**1. Introduction**

Alloying Mg with Li can produce the lightest Mg-Li alloy; its density (1.25–1.65 g/cm3) is close to that of plastic, with only 3/5–3/4 of that of conventional Mg alloy and 1/2–2/3 of that of Al alloy. The Mg-Li alloy exhibits a unique phase transition with the change in Li content, i.e., the alloy is an α-Mg single-phase alloy with an hcp structure when the Li content is lower than 5.7 wt.%, the alloy is a β-Li single-phase with a bcc structure when the Li content is higher than 10.3 wt.%, and the alloy is an α-Mg+β-Li dual-phase alloy with an hcp+bcc structure when the Li content is between 5.7 wt.% and 10.3 wt.% [1]. The addition of Li can reduce the c/a axial ratio of the Mg lattice and even change the crystal structure from an hcp structure to a bcc structure, which makes the Mg-Li alloy exhibit incomparable plastic deformation ability compared to a conventional Mg alloy [1,2]. In addition, the Mg-Li alloy also shows the advantages of high specific strength, excellent damping, good electromagnetic shielding performance, and weak anisotropy, being lightweight and having good prospects in the fields of energy saving, and emission reduction, especially in aerospace and electron industries [3,4]. Taking aerospace satellites as an example, the application of the Mg-Li alloy can achieve a weight reduction effect of about 20–50%, significantly improving the payload of the satellite, and thus generating huge economic benefits.

However, since the binary Mg-Li alloy exhibits low strength, it is necessary to add alloying elements to strengthen the binary Mg-Li alloy. Al and Zn are the common alloying elements for a Mg-Li alloy; they can improve the strength of the Mg-Li alloy by solid solution strengthening and second phase strengthening, owing to their high solid solubility in the Mg matrix and ease of combining with other elements to form compounds [5]. Previous research has shown that Al and Zn have similar effects on Mg-Li alloy; that is, the alloy strength increases while the elongation decreases with the increase in Al content or Zn content [5,6]. In the Mg-Li-Al alloy, the Al content is generally less than 5 wt.%–6 wt.%, above which the alloy strength does not increase significantly and the elongation loss is

**Citation:** Sun, Y.; Zhang, F.; Ren, J.; Song, G. Effect of Li Content on the Microstructure and Mechanical Properties of as-Homogenized Mg-Li-Al-Zn-Zr Alloys. *Alloys* **2023**, *2*, 89–99. https://doi.org/ 10.3390/alloys2020006

Academic Editor: Andrea Di Schino

Received: 14 March 2023 Revised: 12 April 2023 Accepted: 20 April 2023 Published: 28 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

obvious. When the Zn content exceeds 2.5 wt.%, it has a negative effect on the corrosion resistance of the Mg-Li alloy, so the Zn content is usually controlled within 2.5 wt.% [7]. Zr is often used as a grain refiner to improve the microstructure and mechanical properties of an alloy, and its addition amount is generally 0.1 wt.%–0.3 wt.%. In recent years, due to the excellent mechanical properties of Mg-Li-Al-Zn (LAZ) alloys, much research has have focused on improving the mechanical properties of LAZ532 and LAZ832 alloys through alloying with rare earth elements [8–11]. Cui et al. [8] revealed that the addition of Y element into the LAZ532 alloy could form an Al2Y strengthening phase, reduce the AlLi softening phase, and refine grains, and the tensile strength was increased by 32.66% when the Y content was 0.8 wt.%. Zhu et al. [9] indicated that Y and Nd had a synergistic strengthening effect on the LAZ532 alloy, and the grain refinement effect was the best and the mechanical properties reached their maximum values (UTS = 231 MPa, δ = 16%) when adding 1.2 wt.% Y and 0.8 wt.% Nd simultaneously. Zhao et al. [10] showed that the LAZ832-0.5Y alloy had the best tensile strength of 218.5 MPa and elongation of 16.9%, which was mainly attributed to the grain refinement and second phase strengthening effect caused by the addition of Y. However, there are few systematic studies on the microstructure and mechanical properties of Mg-Li-Al-Zn alloys with different Li contents.

In this work, the microstructure and mechanical properties of as-homogenized Mg*x*Li-3Al-2Zn-0.2Zr alloys (*x* = 5, 7, 8, 9, 11 wt.%) were investigated, and the fundamental reason for the variation of mechanical properties was explained.

#### **2. Experimental Procedures**

The alloys used in this work were prepared by melting pure Mg (>99.9 wt.%), pure Li (>99.9 wt.%), pure Al (>99.9 wt.%), pure Zn (>99.9 wt.%), and the Mg-Zr (30 wt.%) master alloy in a vacuum induction melting furnace under pure argon atmosphere. The melts were held at 730 ◦C for 10 min, and then poured into a stainless steel mold with a diameter of 80 mm and cooled to room temperature. After that, the cast ingots were homogenized at 280 ◦C for 24 h to obtain the experimental alloys with uniform composition and microstructure. Choosing the homogenization temperature of 280 ◦C not only ensured that the alloys could be completely homogenized, but also ensured that the alloys were kept warm for a long time without any combustion hazards. The densities of as-homogenized alloys were determined with an electronic density balance (AEL-200, Mettler Toledo, Greifensee, Switzerland). The phase composition was analyzed using an X-ray diffractometer (XRD, D/Max 2500, Rigaku, Tokyo, Japan) using monochromatic Cu Kα radiation, and the 2θ was in the range of 20◦ to 80◦ with a step size of 0.02◦ and a scan rate of 4◦/min. The microstructure was observed with a scanning electron microscope (SEM, Quanta-200, FEI, Eindhoven, The Netherlands), an electron probe microanalyser (EPMA, JXA-8230, JEOL, Tokyo, Japan), and a transmission electron microscope (TEM, Tecnai G20ST, FEI, Eindhoven, The Netherlands). The samples for TEM observation were firstly ground to 80 μm, then thinned to perforation with a twin-jet electropolisher, and finally thinned for 20 min using an ion-beam thinner at a small angle. The electrolyte for twin-jet electropolishing was a mixed solution containing 8 vol.% perchloric acid and 92 vol.% alcohol, and the specific experimental parameters were a temperature of about −30 ◦C, working voltage of 12–20 V, and working current of 12–16 mA. The tensile tests were carried out with a tensile tester (MTS-858, MTS, Costa Mesa, CA, USA) at a tensile rate of 2 mm/min, and the dimensions of samples for tensile tests were 30 mm in gauge length and 6 mm in diameter. The hardness was measured with Vickers hardness tester (HV-50AP, Mitutoyo, Kanagawa, Japan) with a loading force of 98 N and a holding time of 10 s.
