Abstract
Abstract Metals with a bimodal grain size distribution have been found to have both high strength and good ductility. However, the coordinated deformation mechanisms underneath the ultrafine-grains (UFGs) and coarse grains (CGs) still remain undiscovered yet. In present work, a bimodal Cu with 80% volume fraction of recrystallized micro-grains was prepared by the annealing of equal-channel angular pressing (ECAP) processed ultrafine grained Cu at 473 K for 40 min. The bimodal Cu has an optimal strength-ductility combination (yield strength of 220 MPa and ductility of 34%), a larger shear fracture angle of 83° and a larger area reduction of 78% compared with the as-ECAPed UFG Cu (yield strength of 410 MPa, ductility of 16%, shear fracture angle of 70°, area reduction of 69%). Grain refinement of recrystallized micro-grains and detwinning of annealing growth twins were observed in the fractured bimodal Cu tensile specimen. The underlying deformation mechanisms for grain refinement and detwinning were analyzed and discussed.
Highlights
Bulk ultrafine-grain (UFG) materials possess high strength but low ductility, which has evolved into a seemingly insurmountable obstacle for widespread technological applications of these new materials [1,2,3,4,5,6,7,8]
The bimodal Cu has an optimal strength-ductility combination, a larger shear fracture angle of 83∘ and a larger area reduction of 78% compared with the as-ECAPed UFG Cu
A bimodal Cu with 80% volume fraction of recrystallized micro-grains was prepared by the annealing of as-ECAPed Cu at 473 K for 40 min
Summary
Bulk ultrafine-grain (UFG) materials possess high strength but low ductility, which has evolved into a seemingly insurmountable obstacle for widespread technological applications of these new materials [1,2,3,4,5,6,7,8]. Inspection of the published literature shows that there is only a limited number of reports on deformation and fracture mechanisms of bimodal metals and alloys [11, 12, 23,24,25,26]. Wang et al applied finite element modeling in combination with post-mortem transmission electron microscopy analysis [12]. They reported that during deformation, the CGs, which are embedded in the heterogeneous microstructure, experience multi-axial stress state conditions, consisting of triaxial strain components and large strain gradients. Scanning electron microscopy revealed that voids near the tensile fracture surfaces tend to initiate both in the UFG matrix as well as at the UFG and CG interfaces
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