Role of Solute in the Texture Modification During Hot Deformation of Mg-Rare Earth Alloys

Abstract

Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing, the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer alloys ( ‡0.17 at. pct) show the weakening effect, whereas the most dilute alloy (£0.03 at. pct) does not. Electron backscattered diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation (GND) content in dilute, hotrolled alloys contain primarily basal hai dislocations. At higher concentrations, the dislocations are predominantly prismatic hai type. This change in the GND content suggests a change in dynamic recrystallization (DRX) mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering. Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening phenomenon in Mg rare earth alloys.

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

“…This observation led to the suggestion that segregation is likely to be important in enabling much higher concentrations than added in the bulk to be achieved locally. [16,17] Once a critical level of RE is achieved, further texture weakening does not occur with more RE addition. [18] It was also shown that the potency of an element in weakening the strong basal texture scales inversely with the solubility of the element; i.e., for elements with a higher solubility, more RE is needed to produce a texture change.…”

“…This is because as already discussed, the ''RE texture'' only occurs on recrystallization and unrecrystallized grains typically show textures that are very similar to those in non-RE alloys. [8,14,17] An essential characteristic of RE additions in producing the observed texture change is a potent effect on recrystallization. In particular, RE additions can result in inhibition of dynamic recrystallization (DRX) and a change in DRX mechanism.…”

“…In particular, RE additions can result in inhibition of dynamic recrystallization (DRX) and a change in DRX mechanism. [17,22] Stacking fault energy is clearly important in recovery and recrystallization processes, so the effect of RE additions on SFE is likely to be equally as critical for the recrystallization behavior as it is for the deformation behavior. However, other factors are undoubtedly also important, including the tendency for RE elements to segregate in the microstructure.…”

“…These data have come from both atom probe [16] and energy-dispersive X-ray spectroscopy (EDS) measurements. [17] This segregation has been speculated to be responsible for the strong suppression of DRX in Mg-Y alloys. [17,22] It was noteworthy that no such segregation was measured for Zn, a conventional alloying addition that does not produce a texture change.…”

“…[17,22] It was noteworthy that no such segregation was measured for Zn, a conventional alloying addition that does not produce a texture change. [17] One factor that is likely to be important in producing the observed segregation of RE elements is the large size misfit between RE and magnesium atoms.…”