Microstructure and superior mechanical properties of a multi-axially forged WE magnesium alloy

Highlights

• High-temperature deformation behavior of a WE magnesium alloy was investigated through MAF.

• PSN occurred due to the presence of Mg₂₄Y₅ eutectic phases.

• A texture weakening was realized by applying MAF.

• The excellent room temperature ductility was achieved as a result of texture weakening.

Abstract

A magnesium alloy containing rare earth elements (Y and mish metals) was severely deformed through multi-axial forging (MAF) at 400 °C and the obtained microstructure, texture, and mechanical properties were characterized after applying successive MAF passes. The results showed that the volume fraction of the recrystallized grains was noticeably increased by increasing the MAF passes, where a homogeneous microstructure with a mean grain size of 4.8 μm was obtained after five MAF passes. A strengthened basal texture was developed after the first MAF pass. Further deformation to three and five MAF passes resulted in a texture weakening, where pyramidal planes tend to rotate parallel to the forging direction. The contribution of particle stimulated nucleation (PSN) was suggested to play an effective role in the texture weakening. The basal texture and grain refinement, which were achieved after applying one pass MAF, led to a combined improvement in strength and ductility, including 460 MPa ultimate strength and 12.4% ductility. As the number of deformation passes increased to three and five, the flow stress of the material was reduced, while the ductility considerably enhanced. The latter findings were discussed relying on the fact that the texture weakening after multi-pass MAF could make the deformation easier through activating more slip systems.

Introduction

Despite many fascinating properties, the applications of wrought magnesium alloys are still restricted due to: (i) their hexagonal close-packed crystal structure with an insufficient number of operative slip systems at room temperature, and (ii) the development of strong crystallographic texture during deformation processes, which lead to even lower formability of wrought magnesium alloys [1], [2], [3]. Accordingly, the introduction of random or weak texture may effectively counteract the aforementioned drawbacks [4], [5]. In this respect it has been realized that the addition of rare earth (RE) alloying elements to magnesium might assist developing a randomized texture [6], [7], [8], [9]. The main roles of REs in texture weakening are changing the magnesium stacking fault energy (SFE) and enhancing the solute drag effects on the grain boundary and/or dislocation movements [3], [8], [10].

In addition to texture modification, the grain refinement through severe plastic deformation (SPD) is considered as a promising way to improve the mechanical properties of magnesium alloys [11]. Different SPD techniques such as accumulative roll-bonding (ARB) [12], accumulative back extrusion (ABE) [13], [14], friction stir processing (FSP) [15], differential speed rolling (DSR) [16], equal channel angular pressing (ECAP) [17], high-pressure torsion (HPT) [18], and multi-axial forging (MAF) [19], have been already employed to generate the fine-grained magnesium alloys. Biswas and Suwas reported the tensile properties improvements in Mg-Al-Mn alloy through MAF processing [19]. The corresponding results were justified considering the evolution of submicron grain size and a weak texture during multi-axial forging. Kim et al. processed an AZ91 magnesium alloy by DSR and reported achieving submicron grains size (0.3–0.5 μm) and excellent mechanical properties (ultimate tensile strength of 394 MPa and tensile elongations of 9–11%) [16].

Among several SPD methods, MAF is considered as the one which may be performed using conventional deformation tools and also scaled easily to produce large industrial products [20]. MAF has been previously employed to produce fine-grained microstructure in different magnesium alloys [21], [22]. An ultra fine grained (UFGed) AZ61 Mg alloy with a mean grain size of 0.8 μm and a good balance of strength and ductility was produced by Miura et al. through applying MAF under decreasing temperature conditions [21]. Analogous results were also reported by Qiang et al. [23]. Besides the grain refining effect, MAF may also promote the occurrence of texture weakening in magnesium alloys [19].

Incorporating both aforementioned approaches, including alloying with RE elements and MAF processing, may assist achieving more desired improvement in mechanical properties of magnesium alloys. However, limited researches could be found in the literature dealing with MAF processing of Mg alloys containing rare earth elements. In the present work, the effects of MAF processing on the microstructure and texture evolution of a WE magnesium alloy containing Y and mish metals are addressed. Moreover, the mechanical properties of the processed material are evaluated using a miniaturized tensile testing method.