Surface-effect territory in small volume creep deformation
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ng Huangb) State-Key Laboratory for Mechanical Behavior of Material, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
Tianjian Lu MOE Key Laboratory for Strength and Vibration, School of Aerospace, Xian Jiaotong University, Xi’an 710049, People’s Republic of China (Received 28 February 2009; accepted 13 July 2009)
It is yet unclear how far surface effects can dominate small volume creep deformation in the surface layer of a metallic solid. We report experimental results of the apparent activation volume of single, ultrafine-grained, and nanocrystalline Cu over a range of nanoscale displacements. The dependence of the apparent activation volume on the depth and grain size was determined using nanoindentation creep tests. The surface-affected deformation regimen, within which interfacial diffusion between the nanoindenter tip and the sample totally dominates the creep behavior, was quantitatively determined to be below 12 nm. As the initial creep depth is increased, the dominant mechanism is shifted from interfacial diffusion to grain-boundary diffusion as the contribution of the surface effects gradually vanishes when the indenter penetrates deeper into the sample (i.e., further away from the external surface). I. INTRODUCTION A. Diffusion
Small volume creep deformation near a surface layer remains a distinct matter, although creep deformation of crystal metals has been extensively studied.1,2 Nanoindentation is one of a few effective experimental techniques for probing small-scale creep deformation. Several characteristic parameters of nanoindentation creep such as stress exponent,3,4 strain rate,5 and activation volume6–8 have been found to exhibit a strong dependence on indentation depth. However, the interpretation of these findings has been inconsistent as some researchers attributed the depth dependence to surface effects,3–5 whereas others deemed the variation of hardness as the prevailing factor.6,7,9 This inconsistency may simply stem from the different deformation scales considered in these studies. For example, the penetration depth is generally larger in the studies where hardness is considered more crucial than that in the studies where surface effects are considered to be more important. In addition to the inconsistency described above, the critical depth below which the surface effect totally dominates the creep deformation is Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2009.0416 J. Mater. Res., Vol. 24, No. 11, Nov 2009
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unclear. This missing information, we believe, is crucial for improving the present physical understanding of the observed strong depth dependence in small volume creep deformation. In a previous study,5 by evaluating the strain rate sensitivity in single-grained (SG) Cu and ultrafine-grained (UFG) Cu, we have shown that interfacial diffusion between the nanoindenter tip and the test sample is the dominant mechanism i
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