The Contribution of Dislocation Density and Velocity to the Strain Rate and Size Effect Using Transient Indentation Methods and Activation Volume Analysis

Generally, there are 14 different types of crystal unit cell structures or lattices found in nature. Most metals and many other solids have unit cell structures described as body center cubic (BCC), face centered cubic (FCC) or hexagonal close packed (HCP).

There has been significant research on establishing why FCC structured metals tend to exhibit a size effect under specific loading conditions including indentation, torsion and the fundamental mechanism responsible for this behavior. So far, it has already been proven that the indentation size effect (ISE) is governed by dislocation-based mechanisms in crystalline materials and is caused by the accumulation of geometrically necessary dislocations resulting from a large strain gradient for shallow indents. In addition, the stacking fault energy (SFE) of a material, especially metals, has been found to be directly related to its capacity to work harden. Nonetheless, from the available plethora of literature, no work exists that shows the correlation between SFE, hardness, activation volume and ISE for silver (low SFE), nickel (intermediate SFE) and aluminum (high SFE). Therefore, there is need for detailed experiments to be undertaken so as to resolve the aforementioned issue.

Recently, Old Dominion University scientists: Dr. D. E. Stegall and Professor Abdelmageed Elmustafa from the Department of Mechanical and Aerospace Engineering carryout detailed experimentation for some of the more fundamental variables so a to better comprehend previous observations of the same. Specifically, they examined the strain rate, dislocation densities, and dislocation velocity. Additionally, they sought to establish how the coupled relationships between each and every variable contribute to ISE. Their work is currently published in the research journal, Metallurgical and Materials Transactions A.

In brief, the researchers started by taking measurements of the creep parameters using the constant load indentation experiments that were conducted using a nanoindentation system which was properly isolated from vibrational noise. The samples used were properly prepared according to ASTM standards and kept in a desiccator. Lastly, they performed two types of testing to measure the strain rate sensitivity, constant load creep experiments, and constant strain repeated load relaxation tests.

The authors observed that the dislocation velocity which was calculated based on Orowan’s relation decreased with the increasing hardness at shallow depth of indentation. Moreover, they noted that the accumulation of geometrically necessary dislocations (GNDs) resulting from strain gradient at shallow depth of indentation resulted in the decrease of the dislocation velocity with the hardness at shallow depth of indentation. The researchers also realized that at the depths corresponding to the bulk hardness where statistically stored dislocations dominate the crystal plasticity and the influence of the GNDs diminishes, the change in dislocation velocity was not associated with accumulation of GNDs.

In summary, Elmustafa-Stegall study presented detailed experimental procedures with regard to theoretical hypothesis that stipulate ISE is driven by a dislocation mechanism, specifically the increase in the geometrically necessary dislocation density at shallow depth of indentation due to the presence of a large strain gradient. Altogether, by using the Orowan’s relation, they observed that for silver and nickel, the dislocation velocity when plotted vs hardness, exhibited a bilinear behavior. All in all, their observations are in good agreement with related literature and offer a new technique to carry out detailed experiments for other metals.