Strengthening Mechanisms of Heterogeneous Nano-Structured Cu-Zn-Si Alloy Bars Fabricated by Caliber Rolling

Hot-extruded Cu-Zn-Si alloy bars with various chemical compositions were cold-caliber rolled down to 91.7% reduction at maximum. Heterogeneous nano-structure, in which coarse initial grains were subdivided mainly by mechanical twins and shear bands, was gradually developed with increasing reduction. The as-rolled bars exhibited an extraordinarily high tensile strength of 988 MPa at best with a reasonable ductility of 5.9%. The tensile strength was further slightly raised to 995 MPa by low-temperature annealing. 3D atom-probe analyses revealed dense Si segregation at twin boundaries and increase in the amount of segregation after annealing. Multi-scale simulation indicated that strain field formed by the Si segregation at twin boundary obstructed dislocation glide to develop geometrically necessary dislocations, which resulted in higher yield stress and work-hardening rate to cause higher tensile strength. It was found that strengthening mechanisms of heterogeneous nano-structured Cu-Zn-Si alloy bars are, therefore, complicatedly combined ones of grain refinement, work hardening, solid-solution hardening and grain-boundary segregation.