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1
INTRODUCTION TO
MAGNESIUM

This chapter introduces magnesium as an energy-efficient material that has the potential
to replace steel, aluminum alloys, and some plastic-based materials. This is possible
for a design engineer as the specific strength and stiffness of magnesium exceeds that of
most commonly used metals and some plastic-based materials. Serving engineering ap-
plications since 1920s, magnesium was not the material of choice for many applications
due to its high cost till about two decades back. Interest in magnesium-based materials
is recently revived primarily because of its gradually reducing cost and the resolve of the
scientists, researchers, and engineers to cut down energy consumption and greenhouse
gas emissions.

1.1. INTRODUCTION

Over the years, with the increasing demand for economical use of scarce energy re-
sources, skyrocketing crude oil prices (see Figure 1.1) [1], and ever-stricter control over
emissions to lower environmental impact, industries are constantly searching for new,
advanced materials as alternatives to “conventional” materials. The spike in crude oil

Magnesium, Magnesium Alloys, & Magnesium Composites, by Manoj Gupta and Nai Mui Ling, Sharon


C 2010 John Wiley & Sons, Inc.

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2 INTRODUCTION TO MAGNESIUM

World crude oil prices

Weekly all countries spot price FOB weighted by estimated export volume.
160

140
US$136.32 (JULY 18, 2008)
120
US dollars per barrel

100

80

60

40

20

0
1978 1983 1988 1993 1998 2003 2008

Figure 1.1. World crude oil prices. (Energy Information Administration, US)

price in July 2008 (see Figure 1.1) revealed the future trend of oil prices. Owing to
this price rise, coupled with the depletion of energy resources with time, the choice of
lightweight metals is the key and unavoidable solution for the future. Magnesium is
one such promising lightweight metal, which is currently underutilized for engineering
applications.
Magnesium is the sixth most abundant element in the earth’s crust, representing
2.7% of the earth’s crust [2]. Although magnesium is not found in its elemental form,
magnesium compounds can be found worldwide. The most common compounds are
magnesite (MgCO3 ), dolomite (MgCO3 ·CaCO3 ), carnallite (KCl·MgCl2 ·6H2 O), and
also seawater [3]. Magnesium is the third most abundant dissolved mineral in the

TAB L E 1.1. Density of commonly used structural materials [7, 8].

Materials Density (g/cm3 )

Steel (cast iron) 7.2

Titanium 4.51
Aluminum 2.71
Magnesium 1.74
Structural plastica 1.0–1.7
a The density value is dependent on the type and amount of reinforcements.
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INTRODUCTION 3

Figure 1.2. Specific strength of various structural materials. (Data extracted from [8].)

seawater (1.1 kg/m3 ). Magnesium is the lightest of all structural metals. It has a density
of 1.74 g/cm3 , which is approximately one-fourth the density of steel and two-thirds
that of aluminum (see Table 1.1) [4–6]. Because of its low density and high specific
mechanical properties (Figures 1.2 and 1.3), magnesium-based materials are actively
pursued by companies for weight-critical applications.

Figure 1.3. Specific stiffness of various structural materials. (Data extracted from [8].)
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4 INTRODUCTION TO MAGNESIUM

1.2. CHARACTERISTICS OF PURE MAGNESIUM [3]

1.2.1. Atomic Properties and Crystal Structure

Symbol Mg
Element classification Alkaline earth metal
Atomic number 12
Atomic weight 24.3050(6)
Atomic volume 14.0 cm3 /mol
Atomic radius 0.160 nm
Ionic radius 0.072 nm
Orbital electron states in free atoms 1s2 , 2s2 , 2p6 , 3s2
Electrons per shell 2, 8, 2
Most common valence 2+
Crystal structure Hexagonal close-packed (HCP)

1.2.2. Physical Properties

Density (at 20◦ C) 1.738 g/cm3
Melting point (650 ± 1)◦ C
Boiling point 1090◦ C
Linear coefficient of thermal expansion
At 20–100◦ C 26.1 × 10−6 ◦ C−1
At 20–200◦ C 27.1 × 10−6 ◦ C−1
At 20–300◦ C 28.0 × 10−6 ◦ C−1
At 20–400◦ C 29.0 × 10−6 ◦ C−1
At 20–500◦ C 29.9 × 10−6 ◦ C−1
Thermal conductivity (at 27◦ C) 156 W m−1 K−1
Specific heat capacity (at 20◦ C) 1.025 kJ kg−1 K−1
Latent heat of fusion 360–377 kJ kg−1
Latent heat of vaporization 5150–5400 kJ kg−1
Latent heat of sublimation (at 25◦ C) 6113–6238 kJ kg−1
Heat of combustion 24.9–25.2 MJ kg−1
Coefficient of self-diffusion
At 468◦ C 4.4 × 10−10 cm2 s−1
At 551◦ C 3.6 × 10−9 cm2 s−1
At 627◦ C 2.1 × 10−8 cm2 s−1

1.2.3. Electrical Properties

Electrical conductivity 38.6% IACS
Electrical resistivity (polycrystalline magnesium)
At 20◦ C 44.5 n
m
At 316◦ C 92.8 n
m
At 593◦ C 139.5 n
m
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APPLICATIONS 5

TAB L E 1.2. Mechanical properties of pure Mg at 20◦ C [3, 9–11].

Pure Annealed Hand-Rolled Sand
Magnesium Sheet Sheet Cast Extruded PM-Extruded DMD-Extruded

0.2% 69–83 105–115 21 34–55 92 ± 12a 74 ± 4b

Compressive
yield strength
(MPa)
0.2% Tensile 90–105 115–140 21 69–105 132 ± 7c 97 ± 2d
yield strength
(MPa)
Ultimate tensile 160–195 180–220 90 165–205 193 ± 2c 173 ± 1d
strength
(MPa)
Hardness HBe 40–41 45–47 30 35 — —
a PM: powder metallurgy method, extruded at 350◦ C, extrusion ratio 20.25:1 [9].
b DMD: disintegrated melt deposition method, extruded at 350◦ C, extrusion ratio 20.25:1 [11].
c PM: powder metallurgy method, extruded at 250◦ C, extrusion ratio 20.25:1 [10].
d DMD: disintegrated melt deposition method, extruded at 250◦ C, extrusion ratio 20.25:1 [10].
e Using 10-mm diameter ball, 500-kg load.

1.2.4. Mechanical Properties

Table 1.2 shows the room-temperature mechanical properties of pure magnesium pro-
cessed under different conditions.

1.3. APPLICATIONS

1.3.1. Automotive Applications

In the 1920s, magnesium parts made their way into racing cars. However, it was not until
the 1930s that magnesium was used in commercial vehicles such as the Volkswagen
(VW) Beetle. The VW Beetle, back then, contained more than 20 kg of magnesium
alloy in the transmission housing and the crankcase.
Over the past decade, the increasing environmental and legislative pressures on the
automotive industry to produce lighter, higher fuel efficiency, and higher performance
vehicles have resulted in the surge in the use of magnesium. Widely used conventional
steel parts are being replaced by new advanced materials such as magnesium, aluminum,
and metal matrix composites. Leading automobile makers such as Audi, Volkswagen,
DaimlerChrysler (Mercedes-Benz), Toyota, Ford, BMW, Jaguar, Fiat, Hyundai, and Kia
Motors Corporation have used magnesium-based materials in their automotive parts.
Figure 1.4 shows some of the actual magnesium automotive components. Figure 1.5
shows the NUS-FSAE Car using Mg alloy in wheel assembly. This car is built by a group
of mechanical engineering students from the National University of Singapore (NUS),
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6 INTRODUCTION TO MAGNESIUM

(a) (b)

(c)

(d)

Figure 1.4. Magnesium automotive components: (a) magnesium steering wheel core for
Toyota Camry weighing 0.75 kg, (b) seat support for Jaguar and Fiat models weighing 2.6 kg,
(c) rear transfer case made from AZ91D weighing 2.7 kg, and (d) AZ91 magnesium alloy cam
cover for Ford Zetec engine weighing 0.9 kg.

to participate in the FSAE (Formula Society of Automotive Engineers) competition in

the United States.
In the VW Passat and Audi A4, magnesium parts are used in the gearbox housing
[12]. In the Toyota Lexus, Carina, Celica, and Corolla, the steering wheels are made
of magnesium [13]. In the Mercedes-Benz SLK, the fuel tank cover is made of a
magnesium alloy. In Hyundai Azera (Grandeur) and Kia Amanti (Opirus), magnesium
is also used in interior parts such as the seat frame, steering column housing, driver’s air
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APPLICATIONS 7

Figure 1.5. NUS-FSAE Car using Mg alloy in wheel assembly. (Courtesy: Professor K. H. Seah,
National University of Singapore.)

bag housing, steering wheel, and lock body [14]. Hyundai and Kia Motors Corporation
project that the use of a magnesium seat frame translates to a 6 kg weight reduction
per car (∼40% weight reduction by replacing steel with magnesium alloy). Thus, their
annual consumption of magnesium was expected to increase from 670 tons in 2004 to
3700 tons in 2007 [14].

1.3.2. Aerospace Applications

In the aerospace industry, weight reduction is one of the most critical objectives due to
the increasing need for emission reduction and fuel efficiency. The reduction in overall
weight of the aircraft will result in fuel savings, which translates to savings in the
total operational cost. Several weight reduction alternatives such as aluminum, fiber
metal laminates, and low-density structural plastics have been introduced over the years.
However, the limited advancement in the development of aluminum alloys has made
further weight reduction a challenge. Fiber metal laminates are also high-cost materials
and, hence, are only used for primary structures with the highest mechanical properties
requirements. Moreover, low-density structural plastics have low impact and damage
tolerance properties. They also exhibit inferior properties when subjected to temperature
extremes. Thus, all these limitations have made magnesium an attractive alternative.
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8 INTRODUCTION TO MAGNESIUM

Magnesium-based materials have a long history of application in the aerospace

industry. Over the years, magnesium-based materials are extensively used in both civil
and military aircraft. Some applications include the thrust reverser (for Boeing 737,
747, 757, 767), gearbox (Rolls-Royce), engines, and helicopter transmission casings,
etc. Military aircraft, such as the Eurofighter Typhoon, Tornado, and F16, also benefit
from the lightweight characteristics of magnesium alloys for transmission casings.
There is also widespread use of magnesium in spacecraft and missiles due to the
requirement for lightweight materials to reduce the lift-off weight. This is coupled
with its high specific mechanical properties, ease of fabrication, and other attractive
features such as its capability to withstand (i) elevated temperatures, (ii) exposure to
ozone, and (iii) bombardment of high-energy particles and small meteorites. Large
amount of magnesium (in the form of sheets) was used in the Titan, Agena, and Atlas
intercontinental ballistic missiles [15].

1.3.3. Medical Applications

Magnesium alloys were first introduced as orthopedic biomaterials in the first half of
the last century [16]. However, because of its low corrosion resistance, a large amount
of hydrogen accumulates around the implant during the in vivo corrosion process,
confining the widespread use of magnesium-based materials as biomaterials. Despite
this, magnesium still possesses many attractive characteristics that make magnesium-
based materials potential candidates to serve as implants for load-bearing applications
in the medical industry.
Magnesium has a much lighter density than other implant materials (Table 1.3).
It also has greater fracture toughness as compared to hydroxyapatite. Furthermore, as
shown in Table 1.3, its elastic modulus and compressive yield strength values are more
comparable to that of natural bone than the other commonly used metallic implants [17].
Magnesium is also present as a natural ion in the human body, whereby approx-
imately 1 mol of magnesium is stored in a 70 kg adult human body and an estimated
amount of half of the total physical magnesium is present in the bone tissue [17]. It also as-
sists in many human metabolic reactions and is nontoxic to the human body. Magnesium

TAB L E 1.3. Physical and mechanical properties of natural bone and some implant
materials [17].
Fracture Compressive
Density Toughness Elastic Yield Strength
Materials (g/cm3 ) (MPa m1/2 ) Modulus (GPa) (MPa)

Natural bone 1.8–2.1 3–6 3–20 130–180

Ti alloy 4.4–4.5 55–115 110–117 758–1117
Co–Cr alloy 8.3–9.2 — 230 450–1000
Stainless steel 7.9–8.1 50–200 189–205 170–310
Magnesium 1.74–2.0 15–40 41–45 65–100
Hydroxyapatite 3.1 0.7 73–117 600
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APPLICATIONS 9

has good biocompatibility and it is biodegradable in human body fluid by corrosion, thus
eliminating the need for another operation to remove the implant. All these desirable
features make magnesium-based material a promising implant material [17–19].
In order to overcome the corrosion issues that limit the use of magnesium-based
materials in orthopedics application, in recent years, much research efforts are focused
to explore the use of different alloying elements in magnesium and surface treatments
such as protective coatings on magnesium-based materials [17].

1.3.4. Sports Applications

In the sporting industry, it is important that the sports equipment matches up to the ever-
increasing expectations of sports enthusiasts. The excellent specific strength and ability
of magnesium alloys and magnesium composites to form intricate shapes resulted in
many applications in sports-related equipment. For example, magnesium-based materi-
als are used in the handles of archery bows, tennis rackets, and golf clubs (Figure 1.6).

(a) (b)

(c) (d)

Figure 1.6. Magnesium sports equipment: (a) golf club head is cast from high-quality
magnesium (courtesy of www.thegolfdome.ca), (b) in-line skates with magnesium chassis
(courtesy of www.skates.com), (c) tennis racquet with magnesium head (courtesy of www.
courtsidesports.com), and (d) bicycle with magnesium frame (courtesy of www.segalbikes.eu).
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10 INTRODUCTION TO MAGNESIUM

Figure 1.7. Laptop with magnesium alloy

AZ91D casing.

The lightweight and excellent damping characteristics of magnesium-based mate-

rials have also made them a popular material choice in bicycle frames and the chassis of
in-line skates (Figure 1.6). Bicycle frames made from magnesium alloys or composites
are capable of absorbing shock and vibration [15], hence allowing the rider to exert less
energy and enjoy a more comfortable ride.

1.3.5. Electronic Applications

The trend in the electronic equipment industry is to make products more personal
and portable. Hence, it is important that the components that make up the equipment
are lightweight and also durable. Magnesium-based materials meet the necessary re-
quirements as they are as light as plastic, but exhibit great improvement in strength,
heat transfer, and the ability to shield electromagnetic interference and radio frequency
interference, as compared with their plastic counterparts [15]. Hence, as shown in
Figure 1.7, magnesium-based materials are used in housings of cell phones, computers,
laptops, and portable media players (such as the Apple iPod Nano magnesium case).
The ability to form magnesium alloys into complex shapes and the good heat
dissipation and heat transfer characteristics of magnesium alloys also result in the use
of magnesium alloys in heat sinks and the arms of the hard-drive reader [15]. Other
examples of the use of magnesium include the housings of cameras (Figure 1.8) and
digital image projection systems.

1.3.6. Other Applications

Optical Applications. Magnesium is commonly used to make the frame of eye-
wear because of its lightweight property. Other optical equipment that capitalizes on the
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REFERENCES 11

Figure 1.8. Magnesium housing of

digital camera.

lightweight and optical stability attributes of magnesium includes the rifle scopes and
binoculars.

Hand-Held Working Tools. In order to achieve higher working efficiency, it is

desirable that the hand-held working tools are lightweight to allow greater portability.
Hence, the low density of magnesium coupled with its resistance to impact and its
ability to reduce noise and vibration makes it the material of choice for a wide range of
hand-held working tools. Some examples include [15] the following:

(i) Magnesium chain saw housing

(ii) Magnesium housing and cylinder of pneumatic nail gun
(iii) Housings of gear and engine of hand-held tools
(iv) Handles of hand shears
(v) Housing of hand drills

1.4. SUMMARY

This chapter presents the potential of magnesium as an energy-efficient material. Its

lightweight and high specific strength characteristics are favorable properties that have
resulted in many applications in the automotive, aerospace, sports, and electronic indus-
tries. The future applications of magnesium-based materials are unlimited and depend
on the vision and imagination of working engineers.

REFERENCES

1. US Energy Information Administration Website: http://tonto.eia.doe.gov/dnav/

pet/hist/wtotworldw.htm (last accessed on December 20, 2009.).
2. H. Okamoto (1998) In A. A. Nayeb-Hashemi and J. B. Clark (eds) Phase Diagrams
of Binary Magnesium Alloys. Metals Park, OH: ASM International.
3. M. M. Avedesian and H. Baker (ed.) (1999) ASM Specialty Handbook—Magnesium
and Magnesium Alloys. Materials Park, OH: ASM International.
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12 INTRODUCTION TO MAGNESIUM

4. B. L. Mordike and K.U. Kainer (ed.) (1998) Magnesium Alloys and Their Applica-
tions. Frankfurt, Germany: Werkstoff-Informationsgesellschaft mbH.
5. B. L. Mordike and T. Ebert (2001) Magnesium: properties—applications—
potential. Materials Science Engineering A, 302, 37–45.
6. G. Neite, K. Kubota, K. Higashi, and F. Hehmann (2005) In R. W. Cahn, P. Haasen,
and E. J. Kramer (eds) Materials Science and Technology, Vol. 8. Germany: Wiley-
VCH.
7. W. D. Callister (2003) Materials Science and Engineering: An introduction. New
York: Wiley.
8. J. F. King (2007) Magnesium: commodity or exotic? Materials Science and Tech-
nology, 23(1), 1–14.
9. S. K. Thakur, M. Paramsothy, and M. Gupta (2010) Improving tensile and compres-
sive strengths of magnesium by blending it with alumnium. Materials Science and
Technology, 26(1), 115–120.
10. S. F. Hassan and M. Gupta (2006) Effect of type of primary processing on the
microstructure, CTE and mechanical properties of magnesium/alumina nanocom-
posites. Composite Structures, 72, 19–26.
11. M. Paramsothy, M. Gupta, and N. Srikanth (2008) Improving compressive failure
strain and work of fracture of magnesium by integrating it with millimeter length
scale aluminum. Journal of Composite Materials, 42(13), 1297–1307.
12. K. U. Kainer (ed.) (2003) Magnesium—Alloys and Technologies. Weinheim, Cam-
bridge: Wiley-VCH.
13. D. Magers (1995) Einsatzmöglichkeiten von Magnesium im Automobilbau. Leicht-
metalle im Automobilbau (Sonderausgabe der ATZ und MTZ). Stuttgart: Franckh-
Kosmos Verlags-GmbH.
14. J. J. Kim and D. S. Han (2008) Recent development and applications of magnesium
alloys in the Hyundai and Kia Motors Corporation. Materials Transactions, 49,
894–897.
15. H. E. Friedrich and B. L. Mordike (ed.) (2006) Magnesium Technology—Metallurgy,
Design Data, Applications. Springer.
16. E. D. McBride (1938) Absorbable metal in bone surgery. Journal of American
Medical Association, 111(27), 2464–2467.
17. M. P. Staiger, A. M. Pietak, J. Huadmai, and G. Dias (2006) Magnesium and its
alloys as orthopedic biomaterials: a review. Biomaterials, 27, 1728–1734.
18. Y. W. Song, D. Y. Shan, and E. H. Han (2008) Electrodeposition of hydroxyapatite
coating on magnesium alloy for biomaterial application. Materials Letters, 62,
3276–3279.
19. Y. W. Song, D. Y. Shan, R. S. Chen, F. Zhang, and E. H. Han (2009) Biodegradable
behaviors of AZ31 magnesium alloy in simulated body fluid. Materials Science and
Engineering C, 29(3), 1039–1045.