Dislocation Configurations in an Extruded ZK60 Magnesium Alloy
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(Mg) alloy. We mainly employed the weak-beam dark-field (WBDF) technique with 0110 zone axis to unambiguously identify Burgers vectors of various types of dislocations in the microstructure. It was found that nonbasal hai dislocations and hci dislocations were predominant. No hc + ai was observed. Rearrangement of these dislocations, due to dynamic recovery during extrusion, led to cell walls and low-angle grain boundaries with relatively high density dislocations. Dissociation of the hci dislocations with a relatively narrow split distance was observed. The dissociated hci dislocations could form a nodular structure. Fringes of stacking faults from the dislocation dissociation were observed. DOI: 10.1007/s11661-008-9621-3 Ó The Minerals, Metals & Materials Society and ASM International 2008
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INTRODUCTION
IN recent years, magnesium (Mg) and Mg-based alloys have drawn significant attention as attractive lightweight alloys. Among them, the family of ZK60 Mg alloys (based on Mg-6.0Zn-0.5Zr) is of great interest,[1–3] because these alloys possess relatively high strength (compared to other Mg alloys) and ductility. Extruded ZK60 alloys[1,4–6] have also been used as the starting material for severe plastic deformation using, for example, equal channel angular extrusion (ECAE). This processing route that involves heavy plastic deformation is related to the ongoing efforts to improve the mechanical properties of alloys by reducing the grain size down to the ultrafine grain and even nanocrystalline range.[7–16] Despite the extensive investigation on extruded ZK60 alloys, their dislocation structure remains unclear. Extrusion is an important industrial practice to refine the grain structure in Mg and Mg alloys. It can be expected that during extrusion to high strains, the microstructure of the ZK60 alloys experiences drastic changes in terms of the generation of defect structures such as various types of dislocations, twins, and stacking faults. The resultant mechanical properties, such as the strength of the extruded alloys, obviously strongly depend on the dislocations and their configurations in the microstructure. This makes a systematic investigation of the types and configurations of dislocation in extruded ZK60 important for building the foundation for understanding further dislocation evolution in subsequent deformation (e.g., in shaping or mechanical B. LI, Postdoctoral Fellow, E. MA and K.T. RAMESH, Professors, are with the Center for Advanced Metallic and Ceramic Systems, Johns Hopkins University, Baltimore, MD 21218. Contact e-mail: [email protected] Manuscript submitted January 7, 2008. Article published online August 20, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
tests), and for comparisons with the dislocations developed in other processing routes such as ECAE. From the perspective of fundamental research, dislocation slips in hcp Mg and Mg alloys are also of interest, especially because they are much less understood than those in fcc and bcc metals. In-depth analyses of dislocations and dislocation configuratio
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