Effect of Mn content on microstructure, texture and room-temperature mechanical properties of hot-extruded Mg–2Nd–1Gd alloy

Abstract

The effect of Mn content on the microstructure, texture, and room-temperature mechanical properties of hot-extruded Mg–2Nd–1Gd alloy was investigated. The microstructure of hot-extruded Mg–2Nd–1Gd–xMn (x = 0, 0.25wt%, and 0.5wt%) alloys consisted primarily of a fine-grained α-Mg matrix phase and point-like, streamline-distributed Mg41(Nd,Gd)5 phase along the extrusion direction. In the extruded Mg–2Nd–1Gd–0.25Mn and Mg–2Nd–1Gd–0.5Mn alloys, Mn was mainly present as solid–solution Mn atoms and α-Mn particles, respectively. With increasing Mn content, the recrystallization fraction of the Mg–2Nd–1Gd–xMn alloys increased from 79% to 94.3%, and then decreased to 77.8%. Meanwhile, the average grain size first increased from 7.9 to 11.9 μm and then decreased to 7.5 μm. Microstructural characterization revealed that the solid–solution Mn atoms in the extruded Mg–2Nd–1Gd–0.25Mn alloy reduced the segregation of Nd and Gd, thereby weakening the solute drag effect. In contrast, α-Mn particles pinned the grain boundaries and delayed the recrystallization process in the extruded Mg–2Nd–1Gd–0.5Mn alloy. The extruded Mg–2Nd–1Gd and Mg–2Nd–1Gd–0.25Mn alloys exhibited a typical rare-earth texture, whereas the extruded Mg–2Nd–1Gd–0.5Mn alloy displayed a basal texture combined with a rare-earth texture due to the presence of deformed grains. Among the extruded Mg–2Nd–1Gd–xMn alloys, the Mg–2Nd–1Gd–0.5Mn variant exhibited the best room-temperature mechanical properties, with a yield strength of 138.0 MPa, an ultimate tensile strength of 231.1 MPa, and an elongation of 38.8%. Quantitative analysis indicated that grain boundary and dislocation strengthening were the main contributors to the yield strength of the extruded Mg–2Nd–1Gd–0.5Mn alloy, accounting for 44% and 24.1%, respectively.

1. Introduction

Magnesium (Mg) alloys, the lightest metallic structural materials among existing engineering alloys, have significant application potential in the aerospace and automotive industries, where lightweight materials are urgently required [1–3]. Mg rare-earth (RE)-based casting alloys are considered promising due to their strong solid–solution strengthening and age-hardening effects [4–6]. Among them, Mg–Nd–Gd alloys have attracted increasing attention from researchers because of their relatively low cost [7–12]. In general, cast Mg alloys contain casting defects such as porosity, which reduce their mechanical properties [13]. Hot-extrusion deformation can effectively eliminate such defects and refine the grain structure, making it an effective method for developing high-strength and plastic Mg alloys [14–15]. However, the room-temperature strength of extruded Mg–Nd–Gd-based alloys remains relatively low, which limits their broader application. For example, the room-temperature strength of the commercial extruded EV31A alloy (Mg–3Nd–1.5Gd–0.3Zn–0.5Zr) is lower than that of the extruded ZK60 alloy [16–20], preventing optimal strength–ductility matching from being realized. In addition, the excellent high-temperature properties of Mg–RE alloys are their greatest advantage; however, it is necessary to improve their room-temperature mechanical properties. Moreover, the ZK60 alloy has a severe hot-cracking tendency and poor casting properties [21], and its plasticity is much lower than that of Mg–Nd–Gd-based alloys. Therefore, the modification of Mg–Nd–Gd-based alloys is a subject worthy of investigation.

Mn is a common alloying element in Mg–RE-based wrought alloys and offers the advantages of low cost, refined grains, and improved formability [22]. It has proven effective in enhancing the mechanical properties of Mg alloys [23–30]. A series of studies has been conducted on the effects of Mn addition. Fang et al. [23] showed that the grains were significantly refined from 18 to 11 μm after 0.8wt% Mn addition in extruded Mg–10Gd alloy, which was attributed to the inhibition of recrystallization grain growth by fine α-Mn particles. Zhou et al. [24] demonstrated that the addition of 2wt% Mn to extruded Mg–9Gd–4Y–1Zn alloy improved the grain refinement effect, promoted the phase transformation of block-shaped long period stacking ordered phase (LPSO) to lamellar LPSO, and increased the dislocation density. Zhao et al. [25] found that the extrusion direction (ED)-split texture of Mg–1Gd–0.5Zn–xMn sheets was gradually weakened with increasing Mn content, which related to restrained grain growth by fine α-Mn particles. Reportedly, the extruded Mg–3Gd–1Zn alloy with adding 1wt% Mn showed an increase in hardness, shear yield stress, and ultimate shear strength from HV 61, 102 MPa, and 142.8 MPa to HV 66, 128.5 MPa, and 152.4 MPa, respectively [26]. The extruded Mg–1Gd–0.7Mn alloy showed better tensile properties than the Mg–1Gd alloy, with a tensile yield strength of 105 MPa, ultimate tensile strength of 253 MPa, and elongation of 30.2% [27]. However, there are few studies on the effect of Mn on Mg–RE