Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The Role of Grain Boundary Segregation

Abstract

Magnesium alloys that contain certain rare-earth (RE) additions are known to have improved formability and this can be partly attributed to the different texture they display after recrystallization. Previous experimental work has identified segregation of RE to grain boundaries and dislocations as being potentially important in producing this change in behavior. In the present paper, two classical models (Langmuir-McClean and Cahn-Lu¨cke-Stu¨we) are used to explore the likely effect of RE additions on grain boundary solute concentration and drag. It is demonstrated that a wide range of RE elements are predicted to segregate strongly to grain boundaries due to the large atomic size misfit with magnesium. The maximum level of segregation is produced for elements such as Y or Gd that combine a high misfit and high bulk solubility. Segregated Y is predicted to produce a solute drag pressure on migrating boundaries several orders of magnitude greater than that obtained by Al or Zn additions. It is demonstrated that while this drag is predicted to be insufficient to strongly retard static recrystallization under typical annealing conditions, it is expected to suppress dynamic recrystallization by any mechanism requiring boundary migration.

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 2- shear bands

“…Simultaneous precipitation and recrystallisation took place at annealing temperatures of 450 C and 490 C. Only recrystallisation occurred when the sample was annealed at 545 C. Combining these results in the schematic, it was clear that 400 C was below point B, while 450 C and 490 Cwere between points A and B, and 545 C was above point A. Although solute drag can retard dynamic recrystallisation, it cannot inhibit static recrystallisation at high temperatures [22]. Therefore, it was the Zener pinning from the precipitation that retarded the static recrystallisation at 400 C. When the annealing temperature was increased to 450 C and 490 C, recrystallisation occurred in the shear bands, but the grain boundaries were pinned, resulting in a random texture (Fig.…”

Critical Assessment 9: Wrought magnesium alloys

“…2b). 45 Other large atoms such as calcium would be expected to show the same behaviour, although this remains to be confirmed experimentally. The mechanism by which RE additions lead to texture modification and formability improvement is not simple, and has been the subject of intense study.…”

“…In dealing with these challenges it is important to recognise that a completely magnesium structure is never envisaged, so magnesium will always be used in combination with other materials according to the 'right material, right place' philosophy. 66 The greatest challenges to solve are therefore dissimilar material joining of magnesium to other candidate materials such as aluminium alloy or 2 a segregation of rare earth (Y) on grain boundary measured using TITAN ChemiSTEM and b predicted grain boundary concentration of various RE additions for mean alloy composition of 0?22 at.-% (dashed line) using simple classical size based model 45 steel, and protection against corrosion of such multimaterial structures. Magnesium is high in the electrochemical series, but forms an oxide that reduces its corrosion rate to be comparable with that of aluminium in most environments.…”

The Influence of Strain Path on Rare Earth Recrystallization Textures in a Magnesium-Zinc-Rare Earth Alloy

“…[4] Furthermore, there is now good evidence that the strong segregation of RE to sites such as grain boundaries or dislocations helps explain why even very low levels of addition (<0.1 wt pct in some cases) can be effective. [5][6][7][8] These mechanisms have been summarized in a recent review. [9] While weak RE textures in magnesium alloys have been studied for some time, it has only recently been appreciated that tertiary alloying additions such as Zn can produce a significantly different texture (referred to here as the Zn-RE texture).…”