Dislocation nucleation in bcc Ta single crystals studied by nanoindentation

Monika M. Biener, Juergen Biener, Andrea M. Hodge, and Alex V. Hamza

Phys. Rev. B 76, 165422 – Published 18 October 2007

Abstract

The study of dislocation nucleation in close-packed metals by nanoindentation has recently attracted much interest. Here, we address the peculiarities of the incipient plasticity in body centered cubic (bcc) metals using low index Ta single crystals as a model system. The combination of nanoindentation with high-resolution atomic force microscopy provides us with experimental atomic-scale information on the process of dislocation nucleation and multiplication. Our results reveal a unique deformation behavior of bcc Ta at the onset of plasticity, which is distinctly different from that of close-packed metals. Most noticeably, we observe only one rather than a sequence of discontinuities in the load-displacement curves. This and other differences are discussed in the context of the characteristic plastic deformation behavior of bcc metals.

Received 24 July 2007

DOI:https://doi.org/10.1103/PhysRevB.76.165422

Authors & Affiliations

Monika M. Biener, Juergen Biener^*, Andrea M. Hodge, and Alex V. Hamza

Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, P.O. Box 808, L-367 Livermore, California 94550, USA

^*Author to whom correspondence should be addressed; biener2@llnl.gov

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Issue

Vol. 76, Iss. 16 — 15 October 2007

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Images

Figure 1
(Color online) Nanoindentation tests performed on a Ta(001) single crystal surface using a Berkovich tip: (a) a typical load-displacement $(P − h)$ curve with a pronounced discontinuity marking the onset of plasticity. The dashed line is a fit of the initial elastic loading section to the Hertzian contact law. The star symbol marks the condition where the maximum resolved shear stress reaches the theoretical shear stress of Ta. The elastic-plastic part of the $P − h$ curve is fitted to the Taylor dislocation model (dotted line) (Ref. 33). A $P − h$ curve (c) and corresponding AFM image (b) from a test loaded just above the onset of plasticity. (d) Maximum resolved shear stress versus representative strain plot of the nanoindentation data shown in (c). Note the linear elastic response up to a strain of 0.055.Reuse & Permissions
Figure 2
Effect of surface roughness on the load-displacement behavior of Ta(001): Nanoindentation data collected from the same area before (left) and after (right) ion bombardment. Note that both the initial elastic loading regime and the pop-in events are suppressed by increasing the surface roughness via low-energy ion-bombardment.Reuse & Permissions
Figure 3
(Color online) Reproducibility of the experiment and effect of loading rate on the critical load: (a) A narrow distribution of the critical load reveals the homogeneity of the surface finish. (b) The load necessary to induce a pop-in event shifts toward higher loads with increasing loading rate.Reuse & Permissions
Figure 4
(Color online) AFM images from indents produced with a spherical tip: topographic (top) as well as deflection mode (bottom) AFM images from a test loaded just above the onset of plasticity [(a) and (b)] as well as of a deeper indent [(c), (d), and (f)]. (e) In-plane surface orientation as obtained by EBSD; (g) orientation of intersections of the slip planes with the [001] surface plane for both the {110}⟨111⟩ and the {112}⟨111⟩ slip system. Note that the majority of the slip lines in the AFM images are consistent with the activation of the {110}⟨111⟩ slip system.Reuse & Permissions
Figure 5
(Color online) Deflection mode AFM images from spherical indents on (a) Ta(110) and (c) Ta(111) single crystals. The corresponding in-plane surface orientations as well as the in-plane (solid) and out-of-plane (dashed) ⟨111⟩ slip directions are displayed in (b) and (d). The maximum pileup is always observed along the ⟨111⟩ slip directions of the bcc crystal structure.Reuse & Permissions

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