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1 | The Importance of Magnesium and Its Alloys in Modern Technology and Methods of Shaping Their Structure and Properties Leszek A. Dobrzański, George E. Totten, and Menachem Bamberger |
CONTENTS
1.1 General Characteristics of Magnesium
1.2 Acquisition of Magnesium from Natural Sources and Scale of Magnesium Production
1.3 General Characteristics of Magnesium Alloys and Their Application Areas
1.4 Overview of Issues Relating to Magnesium and Its Alloys Discussed in This Book
1.5 General Remarks
References
“Learning naturally is a true pleasure;
how unfortunate then it is
that in most schools it is made a pain.”
Sir Humphry Davy (1778–1829)
1.1 GENERAL CHARACTERISTICS OF MAGNESIUM
How greatly amazed would have been Sir Humphry Davy (1778–1829), a Cornish chemist and inventor, the Fellow and President of the Royal Society, Member of the Royal Irish Academy and Fellow of the Geological Society, to whose memory we dedicate this book, and who was the first to isolate, with electrochemical methods, in 1807–1808, a series of elements, including magnesium [1], had he known how great technical importance was gained by this chemical element after 210 years since discovery. Magnesium’s atomic number is 12. It occurs in the solid state as a substance with a silver-grey colour, metallic gloss, with a close physical similarity to the other five elements in the second column of the periodic table. It belongs to alkaline earth metals, which also include Ca, Sr, Ba, Ra and Be. All the elements of the group have the same electron configuration in the outer shell of electrons and a similar crystalline structure [2,3,4].
Magnesium is one of the lightest metals with a density of 1.74 g/cm3 and the atomic mass of 24.31. The melting point and the boiling point of magnesium is, respectively, 650°C and 1107°C. A linear coefficient of thermal expansion of magnesium at room temperature is equal to 26 10−6 1/K [2,3,4]. The strength and plastic properties of pure magnesium are relatively low and dependent upon its purity. Tensile strength Rm in the cast state is 80–120 MPa, the yield point Re = 20 MPa, elongation A = 4%–6%, and hardness 30 HBW [2,3,4,5,6]. It should be noted that a relatively high specific strength of magnesium, which is 4.6–6.8 km, is very favourable, and in the case of structural alloys of this element, reaches up to 19 km in comparison with the non-alloy steel, which reaches 6–7 km, and after quenching and tempering 11.5 km, and over 19 km only after patenting in the form of very fine wires having a diameter of not more than 5 mm. The properties of pure magnesium are summarised in Table 1.1.
Magnesium crystallises in a compact hexagonal lattice A3 [7,8,9,10], with the parameters a = 0.321 nm and c = 0.521 nm. Below 225°C, slide in the magnesium structure is only possible in the base plane {0001} <1120>, together with twinning in the plane {1012} <1011>. Pure magnesium and conventional cast magnesium alloys have a tendency to brittleness, characterised by an intercrystalline fracture and a local transcrystalline fracture in the twin zones or in the base plane {0001} with large grains. A new base plane is activated above 225°C {1011}, causing good magnesium deformability. The temperature range, in which magnesium can be most easily subjected to plastic working, is 350°C–450°C [4].
Magnesium rather easily oxidises in air, but, as in the case of aluminium, the corrosion process is inhibited by passivation. In contrast to aluminium (PBR = 1.28), magnesium, however, has an unfavourable Pilling–Bedworth ratio PBR = 0.80 [11,12], as a result of which the passivation coating is less effective. Magnesium is also passivated in concentrated (98%) sulphuric acid and in the presence of iodine vapours. A passivation layer of the poorly soluble magnesium fluoride protects it to the temperature of 600°C, and also against the activity of hydrofluoric acid [13]. Magnesium reacts slowly with hot water with temperature of >70°C, forming magnesium hydroxide. It is completely resistant to the activity of alkali, and vigorously reacts with acids, by creating corresponding salts and releasing hydrogen [10]. Magnesium is a flammable substance, the flash point is approx. 760°C. Magnesium dust is pyrophoric, its flash point is approx. 470°C. Magnesium in the air burns with a blinding white flame, whose temperature is 3000°C–3100°C. The main product is magnesium oxide, accompanied by magnesium nitride. Incineration is also maintained in an atmosphere of water vapour and carbon dioxide. Magnesium dissolves when heated in methanol and ethanol by producing relevant magnesium alcoholates. Such reactions are initiated by iodine and inhibited by water with the fraction of over 1%. They are used to produce alcoholates and to obtain the so-called absolute ethanol, i.e., the product with a very small fraction of water.
TABLE 1.1
Comparison of Selected Properties of Pure Magnesium
Properties | Mg |
Atomic number | 12 |
Atomic mass | 24.3056 |
Crystalline structure | A3 |
Density in 20°C, g/cm3 | 1.738 |
Melting point, °C | 650 |
Boiling point, °C | 1107 |
Thermal expansion coefficient, 10−6 1/K | 25.2 |
Specific heat capacity, kJ/(kg·K) | 1.025 |
Heat conductivity, W/(m·K) | 418 |
Resistivity, nΩ·m | 44.5 |
Tensile strength, Rm, MPa | 180 ÷ 220a |
Yield point Rp, 0.2, MPa | 115 ÷ 140a |
Elongation, % | 2 ÷ 10a |
Elastic modulus, GPa | 40 |
Hardness, HBW | 45 ÷ 47a |
Source: Dobrzański, L.A., OAlib. Annal VII, 2, 1–982, 2017.
Magnesium is the eleventh most abundantly occurring element in the human body by mass [14], and is essential for all cells and for about 300 enzymes [15,16,17,18,19]. Magnesium ions play an important role in maintaining the osmotic pressure of blood and other tissues and in maintaining the proper structure of ribosomes. It is a component of bone, reduces the degree of hydration of cellular colloids, participates in the transmission of signals in the nervous system. The demand for magnesium in adults is 300–400 mg per day [15] and although in the natural environment it is richly present in foods consumed by man, there is less and less of it due to chemical fertilisation of soil with compounds containing potassium, and due to the use of excessive amounts of food preservatives. Other causes of magnesium deficiency are alcohol abuse, the use of hormonal contraceptives, stress, excessive consumption of fats and renal failure. One of the most common symptoms of magnesium deficiency is muscle spasms. Aches, poor digestion, worries and difficulty in sleeping are also common. It can also be a foundational cause of the restless leg syndrome [20,21,22,23,24]. Magnesium ions interact with polyphosphate compounds, such as deoxyribonucleic acid (DNA), adenosine triphosphate (ATP) and ribonucleic acid (RNA). The activity of magnesium ions is required for hundreds of enzymes [19]. Magnesium compounds are used in medicine as conventional laxatives, antacids and for stabilising the abnormal stimulation of nerves or vasoconstriction.
Magnesium is the ninth element most abundant in the universe and the eighth most abundant in the earth crust [25], but the fourth most commonly occurring element on Earth, after iron, oxygen and silicon [26], constituting 13% of the planet mass and constitutes about 2% of the crust of the Earth. It is the third element most commonly dissolved in sea water, after soda and chlorine [25], which averages about 0.13% magnesium by weight. When it is harvested from this resource, it becomes the third most-plentiful element on our planet [25,27].
1.2 ACQUISITION OF MAGNESIUM FROM NATURAL SOURCES AND SCALE OF MAGNESIUM PRODUCTION
Magnesium occurs naturally only in combination with other elements, where invariably it has the oxidation state+2. It is present in large deposits of magnesite, dolomite and other minerals and in mineral waters, where the magnesium ion is soluble. Although magnesium is present in over 60 minerals, only dolomite [MgCO3*CaCO3], magnesite [MgCO3], brucite [Mg (OH)2], carnallite [MgCl2 * KCl * 6H2O], talc [Mg3Si4O10(OH)2] and olivine [(Mg, Fe)2SiO4] and also sea water [Mg2+(aq)] have commercial significance. Resources from which magnesium compounds can be recovered range from large to virtually unlimited and are globally widespread. Identified world magnesite and brucite resources total 12 billion tons and several million tons, respectively. Resources of dolomite, forsterite, magnesium-bearing evaporite minerals and magnesia-bearing brines are estimated to constitute a resource of billions of tons. Magnesium hydroxide can be recovered from seawater. Australia holds the highest levels of magnesium reserves in the world today, accounting for 30.4% of the total available raw mineral. China has the second-most reserves, with 28.3% of the available raw mineral. They are followed by North Korea and Russia, with 19.6% and 17.4% of th...
