of the outer arc angle on materials flow in the ECAP process is investigated by etched-grids, which shows that the homogeneity of shear deformation decreases with increasing the outer arc angles in the ECAP process. Finite volume method is conducted to simulate the ECAP load and deformation temperature increment in the ECAP process, which shows that increasing outer arc angle will lead to depress ECAP load as φ =90 ° . And the pressing speed has little effect on ECAP load anddeformation temperature increment. Bulk ultafine-grained copper with the size of 12 X 12 X 80mm is successfully obtained by self-designed ECAP instrument. (2) Grain size of pure copper can be refined as increasing the number of ECAP passes. The average grain size is effectively refined from the initial -lOOum to ~0.2um after four passes, and then be saturated. (3) Tensile strength of copper increases abruptly with increasing the number of ECAP passes. After one ECAP pass, tensile strength of copper arrives at 300MPa, which is ~2 times than that of as-annealed state, however, the ductility (elongation to failure) is reduced from the initial 54% to 23%. After four ECAP passes, tensile strength reaches 420MPa, which is ~3 times than that of as-annealed state, the ductility is -30% and then be saturated. (4) When ultrafine-grained copper produced by ECAP is rolled at room temperature and low strain rate (e ? 2 s'1), deformation twins (the width ~0.4um) are found inside the matrix of ultrafine grains. However, no reports on deformation twins have been found in pure copper at the same rolled conditions so far. Meanwhile the factors for the formation of deformation twins are discussed. Based on the critical stress for twins formation a'"'", grain size d and deformation temperature increment in the cold rolling process, it shows that (a) deformation twins can't come into being as of is larger than lum; (b) The formation of deformation twins is easy and independent on temperature as d is smaller than 0.2uxn; and (c) The temperature for the formation of deformation twins rises when d is reduced among of 0.2um to lum. (5) Tensile strength of ECAP-ed copper after cold rolling almost linearly increases with increasing the rolling reduction. Retaining the same ductility, yield strength of twins/ultrafine-grained copper is much larger than that of copper after ECAP. Yield strength of copper after two ECAP passes is 348MPa, however, yield strength of twins/ultrafine-grained copper is 422MPa after two ECAP passes and 83% CR deformation. (6) The saturation of strain-hardening and strain-softening occurs in copper subjected to ECAP and ECAP+CR deformation, namely, the flow stress increases with increasing strain, and then be saturated, which can be contributed to the saturation of grain size, the width of deformation twins and dislocation density after SPD. (7) The experimental saturated yield strength of copper after ECAP is 378MPa, which is consistent with reported saturation for pure copper (about 380~390MPa). The experimental saturated yield strength of copper after ECAP+CR deformation is 422MPa. Both of them are larger than that of Voce's extrapolated saturation for copper ~305MPa. The Voce model is validated reasonable to describe the relationship between yield strength and strain over a wide range of strain by experimental results from ECAP, ECAP+CR deformation. (8) Considering the composite strengthening effects of grain refinement and twin, a composite...
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