**1. Introduction**

Magnesium alloys are the lightest structural metal view to the very low density of 1.74 g/cm3 and designed as Green Structure Metal [1]. Taking into account its very negative potential (−2.34 V), Mg is a reducing agent and is able to combine with oxygen, sulfur and halogen compounds. This reducing power finds its interest in the production of sacrificial anodes that prevents corrosion. Meanwhile, this reducing agent constitutes a major barrier to the use of Mg as a structural material. In addition to this undesirable property, poor wear resistance of pure Mg hinders its use for different applications.

That's why, pure Mg is combined with other metal elements to improve their properties even at high temperatures, namely manganese, aluminum, zinc, silicon, copper, zirconium and rare-earth metals. Mg alloys, non-ferrous material, are characterized with low density, high ductility, strength and acceptable corrosion resistance.

The lightness is the main reason for the interest in the civil and military transport sector for Mg, in which lightweight structures are required. When compared with metallic structure namely, aluminum Al and iron Fe, the density of Mg is much lower than those of these metals [2]. In reverse, Mg exhibits similar specific mechanical properties, mainly excellent castability and machinability compared to a metal which is durable [3, 4]. When used as alloying element in metallic material, Mg enhances the mechanical properties of Aluminum and the malleability of the iron.

Compared to that of metallic structures, Mg alloys show higher weight/strength ratio. They possess an elastic modulus of 45 GPa and tensile strength of 160–365 MPa [5]. Based on the above reasons, Mg alloys have been widely used in the aerospace industry, mechanic manufacture and automotive industry. Indeed, the replacement

in the three major components (body, power train and chassis) of a vehicle by Mg alloys lead to weight reduction of 20–70% [6].

Mg alloys provides an excellent property of damping vibration and heat dissipation property which is an important factor for different automobile and aerospace industries. As well known, the vibration is a kind of loss and affects the efficiency of the vehicle.

Special attention is paid to Mg-based materials for clinical applications (orthopedic applications, critical wounds …) owing to its density that is very close to that of human bone (1.75 g/cm3 ), higher specific strength and low elastic modulus. Furthermore, Mg is biocompatible as it is essential for several biological reactions and as a co-factor for enzymes.

Quite opposite to the conventionally used metallic materials such as stainless steel and Ti alloys that exhibit stress shielding and metal ion releases, Mg is biodegradable. That is to say, Mg entirely degrades in the human body preventing then the need for second surgical procedure to remove the implants material [7]. This has received a widespread attention from the scientific and medical community [8]. However, the biodegradability of implanted Mg alloys is hindered by an accelerated degradation rate in chloride-abundant environments like human body [9]. Generally, the period of bone remodeling is about 3 to 6 months. To be wary of this suggestion, the rate of degradation must be controlled by suitable surface modification in order to enhance the duration of effectiveness of the implantable material.

Researchers have been working on synthesis and characterization of Mg-based biomaterials with a variety of composition in order to control the degradation rate of Mg that leads to a loss of mechanical properties and contamination in the body. The alloying elements affect the characteristics and performance of Mg alloys.

This paper is a comprehensive review that compiles the recent literature on the important alloying elements and their impacts on the properties of Mg alloys.

## **2. Designation and types of Mg alloys**

According to the addition elements, Mg alloys are designated in different ways. Alloys are designated by letters corresponding to their main addition elements followed respectively by the percentage of each element. The American Society for Testing and Materials ASTM developed a method to designate Mg alloys which are named by their main alloying elements. The first two letters indicate the alloying elements used in the greatest quantity. One or two letters are followed by numbers which represent the percentage by weight of the elements rounded to the nearest whole number. The ASTM code for alloying elements is as follows, aluminum is designated by the letter A, zinc by the letter Z, manganese by the letter M, silicon by the letter S, yttrium by the letter W, zirconium by the letter K, silver by the letter Q, thorium by the letter H. The most common families are: AZ (example AZ31), AM (example AM60), AS (example AS41), WE (example WE43) and AE (example AE42). For instance, AZ91 Mg alloy contains 9% of Al and 1% of Zn and the rest by pure Mg.

Based on the process of operation, Mg alloys can be categorized into two groups: cast alloys and wrought alloys.

#### **2.1 Cast Mg alloys**

Cast alloys are basically made by pouring the molten liquid metal into a mold, within which it solidifies into the required shape. Depending on the chemical composition, there are two groups of cast Mg alloys [10].

#### *Alloying Elements of Magnesium Alloys: A Literature Review DOI: http://dx.doi.org/10.5772/intechopen.96232*

The first group includes Mg-Al alloys in which Al amount does not exceed 10% with an addition of Zn and Mn. These alloys are characterized by a low cost of manufacture. Indeed, this criterion is recommended for the industrialization and the commercialization of high performance of Mg alloys. The most commonly used is AM50 alloy mainly for die casting, and AZ91 for sand and die casting method. Meanwhile, their disadvantage is a low operating temperature-below 120 °C. The aging hardenability of these Mg-Al based alloys, such as AM60 alloy, is relatively poor. That's why, these alloys are prepared by high pressure die casting with relatively high cooling rate. The strength, the castability, the workability, the corrosion resistance and the weldability of these commercial AZ91 and AM60 alloys are not satisfying, but can be improved by introducing alloying elements. For instance, AZ91 alloy's strength is relatively high; however, its ductility is not so good due to the high Al content. Whereas, AM60 alloy has high ductility, but its strength is relatively low. Various alloying elements such as Ce, Nd, Y, Si, Ca, Ti, B, Sr., Sb, Bi, Pr have been used to enhance the operating temperature and the mechanical properties of modified AZ91 alloy [11]. Among these alloying elements Ce, Nd, Y, Bi and Sb are effective to improve the tensile properties of AZ91 alloy. For the modified AM60 alloys, various alloying elements, Ti, Nd, Sn and Ce elements are relatively effective to further enhance the mechanical properties of AM60 alloy. For instance, the tensile strength of 280 MPa and elongation of 11% could be obtained [12].

The second group of alloys is free from Al, but containing mostly Zn, rhenium RE and Y with an addition of Zr. Such alloys are recommended for high temperature until 250 °C, but the cost of their production increases due to the cost of alloy additions. The common commercial WE43 and WE54 alloys can be cast by sand casting process with a low cooling rate. Accordingly, the mechanical properties are further improved by solution and aging treatment. These alloys are largely used as sand castings. The strength of WE54 is basically attained via precipitation strengthening. Depending on the aging temperature and time, the precipitating sequence in WE alloys has been reported to involve the formation of phases β″, β′, and β. The equilibrium β phase is isomorphic to the Mg5Gd phase and is identified as a Mg14Nd2Y phase [13].

The mechanical properties of the cast alloys are determined on poured test bars according to standard ASTM procedures.

#### **2.2 Wrought alloys**

Wrought alloys are destined to mechanical working, such as forging, extrusion and rolling operations to shaping. Al, Mn and Zn are also the main alloying elements. Wrought alloys of Mg are sorted into heat treatable and non-heat treatable alloys. The use of wrought Mg alloys is limited less than 10%. This is due to the poor cold workability of the hexagonal close packed (HCP) crystal structure of Mg, generating low formability at room temperature. New Mg alloys have been developed to enhance their strength by modifying the existing alloy [14] or by grain refinement. It was also reported that advanced processing, such as hot extrusion, rolling, forging are able to refine the microstructure and improve the mechanical properties of Mg alloys. Many commercial wrought Mg alloys have been developed, such as AZ system, ZK system… Compared with wrought Mg alloys, casting Mg alloys have economical advantages due to their shorter processing cycle. Therefore, casting Mg alloys obtain more incremental use of almost 90% of total application products.

Based on literature data, new Mg alloys with high strength can be developed when modified the present commercial cast and wrought alloys [15] by strengthening mechanisms: alloying, grain refinement, precipitation and texture strengthening effect. To overcome the weakness of pure Mg, different elements have to be alloying with pure Mg to obtain Mg alloys with desired mechanical properties. Mg alloys shows an excellent specific strength and stiffness with dimensional stability due to its hexagonal crystal structure and to its atomic size (about 320 nm). Much progress has been achieved in strengthening of Mg alloys through solid solution strengthening using different alloying elements. The effects of such elements on the microstructure and mechanical properties are described.
