The Significance Of Alloy Compositions.
When one picks up a handbook on metals and finds a list of over 4,000 different compositions of standard and proprietary non-ferrous alloys alone, one wonders whether there has not been a lot of unnecessary effort on the part of metallurgists, engineers and craftsmen to produce the maximum possible number of different alloys. The fact remains, however, that by suitable combinations of metals with each other or with non-metallic elements, it is possible to produce materials with a very wide variety of physical and mechanical properties.
The common metals—iron, copper, zinc, lead, tin, aluminium, antimony, magnesium, nickel, manganese, etc., in the pure state provide us with a very limited range of properties, and it is a fact that, apart from the pure copper used for electrical purposes, relatively little metal finds ultimate industrial use in an unalloyed form.
All pure metals in the solid state are essentially crystalline and exhibit to some degree the characteristic properties of elasticity and plastic deformation under varying conditions of stress. Most of the common metals crystallise in one of three simple crystal lattice types, face centred cubic (F.C.C.), body centred cubic (B.C.C.), and close packed hexagonal (C.P.Hex.) These simple crystalline forms allow deformation of the crystals to occur under severe stress, by block-like movement of adjacent sections of the crystals. This movement or slip within the crystals takes place along certain planes of atoms in the lattice. When it occurs it causes a disarrangement of the atoms along the slip plane which increases the resistance to further slip and eventually causes movement in that plane to stop, and, on further increase of stress, slip will occur along another plane until all the potential planes have slipped to their limit. During this process the metal will be plastically deformed and at its completion the resistance of the metal to further deformation will be considerably greater per unit of cross-section than that of the original metal, and we say the metal is work hardened. Such deformation leaves the atoms of the metal in a state of strain, but in many cases this state can be preserved indefinitely at ordinary temperatures. The hardness and strength of many metals can thus be increased by cold working processes, such as cold drawing, rolling, spinning, etc.
Generally slip occurs in the most densely populated atomic planes of a crystal and in the direction of the most closely packed line within that plane. In the F.C.C. lattice slip occurs along four such families of planes. In the B.C.C. there are three different types of planes along which slip occurs, although two of these are not planes of highest atomic population. In the hexagonal lattice, slip occurs along one family of planes only—those parallel to the base of the hexagonal prism.
The degree of plastic deformation that pure metals will undergo without fracture bears a close relation to their crystal structure, e.g., the metals aluminium, copper, lead, nickel, silver, gold, platinum, rhodium, palladium, iridium which crystallise with a F.C.C. lattice will be recognised as being among the most ductile; lithium, iron, chromium, vanadium, molybdenum, tungsten, tantalum have B.C.C. lattices and are, on the whole, harder and less ductile, while beryllium, magnesium, antimony, bismuth, zinc, titanium and cobalt, which are all relatively hard and brittle, have hexagonal close-packed or rhombohedral hexagonal lattices.
A F.C.C. lattice, therefore, appears to be a very desirable feature in a metal in which the property of ductility is important, as in the cold-forming operations of drawing, forming, rolling, spinning, and we find the pure metals with a F.C.C. lattice—aluminium, copper, lead, silver, gold, platinum—perform very satisfactorily. Their range of other properties such as hardness, strength, and cost is, however, somewhat limited. Alloys have, therefore, been developed which are similarly ductile, but which are cheaper or more suitable in other ways than the pure metals. This can be illustrated by reference to the addition of tin or zinc to copper. When tin or zinc is added to copper, the resulting alloy crystallises with the F.C.C. lattice of copper, the added elements entering at random into the copper lattice forming a solid solution. In entering the lattice the solute atoms cause some distortion of the lattice which offers resistance to slip along the slip planes, and the alloys are therefore harder and stronger. About 10%
of tin or 35% of zinc can be added to copper to form single-phase solid-solution types of alloys which have excellent ductility and give a wide range of mechanical properties.
The effect of increased distortion of the copper lattice on tensile strength is evident from the following table which gives the minimum tensile properties required by the appropriate A.S.T.M. specifications for a range of alloys of copper with zinc and tin. The second column shows the increase in strength that can be achieved with the same alloys by the work-hardening process referred to above.
|Metal||Minimum Ultimate Tensile Stress, tons/sq. in|
|95 copper, 5 zinc||16.5||27.2|
When a mixture of metals crystallises from the liquid state, solid solutions are not always formed. In some cases the constituents are entirely insoluble in the solid state, in which case the properties of the resulting alloy is the sum of the properties of the constituents as in any other mechanical mixture. Sometimes these mixtures may be relatively simple, e.g., in the copper-lead alloys, and sometimes they are complicated by eutectic formation, e.g., lead-antimony alloys. In other cases definite intermetallic compounds may be formed. In combinations of metals it is possible to get all manner of combinations of these structural types each of which has characteristic properties which it confers on alloys. Solid solutions, as we have seen, are relatively soft and ductile; eutectics have low melting points and are inclined to be brittle—their low melting points make them suitable for soldering and brazing alloys; intermetallic compounds are usually very much harder than their constituent elements and are generally brittle. They lessen the shock resistance, increase compression strength and improve machinability.
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Machinability is a property of considerable industrial importance and is governed largely by composition. In many articles the cost of machining far exceeds the cost of the metal, and even if a greater quantity of metal has to be used for a particular part owing to its inferior strength, the increase in cost of the extra metal may be far outweighed by the saving in machining if the weaker metal has better machining characteristics. The term “machinability” covers a number of factors including life of the cutting tool, power required, speed at which cutting can take place, and the finish of the machined surface. From the point of view of speed of cutting and surface finish the ductile metals machine poorly. Under the action of the cutting edge the metal tends to flow plastically and to build up on the tool rather than to chip away cleanly— the metal is torn off rather than sheared off, and the surface finish is rough and distorted, and power consumption is high. It may seem anomalous that harder metals should machine easier than softer, but a little consideration of the action at the cutting edge will help to explain this. Machinability can be greatly improved by the introduction of a second phase of contrasting properties into the metal structure. In the case of non-ferrous alloys, this is frequently accomplished by the addition of lead. While lead is soluble in copper and copper alloys in the liquid state, it is almost completely insoluble in the solid. Small amounts of lead added to copper alloys form a finely dispersed second phase throughout the metal. The effect of this distributed lead is to cause the chips formed in machining to break away cleanly from the metal surface and to leave the tool clear. Lead also seems to have a lubricating effect. As little as 0.5% in 88/10/2 gunmetal will render the alloy free machining. Two per cent. added to 60/40 brass increases machinability from 30 to 80 and allows a surface speed of 400 feet per minute instead of 150.
In the case of aluminium alloys lead is sometimes used, but as most of the aluminium alloys have a multi-phase structure which includes a proportion of intermetallic compounds, their machinability is reasonably good. The machinability of steels increases as the carbon content rises, it being rather difficult
to obtain a good finish on steel with less than 0.1% carbon. This again is due to the presence in the higher carbon alloys of two phases—ferrite and pearlite—with very different physical properties. In the case of low-carbon free-cutting steel the improvement is effected by the introduction of high manganese and sulphur into the steel. This gives rise to a dispersion of manganese sulphide throughout the body of the metal which acts similarly to lead in brass in embrittling the machined chips. Lead is also sometimes used in steel.
Another property of great industrial importance is the capacity of alloys to harden by heat treatment. The hardening of steels by heat-treatment is well known, but the process has been shown capable of development in quite a range of non-ferrous alloys. The practical details of the hardening and softening processes may appear to vary considerably, but fundamentally all forms of hardening by heat-treatment depend on the capacity of the crystal lattices of the metals concerned to take in proportions of atoms of other elements at one temperature and to reject them at another, i.e., they depend on variations in solid solubilities at different temperatures.
In a steel that has been cooled slowly from a high temperature, there are two principal constituents—ferrite, which consists of almost pure iron, and cementite, an iron carbide Fe3C, which may either be intimately associated with a proportion of ferrite in the eutectoid grains called pearlite, or may be present as free cementite, depending on the carbon content. This cementite is a very hard, brittle substance and its presence in annealed steels is responsible for the considerable increase in hardness which is observed with increasing carbon content and the improved machinability already referred to. It is present in sufficiently large particles to be readily seen under the microscope. When the steel is heated above its critical temperature, the entire crystalline nature of the metal is changed. The iron no longer crystallises with the B.C.C. lattice of ferrite, but changes to the F.C.C. form which has the power to take into solid solution up to 1.7% of carbon. The cementite, therefore, entirely disappears from the microstructure and a single phase alloy is formed called austenite. If this austenite is now quenched, it immediately transforms back to the stable low-temperature B.C.C. form in which the cementite is insoluble, the latter is therefore thrown out of solution in the form of sub-microscopic crystals which are so small and so entangled in the lattices of the reformed ferrite that they offer enormous resistance to movement in the slip planes of the crystals and the metal is hardened.
Among the non-ferrous alloys that have been developed which are capable of being hardened by heat-treatment may be mentioned the aluminium alloys with copper, silicon, and magnesium; the magnesium alloys with aluminium and zinc; and the copper alloys with beryllium, silicon, and nickel. A study of these alloys shows that they have the common feature of possessing a phase of the intermetallic compound type which crystallises out from the main body of the metal under equilibrium conditions at ordinary temperatures, but which can. be dissolved in the solid state at a high temperature and subsequently reprecipilated in a form sufficiently finely dispersed to cause interference with the normal movements of the main crystals by plastic deformation. In some cases this precipitation takes place instantaneously on quenching, as in plain carbon steels; in others gradually, as in the age hardening of aluminium alloys; in others, such as beryllium-copper, on a further low-temperature heat-treatment; while in others the precipitation may occur under the action of cold working, which accounts for the apparently phenomenal work-hardening that occurs in austenitic stainless steels.
Corrosion resistance is another property which is affected by structural considerations. The danger of forming corrosion cells is less in the case of pure metals and single solid-solution alloys than in multi-phase alloys, particularly if the electropotentials of the phases are very different. This explains the general superiority under corrosive conditions of the pure metals, of alpha brass as compared with the alpha-beta alloy, of wrought iron as compared with mild steel, and the poor resistance of the aluminium-copper alloys, in which a copper-rich phase separates out from a matrix of relatively pure aluminium.
I have endeavoured to sketch briefly some of the simpler relations between composition and properties in metal alloys to give some indication of the more modern approach to the problem of finding the most suitable metal for any particular application. Much research is being done in the development of new
alloys, but it is no longer entirely by the methods of trial and error. While there are still many gaps to be filled in our knowledge of the metal state, much data have been accumulated on the atomic and chemical properties of the elements, from which it is possible to predict to a considerable degree their behaviour on alloy formation. Thus we know that for the formation of appreciable solid solutions, it is essential that the atomic diameter of the solute should be within 15% of that of the solvent, and that the two should not differ appreciably in electrovalency. Precipitation hardening is liable to occur where the atomic diameter is outside the favourable zone and only restricted solid solution can occur. Intermetallic compounds are likely to be formed between elements differing widely in electrovalency. Armed with information of this type, a metallurgist is in an infinitely better position to formulate alloys with a combination of properties he desires than was his predecessor.