Size-dependent deformation mechanisms and strain-rate sensitivity in nanostructured Cu/X (X=Cr, Zr) multilayer films

Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Sig...

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Bibliographic Details
Published in:	Acta materialia 2012-05, Vol.60 (9), p.3677-3689
Main Authors:	Niu, J.J., Zhang, J.Y., Liu, G., Zhang, P., Lei, S.Y., Zhang, G.J., Sun, J.
Format:	Article
Language:	eng
Subjects:	ACTIVATION Applied sciences COMPOSITES Copper Cross-disciplinary physics: materials science rheology DEFORMATION MECHANISMS DISLOCATIONS Exact sciences and technology HARDNESS Materials science MATHEMATICAL ANALYSIS Mathematical models Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology Metals. Metallurgy Methods of deposition of films and coatings film growth and epitaxy MICROSTRUCTURES Multilayer films Nanocomposites Nanomaterials Nanostructure Physics Size effect STRAIN Strain-rate sensitivity
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Summary:	Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Significant size effects were found for all the three plastic deformation characteristics, i.e. enhanced hardness and activation volume but reduced strain-rate sensitivity with decreasing the dimension length L. Microstructure evolution was statistically examined to rationalize these size dependences. It was crucially observed that abundant nanotwins existed in the Cu grains, though nanotwin formation was depressed with smaller L. This inverse size-dependent nanotwin formation is responsible for the reduction in strain-rate sensitivity, because the negative effect induced by the decreased nanotwins predominates over the positive effect coming from the raised interfaces/boundaries. A mechanistic model is modified to account for the interface effect as well as the nanotwin effect, which yields calculations of strain-rate sensitivity in broad agreement with the experimental results when L is larger than about 20nm. Below this critical length size, there are discrepancies between the calculations and the experimental results, due to the change in deformation mechanism from dislocation nucleation/slip in confined layers to dislocation crossing interfaces. A confined layer slip model is also modified by considering the nanotwin strengthening to quantitatively describe the L-dependent hardness. In addition, the effects of constituent phases and their relative content on the activation volume and strain-rate sensitivity of NMFs are discussed with regard to variation in η.
ISSN:	1359-6454 1873-2453