A TEM study of microstructural change in Mg alloys during creep

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A TEM study of microstructural change in Mg alloys during creep

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Publication Conference Paper, peer reviewed
Title A TEM study of microstructural change in Mg alloys during creep
Author(s) Wei, Liu-Ying ; Wei, Kun ; Warren, Richard
Date 2007
Editor(s) Nebesarova, Jana; Hozak, Pavel
English abstract
M5-10 A TEM Study of Microstructural Change in Mg Alloys during Creep Wei L.-Y. Wei K. and Warren R. Division of Materials Science, School of Technology and Society Malmö University, 205 06 Malmö Sweden liu-ying.wei@ts.mah.se Due to their low densities and a good combination of castability and different properties, magnesium alloys have been attractive materials for automotive industry for weight saving. Die-cast AZ91 is one of the most widely used alloys. It exhibits excellent die castability, good strength properties and a fair ductility but its mechanical properties, especially the creep resistance, decrease on exposure at temperature above 120ºC [1]. Softening and coarsening of the interdendritic phase β-Al12Mg17 in AZ91 can readily occur at elevated temperatures due to its low melting temperature (Tm = 437°C) [2]. Rare earth in magnesium alloys usually brings about alloy strengthening through the formation of thermally stable intergranular phases and the complete suppression of the β-Al12Mg17 phase formation. In the present work tensile creep behavior of the die-cast magnesium alloys AZ91 and AE42 and their microstructural change during plastic deformation in creep have been studied. The objective of the work is to study the active creep mechanisms on the basis of the creep data in combination with the microstructural investigations. The SEM micrographs in Fig. 1 (a-b) illustrate the as-cast microstructures of the two die cast alloys showing a typical dendritic solidification structure with a fine-scale grain size. Segregation of Al adjacent to the dendrite boundaries can be seen clearly in AZ91D. Phases with a bright contrast and surrounded by high Al-segregated eutectic α-Mg are β-Al12Mg17 phase. Segregation of Al seems not obvious in AE42. The dominant interdendritic phase is Al11RE3. Mn-containing phases also formed during solidification in both AZ91 and AE42. The TEM micrographs in Fig. 2 (a-c) illustrate the structural changes during creep in AZ91 and AE42. Fig. 2 (a) shows the formation of denuded zones around the dendrite boundaries and the formation of discontinuous precipitates Al12Mg17. TEM/XEDS revealed that the Al content in the denuded zones decreased after creep test, from the original 5.5% to 2.5% (wt%). Fig. 2 (b-c) illustrates the formation of dislocation sources in the (0001) basal planes under a relatively high stress and the movement of dislocations from the sources towards the dendrite boundaries. The existence of the supersaturated Al-segregated zones adjacent to the dendrite boundaries is the main reason for the recovery of AZ91 at operating temperatures above 100ºC. Under loading plastic deformation occurred by the diffusion of Al atoms along the boundaries from compressive to tensile grain boundaries that caused the formation of the denuded grain boundaries and as a consequence the formation of the discontinuous precipitates Al12Mg17. Dislocation climb process also operated and tended to become a dominant process at stress above 60MPa.The low level of Al in AE42 suppresses the formation of Al supersaturated zones adjacent to the dendrite boundaries that, combining with the formation of the thermally stable interdendritic phase Al11RE3 (Tm~640ºC), provides AE42 a much better creep performance. Diffusion accompanying with the dislocation climb also occurred and became more dominant at a relatively high stress. Figure 1 SEM micrographs of the die cast alloys (a) AZ91D; (b) AE42. Fig. 2 TEM micrographs of the crept samples, (a-b) AZ91 after creep test at 100ºC/100MPa (n=3), (c) AE42 after creep at 175ºC/60MPa (n=2-7). References: 1. I.J. Polmer, 2nd Mg conference, DGM Informationsgesellschaft, Germany (1992), 201 2. M.S. Dargush, G.L. Dunlop, K. Pettersen, in: W. Higgins (Ed.), Trans. of 19th Intern. Die Casting Cong., North American Die Casting Association, Rosemont, IL, 1997, 131–137. Acknowledgements: Norsk Hydro ASA is gratefully acknowledged for providing the test bars
Link http://www.univie.ac.at/asem/8mcm_2007_prag_proceedings/pdf/material_sciences/M5/M5-10.pdf (external link to publication)
Host/Issue 8th Multinational Congress on Microscopy : June 17-21, 2007, Prague, Czech Republic : proceedings
ISBN 9788023993974
Language eng (iso)
Subject(s) Technology
Research Subject Categories::TECHNOLOGY
Note 8th Multinational Congress on Microscopy (MCM) June 17-21, 2007, Prague, Czech Republic
Handle http://hdl.handle.net/2043/7553 (link to this page)

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