Possible self-organized criticality in the Portevin-Le Chatelier effect during decomposition of solid solution alloys
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Possible self-organized criticality in the Portevin–Le Chatelier effect during decomposition of solid solution alloys Nguyen Q. Chinh, Tivadar Győ ri, Jenő Gubicza, and János Lendvai, Department of Materials Physics, Eötvös University Budapest, 1117 Budapest, Pázmany P. sétány 1/A, Hungary Terence G. Langdon, Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1453; Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK Address all correspondence to Nguyen Q. Chinh at [email protected] (Received 3 September 2011; accepted 15 December 2011)
Abstract Spatial fluctuations of the microstructure suggest possible self-organized criticality in the Portevin–Le Chatelier plastic instability occurring in age-hardenable alloys. The discontinuous yielding found in a supersaturated Al alloy can be characterized by a universal power-law spectrum that is independent of the experimental conditions. The result provides an explanation for the formation of unexpected detrimental strain localizations when samples are severely deformed, giving a framework for studying the simultaneous effects of solute atoms and precipitates in the decomposition of solid solutions.
The Portevin–Le Chatelier (PLC) effect[1–5] is a typical nonlinear phenomenon of plastic instabilities that is attributed to the interaction of dislocations with solute atoms in metal alloys.[6] Although the role of the alloying atoms in stable solid solutions is well established, the situation is less clear for the decomposition of supersaturated solid solutions, where it is necessary to consider the simultaneous effects of a decreasing concentration of solute atoms and the formation of precipitates. This report describes the self-organized features of the plastic instabilities occurring in a supersaturated AlZnMg alloy, which is widely used in the aluminum industry. The alloy, having a composition in wt% of Al–4.8 Zn–1.2 Mg– 0.14 Zr, is one of the so-called age-hardenable alloys that generally have high strength due to the nanometer-size precipitates formed in the thermodynamically non-equilibrium system after homogenization.[7,8] Storing the samples at room temperature (RT) leads to the formation of Guinier–Preston (GP) zones in a natural ageing process.[7,8] These nanoparticles effectively hinder the motion of dislocations within the matrix, thereby increasing the strength of the material. In the present case, for example, the Vickers hardness, HV, of the alloy changed by 50% from 40 to 60 HV after storing at RT for 3 h.[9] Figure 1 shows the typical surface topographies of supersaturated billets after homogenization and storing at RT for the same period of 3 h and then deforming plastically in one pass using the equal-channel angular pressing (ECAP) technique, which is a well-established severe plastic deformation (SPD) method for the fabrication of bulk nanostructured metals and alloys.[10] Prior to storage
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