Nucleation and preferential growth mechanism of recrystallization texture in high purity binary magnesium-rare earth alloys

Abstract

We investigated the effect of rare-earth element on recrystallization textures using Mg-Ce and Mg-Gd binary alloys at various concentrations of Ce and Gd, and employing electron backscattered diffraction – EBSD technique. Our results indicate that the rare earth element concentration required to noticeably alter recrystallization texture depends primarily on the processing conditions, and cannot be regarded only from the concentration standpoint. Rare earth element additions and stored energy from deformation work in combination to alter texture in rare earth element containing magnesium alloys. The nucleation stage of dynamic recrystallization was explored in samples with partially recrystallized microstructures, revealing necklace structures of small grains forming continuously along the boundaries of pancake-shaped grains in the extrusion direction. Stochastic relaxation of basal , pyramidal , and prismatic dislocations into low-angle grain boundaries produced chains of new dynamically recrystallized embryos with nearly random orientations. A remarkable grain-size driven textural transition occurs from a predominant band in the inverse pole figures where the -axis is oriented 35–65°away from the extrusion direction to a hot spot between texture components. EBSD observations showed that Mg-Gd alloys have a slower textural transition rate due to the stronger grain refining potential of Gd compared to Ce. These results suggest that the formation of band-like texture patterns in the inverse pole figures is associated with the recovery of basal/ dislocations, while transition to a hot spot in texture is a result of preferential grain growth, which was made possible by an effect related to the presence of rare earth elements in the grain boundaries. We investigated the growth behavior during dynamic and static recrystallization. Schmid factor analysis showed a lower net activation of dislocations in grains compared to grains. Lower dislocation densities within RE grains favor their growth by setting the boundary migration direction toward grains with higher dislocation density, thereby decreasing the system energy.

Introduction

Over the past two decades, industries have shown an ever increasing demand for low density alloys, spurring a strong interest in the lightest metal on earth – magnesium (Mg) [1], [2], [3], [4], [5], [6], [7], [8]. Despite its favorable specific strength, poor low temperature formability has dramatically restricted the wide use of wrought magnesium alloys in critical safety components [9]. This inadequacy is directly associated with the limited number of active slip systems in directions other than those contained in the basal plane of the hexagonal close packed (HCP) structure [10], [11], [12], [13], [14], [15], [16]. Moreover, rolling and extrusion systematically lead to very sharp textures, intensifying strain-path anisotropy, asymmetry and damage propensity [17], [18], [19], [20]. For instance, a typical fiber texture develops during the extrusion process, which may transform to fiber [21]. This type of texture evolution aligns the basal planes parallel to extrusion direction (ED) [10]. A texture is subject to strong plastic anisotropy as compressive deformation along the ED activates profuse $\left\{10\overline{1}2\right\}$ twinning, while tension along ED would hardly cause any twinning of the same type. Usually, strong slip-twin interactions take place at the twin boundaries leading to hot stress spot and acceleration of damage [12].

In contrast to traditional Mg alloys such as AZ31B, rare earth (RE) element containing Mg alloys express ameliorated formability [21], [22], [23], [24], [25]. RE addition activates non-basal dislocations (e.g. dislocations) at low temperatures, which mitigates the need of twinning and thus improves ductility. This has been attributed to an effect that tends to decrease the difference in the critical resolved shear stress (CRSS) values of the available slip systems [5], [26], [27], [28]. Moreover, RE elements tend to decrease the energy of the I₁ stacking fault (ISF₁), which serves as a nucleation site for dislocations [28], [29]. However, although RE addition offers the prospect of improved mechanical properties, understanding of the mechanisms by which they can enhance ductility is still lagging. Moreover, the expense of RE elements means that only very small concentrations of them can be present in cost-effective alloys, and such alloys have not met yet ductility requirements in critical safety components. Therefore, a more rigorous understanding of the effect of RE elements on dynamic recrystallization is critical to improve their texture modification action.

Alloying with RE elements weakens the tendency of magnesium to retain a sharp texture during recrystallization [23], [30], [31], [32]. This tendency is prevalent for both single RE element additions and cheaper RE alloy additions such as Mischmetal (MM) [33], [34], [35]. Hantzsche et al. [36] found that binary Mg-RE alloys with concentrations above the threshold limit intrinsic to each RE element (e.g. Ce and Nd) develop weaker texture intensities during rolling, while more dilute alloys retained their sharp basal texture that is typical of conventional Mg alloys [8]. In other words, a sudden drop from a strong basal texture to weak texture intensities was observed as the RE concentration exceeded a threshold limit [36]. A number of authors correlated the threshold limits to solubility limits [36]. However, for all the elements they studied, texture modification started off well below the solubility limit, which rules out precipitate formation as the cause of this correlation [9]. Stanford [24] also found small concentrations are required to induce RE texture. For example, only 400 ppm of Ce is required for texture modification in pure magnesium. On the other hand, Robson et al. [37] pointed out the variation of dominant texture components with extrusion condition in a Mg-RE alloy. In a recent study by the present authors [38], extrusion speed was a crucial factor in developing RE texture, as the driving force for dynamic recrystallization (DRX) is in fact stored strain energy. Therefore, one can perceivably question the dependence of the RE threshold limit on extrusion condition. Is defining an absolute value as a threshold concentration for texture modification realistic? The first aim of the present study is to explore the combined effect of RE concentration and extrusion condition on texture evolutions in binary Mg-RE alloys.

Several approaches have been exploited to understand the mechanisms underlying texture modification associated with RE addition [25], [32], [33], [35], [39], [40]. Although particle stimulated nucleation (PSN) was believed to account for this phenomenon [41], further studies demonstrated texture weakening in solid solution Mg-RE alloys, indicating that the role of PSN is not necessary [33], [34], [35]. Stanford and Barnett [25] observed shear band formation in extruded Mg-RE alloys, within which DRX grains had “RE texture” orientation (i.e. ). However, shear band formation and consequent texture alterations have been reported in traditional Mg alloys [3]. Furthermore, RE texture formation was reported in Mg-RE alloys with no shear banding events, which questioned its necessity as a mechanism for texture modification [34], [35]. Deformation twinning has also been observed to serve as DRX nucleation sites for randomly oriented grains, as RE additions bolster contraction and secondary twin formation [36], [42]. Nevertheless, the effectiveness of such nuclei in terms of grain growth, and hence texture modification, are limited by twin boundaries [43]. In a recent study by the present authors, boundaries with tilt character between bands of deformed grains were suggested to be the main contributor for widespread DRX nucleation [38]. Stanford [24] suggested that a strong interaction of solute atoms with dislocations and grain boundaries may account for the remarkable influence of RE elements on the recrystallization texture of Mg-RE alloys. Employing intragranular misorientation axis (IGMA) analysis, Hadorn et al. [35] determined that the population of geometrically necessary dislocations (GNDs) shifts from a predominantly basal type to non-basal type dislocations by increasing the RE content [34]. Therefore, a change in recrystallization mechanism was envisaged. They also reported a significant segregation of yttrium (Y) to grain boundaries. Segregation of RE elements to grain boundaries and dislocation cores suppress recrystallization kinetics [30], [35]. This can potentially diversify crystal orientations of grain nuclei by boosting the available time period for basal and non-basal dislocations to migrate into DRX nuclei and induce random crystal rotations. However, these mechanisms are yet to be demonstrated and described in detail. The second goal of this study is to directly observe the occurrence of such phenomenon and to explicate formation of random nuclei in binary Mg-RE alloys.

It is well understood that growth of grains with new orientations, following DRX nucleation, is essential for modification of recrystallization texture [17], [44], [45]. The growth advantage of $\left[0001\right]\left|\right|ND$ grains (i.e. -axis nearly normal to rolling direction) in rolled sheets of traditional Mg alloys retains strong basal texture upon recrystallization [46]. A similar tendency for retaining a sharp fiber texture was observed in extruded products [47], [48], [49], [50]. RE addition ought to suppress/alter the preferential growth over the course of static or dynamic recrystallization [9], [17], [34], [45]. In a recent study, the present authors found that Mg-RE alloys with randomized texture possess a broader range of misorientation distribution, which imparts the grains with nearly equal chance for growth [38]. Barrett et al. [51] showed that addition of Y to Mg reduces the gap between energy cusps in the excess potential energy plot for c-axis tilt boundaries, which they hypothesized would eliminate the preference to nucleate special boundaries, instead stabilizing grains with a wide variety of orientations and giving them nearly equal opportunity for growth. This may rationalize the more isotropic growth of DRX nuclei in Mg-RE alloys compared to traditional Mg alloys. One would expect isotropic grain growth to result in indiscriminate growth behavior. However, isotropic grain boundary mobility can only partially solve the RE texture development puzzle, as the growth behavior in Mg-RE alloys seems to act rather selectively. Barrett et al. [52] also found that high mobility of $\left\{13\overline{4}0\right\}$ twin boundary was accountable for the formation of fiber in AM30 Mg alloy during dynamic recrystallization. However, for the case of Mg-RE alloys, one can perceive the existence of all types of grain boundaries due to the diversity of nuclei orientation. Hence, there must be another factor influencing the growth behavior in the presence of RE elements. Does this factor pertain to a geometrical effect operating during thermo-mechanical processing? This question is key to uncover RE effect on texture modification and would constitute our third and last goal in this paper.

In order to address the abovementioned questions, we use extensive and fine electron back scattered diffraction (EBSD) characterization. The reasonability of defining a threshold limit for two RE elements (Ce and Gd) was examined through four extrusion conditions. Additionally, different processing parameters and RE concentrations allowed us to observe microstructural and texture evolutions through the entire restoration phenomena. Nucleation and growth of new orientations was elucidated fastidiously and step-by-step. Schmid factor analysis explains the geometrical effect behind the preferred growth of RE texture.