ALLOYS
An alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements.
The properties of alloys are often greatly different from those of the components. Gold is too soft to be used without a small percentage of copper. The corrosion and oxidation resistance of steel is markedly increased by incorporation of 15 to 18% of chromium and often a few per cent of nickel (stainless steel). The presence of carbon up to 1.5% profoundly affects the properties of steel. Similarly, a low percentage of molybdenum improves the toughness and wear resistance of steel. The hundreds of special alloys available are instances of the tailor-made nature of alloys to meet specific operating conditions. Amorphous alloys (used in transformer coils) are made by quick quenching of the molten metals.
Alloys melting in the range of about 51°C to 260°C, usually contain bismuth, lead, tin, etc. These alloys are called fusible alloys.
A steel containing up to 10% of elements such as chromium, molybdenum, nickel, etc., usually with a low percentage of carbon is known as alloy steel. These added elements improve hardness, wear resistance, toughness and other properties.
Amalgam: A mixture or an alloy of mercury with a number of metals or alloys such as sodium, zinc, gold and silver as well as with some non-metals is known as amalgam.
Dental amalgam: is a mixture of mercury and a silver-tin alloy.
Mercury does not form an amalgam with iron and hence mercury is commonly stored in iron bottles.
Purpose of making alloys
Alloys are made to change the property of their major constituents to achieve a specific objective.
For example:
- Strength of iron is increased by making steel.
- Brass is stronger than its components copper and zinc.
- Aluminium bronze looks like gold.
- Nichrome, an alloy of Ni, Fe and Cr, is more resistant to electricity.
- Carboloy, an alloy of tungsten, carbon and cobalt, is as hard as diamond.
- Bell metal (Cu- 78%, Sn-22%) is more sonorous than copper or tin.
- Alnico, an alloy containing aluminium. nickel and cobalt, can lift 60 times its own mass.
- Solder, an alloy of lead and tin. Tin lowers melting point of alloy, it melts at 180°C. It is useful for making electrical connections (soldering purposes) because its melting point is lower than the individual melting points of both the component metals.
- Type metal (an alloy of lead-75%, tin-10%, and antimony-15%) has a low melting point and can easily be cast. It is used for printing.
- Sodium amalgam is much less reactive than sodium. It is a liquid alloy at room temperature.
The purpose of an alloy is to improve the specific usefulness of the primary component and not to adulterate or degrade it.
Reasons for alloying
Depending upon the purpose for which the particular alloy is used:
1. To modify appearance and colour.
e.g. Aluminium bronze (Al + Cu) resembles gold as it is bright yellow.
2. To modify chemical activity.
e.g. Sodium amalgam (Na + Hg) is less reactive than sodium.
3. To modify casting ability.
e.g. Type metal (Pb + Sn + Sb) expands on solidification and is easily cast.
4. To lower the melting point.
e.g. Solder (Pb + Sn) melts at 180°C which is lower than the melting point of lead or tin.
5. To increase hardness and tensile strength.
e.g. Brass (Cu + Zn) is harder than copper. Duralumin (Al + Cu) has a strength up to six times greater than pure aluminium.
6. To increase resistance to electricity.
e.g. Nichrome (Ni + Fe + Cr) has more resistance (can produce much heat) to electricity than copper.
Method of making alloys
1. By fusing the metals together. For example, brass is made by melting zinc and copper.
2. By compressing finely divided metals.
For example, wood metal: an alloy of lead, tin, bismuth and cadmium powder is a fusible alloy. This alloy is used in automatic sprinkler which provides a spray of water to prevent fires from spreading.
Alloys as solid solutions
Alloys can be considered solid solutions in which the metal with a high concentration is solvent and other metals are solute. For example, brass is a solid of zinc (solute) in copper (solvent).
All Important Definitions Of Metallurgy
1. Metals: Mostly solids (Hg is liquid), brilliant lustre, high density, a good conductor of heat and electricity, malleable, ductile, tenacious.
They usually form basic oxides, are non-volatile and electrovalent chloride, have electropositive nature, react with date acids to liberate hydrogen (except Cu, Ag and Au), form positively charged ions which get deposited on the cathode on electrolysis of fused salts or their aqueous solutions. Some oxides like Al,0, and PbO are amphoteric
2. Non-Metals: They usually form acidic oxides, are volatile and covalent chlorides, do not react with dilute acids to liberate hydrogen gas, usually form negative ions which get discharged at the anode, and have electronegative nature. Some oxides like CO. NO and H2O are neutral.
3. Metal Activity series: K> Na > Ca> Mg> Al > Za > Fe > Pb > H Cu> g> Ag>Pt> Au.
- Metals below hydrogen, cannot displace hydrogen from acids.
- Metals, which are higher in the series, can displace metals below them.
- The higher the position, the more active is the metal.
- Hydrogen has an electropositive character and so placed among the metals
4. Minerals: Naturally occurring materials from which, metals can be extracted.
5. Ores: Minerals from which, the metals may be extracted economically in reasonably pure conditions.
6. Flux: A substance, added to the furnace during smelting to remove the gangue.
7. Gangue: Rocky and earthly impurities present in ores: Gangue may be acidic (like Si0.) or basic (like Fes & Slag: A substance formed when flux reacts with gangue
Sio + CaCO, → Casio, + CO₂
gangue flux slag
9. Roasting: Heating the concentrated ore in the presence of air to remove volatile impurities like S. As, moisture, etc. and to render the ore porous.
10. Smelting: The process of reducing roasted ore and removing the gangue.
11. Methods of Concentration of Ores
- Froth floatation: Finely ground sulphide ores water + pine oil + Airblast. Ore particles come up with the froth and float on the surface. Rocky matter settles to the bottom eg zinc blende is concentrated by froth floatation.
- Electromagnetic Separation: Magnetic ores are separated by this process like iron ores.
- Gravity Separation : Heavier ores are separated.
12. Roasting: Concentrated ore heated in the air without melting to drive off S. A, P. C and moisture. It makes the ore porous and even converts the sulphide ores to respective oxides.
2ZnS + 3O2 ⟶ 2Zn0 + 2SO2
Calcination: Concentrated ore heated in either absence of air or limited supply of air. It also makes the ore porous, removes volatile impurities and converts the carb--nate ore into respective oxides.
ZnCo3 ⟶ ZnO + CO2
13. Reduction of metallic oxides: Less reactive metals like copper, zinc, lead and iron can be reduced by heating the metallic oxide by carbon, carbon monoxide or hydrogen. Reactive metals like K. Na, Ca, Mg and Al have been reduced by electrolysis
14. Refining of metals
- Distillation: Volatile metals like Zn, Cd and Hg are refined by distillation
- Liquation: Easily fusible metals like lead and tin may be refined by liquation
- Oxidation: Easily oxidisable impurities metals. like Cu and Sn may be removed by passing air blast through molten impure metals.
- Electrolytic refining: Many metals like C_{u} Al, Zn, Ag and Au may be refined using the impure metals as anodes, pure metal strip as cathode and metal salt solution as electrolyte.
16. Alloy: Alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements For example Duralumin (Al+Cu) Stainless steel (Fe+Cr+Ni+C), Brass (Cu+Zn), Bronze (Cu+Su+Zn), Solder or fuse metal (Pb + Sa).
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