Jaafar El-Awady, HEMI Fellow and associate professor of mechanical engineering, has co-authored research that reveals more clues about the microscopic mechanisms that govern the strength of metals.
In the paper recently published in Nature Communications, titled “Strain rate dependency of dislocation plasticity,” El-Awady and team present a new understanding of the effects of strain rate and dislocation density on the deformation behavior of aluminum and cooper.
Metals like aluminum and cooper are commonly used engineering materials, thanks in part to their strength and damage tolerance. Why, then, do metals sometimes deform or break unexpectedly due to stress? Understanding why metal components fail will inform the design of stronger metals, improving their performance in a broad range of applications.
In materials research, the flow strength refers to a material’s ability to resist deformation under an applied load. Metal properties are loading rate sensitive, meaning they react differently across various rates of loading. Dislocations, or line defects, in metals also move and localize under loading, which is a precursor to crack initiation and failure. Thus, researchers hypothesize that dislocation density, or the number of dislocations per unit area in a material, may affect the loading rate sensitivity of the material. However, the relationship between the two mechanisms is still not well understood.
To solve this problem, the team performed a large set of three-dimensional discrete dislocation plasticity simulations to predict the metal yield strength at different initial dislocation densities and strain rates. The simulation results showed that the material response can be divided into two regimes. In one regime, dislocation-dislocation interactions control the material flow strength; in the other, strain rate induced hardening dominates. Additionally, the transition between both regimes is mainly controlled by the dislocation density in the material.
Based on their observations, the team was able to develop a unified law that correlates the material flow strength with the loading strain rate and the initial dislocation density in the material. This unified law is shown to be in excellent agreement with a large set of experimental results from previous studies.
The study provides a fundamental understanding of the co-interaction of different mechanisms in materials and how they influence the material strength, said El-Awady. These findings could help researchers design metals better suited for high strain rate loading applications, by tailoring the initial dislocation density in the material to achieve a desired strength.
In addition to El-Awady, the research team includes first author Haidong Fan, a former Johns Hopkins postdoc and current professor at Sichuan University, Qingyuan Wang, Dierk Raabe, and Michael Zaiser.
Excerpted from the Department of Mechanical Engineering >>