Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The Role of Grain Boundary Segregation
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HE benefits that magnesium can offer to transport applications in enabling mass reduction and improving fuel efficiency are hampered by the poor ambient temperature formability of standard wrought magnesium alloys. This means that processes such as cold stamping cannot be directly applied to form magnesium parts. This represents a major barrier to the wider adoption of wrought magnesium alloys. Reducing the critical temperature required to form magnesium alloys is therefore now a topic of intense world-wide research activity (e.g., References 1, 2). It is clearly understood that part of the cause of the formability problem is the very strong basal textures that tend to be developed in magnesium sheet or extrusions. Reducing the texture strength or changing the texture is therefore one route to improving formability.[2–4] This can be achieved by changing the processing route; for example, equal channel angular extrusion, shear rolling, or other novel metal forming methods have been successfully used to alter the texture in magnesium and provide improvements in ductility.[5,6] However, alloying is also another attractive route to changing texture. Several alloying additions including Li, Ca, and a range of rare-earth (RE) metals have been shown to be effective in producing a texture change (e.g., References 2, 3, 7–9).
The beneficial effects of RE on the ductility of magnesium alloys was first reported over 60 years ago[10] and since then the so called ‘‘RE’’ effect has received considerable attention from the scientific community. The early work on Mg-RE alloys is well documented by Rokhlin.[11] In the past decade there has been an intense resurgence of scientific interest in understanding the RE effect in magnesium. The application of new techniques to this problem such as electron backscatter diffraction (EBSD) and atomistic modelling has enabled new insights to be obtained into these materials. Despite these efforts, there is not yet a complete and consistent scientific consensus as to the mechanisms that lead to both texture weakening and improved formability in Mg-RE alloys. In this paper, a brief overview of the history of developments in understanding the Mg-RE effect is given first. This is not intended to be comprehensive in the manner of a review, but is instead used to highlight some of the key steps in reaching the current understanding of these alloys. The remainder of this paper is devoted to exploring one aspect of the RE effect; the role of grain boundary segregation and solute drag. Simple classical models are used to demonstrate that solute drag is likely to have a critical role in producing the RE effect.
II. JOSEPH D. ROBSON, Reader, is with the Physical Metallurgy, Manchester Materials Science Centre, University of Manchester, Grosvenor Street, Manchester M1 7HS, UK. Contact e-mail: [email protected] Manuscript submitted June 18, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
UNDERSTANDING THE RARE-EARTH EFFECT: BACKGROUND
Although a great deal of work was carried out on Mg-RE alloys in the period
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