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ISSN (1897-3310) Volume 7 Issue 3/2007 11 – 16

FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

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Ultralight Magnesium-Lithium Alloys a b

A. Białobrzeski a,b, K. Saja a, K. Hubner a

Foundry Research Institute, Zakopiańska Str., 7330-418 Kraków, Poland University of Bielsko-Biała, Willowa Str. 2, 43-309 Bielsko-Biała, Poland Received on: 02.04.2007; Approved for printing on: 27.04.2007

Abstract The article gives basic information on the chief constituents of Mg-Li alloys and on their expected properties. A schematic representation and technical performance of a pilot stand for melting and pouring of reactive ultralight magnesium-based alloys have been presented. The preliminary data regarding the manufactured magnesium alloys with about 2-3 % Li and about 10 % Li have been given in the form of microstructures and chemical compositions. Keywords: Magnesium-lithium alloys, Microstructural examinations

1. Introduction Magnesium-lithium alloys belong to the family of ultralight materials. Their specific gravity with maximum lithium content applicable nowadays (14 -16 % max.) amounts to 1,35-1,45 g/cm3, which means that it is not much higher than the specific gravity of water (∼1,0 g/cm3).[1,3,4] Magnesium (Mg, Lat. magnesium) of the atomic number 12 is characterised by a hexagonal crystallographic structure, and at ambient temperature it has the density of 1,738 g/cm3. The melting point of magnesium is 650 0C, and the evaporation temperature is 1107,0 0C [12]. Magnesium was discovered in year 1808 by the British scientist Sir Humprhrey Davy. Metallic magnesium was obtained in 1828 by A.A. Bussy through reduction with potassium of molten magnesium chloride; in 1833 W. Faraday obtained magnesium during electrolysis of molten chlorides. In the Earth’s crust there is about 2,33 wt.% of magnesium contained in almost 200 of the well-known minerals, mainly in the form of carbonates, magnesites and potassium-magnesium salts. Another source of magnesium is seawater; magnesium is also found in the water of some salt lakes. In one ton of the seawater are dissolved about 4 kg of magnesium. Magnesium is a very active metal - at a temperature of about 550 0C in the air it ignites with a flashing flare; in the atmosphere of chlorine is ignites even at room temperature. It is resistant to the effect of some acids, soda, alkalies, hydrocarbons, and mineral oils, and it

dissolves in seawater. It does not practically enter into reaction with cold fresh water, but very effectively displaces hydrogen from hot water. Magnesium is sometimes called “the element of life” as it forms one of the components of chlorophyl (which contains about 2% Mg), indispensable in the process of photosynthesis. The mass of a grown up man contains about 25 g of magnesium. It is the element necessary in correct functioning of the man’s nervous and cardio-vascular system. Magnesium is obtained in an electrothermal process or during electrolysis. In former case the metal is obtained directly from magnesium oxide through its reduction with metals under the conditions of reduced pressure (the methods developed by Magnethers, Knapsack-Giesheim). Electrolytic methods (Dow, Magnola) consist in electrolysis of molten mixture of salts, like anhydrous chlorides of magnesium, calcium, potassium and sodium, and fluorides of calcium and sodium, obtaining finally magnesium of 99,99 % purity. Of great technical importance are magnesium alloys with aluminium, zinc and manganese, where Al and Zn increase the alloy strength, while Mn improves the corrosion resistance. The alloying additions which increase the strength at elevated temperatures and plastic properties as well as resistance to the oxidising effect are beryllium, calcium, cerium, cadmium, and titanium. Iron, silicon and nickel deteriorate the mechanical properties of alloys and their corrosion resistance. Castings made from magnesium alloys weigh by about 20-30 % less than castings made from aluminium alloys and by 50-75 % less than analogical castings made from ferrous alloys. The price of 1 kg of magnesium is at a level of 2 USD.

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Magnesium alloys offer a very advantageous strength-to-density and/or elastic modulus-to-density ratio, which means that they can transfer static and dynamic loads in a way similar as alloys of iron and aluminium do, offering - moreover - a good damping capacity. They are resistant to corrosion (provided the level of contamination with iron, nickel and copper is effectively reduced), and are resistant to alternate mechanical loads, also at elevated temperatures. At present, the increasing degree of magnesium alloys application in automotive industry is, on one hand, dictated by current price of these alloys while, on the other, it is steered by the legal regulations which claim that it is necessary to reduce fuel consumption in standard vehicles, e.g. in USA to a level of 5,9 l/100 km in city traffic, and in EU to a level of 3 l/100km. The most effective way to achieve this goal is by reducing the weight of a car. It is estimated that reducing the weight of a car by 10 % results in 5,5 % reduction of fuel consumption which, in turn, enables reducing the level of exhaust gas.The weight of magnesium alloy elements used in modern cars may reach about 50 kg. Magnesium alloys are at present used also in aircraft (construction of airframe parts) and aerospace industry as well as in electronics (casings of notebooks, cell telephones, etc.), in nuclear installations, e.g. neutron-absorbing reactor housings, in manufacture of household appliances, etc. Castings are mainly made on the pressure die casting machines (about 90%). Another field of magnesium applications are the metallurgical processes, e.g. spheroidising treatment of cast iron and deoxidising of metal bath. Lithium (Li) of the atomic number 3 crystallises in body-centered cubic lattice and at ambient temperature has the density of 0,53 g/cm3. The melting point of lithium is 180,54 0C, and the temperature of evaporation is 1347,0 0C [12]. This elements was discovered by the Swedish chemist Jochan Arfvedson in 1817 when making analysis of the mineral petalite. It was called by him lithium (Gr. litheos - stone). In 1855, the German chemist Bunsen and, independent of him, the English physician Matissen obtained pure, metallic lithium in the electrolysis of molten lithium chloride. The Earth’s crust contains about 6,5⋅10-3 wt.% Li. The main world deposits of lithium in the form of mineral are located in Australia, Chile, China, and also in Canada, USA, Russia, Zimbabwe and in a few other places [10]. The price of 1 kg of lithium is at a level of 30 USD [5,11]. Lithium is a soft, silvery-white metal almost two times lighter than water. It enters into very violent reaction with nitrogen and oxygen. At a temperature above 200 0C it burns with white flame. Under standard conditions lithium forms on its surface a layer of Li3N, mainly because at low temperatures it is characterised by greater affinity to nitrogen than to oxygen. At higher temperatures the surface of lithium is coated with LiO2. Lithium decomposes water with evolution of H2 and LiOH. Because of its high chemical reactivity, lithium in the form of granules, rods, or ribbons must be stored under the cover of paraffin oil or kerosene, in air-tight vacuum sealed containers, or protected by the atmosphere of noble gases. Some master alloys containing lithium, e.g. Al-Li, are also produced. Lithium compounds fix large volumes of hydrogen, e.g. in 1 kg of lithium hydroxide is fixed 2800 litres of hydrogen. This property of litium compounds

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is effectively utilised in air purification installations, e.g. in submarines, in masks and air conditioners. Some organic compounds of lithium, e.g. stearate, palmitate) preserve their physico-chemical properties within a large range of temperatures and form a base constituent of lubricants, useful specially at low temperatures (lithium lubricants). Lithium is also used in glass industry (it reduces the glass solubility and improves optical properties of glasses). Lithium fluoride is known to have the highest, among all the materials known in optics, transparency for ultraviolet radiation. Lenses made from monocrystals of this salt are used in construction of special telescopes. In nuclear technology, lithium deuteride is used as fuel in the, so called, lithium reactors. Lithium is also used as a moderator controlling the intensity of nuclear reactions. In nuclear power engineering it is used as a carrier of thermal energy in the primary circuits of heat exchangers operating in reactors of the power plants. Lithium is also one of the two most effective propellants in rockets (solid propellant rockets), since its calorific value is at a level of 10 000 kcal/kg. Only beryllium (Be) is characterised by a higher calorific values of 15 000 kcal/kg. For comparison, the calorific value of kerosene (also used as a propellant in liquid propellant rockets) is only 2 300 kcal/kg, while that of nitroglycerine (known to be one the strongest explosives) is at a level of 1 500 kcal/kg. There are patents (USA) for solid rocket propellants containing 51-68% lithium. First attempts at using lithium as a constituent in metal alloys were made during the First World War. During that period the German industry suffered from the defficiency of tin, necessary to produce bearings. An alloy of lead and lithium - developed by Jan Czochralski - has proved to be an excellent bearing metal, used mainly for the wheels of railway cars - hence it name “Bahnmetal”. There are known lithium alloys in combination with aluminium, beryllium, copper, zinc, silver, etc. Particularly bright future opens for alloys from the magnesium-lithium family. If attempts at making a permanent magnesium-based alloys with lithium content above 50 % ended in success, then the density of such alloys would be close to the density of water. The fact that an element is commonly encountered in the Earth’s crust does not necessarily mean that this common occurrence remains in straight relationship with its price, which also depends on the volume and cost of production. The price of lithium is high, still it is considerably lower than the price of such transient elements (present in magnesium-based alloys) as Sc or Y. The lanthanides widely used in research of basic magnesium alloys are more expensive than lithium by one order of magnitude. Over the past few years, magnesium-lithium alloys with the alloying additions of Al, Cd., Zn and Ag have been raising a very vivid interest. The solubility of lithium in magnesium characterised by a hexagonal structure is very low and amounts to about 5 wt.%, while magnesium forms the solid solution β in a very wide range of values, since up to 90 wt.% of this element is successfully dissolving in lithium of the body-centered cubic structure (Fig. 1).

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2. Experimental The first stage of the experimental work was related with designing and development of technical guidelines for construction of a pilot stand which should enable manufacture of magnesium-lithium alloys. Taking into account high reactivity of these alloys, both melting process and pouring of pilot castings had to be conducted under a controlled protective gas atmosphere maintained in both the melting space (crucible) and pouring chamber. This forced the use of a stoppered crucible, connected directly with the pouring chamber. A resistance furnace of 1,5 kW power provided with a crucible holding approximately 3 kg of Mg alloys was used. The protective atmosphere was a mixture of Ar and FS6. The applied system of temperature control in crucible enabled its stabilisation at an accuracy of +/-3 0C.

Fig. 1. Phase equilibrium diagram for Mg-Li alloys and change in alloy density as a function of the changing lithium content in Mg-Li alloy according to [8,9] Lithium has a favourable effect on the deformability of magnesium alloys replacing the hardly deformable hexagonal αMg (hcp) lattice with a body-centered cubic β-Li lattice (bcc), which reduces mechanical properties due to the appearance of β phase. An optimum combination of the properties is obtained in the range of binary α+β alloys containing over 10 wt.% lithium. Figure 1 also shows the change in Mg-Li alloy density in function of the alloy chenical composition. From the plotted curve it follows that it is possible to obtain an alloy of the density lower even than 1 [g/cm3]. Alloys with lithium content comprised in this range are in as-cast condition characterised by the elongation of 60 %. Adding aluminium to Mg-Li alloys [9] results in appearance of a hexagonal δ phase in the structure, which is a solid solution of Al in Mg characterised by a limited deformability, of a ductile λ phase, which is a solid solution of Al in Li characterised by a body-centered cubic lattice, of a hard (allowing precipitation hardening) intermetallic aluminiumlithium compound, and of η phase characterised by a B 2 structure. The toughness of such alloys increases with increasing content of δ+λ eutectic. Sometimes the alloys also contain a metastable Li2MgAl phase [ 2, 10,13]. In spite of quite abundant reference literature, the majority of the available publications on Mg-Li alloys are of a strictly academic nature. On the other hand, information on the physicochemical properties or technological processes of making magnesium-lithium alloys is but only very scarce.

Fig 2. Schematic diagram of pilot stand for melting and pouring of ultralight alloys

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Magnesium alloys are very plastic and this causes serious technical problems with correct preparation of metallographic sections. The sections were etched in 4 % HNO3 and examined under polarised light. On successive drawings the microstructures obtained in Mg-Li alloys are shown.

Fig 3. Schematic diagram of pilot stand for melting and pouring of ultralight alloys with well visible pouring chamber and dies in position Figure 2 shows a schematic diagram of the pilot stand, while. The task of the first stage of the research was mastering the technology of making Mg-Li alloys and development of basic technological parameters of the melting and pouring process. The alloys were obtained from pure constituents, i.e. from magnesium and lithium, where lithium was used in the form of granules and ribbons. The first melts were conducted starting with the lowest lithium content, i.e. 2-3%; in the next melts this level was raised to about 10% Li. From thus obtained alloys, the keel blocks commonly used for mechanical tests were cast in the atmosphere of protective gas. From these castings the specimens for metallographic examinations were cut out. In view of the high reactivity of these alloys, preparation of the metallographic sections required application of special methods [9,10].

Fig. 5. Microstructure of Mg-Li alloy containing 1,77 % Li; α-Mg-Li matrix (hcp) 100x, section etched in 4 % HNO3 , polarised light

Fig. 6. Microstructure of Mg-Li alloy containing 2,12 % Li; α-Mg-Li matrix (hcp) 25x, section etched in 4 % HNO3 , polarised light

Fig. 4. Microstructure of Mg-Li alloy containing 1,77 % Li; α-Mg-Li matrix (hcp) 25x, section etched in 4 % HNO3 polarised light Fig. 7. Microstructure of Mg-Li alloy containing 2,12 % Li; α-MgLi matrix (hcp) 100x, section etched in 4 % HNO3 , polarised light

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Fig. 8. Microstructure of Mg-Li alloy containing 3,01 % Li; β-MgLi matrix (bcc), 25x, section etched in 4 % HNO3 , polarised light

Fig. 11. Microstructure of Mg-Li alloy containing 10,7 % Li; β-MgLi matrix (bcc) 100x, section etched in 4 % HNO3 , polarised light

Since literature gives practically no information on microstructural examinations of α-Mg-Li alloys by the methods of optical microscopy, it is difficult to explain the appearance of these alloys in polarised light. One can only suppose that this is caused [11,12,13] by the presence of a hexagonal lattice of αMgLi solution (hcp). The alloys microhardness was also measured by the Vickers method. The three examined lithium alloys are characterised by the hardness at a level of 51,0- 61,0 units. Table 1. The results of Vickers microhardness measurements – f orce of 50 G Fig. 9. Microstructure of Mg-Li alloy containing 3,01 % Li; β-MgLi matrix (bcc) 100x, section etched in 4 % HNO3 , polarised light

Fig. 10. Microstructure of Mg-Li alloy containing 10,7 % Li; β-MgLi matrix (bcc), 25x, section etched in 4 % HNO3 , polarised light

No.

Specimen

Average microhardness

1

MgLi-1,77

51,00

2

MgLi--2,12

59,07

3

MgLi-10,7

61,70

From analysis of the data given in literature [9] it follows that for Mg-12Li alloy the hardness of about 50 HV (at the force of 500G) was obtained, while according to [11] the MgLi-8,7 alloy offered the hardness of 36 BHN. From these data it follows that Mg-Li alloys are very plastic, which is confirmed by the results of research described in [7], where an increase in lithium content from 0 to 16% was reported to increase the impact strength of Mg-Li alloy from about 7 [J/cm2] to 43 [J/cm2]!

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3. Conclusions -

on a unique pilot stand, binary alloys of Mg-Li were obtained, - unique photographs of the microstructure of these alloys were taken; on these microphotographs will be based further advanced researches on Mg-Li alloys with elevated lithium content. The study was conducted under a Commissioned Research Project no. PBZ – KBN – 114/T 08/2004.

References [1] Gulayev B.B. Sintiez splavov (Osnovnyje principy. Vybor komponientov). “Mietallurgiya”, Moskva, 1984. [2] Górny Z., Sobczak J.: Nowoczesne tworzywa odlewnicze na bazie metali nieżelaznych. Wyd. ZA-PIS, Kraków, 2005. [3] Ashby M.F., Jonem D.R.H.: Materiały inżynierskie – właściwości i zastosowania. Wydawnictwa NaukowoTechniczne. Warszawa, 1995. [4] Ashby M.F.: Dobór materiałów – projektowanie inżynierskie. Wydawnictwa Naukowo-Techniczne, Warszawa 1998.

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[5] Ober J.A.: LITHIUM. US. Geological Survey. Mineral Commodity Summaries, January 1998, p.98-98. [6] Svoystva eliemientov: Sprav. izd./pod red. Drica M.E., “Mietallurgiya”, Moskva 1985. [7] Bach F.W., Niemeyer M., Haferkamp H. Density reduced magnesium alloys with increased ductility SFB 390 Project A: http://www.iw.unihannover.de/sfb/sfb390/englisch/a4_e.html [8] ASM Binary Alloy Phase Diagrams 1996 ASM International [9] Song G.S., Staiger M., Kral M., Some new characteristic of the strengthening phase in β-phase magnesium-lithium alloys containing aluminum and beryllium Materials Science and Engineering A 371 (2004) 371-376. [10] Padfield T.B., Sachs Z.F., Metallography and microstructure of magnesium and its alloys ASM Handbook Metallography and Microstructure vol 9, 2004. [11] Sanschagrin A. Tremblay R., Angers R., Dibe D., Mechanical properties and microstructure of new magnesium-lithium base alloys - Materials Science and Engineering A 220 (1996) 69-71. [12] Conn G.K.T., Bradshaw F.J., Polarized light in metallography Butterworths Scientific Publications 1952. [13] Vander Voort G.F. Color Metallography ASM Handbook Metallography and Microstructure vol 9, 2004.

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