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Integrated Experimental, Atomistic, and Microstructurally Based Finite Element Investigation of the Dynamic Compressive Behavior of 2139 Aluminum

[+] Author and Article Information
K. Elkhodary, M. A. Zikry

Department of Mechanical and Aerospace Engineering, North Carolina State University, 2412 Broughton Hall, Box 7910, Raleigh, NC 27695-7910

Lipeng Sun, Douglas L. Irving, Donald W. Brenner

Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27587-7907

G. Ravichandran

Graduate Aeronautical Laboratories, California Institute of Technology, Aeronautics and Mechanical Engineering, Mail Stop 105-50, Pasadena, CA 91125

J. Appl. Mech 76(5), 051306 (Jun 15, 2009) (9 pages) doi:10.1115/1.3129769 History: Received June 05, 2008; Revised December 29, 2008; Published June 15, 2009

Abstract

The objective of this study was to identify the microstructural mechanisms related to the high strength and ductile behavior of 2139-Al, and how dynamic conditions would affect the overall behavior of this alloy. Three interrelated approaches, which span a spectrum of spatial and temporal scales, were used: (i) The mechanical response was obtained using the split Hopkinson pressure bar, for strain-rates ranging from $1.0×10−3 s$ to $1.0×104 s−1$. (ii) First principles density functional theory calculations were undertaken to characterize the structure of the interface and to better understand the role played by Ag in promoting the formation of the $Ω$ phase for several $Ω-Al$ interface structures. (iii) A specialized microstructurally based finite element analysis and a dislocation-density based multiple-slip formulation that accounts for an explicit crystallographic and morphological representation of $Ω$ and $θ′$ precipitates and their rational orientation relations were conducted. The predictions from the microstructural finite element model indicated that the precipitates continue to harden and also act as physical barriers that impede the matrix from forming large connected zones of intense plastic strain. As the microstructural FE predictions indicated, and consistent with the experimental observations, the combined effects of $θ′$ and $Ω$, acting on different crystallographic orientations, enhance the strength and ductility, and reduce the susceptibility of 2139-Al to shear strain localization due to dynamic compressive loads.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

Figure 1

True-stress true-strain curves for 2139-Al under varied loading rates. The quasistatic stress-strain curve, obtained from the MTS servohydraulic machine under displacement control, shows considerable hardening. The high strain-rate curves, obtained from the split Hopkinson pressure bar, show extensive ductility (up to 80%) and slight stress softening of the 2139-Al alloy.

Figure 2

Strain-rate sensitivity of flow stress in 2139-Al at an equivalent strain of 0.06 exhibiting considerable material rate sensitivity, particularly at strain rates beyond 103 s−1

Figure 3

Superposition of the atomic arrangement (left) and the electron density (right) from the first principles calculations over the experimental Z contrast HRTEM image from Ref. 1 of the Al-Ω interface

Figure 4

Illustration of slip vector (Vj) transformation sequence ((a)–(e)) to the element axes. (a) The slip system is identified in fractional coordinates. (b) The slip system vectors are transformed from precipitate space to matrix space. (c) The precipitate vectors are aligned with the matrix vectors in accordance with the orientation relationships. (d) The oriented slip vectors are mapped to the axes of the polycrystalline aggregate. (e) The vectors are mapped from the polycrystalline aggregate to the element axes.

Figure 5

An 18 grain aggregate with θ′ and Ω precipitates, subject to an applied strain rate of 104 s−1 on the upper surface and with symmetry boundary conditions at the left and bottom edges

Figure 6

(a) Contour plot of plastic slip at a nominal strain of 25%. (b) Plastic slip comparison between 2139-Al and precipitate-free Al along path shown in (a).

Figure 7

Adiabatic temperature increase comparison between 2139-Al and precipitate-free Al along a selected path, showing the temperature build up to be the lowest inside the Ω precipitates

Figure 8

(a) Contour plot of immobile dislocation density normalized by the initial density for (111)[01¯1] slip system in the matrix and (110)[1¯12] in the precipitates at a nominal strain of 25% (i.e., most active slip systems). (b) Comparison of dislocation densities for most active slip systems between 2139-Al and precipitate-free Al along path shown in (a).

Figure 9

(a) Contour plot of reference shear stress normalized by static yield at a nominal strain of 25%. (b) Comparison of reference shear stress (normalized by static yield) between 2139-Al and precipitate-free Al along the path shown in (a).

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