Ball milling of ductile metals

Abstract

Pure copper powder was employed to study the effects of ball milling on the development of the structure and properties of ductile metals. The 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 spheres 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 spheres. The average grain size of such spheres in 10–100 nm. The increase in lattice strain is about 0.2%. The microhardness increases from 45 MPa (unmilled) to 220 MPa (milled for 20 h). High-resolution transmission electron microscopy (HRTEM) investigations show the following: (a) the deformation of hall-milled copper proceeds by [112](111) twinning or high-order twinning: (b) the [112](111) twins are thickned by passage of (a/6)[112] twinning partial dislocations; (c) subgrains tend to form in the twins. In addition to twinning, dislocation slip plays an important role in the deformation process; the mobility of 60° dislocations and their pile-up in the crystals can lead to the formation of subgrains. Crystal refinement leads to an increase in the number of grain boundaries; both low-angle and high-angle grain boundaries with local strain and a high density of dislocations are observed. The estimated mean dislocation density is more than 1014 m−2, which is hardly even reached in plastically deformed metals. The different kinds of structural defects which exists in the grain boundaries and within the crystals may result in increased strength and microhardness, increased free energy and changes in other properties of ball-milled materials.