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Research Papers

Dynamic Dislocation-Defect Analysis and SAXS Study of Nanovoid Formation in Aluminum Alloys

[+] Author and Article Information
L. Westfall, B. J. Diak, S. Saimoto

Department of Mechanical and Materials Engineering,  Queen’s University, Kingston, ON, K7L 3N6, Canada

M. A. Singh

Department of Physics, Engineering Physics and Astronomy,  Queen’s University, Kingston, ON, K7L 3N6, Canada

J. Eng. Mater. Technol 130(2), 021011 (Mar 13, 2008) (7 pages) doi:10.1115/1.2841619 History: Received August 04, 2007; Revised January 14, 2008; Published March 13, 2008

Crystalline defects other than the essential dislocations are produced by dislocation intersections resulting in debris, which can transform into loops, point defects, and∕or nanovoids. The stress concentrations ahead of slip clusters promote void formation leading to incipient cracks. To evaluate the progression of these processes during deformation, dynamic dislocation-defect analysis was applied to nominally pure aluminum, Al–Mg, and Al–Cu alloys. In the case of nanovoid formation, small angle X-ray scattering (SAXS) was used to quantitatively assess if the void size and its volume fraction can be determined to directly correlate with the measured thermodynamic response values. The SAXS signal from the nanovoids in nominally pure aluminum is distinctly measurable. On the other hand, thermomechanical processing of even nominally pure aluminum results in the formation of nanoprecipitates, which requires future calibration.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Changes of the attenuation coefficient measured after quenching the specimens from 573K to cold water temperature and then to liquid-nitrogen temperature. The measurement for the nonquenched specimen is also shown. Broken curves are estimated jump frequency (f) for activation energies of 0.28eV∕at. and 0.61eV∕at. After Ref. 4.

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Figure 2

Load-displacement (L‐d) response at 300K to down- and up-1∕10 rate change for free-GB and sealed-GB AA1100 alloys as denoted. The kinetics of pinning was examined at 195–267K and found to be due to vacancies.

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Figure 3

Load-displacement (L‐d) response of reverted (random solution) specimen of Al–4wt% Cu alloy tested at 195K at a tensile strain rate of 6×10−5s−1 with denoted 14 down- and up-rate changes. Compared to pure Al tested at 195K(5), considerable point defect pinning takes place at strains below 2%, suggesting that point defect retention is more efficient in Al–Cu compared to pure Al. The extrapolated line from the steady-state flow stress for the up-change depicting flow stress change due to depinning is almost comparable to pinning.

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Figure 4

Load-displacement (L‐d) response to 14 rate changes for AA5754 at strains of 4.45%. The backextrapolation shows that engineering strain-rate sensitivity is near zero. At higher strains, it was found to be slightly negative.

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Figure 5

Stress-strain curves for the free and sealed GBs at 300K with the associated logλ versus logτ plot in the inset. The flow stress of the sealed GB is larger than that for free GB at high strains. After Ref. 5.

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Figure 6

Comparison of load-displacement (L‐d) response for superpure (99.999) and nominally pure (99.971) Al alloys of similar large grain sizes but with one (bottom) containing nanovoids, which manifests a decrease in flow stress. The contrary effect to pinning indicates that the excess point defects are being swept into the nanovoids whenever the mobile dislocations encounter them, thus enlarging the void size. Strain increases to the left in both plots.

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Figure 7

Stress-strain plots for the superpure and quenched-aged nominally pure aluminum tested at 195K and 6×10−5s−1. Note that the specimen containing nanovoids necks at a lower strain as detected by the stress drop.

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Figure 8

Nanovoid (bright region) found within the tangled cell wall of a surface grain in the deformed neck of the quenched-aged specimen of Fig. 7. The existence of the dense dislocations was confirmed by tilting.

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Figure 9

Haasen plot for the specimens depicted in Fig. 7 whereby the nanovoid specimen (pure Al quenched denoted RC2-11) manifests a lower slope than that for the superpure one. Note that the origin of the RC2-11 data is offset vertically 0.0002 for clarity.

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Figure 10

SAXS profile of pure aluminum quenched from 550°C with a slow cooled reference

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Figure 11

TEM micrograph of a typical void found in the quenched aluminum SAXS sample shown in Fig. 1. The image is underfocused.

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Figure 12

SAXS profiles of pure (99.985) aluminum rapidly quenched from various temperatures with a slow cooled reference

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Figure 13

Incomplete invariant, Qeff of quenched aluminum samples plotted against quench temperature. Qeff is proportional to the volume fraction of electron density heterogeneities in a limited range of length scales.

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Figure 14

Comparison of SAXS profiles for deformed free and sealed GBs of AA1100, showing an increased volume of defects when GBs are sealed.

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