BALL-MILLING OF DUCTILE METALS

Pure copper powder was employed to study the effects of ball milling o n the development of the structure and properties of ductile metals. T he results indicate that larger spheres with diameters of about 2-2.5 mm are created after 20 h of ball milling. The formation of such spher es is mainly due to sphere-to-flake or sphere-to-sphere welding. This welding is not complete, leaving large pores and curved voids in the s pheres. The average grain size of such spheres is 10-100 nm. The incre ase in lattice strain is about 0.2%. The microhardness increases from 45 MPa (unmilled) to 220 MPa (milled for 20 h). High-resolution transm ission electron microscopy (HRTEM) investigations show the following: (a) the deformation of ball-milled copper proceeds by [112](11 $$($) o ver bar 1) twinning or high-order twinning; (b) the [112](11 $$($) ove r bar 1) twins are thickened by passage of (a/6)[112] twinning partial dislocations; (c) subgrains lend to form in the twins. In addition to twinning, dislocation slip plays an important role in the deformation process; the mobility of 60 degrees dislocations and their pile-up in the crystals can lead to the formation of subgrains. Crystal refineme nt leads to an increase in the number of grain boundaries; both low-an gle and high-angle grain boundaries with local strain and a high densi ty of dislocations are observed. The estimated mean dislocation densit y is more than 10(14) m(-2), which is hardly ever reached in plastical ly deformed metals. The different kinds of structural defects which ex ist in the grain boundaries and within the crystals may result in incr eased strength and microhardness, increased free energy and changes in other properties of ball-milled materials.