Text B. Physical and Mechanical Properties of Metals and Alloys

We know that metallurgy is the science of nature, structure and properties of metals and alloys. Metalls and alloys being subjected to mechanical, thermal, magnetic treatment may improve their properties.

Metal possess (a number of) physical and mechanical properties such as metallic lustre, heat and electrical conductivity, magnetism, hardness, ductility and others.

Let’s consider some of these properties.

Colour. Most of the metals are silvery white or grey in colour. Copper is (the only) red metal, and gold is (the only) yellow one, although (a number of) copper-base alloys are also yellow. All solid metals may have white, grey, red, brown, and black metallic lustre.

Electrical conductivity. Metals conduct electricity better through the crystals than across the crystal boundaries. The electrical resistance of metals and alloys is increased by decreasing the size of the crystals and therefore increasing the number of crystal boundaries. Their electrical resistance increases with the increase of impurities and temperature.

Magnetism. The property of a metal (or an alloy) to exert forces may be called magnetism. Some metals and alloys possess this property. Strong permanent magnets are able to exert forces many times greater than their own weight. Iron, cobalt and nickel possess magnetism at room temperature and they become non-magnetic when heated to a certain temperature.

Melting and Boiling Points. The temperature at which a metal melts, called its melting point, is a measure of its fusibility. The term “boiling point” refers to the temperature at which the metal boils under normal atmospheric pressure (760 mm); it is a measure of volatility.

Strength. The strength of a material is the property of resistance to external loads or stresses without incurring structural damage. The strongest substance known is the tungsten wire of incandescent electric lights. Pure iron is weak, but when alloyed with carbon to make steel, the steel may be stronger than any of the pure metals except tungsten.

Hardness. Hardness of metals is one of the most important and useful of all mechanical properties. It may be defined as resistance to deformation. This resistance is related to the cohesion of the atoms of the body of the metal.

Toughness. Toughness is the property of absorbing considerable energy before fracture.

Elasticity. The property of regaining the original dimensions upon removal of the external load is known as elasticity. The elasticity of a metallic substance is a function of a resistance of its atoms to separation or compression, and thus it is fundamental property of the material.

Ductility. It is the capacity of a metal to be permanently deformed in tension without breaking.

Not all metals possess equal ductility and malleability (or other properties). But there are (some) properties which are recognized as metallic. An engineer must know these properties. The correct choice of a metal or alloy can improve the quality of production.

Exercise 1. Translate the words of the same root. Define speech parts:

To improve - improvement; to possess - possession - possessive; to resist - resistant - resistance; to volatilize - volatile - volatility; to load - loading - loader; to absorb - absorbtion - absorber.

Exercise 2. Remember the following definitions:

1) The property of metals (or an alloy) to exert forces is called magnetism.

2) Hardness may be defined as resistance to deformation.

3) The strength of a material is the property of resistance to external loads or stresses without incurring structural damage.

4) The temperature at which a metal melts is called its melting point.

5) Toughness is the property of absorbing energy before fracture.

Exercise 3. Determine the properties according its definition:

1) Resistance to deformation is...

2) Ability of a metal to conduct electricity is...

3) Property of absorbing energy before fracture is...

4) The temperature at which a metal melts is...

Exercise 4. Answer the following questions to the Text B:

1) What kinds of treatment are metals and alloys subjected to?

2) What physical properties do metals and alloys possess?

3) What mechanical properties do metals possess?

4) What colour do most of the metals possess?

5) Does the choice of a metal for various purposes depend on its physical, mechanical and chemical properties?

6) What is called magnetism?

7) Is hardness of metals one of the most important and useful of all mechanical properties?

8) Do all metals possess equal ductility and malleability?

9) By what characteristic feature do metals differ from other substances?

10) Why (What for) must an engineer know the qualities of metals and alloys?

Exercise 5. Define which of these statements is right or wrong, using phrases: It is right (wrong), I agree (disagree) with you:

1) Metalls and alloys being subjected to mechanical, thermal, magnetic treatment may improve their properties.

2) All solid metals have the same colour.

3) The electrical resistance increases with the increase in impurities and temperature.

4) Properties of metals and alloys don’t make them useful for many purposes of construction.

Exercise 6. Look through the Text B again and express the main points of it in three sentences:

1) Metallurgy is the science...

2) Metals possess...

3) The choice of a metal depends on...

Date: 2016-03-03; view: 2108