| Summary: | 博士 === 國立中山大學 === 材料科學研究所 === 91 === It has been shown that alloys with submicron-grained structure can be produced by severe plastic deformation (SPD). However, our understanding about the characteristics of mechanical behaviors of these materials is still limited. According to the literature, many alloys exhibit quite different mechanical properties as the grain size decreasing to submicrometer range. In this study, commercial purity aluminum (AA1050) of grain size ranging from 0.35 to ~ 45 mm was obtained by the proper combination of equal-channel angular extrusion (ECAE) and annealing treatment. The influences of grain size, testing temperature and boundary character on the mechanical properties were studied in this work.
Generally speaking, the materials of grain sizes below 1mm have quite different mechanical properties than those of coarser grain sizes. In tensile tests, they exhibited yield drop immediately followed by work softening at RT, while they showed Lüders extension followed by work hardening at 77K. In addition, their yield strength at RT was about 20% higher in compression than in tension. The submicron-grained aluminum has much higher strength but lower tensile ductility than large grained aluminum at room temperature, while it exhibits both high strength and good ductility at 77K. This finding suggests that the poor tensile ductility of submicron-grained alloys at room temperature may be improved by reducing the dynamic recovery rate.
The Hall-Petch slope in the submicrometer grain size range showed positive deviation from that extended from coarser grains at both room temperature and 77K. This might be arisen from the phenomenon of inhomogeneous yielding as grain size below 1 mm. In addition, the grain boundary character distribution was found to have influence on the tensile properties of matrials of submicrometer grain sizes.
As the grain size increases to the range between 1 mm and 4 mm, the tensile deformation at RT proceeds by the propagation of Lüders band initially, and followed by strain hardening. For materials of grain sizes greater than 4 mm, a normal strain hardening behavior of coarse-grained aluminum resumes.
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