Deformation behavior and indentation size effect in amorphous and crystallized Pd 40 Cu 30 Ni 10 P 20 alloy
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K.C. Chan Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, People’s Republic of China (Received 17 September 2008; accepted 17 November 2008)
The deformation behavior and indentation size effect (ISE) in amorphous and crystallized Pd40Cu30Ni10P20 alloy were comparatively studied through instrumented nanoindentation. It was found that the two alloys showed different deformation behaviors, the amorphous alloy exhibited conspicuous pop-in events in the load-depth (P-h) curve, while the crystallized alloy showed a relatively smooth P-h curve. In addition, the indentation hardness was observed to decrease with increasing penetration depth in the two alloys, exhibiting a significant ISE. However, the crystallized alloy displayed a sharper reduction of hardness with indentation depth as compared to the amorphous alloy, indicating a more significant indentation size effect in the crystalline alloy. The structure difference and friction factor associated with the surface residual stress are taken into account to interpret the difference in the deformation behavior and indentation size effect of the two alloys.
I. INTRODUCTION
Instrumented microindentation and nanoindentation techniques have been extensively used for the micromechanical characterization of materials due to their high time resolution and spatial resolution. However, the hardness is often found to decrease as the depth of penetration increases during indentation, that is, the so-called indentation size effect (ISE).1 Upit and Varchenya2 first investigated the ISE in single crystals and related it to the effect of free surface on the behavior of dislocations. Based on the Taylor dislocation and geometrically necessary dislocations (GNDs) induced by imposed strain gradients underneath an indenter, Nix and Gao3–5 developed the strain gradient plasticity (SGP) model that established a linear relationship between the square of hardness (H2) and the reciprocal of the indentation depth (h–1). However, this mode is only applicable for indentation depth between 0.1–10 mm, but became invalid for an indentation depth of less than 100 nm.6,7 The improvements to the model were made by modifying the GND storage volume,8 considering the effect of work hardening9 and the limitation of the maximum allowable density of GNDs10 in the nanoscale. On the other hand, the role of friction between the indenter a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0222 J. Mater. Res., Vol. 24, No. 5, May 2009
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facets and the test specimen has been proven crucial in crystalline materials, and the contribution of friction to the ISE is inversely related to the indentation size.11,12 Other factors, such as surface effect,13 structural nonuniformity of the deformed volume,14 mixed elastic and plastic deformation response of material,15 and so on, have also been proposed to illustrate the complicated nature of the ISE. Although most of t
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