High tensile ductility via enhanced strain hardening in ultrafine-grained Cu

► Ultra-fine grained (UFG) pure Cu was produced by friction stir processing (FSP). ► FSP Cu consisted of equiaxed grains with predominant high angle grain boundaries. ► Some twin boundaries were observed in fine grains with low dislocation density. ► Enhanced strain hardening capacity was achieved in the special FSP microstructure. Sound tensile properties with high strength/ductility were obtained in FSP Cu. Low tensile ductility owing to the insufficient strain hardening is the main drawback for ultrafine-grained (UFG) materials, which restricts their practical applications. Here, via a simple friction stir processing technique with additional cooling, we prepared UFG Cu with high strength and tensile ductility. Enhanced strain hardening capacity, which is effective in blocking and accumulating dislocations, was achieved in the present recrystallized UFG microstructure. The enhanced strain hardening capacity is attributed primarily to the low dislocation density, and the presence of large fraction of high angle grain boundaries and a certain amount of coherent twin boundaries. This work provides a strategy for designing UFG materials with good mechanical properties.