Unit 9. METALS AND THEIR ALLOYS.
Task for selfstudy:
- read the texts
- write down all special terms to each of the texts
- answer the questions
- make a report on any metal you like
ALLOYS
Pure metals are comparatively seldom used; in engineering, application is made chiefly of alloys which consist of two or more metals, or of metals and metalloids.
Alloys are metallic solids, complex in composition,
formed as a result of the freezing of the melt — the liquid
solution of two or more metals, or metals and metalloids.
Each constituent of an alloy is called a component. Alloys may be binary (two-component), ternary (three-component), etc.
The ability of various metals to form alloys differs greatly and, therefore, the structure of various alloys after solidification may also be very diverse.
In the liquid state, alloys are entirely homogeneous and from the physical point of view constitute a single phase. Nonhomogeneity may appear when an alloy is transformed from the liquid to the solid state, i.e. several solid phases are formed. After solidification, alloys may consist of one, two or more phases depending upon the nature of their components. Certain metals are not mutually soluble in the liquid stale; they form two layers with different specific weights (e.g., lead and iron, lead and zinc, etc.). It is difficult to form an alloy in such cases since it is necessary to mix the metals into each other.
ALUMINIUM AND ITS ALLOYS
Next to oxygen, aluminium is the most abundant element in nature: about 7.45 per cent of the earth's crust consists of aluminium.
Aluminium is extracted from rock with a high alumina content. The most important sources are bauxite, kaolin, nepheline and alunile.
Bauxite is the principal source of aluminium. The less silica in a bauxite the higher its quality as an aluminium ore. Kaolin clays are very abundant in nature but the extraction of aluminium from these ores presents difficulties due to the considerable amount of silica present.
The most important properties of aluminium are its low specific gravity (2.7), high electrical and thermal conductivities, high ductility, and corrosion resistance in various media.
Pure aluminium has only few applications; it is used for the manufacture of electrical wire, chemical apparatus, household utensils and for coating other metals.
Aluminium alloys are more widely used in industry. Wrought aluminium alloys have a high mechanical strength which in some cases approaches the strength of steel. Wrought aluminium alloys are further classified as non-heat-treatable and heat-treatable alloys. .Wrought aluminium alloys also include complex alloys of aluminium with copper, nickel, iron, silicon and other alloying elements. Complex wrought aluminium alloys of the duralumin (dural) type and certain others have found most extensive application in many industries.
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Several grades of duralumin are available in the Russia. They are identified by the Russian letter Д followed by a figure indicating the number of the alloy in the series. Duralumin, grade Д-1 can be obtained in the form of sheets, bar stock and tubing; grades Д-6 and Д-16 аre usually produced in the form of bars, and grade Д-ЗП is made as wire for rivets.
Answer the following questions:
1. What elements are the most abundant in nature?
2. What are the most important sources of aluminium?
3. What are the most important properties of aluminium?
4. Is pure aluminium widely used?
5. Do wrought aluminium alloys have a high mechanical strength?
6. How are wrought aluminium alloys further classified?
7. What complex alloys do wrought aluminium alloys also include?
8. What aluminium alloys have found most extensiveapplication in many industries?
9. How are various grades of duralumin identified?
MAGNESIUM AND ITS ALLOYS
Magnesium has a specific gravity of approximately 1.7; its alloys are the lightest of all engineering metals employed.
The melting point of magnesium is 650° C; its boiling point is 1007° C. Magnesium is very inflammable and burns with a dazzlingflame, developing a great deal of heat.
The mechanical properties of magnesium, especially the tensile strength, are very low and therefore pure magnesium is not employed in engineering.
The alloys of magnesium possess much better mechanical properties which ensure their wide application.
The principal alloying elements in magnesium alloys are aluminium, zinc and manganese. Aluminium, added in amounts up to 11 per cent, increases the hardness, tensile strength and fluidity of the alloy. Up to 2 per cent zinc is added to improve the ductility (relative elongation) and castability. The addition of 0.1-0.5 per cent manganese raises the corrosion resistance of magnesium alloys.
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Small additions of cerium, zirconium and beryllium enable a fine-grained structure to be obtained, they also increase the ductility and oxidation resistance of the alloys at elevated temperatures.
Magnesium alloys are classified into two groups: wrought alloys, grades MA1, MA2, casting alloys, grades MЛ4, MЛ5.
Wrought magnesium alloys MA1 and MA2 are chiefly used for hot smith and closed-die forged machine pants. They are less frequently used as sheets, tubing or bar stock.
Magnesium casting alloys MЛ4 and MЛ5 are widely used as foundry material though their castability is inferior to that of aluminium-base alloys.
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