Solute concentration dependence of strength and plastic instabilities in Al-Mg alloys
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haracteristics of the dynamic strain aging (DSA) in the Portevin-Le Chatelier effect are experimentally investigated by dynamic indentation tests and numerically analyzed by using literature models. Experimental results obtained on Al–Mg alloys show that the occurrence and development of the plastic instabilities—serrated indentation— depend strongly on the solute content. During dynamic microindentation tests the amplitude of microhardness drops—similarly to the global hardness—and is changing as a power law function of Mg solute content with an exponent of 2/3. It has been shown that the term describing the effect of DSA in serrated flow is not proportional but rather a power expression of the local solute concentration, Cs, on the dislocation line with the exponent of 1/2. Together with this, the kinetics of solute segregation during DSA is controlled by the pipe diffusion.
I. INTRODUCTION
During plastic deformation of solid solution hardened materials in a certain regime of temperature (T), strain . rate (⑀ ), and solute concentration (C), dynamic strain aging (DSA) may take place,1,2 which is a dynamic interaction between mobile dislocations and diffusing solute atoms. As a consequence of DSA, some characteristic physical parameters do not change monotonously, but local fluctuations, which are discussed in the literature under various names such as plastic instabilities, Portevin-Le Chatelier (PLC) effect, serrated yielding, or jerky flow appear. Although the phenomenon of serrated flow has been observed and investigated mainly by tensile and compression tests,3–6 it was also studied in other modes of deformation.7 Recently, the phenomenon as discontinuous indentation in depth-sensing measurements was investigated intensively in crystalline alloys.8–15 In unidirectional deformation at constant strain rate, jerky flow occurs when the strain reaches a critical value, ⑀c (onset strain), beyond which stress drops; ⌬ appears repeatedly on the originally smooth stress-strain ( − ⑀) curve. Macroscopic treatments of the PLC effect revealed that negative strain rate sensitivity (SRS) is a necessary condition of the appearance of discontinuous yielding.16–22 Depending on the testing conditions, nega-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0040 J. Mater. Res., Vol. 20, No. 2, Feb 2005
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tive SRS may occur at a certain part of the N-shaped . stress-strain rate ( − ⑀ ) function. Several models have been developed both on the solute-dislocation interaction16,23–26 and on the explicit expression of the men. tioned N-shaped − ⑀ function.20–22 On the basis of these theories, the main characteristic features of the plastic instabilities (the onset point, ⑀c, and stress drops, ⌬) can be explained. Taking into account atomistic treatments such as the segregation of solute atoms on dislocations directed by diffusion, the solute concentration dependence of the PLC effect has been interpreted also.16,25,26 Several
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