Mapping of the Initial Volume at the Onset of Plasticity in Nanoindentation
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Mapping of the Initial Volume at the Onset of Plasticity in Nanoindentation T.T. Zhu1, K.M.Y. P'ng1, M. Hopkinson2, A.J. Bushby1, and D.J. Dunstan1 1 Centre for Materials Research, Queen Mary University of London, London, E1 4NS, United Kingdom 2 Department of Electronic and Electric Engineering, University of Sheffield, Sheffield, S1 3JD, United Kingdom
ABSTRACT Understanding the finite volume throughout which plastic deformation begins is necessary to understand the mechanics of small-scale deformation. In indentation using spherical indenters, conventional yield criteria predict that yield starts at a point on the axis and at a depth of half the contact radius. However, Jayaweera et al (Proc. Roy. Soc. 2003) [4]concluded that yield occurs over a finite volume at least 100 nm thick. Semiconductor superlattice structures, in which the stress and thickness of individual layers can be varied and in which known internal stresses can be incorporated, open up new possibilities for investigation that cannot be achieved by varying external stresses on a homogenous specimen. We have designed samples with bands of highly strained InGaAs superlattice, which are essentially bands of low yield-stress material devoid of other metallurgical artifacts. These bands are placed at different depths in a series of samples. Spherical indenters with a range of radii were used to determine the elastic-plastic transition. The stress field from different sized indenters interacts with the low yield-stress material at different depths below the surface to map out the size of the initial yield volume.
INTRODUCTION Small-scale mechanical behaviour is at the cutting-edge of research in materials science and applied mechanics. Plastic yield strength is one of the most fundamental subjects in materials science and engineering field. Also, it is a very useful in materials designing and applications, since it certainly helps to predict the reliability of materials. The plastic yield strength is not only dependent on the material structure but also on the loading configuration. Nanoindentation testing is an excellent, usually non-destructive method for testing the mechanical properties of materials at small scales. The yield behaviour in nanoindentation is a very popular and attractive topic nowadays. [1-4] With spherical indenters, the stress fields can be obtained using Hertzian contact mechanics and yield is predicted to initiate at the point where a suitable some yield criterion is first exceeded. This point is on the indenter axis and at a depth of half the contact radius. However, on the basis indirect experimental evidence, Jayaweera et al. [4] proposed that yield in spherical nanoindentation should initiate throughout a finite volume at least 100nm across, perhaps as large as a micron. In this paper, a unique experimental method is demonstrated to observe directly the real initial volume of
yield for spherical indentation. Results indicate that the size of the initial volume varies for different radius indenters and also i
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