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Text A. Metals, Alloys, Plastics

The discovery of metal was one of the most significant achievements of prehistoric times. Pieces of hammered copper and lead dating from about 6000 BC are known to have been found in southern Turkey.

There are different kinds of metals. They can be divided into various groups: ferrous and nonferrous; base, noble and precious. Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be precious metals, often due to perceived rarity. Examples include tantalum, gold, platinum, and rhodium. Certain elements resemble metals, but are brittle and do not conduct heat and electricity as well. These elements are called semimetals, or metalloids.

The physical properties of most metals include high density, and high strength. Most metals also have great plasticity: they can change their shape without breaking. Metals also have considerable elasticity. A metal spring can be stretched, for example, but when the load is removed, it contracts to its original length. About three quarters of all the known chemical elements are metals. There is such a variety that not all metals can be described by a single definition. Sodium and potassium, for example, are soft enough to be cut with a knife. Gallium's melting point is so low (85.6 º F, or 29.8 º C) that it melts in the palm of a hand. Several metals, including uranium, thorium, and radium, are radioactive.

The electronic structure of metals explains many of their properties.

Heat is a phenomenon involving whole atoms. In solid metals the atoms are arranged in a regular structure. When the metal is heated, the atoms vibrate, influencing their neighbours to vibrate as well. Metals are malleable and ductile because sheets of these atoms can slip gradually past one another.

Why does man use metals still so much today when there are other materials, especially plastics, which are available? A material is generally used because it offers the required strength, and other properties, at minimum cost. Appearance is also an important factor. The main advantage of metals is their strength and toughness. Concrete may be cheaper and is often used in building, but even concrete depends on its core of steel for strength.

Plastics are lighter and more corrosion-resistant, but they are not usually as strong. Another problem with plastics is what to do with them after use. Metal objects can often be broken down and the metals recycled; plastics can only be dumped or burned.

Not all metals are strong, however. Copper and aluminium, for example, are both fairly weak, but if they are mixed together, the result is an alloy called aluminium bronze, which is much stronger than either pure copper or pure aluminium. Engineers often require materials that have properties that none of the available metals have. To fill these needs, metallurgists create alloys (metals combined with other metals, with non-metals, or both) to extend the range of a metal's properties. Steel, for example, is an alloy of iron and carbon. Alloying is an important method of obtaining whatever special properties are required: strength, toughness, resistance to wear, magnetic properties, high electrical resistance or corrosion resistance.

The properties of a metal can be further improved by use of heat treatment. Heat treatment is the term given to a number of different procedures in which the properties of metals and alloys are changed. It usually consists of heating the metal or alloy to a selected temperature below its melting point and then cooling it at a certain rate to obtain those properties which are required. For example, hardening is used to make metals harder. Tempering makes them softer and less brittle. Annealingis carried out to make a metal soft so that it can be produced to meet every kind of engineering specification and requirement.

Powder metallurgy, a method of forming metal, has developed rapidly since the 1950s. In this process, the metals to be used are first made into powders. They are then mixed and compressed into the desired shape and heated to form the solid part. Powder metallurgy is used to make small parts that could not be economically mass-produced in any other way.

When Concorde (the British-French supersonic plane) was built, a material was needed which could withstand extreme aerodynamic conditions and would have a life of at least 45,000 flying hours. To achieve this a special aluminium alloy was developed which is tough and lightweight and was used in over 70% of Concorde’s structure. Another 16% is made of high-strength steel, and titanium alloys were used in the engine surrounds to withstand temperatures of 4000 degrees centigrade.

Date: 2015-12-18; view: 791

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