Parameters Influencing Steady-State Grain Size of Pure Metals Processed by High-Pressure Torsion

Abstract

High purity elements such as magnesium, aluminum, silicon, titanium, vanadium, iron, nickel, copper, zinc, zirconium, molybdenum, palladium, silver, indium, tin, hafnium, gold and lead were processed by high-pressure torsion and subsequently evaluated by microstructural examinations and Vickers microhardness measurement. The grain size at the steady state, where the grain size and hardness remain unchanged with straining, was determined using either transmission electron microscopy, electron back-scatter diffraction analysis and/or optical microscopy. It is found that the steady state grain sizes are at the submicrometer level in elements with metallic bonding and at the nanometer level in elements with covalent bonding. The correlations between the steady-state grain size and the physical properties of metals are examined and it is found that the atomic bond energy and the homologous temperature are important parameters influencing the steady-state grain size after processing by HPT. A linear correlation between the hardness and grain size at the steady state is achieved by plotting the hardness normalized by the shear modulus against the grain size normalized by the Burgers vector in the logarithmic scale.