Hot deformation behavior and microstructure evolution of Mg–Gd–Sm(–Zn)–Zr alloys

Abstract

The dynamic recrystallization (DRX) and dynamic precipitation of Mg–5Gd–3Sm(–1Zn)–0.5Zr alloys after hot compression deformation were analyzed by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Furthermore, the DRX mechanisms were investigated by calculating the deformation activation energy, establishing the constitutive equation, and creating a critical strain model. The results indicate that the presence of Zn element enhanced the production of DRX, considerably reduced the strength of {0001} plane texture, and boosted the Schmidt factor of nonbasal plane slip. The Mg–5Gd–3Sm–0.5Zr alloy had a low degree of DRX, manifested as a monolayer of DRX grains at the grain boundaries, and dominated by the discontinuous DRX mechanism. However, the Mg–5Gd–3Sm–1Zn–0.5Zr alloy had a high degree of DRX, which occurred in the form of multilayered DRX grains by the main mechanism of continuous DRX. Compared with the Mg–5Gd–3Sm–0.5Zr alloy, in addition to the Mg5(Gd,Sm) phase, the Mg–5Gd–3Sm–1Zn–0.5Zr alloy also introduced a new dynamic precipitation phase called (Mg,Zn)3(Gd,Sm) phase. The dynamic precipitation phase prevented grain boundary migration and dislocation motion, which promoted DRX nucleation and prevented the growth of recrystallized grains.

1. Introduction

The density of magnesium is 1.74 g/cm3, which is approximately 3/5 of the density of aluminum. Magnesium (Mg) alloys have the advantages of strong seismic and noise reduction ability, excellent die-casting performance, and easy cutting processing. It has good development trends in the automotive industry, aerospace, biomedical sector, and 3C products (computer, communication, and consumer electronics) [1–4]. However, Mg alloys have poor mechanical properties, difficult deformation ability, and weak corrosion resistance. The general light metal used is aluminum alloy in the engineering area. Many scholars have used alloying and heat treatment methods to improve the properties of Mg alloys [5–6]. Among them, high-temperature resistant and corrosion-resistant Mg–RE alloys have become the research objects of many scholars [7–8].

Mg alloy has a typical close-packed hexagonal (hcp) structure. At room temperature, only three slip systems can be started [9], which leads to poor plastic deformation ability. Although alloying and heat treatment methods can enhance the properties, they still have certain limitations. Many scholars have adopted the hot deformation method to study changes in the properties and structure of Mg alloys [10]. High temperatures are favorable for material deformation and help eliminate defects in materials during casting [11].

Many studies have shown that the properties of deformed Mg alloys are affected by the dynamic precipitation and dynamic recrystallization [12–13]. The dynamic precipitated phase can improve the strength of the alloy by hindering the movement of dislocations. Wang et al. [14] found the dynamic precipitated phase at the grain boundary of the hot compressed Mg–Gd–Nd–Zr alloy, and pointed out that a dynamic precipitated phase with a size of more than 200 nm can promote the generation of DRX. The formation of DRX depends on dislocation motion. However, dislocation motion is affected by deformation conditions. Pei et al. [15] found that after compression at strain rates of 0.1 and 1 s−1, deformation bands appear in the interior of Mg–Gd–Sm–Y–Zr alloy grains. These deformation bands, like grain boundaries, hinder dislocations and form DRX grains in the form of discontinuous DRX (DDRX). In the early stage of recrystallization, the nucleation mechanism is the DDRX mechanism. When the strain increases, the continuous DRX (CDRX) mechanism gradually becomes dominant. Ding et al. [16] focused on the relationship between a high strain rate and DRX. They believed that at a high strain rate of 50 s−1, the occurrence of DRX can be advanced. Due to the accelerated deformation process, the dislocation density of the alloy and the speed of dislocation movement increases, resulting in an increase in the degree of DRX for the Mg–8Gd–1Er–0.5Zr alloy, and the main mechanism of DRX is CDRX and twinning induced DRX (TDRX). Guo et al. [17] found that in Mg–9Gd–4Y–2Zn–0.5Zr alloy, the bulk long-period stacking ordered (LPSO) phase can stimulate DRX via particle-stimulated nucleation.