Occurrence of plastic instabilities in dynamic microhardness testing
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Occurrence of plastic instabilities in dynamic microhardness testing G. B´erces, N. Q. Chinh, A. Juh´asz, and J. Lendvai Department of General Physics, E¨otv¨os University, Budapest, H-1088 M´uzeum krt. 6-8, Budapest, Hungary (Received 20 May 1997; accepted 20 November 1997)
Plastic instabilities were observed to appear during dynamic ultramicrohardness testing of a solid solution Al–3.3 wt. % Mg alloy. The tests were carried out at room temperature with a Vickers hardness indenter in a computer-controlled dynamic ultramicrohardness testing machine. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appear as characteristic steps in the continuously recorded load-indentation depth curves. The physical basis for the occurrence of the instabilities is the interaction between moving dislocations and solute atoms, a phenomenon termed in the literature as serrated yielding, jerky flow, or Portevin-Le Chˆatelier effect. The instabilities start at a critical load, Fc , in the depth-load curve. Varying the loading rate, m, by two orders of magnitude Fc was found to increase linearly with the loading rate.
The phenomenon of repeated yielding during plastic deformation has been widely observed, and discussed in the literature under various names like PortevinLe Chˆatelier (PLC) effect, serrated yielding, jerky flow, etc. The phenomenon has been observed most often on metallic solid solution (both substitutional and interstitial)1,2 alloys, but it appears also in precipitation strengthened alloys in underaged condition.3 In unidirectional deformation the most characteristic features of the effect are the appearance of serration (stress drops or steps) on the stress-strain curves and traces of strain localization readily observable on the specimen surface. The PLC effect is a class of plastic instabilities which has been subdivided according to the serration type, depending primarily on temperature and strain rate.4,5 The PLC effect is often accompanied by the development and repeated propagation of deformation bands along the specimen. The physical basis for the appearance of the PLC effect is a negative strain rate sensitivity which originates from dynamic strain aging, i.e., the interaction between dislocations and diffusing solute atoms.1 The PLC effect has been observed in different modes of deformation, like tension, compression, torsion, fatigue, etc. Acoustic emission was also successfully applied to study the mechanism of the PLC effect.6 At the points were serrations appear in the plastic stressstrain (s-e) curves, the rate of work hardening, ≠sy≠e, abruptly decreases, and in some cases it even becomes J. Mater. Res., Vol. 13, No. 6, Jun 1998
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negative. This suggests that a similar effect can be expected in depth-sensitive microhardness tests applied in the present investigation. The experiments were carried out on an Al– 3.3 wt. % Mg alloy by using a Vicker
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