In this article we will discuss about:- 1. Meaning of Alloys 2. Metals Employed for Making Alloys 3. Precautions 4. Theory 5. Modes of Formation 6. Classification 7. Types 8. Alloys of Lead 9. Properties 10. Treatment.
Meaning of Alloys:
Alloy is a mixture with metallic properties and is composed of two or more elements, of which at least one is a metal.
In course of development of metallography it has been found that some alloys are definite compounds, some are solid solutions of one metal in another, some are mere mechanical mixtures and others are combinations of these conditions.
Microscopic and X-ray studies of alloys together with spectroscopic analysis have helped metallurgists to carry out systematic study and development of useful alloys.
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A metal can be changed with respect to its physical and chemical properties such as hardness, tensile strength and resistance to corrosion, by simply adding a small quantity of one or more elements into it. This basic fact has given great impetus to the development of useful alloys continuously.
Metals Employed for Making Alloys:
Important metals used for making alloys are:
i. Copper,
ii. Lead,
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iii. Zinc,
iv. Nickel,
v. Aluminium,
vi. Chromium,
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vii. Tungsten.
To some extent, antinomy and bismuth are also added.
Precautions Necessary for Making Alloys:
The following precautions are essential for making alloys:
i. When two elements which are to be mixed have got widely different melting points or when one of the elements is easily oxidised or volatilised, then the minor element is first mixed into the major element in known excess quantity.
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This mixture whose composition is known is then added into the required quantity of the major element. For example, if any alloy has to be formed by adding small quantities of silicon to copper, at first a known excess quantity of silicon is added into copper. This mixture is then mixed with the required quantity of copper.
ii. In preparing alloys, it is essential that oxidation be prevented or minimised as far as possible. This is done either by covering the metals with fine charcoal or by using electric induction furnaces for melting where furnace conditions can be accurately controlled.
iii. The least fusible metal is melted first and the more fusible is added afterwards.
iv. Heaviest metal is added last of all in order to prevent its settling at the bottom.
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v. Sufficient stirring with a graphite, wooden or iron rod is necessary before casting.
vi. For casting of an alloy, suitable moulds are necessary. If moulds are made of metal, surface should be coated with a mould wash for preventing the casting from sticking to the mould surface; otherwise the smoothness of the surface of the cast will be spoiled. Mould wash should be such that it does not give off any gas.
vii. Metal when poured into another metal should not be at very high temperature. Generally it is added at a temperature about 100°C above the melting point of the alloy.
Theory of Alloys:
Theory of alloys can be viewed and studied from two aspects. These two aspects are atomic structure and phase aspects.
Both these aspects are described below:
i. Atomic Structure:
Atoms in an alloy are arranged in a periodic manner in a crystal space. Different alloys have different crystal structures, and in crystal structure of different alloys the distances between atom centres in the lattice are also different. The differences in properties of different alloys can be related to the difference in their crystal structure.
For example, metals and alloys which have body- centred cubic-crystal structure (such as copper and silver) are more ductile but not so strong as the metals and alloys which have face-centred cubic crystal structure (such as iron and tungsten). Alloys having hexagonal and other non-cubic crystal structure have not so good a malleability as metals and alloys having cubic crystal structure.
ii. Phase Aspect of Alloys:
A phase in an alloy or metal is the region having the same crystal structure, composition and interatomic distances. A solid metal or alloy consists of one or more phases. The distribution of phases in an alloy determines the properties of an alloy.
For example, in age-hardened alloys, it is the fine dispersion of the second phase throughout the alloy that causes strengthening of the alloy. However, when the second phase is distributed as coarse particles by annealing, the strength of the alloy is reduced.
Modes of Formation of Alloys:
Following are the modes of formation of alloys:
i. Solubility Alloys:
When different metals are made to alloy, either they dissolve into each other completely or partially or may not dissolve at all and remain insoluble. Good alloys are formed only when the component metals dissolve into each other. For example, copper and zinc are completely soluble in liquid state but partially soluble in solid state.
An alloy containing 10% zinc and 90% copper consists of a single phase, face-centred cubic solid solution. Some of the copper atoms in the crystal structure are replaced or substituted at random by zinc element. This type of solid solutions called substitutional. All metallic solutions are of this type.
Iron and lead are insoluble in both phases, liquid and solid but yet free machining lead steel containing 0.2% lead is made by dispersing lead through the modern steel when ingot is being prepared. Lead remains in suspension while the ingot is freezing quickly. Lead is present as solid particles of lead not as solid solution.
Following are the rules which govern the solubility of the substitutional type alloys:
(a) Complete solubility occurs if the components have the same crystal structure. Example of complete solubility is the alloy of copper and nickel; both the elements have got face centred cubic structures.
(b) The more is the difference between atomic size (radius), the less is the solid solubility. Complete solubility occurs only when the sizes differ up to 15%.
(c) Metals having the same valency show more solid solubility than metals having different valencies. Also metals of lower valency tend to dissolve a metal of high valency to a greater extent.
(d) If the metals are placed farther apart in electrochemical series the tendency to form solid solution is greater.
ii. Compound Formation:
Metals react together to form compounds.
Following kinds of compounds are commonly found:
(a) Valence Compounds:
Normal rules of valency are followed in their formation.
(b) Electron Compounds:
Electron compounds are phases with wide range of homogeneity. Their crystal structure is determined by the number of valence electrons in the alloying elements. So they are called electron compounds.
The electron compounds may have the following three electron atom ratios-3 : 2 (beta brass or beta manganese structure), 21: 13 (gamma brass structure), and 7 : 4 (epsilon brass hexagonal structure). These are the ratios at which electron compounds occur. Examples of beta brass type are CuZn, AuZn, AgZn, FeAl, NiAl, CoAl, Cu3A1, Cu3 Sn. In all these the electron-atom ratio is 3 : 2.
iii. Miscellaneous Compounds:
Among the miscellaneous compounds are included extremely hard metallic carbides, nitrides and borides. In these compounds, small atoms of carbon, nitrogen and boron fit interstitially between the metal atoms in the structure.
Examples are Fe3C (cementite), a very hard carbide used for making edges of razor blades and tools and W2C with 13% cobalt to form heavy duty tungsten carbide are used for cutting tools.
iv. Order-Disorder:
In solid solutions, atoms of different metals are distributed at random on the lattice points. In compounds, atoms of each kind occupy assigned lattice points. There is another class of solid solutions that change to compounds called superlattice compounds.
In these superlattice compounds, atoms go from random places to assigned places. The superlattices are like ionic compounds. Examples are dental alloys (mainly CuAu) which are solid solutions when cooled rapidly but they form superlattice structure when cooled slowly.
Classification of Alloys:
Classification of alloys can be done in the following four ways:
i. Classification based on metallurgical structure:
Alloys are classified according to the fact whether they consist of single phase or of two or more phases. For example, monel metal (2/3 Ni and 1/3 Sn), some brasses (70% Cu, 30% Zn), transformer iron (96% Fe, 4% Si) are single- phase alloys. Annealed steels (phases of ferrite and carbide), Muntz metal (60% Cu and 40% Zn) are two-phase alloys.
ii. Classification Based on Principal Metal in the Alloy:
Alloys are classified according to the principal metal contained in them, for example aluminium alloys, magnesium alloys. These alloys when distinguished by principal metals, have certain distinguishing characteristics too. For example, Al alloys and Mg alloys have low sp. gravity (2.8 for Al alloys and 1.8 for Mg alloys), and as strong as steel.
Copper alloys are characteristic for their corrosion resistance and good pliability. Lead alloys and tin alloys are corrosion-resistant and heat-resistant and are used for bearings and solders. Nickel alloys are notable for their strength.
iii. Classification Based on Method of Fabrication:
Different alloys are used for different types of fabrications. So they are classified according to the type of fabrication for which they are used. For example, there are copper casting alloys and copper wrought alloys. Casting alloys contain 5% each of tin, zinc and lead for getting pressure tightness and easy machinability. Wrought copper alloys contain 5 to 40% of zinc.
iv. Classification Based on the Application of Alloys:
Alloys are made for different purposes and thus these may be classified accordingly. For example, solder alloys containing tin and lead (tin 40 to 60% and lead 60 to 40%), and bearing metal alloys, etc.
Types of Alloys:
A typical shape memory alloy has composition of 55% Ni and 45% Ti. These alloys after being plastically deformed at room temperature into various shapes, return to their original shapes on heating. These have good ductility, corrosion resistance, and electrical conductivity.
These find application in connectors, clamps, fasteners and seals. For space application, they are folded at room temperature to occupy less space and upon reaching their destination they are heated to attain their original shapes.
These alloys do not have a long-range crystalline structure. They have no grain boundaries, and the atoms are randomly and tightly packed. Because their structure resembles that of glasses, these alloys are called metallic glasses.
Such a structure was earlier obtained by extremely rapid solidification of the molten alloy. Amorphous alloys typically contain iron, nickel, and chromium alloyed with carbon, phosphorus, boron, aluminium and silicon. They are available in the form of wire, ribbon, strip, and powder.
These alloys exhibit excellent corrosion resistance, good ductility, high strength, and very low loss from magnetic hysteresis. Thus it is ideally suited to make magnetic steel cores for transformers, generators, motors, lamp ballasts, magnetic amplifiers, etc.
These constitute an important class of materials because of wide ranging mechanical, physical and chemical properties. These have lower density, strength, elastic modulus compared to metals.
Plastics are composed of polymer molecules and various additives. Monomers are linked by polymerisation processes to form larger molecules. Polymer structures can be modified by various means to impart a wide range of properties to plastics.
Elastomers comprise a large family of amorphous polymers having a low glass transition temperature. They have the characteristic capability to undergo a large elastic deformation without rupture. Synthetic rubbers having wide ranging applications have been developed. Silicones have the highest useful temperature range (upto 315°C).
These are compounds of metallic and non-metallic materials. These are characterised by high hardness and compressive strength, high temperature resistance and chemical inertness.
These have superior mechanical properties and yet are light weight. The reinforcing fibres are usually glass, graphite, boron, etc. Epoxies and polyester commonly serve as a matrix material. Reinforced plastics are being developed rapidly.
New developments concern metal-matrix and ceramic-matrix composites and honey comb structures. (Honey comb structure consists of a core of honey comb or other corrugated shapes bonded to two thin outer skins. Ceramic-matrix cutting tools are being developed, made of silicon carbide-reinforced alumina, with greatly improved tool life.
A composite material, as stated above, contains more than one component. The compound materials are incorporated into the composite to take advantage of their attributes, thus obtaining improved material. They become cohesive structures made by physically combining two or more compatible materials.
Fibre reinforced composites are heterogeneous materials prepared by associating and bonding in a single structure of materials possessing different properties. Due to complementary nature, the composite material possesses additional and superior properties. These thus become ideal materials for structural applications requiring high strength-to-weight and stiffness-to-weight ratios. Fibre reinforced materials exhibit anisotropic properties.
Glass fibres are strong but if notched they fracture readily. By encapsulating them in a polyester resin matrix, they can be protected from damage. Fibres of graphite and boron are also used in composites. Commonly used fibres for composite materials are-glass, silica and boron for amorphous structure, ceramic and metallic for single crystals as well as polycrystals, Carbon and boron (amorphous) materials for multiphase structure and organic material for macromolecular structure.
For two-dimensional structural applications such as in plates, walls, shells, cylinders, pipes etc. a planar reinforcement is much more advantageous as compared to the linear reinforcement.
6. Duplex Composite Components:
Components subjected to severe wear and high contact stresses can be made of duplex composite, the composite layer being located on outer or inner surface depending on the requirement. Aluminium composite alloys reinforced by ceramic have been developed and these have relative high strength to weight ratio, high modulus of elasticity and good wear characteristics.
Silicon carbide particles are incorporated into the surface of aluminium alloy heated to its mushy state and pressure is applied to get a good wetting between the aluminium alloy and the silicon carbide particles.
Experiments can be carried out to determine the semisolid forming conditions. Specimen surrounded by SiC particles is heated upto this temperature for about 45 minutes in order to homogenise the temperature through the specimen. A hydraulic press is used to apply the necessary low pressure for the semi-solid forming process.
There is an optimum combination of temperature and pressure values to obtain optimum mechanical properties. In this way, a composite layer of about 2.5 mm width can be formed with uniformly distributed particles having good bond with aluminium matrix, with no separation or porosity at the composite layer/matrix interface.
Surface composite layer has hardness and wear resistance about 1.75 and 10 times those of as received aluminium matrix alloy.
Alloys of Lead:
1. Antimonial-Lead:
Its composition varies from 6 to 8% of antimony with balance lead. Antimonial lead is highly resistant to sulphuric acid and many chemical solutions containing sulphuric acid. The hardness and strength of antimonial lead is more than that of lead. It has a high tensile strength of about 470 kg/cm2 and elongation of 22%.
Lead containing 13% antimony, 1% tin, 0.5% arsenic and 1% copper has got good casting properties. The castings obtained are quite strong too.
2. Lead-Tin Alloys:
An alloy containing 10 to 25% tin and 90 to 75% lead is used as metallic coating for sheet iron. Such a coating is applied by hot dipping process. Sheet iron which is coated with such an alloy is used for the manufacture of containers. When harder and more resistant coating is required, antimony is also added. Alloys of lead, tin and antimony are used as type metals.
3. Alloys of lead for Cable Industry:
Cables require a flexible covering and sheathing for protection from moisture and oil.
Bearing lead and 1% antimony lead are used for covering electric power and communication cables due to the following reasons:
(i) These are impervious to moisture and oil,
(ii) These can be readily extruded,
(iii) These have got ample strength,
(iv) These are not corroded in all normal atmospheric environments,
(v) These are pliable and sheathing can be reeled and unreeled and can be bent around sharp corners.
An alloy containing about 0.40% of calcium can also be used for sheathing cables. Addition of calcium increases strength without affecting ductility and pliability of lead. Fatigue strength is also increased.
Another alloy used for this purpose has composition of 0.15% arsenic, 0.1% tin and 0.1% bismuth and the balance lead.
Properties of Alloys:
(i) Thermal and electrical conductivities of a solid solution are less than those of the pure metals. According to Mathiessen’s rule when small quantities of an alloying element are added in solid solution to metal, in increase in resistance does not depend upon the temperature.
For mixtures of insoluble phases, the thermal and electric-resistances follow the law of mixtures.
(ii) Density is increased by heavier metal in solid solution and is decreased by a lighter metal. In case of interstitial solid solution, there is little effect on density by the added element.
(iii) Specific heat and co-efficient of thermal expansion are governed by the law of mixtures.
(iv) Melting Point of a metal is converted into a range by the addition of an alloying element. The melting point range of alloy can be higher or lower than the melting point of metal. The greater is the difference in valences between the metals of alloy, the wider is the melting range.
(v) Boiling Point is also converted into a range by the addition of alloying elements.
Treatment of Alloys:
Alloys have to pass through one or more of the following processes before they are converted into finished products:
i. Melting:
Alloys may be melted and in the molten state, hardeners, de-oxidisers, etc. may be added, or the molten metal may be superheated. Lead, aluminium and magnesium alloys are melted in iron pots. Bronze is melted in graphite crucibles. Nickel and steel are melted in refractory lined furnaces. Magnesium alloys are melted with flux which surrounds the entire melt.
ii. Casting:
Castings are made by gravity is sand or metal moulds. Cast iron and bronze are cast in green sand moulds. Magnesium alloys are cast in dry sand mould as molten magnesium reacts with water. Centrifugal force is employed for casting into spinning steel moulds. Pressure is used to force liquid alloys into die casting moulds.
iii. Sintering:
Sintering is employed for blending metallic powders which have been pressed into shape. Either an alloy powder or a mixture of powders of alloying metals is used. In sintering, diffusion takes place. This diffusion homogenises the mixture of powders and the finished product obtained is uniform in composition. For example, for the formation of Alnico magnets, the pressed mixture of iron, nickel, and aluminium powders is sintered upto the melting point of the alloy to ensure good diffusion.
iv. Hot Working:
For making plates, rods and structural shapes, hot rolling is employed. Hot forging is carried out for complicated shapes. Extrusion is adopted for making rods and structural shapes from aluminium, magnesium and copper.
Lower limiting temperature for hot working is the re-crystallisation temperature or a bit higher than that. The maximum hot working temperature is generally below the solidus. Some alloys between certain ranges of temperature are brittle and cannot be hot worked. For example, monel metal cannot be hot worked in the range 650 to 870°C.
v. Cold Working:
Cold working is carried out below re-crystallisation temperature. By cold working, the metal is strengthened. The other advantage of cold working is the convenience of operation as it is impossible to hot work thin metal for foil rolling and wire drawing.
vi. Surface Treatments:
There are numerous surface treatments to which alloys can be subjected. Carburising in which carbon steel is heated up to 900°C in a carburising material is an example of surface treatment. Nitriding is another example of surface treatment, in which alloy steel (C = 0.30%, Cr = 1.3%, Al = 1.3%, Mo = 0.2%) is subjected to an atmosphere of dissociated ammonia for 48 to 96 hours at a temperature of 550°C. A surface coating of aluminium nitride is formed. This coating raises the hardness to above 1000 Brinell.
Other surface treatments are colouring, chromising, and siliconising in which layers about 1.0 mm thick containing 25% Al, 20%; Cr and 14% Si are produced on the surface of mild and other steels. Colorised steels are used for service at high temperatures to 815°C in an atmosphere containing sulphur. Chromised and siliconised steels are used where heat and corrosion resistance are required.
vii. Joining:
Finished materials like machines, pressure vessels, and big structures, etc., can be fabricated by the process of fusion welding. In fusion welding alloys are welded together by bringing them to molten form with or without the addition of filler metal.
In brazing, the filler metal is non-ferrous metal or alloy whose melting point is more than 540°C but less than that of the metals or alloys to be joined.
viii. Heat Treatment:
Heat treatments of following types are commonly used:
(a) Annealing:
When an alloy is cold worked there is increase in tensile strength, hardness, diffraction line width, electric resistivity, coefficient of expansion, and electrode potential, but there is a decrease in ductility, cold workability, density and resistance to notches. When temperature of alloy is raised, all the properties tend to return to normal values. This heating of the alloy for softening is called annealing.
(b) Precipitation Hardening:
It is a heat treatment that increases the strength of an alloy by virtue of the appearance and growth of a second phase in a supersaturated solid solution.
When 4% copper alloy is slowly cooled from 254°C, crystals of theta phase appear when the temperature falls below the saturation curve. These theta crystals increase in size and the copper content of the solid solution decreases during continued slow cooling to room temperature. This appearance of crystals of theta phase is called precipitation hardening.
(c) Martensite Hardening:
It is carried out by the suppression of a eutectoid phase transformation by rapid cooling or quenching. In this way there is a formation of hard unstable phase.
For example, heat treatment of steel consists in the formation of martensite which is a hard unstable phase. Martensite is formed by quenching austenite (solid solution of carbon in face-centred cubic iron) from the temperature above the iron-iron carbide eutectoid at 720°C.