Depth-sensing indentation tests in studying plastic instabilities
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his review surveys the phenomenon of plastic instabilities occurring in depth-sensing indentation measurements. Investigations presented focus on the characterization of Portevin–Le Chaˆtelier type instabilities observed in different metal alloys during indentation. The effect of some important factors such as solute concentration, the formation of Guinier–Preston zones, and grain size and orientation are described and discussed. The phenomenon of plastic instabilities as serrated flow recently observed in bulk metallic glasses is also briefly reviewed.
I. GENERAL APPLICATIONS OF DEPTH-SENSING INDENTATION METHOD
In the last two decades, depth-sensing indentation (DSI) became widely used for the mechanical characterization of solids. The reason for its popularity is that DSI enables the determination of mechanical properties even for a small volume of material without complicated sample preparation. The DSI method is an improvement over conventional hardness measurement. The most important motivations of this development are the following: (i) the optical evaluation of extremely small indentations in hard materials and thin layers is very uncertain, (ii) the DSI enables the measurement of the depth profile of the hardness perpendicular to the surface in gradient materials, (iii) the method allows the study of changes in the hardness during deformation under load (dynamic hardness), and (iv) the penetration or the loading rate, which affect the hardness value, can be selected and controlled during indentation. During the DSI process, an indenter is penetrated into the surface of the material with constant indentation rate or loading rate. In the majority of the measurements, the loading rate is held constant. When the load reaches its selected maximum value, the load decreases with the same rate as for loading and the indenter moves backward. The load-penetration depth curves are recorded in both the loading and the unloading parts of the measurement as schematically shown in Fig. 1. The characteristic features of these curves are used for the determination of mechanical properties of solids. The shape of the loading and unloading curves depend on the geometry of the indenter, the loading rate, the testing temperature, and, of course, the mechanical properties of the material. For the most often used Vickers and Berkovich type indenters, the relationship between the load (F) and the penetration depth (h) for the loading curve is usually approximated by a quadratic function: J. Mater. Res., Vol. 19, No. 1, Jan 2004
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F ⳱ Ch2
,
(1)
where C is a constant depending on the geometry of the indenter and the elastic and plastic properties of the material. This relation has been recovered by numerical simulation for both elastic–perfect plastic and elastic– plastic materials1–3 and confirmed also by experimental results.4–7 Assuming perfect tip shape and that the indentation holds the shape of the indenter, the hardness number (H), defined as the ratio of the load and
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