Understanding the weakening of texture in multiaxial forged ultrafine-grained strong and ductile Mg-2Zn-2Gd alloy

ABSTRACT

Texture is an important aspect that impacts the formability of magnesium (Mg) and its alloys. Recently, the unique characteristics of multiaxial forging of Mg-2Zn-2Gd alloy were discussed, where it was demonstrated that multiaxial forged processed alloy is characterized by high strength and high ductility combination and the texture is significantly weakened or almost texture-free. The objective of the study described here is to obtain some fundamental insights that explain the underlying reasons for the weakening of texture in the multiaxial forged Mg-2Zn-2Gd alloy.

KEYWORDS:

• Magnesium alloy
• rare-earth element
• multiaxial forging
• texture

Introduction

Grain refinement and introducing an ultrafine grain size through a severe plastic deformation process in metals and alloys are considered a viable approach to significantly enhance the mechanical properties beyond what can be achieved by conventional processing methods. The underlying reason is that the severe plastic deformation methods enable stable deformation to be introduced at large strains.

In recent years, severe plastic deformation processing has attracted significant attention to processing bulk ultrafine-grained (UFG) metals and alloys. While there are several severe plastic deformation methods, multiaxial or multidirectional forging is viewed as a plastic deformation process that is generally believed to involve dynamic recrystallization and dynamic precipitation. UFG alloys are envisaged to exhibit outstanding cyclic stability together with a good combination of strength and ductility [1–5]. High strength and high ductility combination governs the formability of structural metals and alloys [6–16].

Magnesium (Mg) with density lower than steel (<78%) and aluminium (<37%) is attractive for use in automotive and aerospace applications [4,5]. However, the hexagonal close-packed structure of Mg with limited number of slip systems makes it difficult to plastically deform and process. This leads to the development of texture [4,6–8]. Recently, it was demonstrated that the Mg alloys strengthened by rare earth (RE) elements, for instance, gadolinium (Gd), can be successfully processed by multiaxial forging (MAF) such that a UFG structure with high strength and high ductility combination is obtained with the promise of a texture-free Mg RE alloy [6–10]. The design, preparation and details of the MAF process for Mg-2Zn-2Gd alloy have been described previously [6–10].

In sequel to the recent studies [6–10], the objective of the study described here is to obtain a perspective and some fundamental insights on the weakening and development of texture-free Mg-2Zn-2Gd alloy processed by MAF by utilizing the recent results and the information documented in the literature. We first briefly revisit the microstructure and mechanical properties of Mg-2Zn-2Gd alloy in the following section prior to the presentation of recent results and discussion on the weakening of texture.

Microstructure and mechanical properties of Mg-2Zn-2Gd alloy processed by multiaxial forging

The as-cast annealed alloy with a coarse-grained (CG: average grain size of 44 ± 5 µm) microstructure is presented in Figure 1(a) together with the microstructure of the UFG-MAF alloy (UFG: average grain size of ~ 416 ± 140 nm) (Figure 1(b)) [6–8]. The mechanical properties were as follows: CG (yield strength: 46 ± 2 MPa, tensile strength: 70 ± 2 MPa, elongation: 7 ± 1%) and UFG (yield strength: 227 ± 2 MPa, tensile strength: 272 ± 1 MPa, elongation: 30 ± 2%). The increase in strength from 46 MPa in the CG alloy to 227 MPa in the UFG Mg-2Zn-2Gd alloy was obtained via combination of UFG structure and nanoscale precipitation of Mg₃Zn₃Gd (Figures 1(c,d)) [6–8].

Figure 1. (a) Microstructure of coarse-grained (CG) and (b) multiaxial forged ultrafine-grained (UFG) Mg-2Zn-2Gd alloy. (c) A representative transmission electron micrograph illustrating Mg₃Zn₃Gd precipitates in the α–mg matrix. (d) Electron diffraction pattern [6–8].

Texture results and discussion on the weakening of texture

The common textures in Mg alloys include basal texture (<0001>∥ND), commonly observed in the rolling deformation process, and double twin texture (<0001>∥RD), which typically forms during the extrusion process. Additionally, under specific processing conditions, such as in high-temperature environment, a random texture can form.

The pole figures in different formats for the CG alloy are presented in Figures 2(a–c). Analyzing the {0001} pole figures in Figure 2(a–c) reveals the presence of basal texture and a strong near-twin texture, as shown in regions R1 and R2 of Figure 2(a), with the texture intensity peak in region R2 at 52.59. There is a decrease in texture on {1–100} plane with a maximum value of ~24. Comprehensive analysis suggests strength on all the slip planes, indicative of significant texture in the CG Mg-2Zn-2Gd alloy.

Figure 2. (a–c) Pole figures in different formats for the CG Mg-2Zn-2Gd alloy.

Continuing the analysis of texture of UFG-MAF Mg-2Zn-2Gd alloy (Figures 3(a–c)) and focusing mainly on the {0001} pole figures, it is evident that compared to the CG alloy (Figures 2(a–c)), the texture distribution in the UFG alloy is more dispersed and random. Both types of textures are present on the {0001} plane, with maximum texture intensity significantly reduced to 8.29. The texture on the remaining planes is weak.

Figure 3. (a–c) Pole figures in different formats for the UFG-MAF Mg-2Zn-2Gd alloy.

In pure Mg, basal slip dominates, which generally leads to basal texture in mechanically worked alloys [17]. The near-absence of texture in the experimental Mg-2Zn-2Gd alloy suggests that a significantly greater spread of grain orientations such that many grains are favourably oriented towards basal slip when the alloy is plastically deformed in the random direction [4].

Some studies indicated that the addition of RE elements to Mg enhances elongation [4,18–21] and the formability is improved because of weakening of texture. For instance, small additions of Gd (~0.1 at.%) increased ductility and reduced yield anisotropy [19,22,23]. The concentration of Gd required to weaken texture was below the solubility limit of Gd in Mg [24–26]. This aspect was considered as a generic rule for the majority of the RE elements and implied that particles are not necessary to induce weakening of texture [4]. Irrespective of the effect of RE addition, the deformation condition and presumably deformation mechanism was also considered to influence the final texture [4]. This viewpoint was based on the observation that there was no increase in ductility when the addition of RE exceeded a critical value.

If we now consider that strong texture was present in the CG Gd-containing Mg alloy (Figure 2) and was almost absent in the UFG multiaxial forged alloy (Figure 3), it then becomes important to consider the role of dynamic recrystallization and dynamic precipitation that occurs during multiaxial deformation and is responsible for the UFG structure. In this regard, it was noted that in both non-Gd and Gd-containing Mg alloys, the orientations of dynamic recrystallized nuclei were spread about the deformed grains [27]. Based on texture studies on static and dynamic recrystallized alloys, it was proposed that RE addition not only weakens texture but also significantly retards dynamic recrystallization because of grain boundary segregation of RE atoms [28–30]. However, it may be noted based on our observations that in the MAF Mg-2Zn-2Gd alloy, the UFG structure would not have been obtained in the absence of dynamic recrystallization. Preliminary studies on our experimental Mg-2Zn-2Gd alloy show the occurrence of continuous dynamic recrystallization with a necklace type structure (Figure 4). This is envisaged to occur via continuous dynamic recrystallization [31,32] during the MAF process. Several extremely fine equiaxed grains formed in the vicinity of the elongated grains. Moreover, these elongated large grains with near <120> orientation were also broken or fragmented.

Figure 4. The occurrence of continuous dynamic recrystallization mechanism with a necklace-type structure in the multiaxial forged Mg-2Zn-2Gd alloy after one-pass. There are several ultrafine grains in the vicinity of elongated grains.

If we now consider the post-deformation of Mg-2Zn-2Gd alloy, besides the basal plane, a high percentage of prismatic and pyramidal dislocation activity was observed (Table 1) [6]. The enhanced activity on the non-basal slip systems is envisaged to impact the recrystallization process such that sub-grain structure forms at the grain boundaries [32,33], as observed in Figure 4, and requires increased number of slip systems (Table 1). It is also likely that slow but continuous dynamic recrystallization led to extremely low intensity of texture. The UFG structure and strong weakening of texture is currently believed to be a cumulative contribution of MAF, activation of non-basal slip systems besides basal slip, continuous dynamic recrystallization and Mg₃Zn₃Gd-nanoparticle-simulated nucleation of new grains. Dynamic precipitation influences nucleation of new grains and restricts grain growth [6]. Slow dynamic recrystallization may have contributed to the weakening of texture. Lastly, grain fragmentation via kinking mechanism during the MAF process cannot be ignored [6–8].

Table 1. Frequency of dislocations observed in the UFG-MAF Mg-2Zn-2Gd alloy after post-deformation. The analysis is based on complete Burgers vector analysis in at least 25 random selected grains [6].

Dislocation type	Dislocation Burgers Vector	Amount (%)
Basal or Prismatic <a>	Perfect b = 1/3 [11$\bar{2}$0] and b = 1/3 [1$\bar{2}$10] Dissociated (SFI1+SFI2) b = 1/3 [1$\bar{2}$10]	32 2
Pyramidal <c+a>	Perfect b = 1/3 [11$\bar{2}$3] screw Perfect b = 1/3 [11$\bar{2}\bar{3}$] screw Perfect b = 1/3 [$\bar{2}11\bar{3}$] mixed Dissociated b = 1/3 [$\bar{2}$113] = 1/6 [$\bar{2}$023] + 1/6 [$\bar{2}$203]	32 2 8 18 6

Conclusions

The addition of the RE element, Gd, significantly weakens the texture in the multiaxial forged UFG Mg-2Zn-2Gd alloy. Other factors that are currently believed to have contributed to the UFG structure and weakening of texture are cumulative effect of severe plastic deformation, activation of slip systems besides basal slip, dynamic recrystallization and Mg₃Zn₃-Gd-nanoparticles-simulated nucleation of sub-grains.

Acknowledgments

The author is grateful to the National Science Foundation for financial support through grant # CMMI 2130586.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Science Foundation USA [CMMI 2130586].