Quantifying solid-state mechanical mixing by high-pressure torsion

A severe plastic deformation process of High-Pressure Torsion (HPT) is presented as a viable tool for mixing of metals in the solid state. At the billet or specimen scale, mixing occurs due to chaotic mass transfer (stirring) which sets in at a certain stage of HPT of layered assemblies of metals. To characterise this process quantitatively, a stirring efficacy index is introduced. It is based on a parameter that characterises the uniformity of the concentration of the constituents of a sample in the bulk in a quantitative way. The methodology is illustrated by considering the process of high-pressure torsion of assemblages of alternating Ni and Al layers. It is shown that up to a threshold shear strain of 120, only a distortion of the layers takes place, while no mechanical mixing occurs. Rupturing of the layers at larger shear strains leads to an increase in the degree of uniformity of the distribution of Al and Ni. Principal differences in stirring of solid metals and liquids are elucidated.