94793

PHYSICAL PROPERTIES OF METALS AND ALLOYS

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Физика

The electrical resistance of metals or alloys is increased by decreasing the size of the crystals and therefore increasing the number of crystal boundaries. In general all metals increase in resistivity with increase in impurities. The resistivity of metals is also increased in most cases by an increase in temperature.

Английский

2015-09-16

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UNIT 9

PHYSICAL PROPERTIES OF METALS AND ALLOYS

The temperature at which a metal melts, is called the melting point, the metals of lower melting points are generally the soft metals and those of high melting the hard metals. The boiling point of a substance depends on the surrounding pressure. The term "boiling point" refers to the temperature at which the metal boils under normal atmospheric pressure.

The electrical conductivity of a substance is the electrical conducting power of a unit length per unit of cross-sectional area. The electrical resistance of metals or alloys is increased by decreasing the size of the crystals and, therefore, increasing the number of crystal boundaries. In general, all metals increase in resistivity with increase in impurities. The resistivity of metals is also increased in most cases by an increase in temperature.

Heat conductivity is measured as the heat-conducting ability of a unit length or thickness of a substance per unit of cross-sectional area.

Magnetism is measured as the magnetic force exerted by a unit volume of a substance under standard magnetising force. Iron, cobalt and nickel are the only metals possessing considerable magnetism at room temperature, and they become non-magnetic when heated to a certain temperature. Strong permanent magnets have been made chiefly of one of several compositions of steel, but in recent years a number of magnet alloys of much greater magnetism, able to exert forces many times their own weights, have been developed.

Porosity, the quality of containing pores is lack of denseness. Density, on the other hand, denotes weight per unit of volume. Some heavy metals, like grey cast iron, are porous enough to leak under heavy hydraulic pressures, whereas some lightweight metals, like aluminium, are dense and compact. Most metals expand on heating and contract on cooling.

Most of the metals are silvery white or grey in colour. Copper is the only red metal, and gold the only yellow one, although a number of copper-base alloys are also yellow. All solid metals have metallic lustre, although the true colour and lustre of many metals are often obscured by a coating of oxide - which may be white, grey, red, brown, bluish, or black.