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

Abstract

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.