Dynamic Evolution of Defect Structures during Spall Failure of Nanocrystalline Al
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Dynamic Evolution of Defect Structures during Spall Failure of Nanocrystalline Al Kathleen Coleman1, Garvit Agarwal1, and Avinash M. Dongare1,* 1
Department of Materials Science and Engineering, and Institute of Materials Science, University of Connecticut, Storrs, CT, United States
ABSTRACT The dynamic evolution and interaction of defects under the conditions of shock loading in nanocrystalline Al with an average grain size of 20 nm is investigated using molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock deformation and failure that correspond to the initial shock compression (I), the propagation of the compression wave (II), the propagation and interaction of the reflected tensile waves (III), and the nucleation, growth, and coalescence of voids (IV). The results suggest that the spall strength of the nanocrystalline Al system is attributed to a high density of Shockley partials and a slightly lower density of twinning partials (twins) in the material experiencing the peak tensile pressures. I.
INTRODUCTION
Nanocrystalline metals, due to their enhanced strength and wear resistance, show significant promise in the development of materials for use in extreme environments. The plastic deformation mechanisms in nanocrystalline metals depend on the interplay between dislocation and grain boundary (GB) processes. Nanocrystalline metals with ultra-fine grain sizes (d ≤ 30 nm) have gained considerable attention due to their increased strengths during deformation at high strain rates (104 s-1) [1]. In addition, shock loading of ultra-fine nanocrystalline metals at speeds that are greater than the speed of sound limits the GB sliding mechanism and thus results in ultra-high strength values [2]. Thus, the ability to limit the GB sliding processes by the use of ultra-high strain rates opens up the possibility of creating ultra-hard metals for use in extreme environments. The response of metals under dynamic loading conditions (shock) is very complex and involves plastic deformation, damage creation and evolution, phase transformation, heat generation and transfer, etc. [3]. The impact tolerance behavior of a material is therefore determined by the ability of the microstructure to distribute load as the material deforms plastically. This ability is related to the evolution of the defects that comprise interfaces, stacking faults, dislocations, twins, vacancies, and interstitials. The interaction and accumulation of these defects and their distribution during shock compression generates a heterogeneity in the microstructure that may result in weak regions in the microstructure wherein voids nucleate to initiate failure under the action of the reflected tensile waves (spallation) [4]. A fundamental understanding of the nucleation, evolution, and accumulation of defect structures that initiate failure is therefore critical to improve the understanding of the deformation and failure behavior of nanocrystalline metals.
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