Synthesis and properties of nanocrystalline metals and alloys prepared by mechanical attrition

Abstract

The grain size in powder samples can be reduced to nanometer scales during heavy cyclic mechanical deformation as produced in a standard ball mill. For pure bcc and hcp metals, intermetallic compounds and solid solutions, nanocrystalline materials can be synthesized at temperatures close to room temperature with a grain size ranging from 5 to 15 nm. During this process three different stages have been observed with (i) the deformation being localized in shear bands, (ii) formation of small angle grain boundaries separating the individual grains and (iii) formation of large angle grain boundaries with a completely random orientation of the nanosized grains. Thermal analysis of these samples reveals excess energies of up 40% of the heat of fusion and excess heat capacities of up to 20% in comparison to the undeformed state thus exceeding by far any values determined for conventional deformation processes and the energy of grain boundaries in fully equilibrated polycrystalline samples. These thermophysical data are in agreement with a theoretical model adopting a free volume approach for the grain boundaries based on the universal equation of state at negative pressure.