Effect of solid solution impurities on dislocation nucleation during nanoindentation
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Dislocation nucleation in solid solutions of face-centered-cubic metallic materials was studied using nanoindentation. The effects of solute impurities in the copper–nickel system on the formation of dislocations in a previously dislocation-free region were demonstrated to be minimal. The shear stress required to nucleate dislocations in copper is approximately 1.6 GPa, while in nickel a 3.9 GPa shear stress is required. Changes in shear stress for nucleation track closely with changes in elastic modulus showing the nucleation stress is approximately 1/30 to 1/20 of the shear modulus. The expected solid-solution strengthening is identified within the same experimental method, demonstrating unambiguously the fact that solid-solution impurities in this system will impact the propagation of dislocations during plastic deformation but not alter the homogeneous nucleation of dislocations in these materials.
Recent work has demonstrated the ability to quantify the stress required to nucleate dislocations in dislocationfree solids.1–5 This method is only possible using small scale mechanical testing; large volume mechanical testing will generally measure the motion of pre-existing dislocations under applied stresses. The study of dislocation nucleation has wide-ranging implications in thin film deformation,6 shock deformation,7 structural nanolayered materials,8 nanocrystalline solids,9 and fracture processes,10 where dislocation nucleation in front of a crack tip can be a controlling factor in failure. Nanoindentation couples well with small-scale mechanical modeling, as it approaches volumes that can be simulated using molecular dynamics11–13 and the embedded atom technique.14 Some of these studies7 have noted the need for further studies evaluating the effects of microstructure and chemistry changes on dislocation nucleation. Nanoindentation, which evaluates the mechanical properties of small volumes of materials, can be used to monitor both the propagation of dislocations (via the hardness of a material), the strain hardening behavior (using out of plane deformation from the resulting impression of an indentation), and the nucleation of dislocations. Since nanoindentation probes a small volume of material, it is possible to evaluate regions of different structures and chemistries in traditionally bulk solids.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0244 J. Mater. Res., Vol. 20, No. 8, Aug 2005
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While structural defects such as interfaces and grain boundaries, particularly in multilayers and nanocrystalline materials, have been simulated as both sources and sinks of dislocations, it is also possible that chemical impurities will impact the nucleation of dislocations under applied stresses. Classical work on solid-solution strengthening clearly demonstrates that impurities will increase the flow stress in bulk materials, but that is only accounting for the propagation, not nucleation of dislocations. Similarl
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