MECHANICAL PROPERTIES OF ULTRAFINE GRAINS BULK COPPER ALLOY DEVELOPED BY CRYOROLLING

Abstract

In the present study, we have reported the microstructures and mechanical properties of bulk Cu-20wt.%Zn alloy subjected to cryorolling (CR) under liquid nitrogen temperature (LN2). Samples were plastically deformed to obtain different reduction in area (RA), i.e. 50%, 75% and 90%. A short time (20 mm) and low temperatures (225- 300°C) annealing was carried out for 90% RA specimens to enhance the ductility of the cryorolled materials. The mechanical properties of cryorolled as well as cryorolled+annealed samples were investigated by tensile testing and hardness measurement. Cryorolled samples with 90% RA shows maximum tensile yield - strength (600 MPa) with an elongation only -2.l%. After annealing at 225°C for 20 min the yield strength was reduced to 452 MPa with an increase in the elongation to - 10%. The increase in tensile yield strength of cryorolled+annealed samples is more than 5 times compared to that of the as-received annealed samples (YS=80 MPa). The increase in mechanical strength is due to the refinement of grain size by the combined effect of suppression of dynamic recoveiy and recrystallization, and accumulation of high density dislocations during cryorolling. The grain size refinement has been confirmed by AFM and TEM analysis. After annealing for short duration the reasonable increase in the ductility is due to the recovery and annihilation of dislocations. Brittle type of fracture was identified for 90% RA cryorolled samples from fractography analysis, whereas presence of dimples in the fractured surface of cryorolled+annealed tensile specimens indicated a ductile failure