Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component elements. Alloy constituents are usually measured by mass.

Theory
Alloying a metal is done by combining it with one or more other metals or non-metals that often enhance its properties. For example, steel is stronger than iron, its primary element. The physical properties, such as density, reactivity, Young's modulus, and electrical and thermal conductivity, of an alloy may not differ greatly from those of its elements, but engineering properties such as tensile strength and shear strength may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the atoms in the alloy, because larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element occur. For example, impurities in semi-conducting ferromagnetic alloys lead to different properties. Some alloys are made by melting and mixing two or more metals. Bronze, an alloy of copper and tin, was the first alloy discovered, during the prehistoric period now known as the bronze age; it was harder than pure copper and originally used to make tools and weapons, but was later superseded by metals and alloys with better properties. In later times bronze has been used for ornaments, bells, statues, and bearings. Brass is an alloy made from copper and zinc.

Solidus - Liquidus - Eutectic
Unlike pure metals, most alloys do not have a single melting point, but a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. However, for most alloys there is a particular proportion of constituents (in rare cases two)—the eutectic mixture—which gives the alloy a unique melting point.

Metallurgy: Making Alloys

The majority of alloys are prepared by mixing metals in the molten state; then the mixture is poured into metal or sand moulds and allowed to solidify. Generally the major ingredient is melted first; then the others are added to it and should completely dissolve. For instance, if a plumber makes solder he may melt his lead, add tin, stir, and cast the alloy into stick form. Some pairs of metals do not dissolve in this way. When this is so it is unlikely that a useful alloy will be formed. Thus if the plumber were to add aluminum, instead of tin, to the lead, the two metals would not dissolve - they would behave like oil and water. When cast, the metals would separate into two layers, the heavy lead below and aluminum above.

One difficulty in making alloys is that metals have different melting points. Thus copper melts at 1,083 C, while zinc melts at 419 C and boils at 907 C So, in making brass, if we just put pieces of copper and zinc in a crucible and heated them above 1,083 C, both the metals would certainly melt. But at that high temperature the liquid zinc would also boil away and the vapor would oxidize in the air. The method adopted in this case is to heat first the metal having the higher melting point, namely the copper. When this is molten, the solid zinc is added and is quickly dissolved in the liquid copper before very much zinc has boiled away. Even so, in the making of brass, allowance has to be made for unavoidable zinc loss which amounts to about one part in twenty of the zinc. Consequently, in weighing out the metals previous to alloying, an extra quantity of zinc has to be added.

Sometimes the making of alloys is complicated because the higher melting point metal is in the smaller proportion. For example, one light alloy contains 92 per cent aluminum (melting point 660 C) with 8 per cent copper (melting point 1,083 C). To manufacture this alloy it would be undesirable to melt the few pounds of copper and add nearly twelve times the weight of aluminum. The metal would have to be heated so much to persuade the large bulk of aluminum to dissolve that gases would be absorbed, leading to unsoundness. In this, as in many other cases, the alloying is done in two stages. First an intermediate 'hardener alloy' is made, containing 50 per cent copper and 50 per cent aluminum, which alloy has a melting point considerably lower than that of copper and, in fact, below that of aluminum. Then the aluminum is melted and the correct amount of the hardener alloy added; thus, to make l00lb of the aluminum-copper alloy we should require 84lb. of aluminum to be melted first and 16lb of hardener alloy to be added to it.

In a few cases, the melting point of the alloy can be worked out approximately by arithmetic. For instance, if copper (melting point 1,083 C) is alloyed with nickel (melting point 1,454 C) a fifty-fifty alloy will melt at about halfway between the two temperatures. Even in this case the behavior of the alloy on melting is not simple. A copper-nickel alloy does not melt or freeze at one fixed and definite temperature, but progressively solidifies over a range of temperature. Thus, if a fifty-fifty copper-nickel alloy is liquefied and then gradually cooled, it starts freezing at 1,312 C, and as the temperature falls, more and more of the alloy becomes solid until finally at 1,248 C it has completely solidified. Except in certain special cases this 'freezing range' occurs in all alloys, but it is not found in pure metals, metallic, or chemical compounds, and in some special alloy compositions, referred to below, all of which melt and freeze at one definite temperature.

The alloying of tin and lead furnishes an example of one of these special cases. Lead melts at 327 C and tin at 232 C. If lead is added to molten tin and the alloy is then cooled, the freezing point of the alloy is found to be lower than the freezing points of both lead and tin (see figure 1). For instance, if a molten alloy containing 90 per cent tin and 10 per cent lead is cooled, the mixture reaches a temperature of 217 C before it begins to solidify. Then, as the alloy cools further, it gradually changes from a completely fluid condition, through a stage when it is like gruel, until it becomes as thick as porridge, and finally, at a temperature as low as 183 C, the whole alloy has become completely solid. By referring to figure 1, it can be seen that with 80 per cent tin, the alloy starts solidifying at 203 C, and finishes only when the temperature has fallen to 183 C (note the recurrence of the 183 C).

What happens at the other end of the series, when tin is added to lead? Once again the freezing point is lowered. An alloy with only 20 per cent tin and the remainder lead starts to freeze at 279 C and completes solidification at the now familiar temperature of 183 C. One particular alloy, containing 62 per cent tin and 38 per cent lead, melts and solidifies entirely at 183 C. Obviously this temperature of 183 C and the 62/38 per cent composition are important in the tin-lead alloy system. Similar effects occur in many other alloy systems and the special composition which has the lowest freezing point of the series and which entirely freezes at that temperature has been given a special name. The particular alloy is known as the 'eutectic' alloy and the freezing temperature (183 C in the case of the tin-lead alloys) is called the eutectic temperature.

By a careful choice of constituents, it is possible to make alloys with unusually low melting points. Such a fusible alloy is a complex eutectic of four or five metals, mixed so that the melting point is depressed until the lowest melting point possible from any mixture of the selected metals is obtained. A familiar fusible alloy, known as Wood's metal, has a composition:

Bismuth - 4 parts
Lead - 2 parts
Tin - 1 part
Cadmium - 1 part

and its melting point is about 158F / 70 C; that is, less than the boiling point of water. Practical jokers have frequently amused themselves by casting this fusible alloy into the shape of a teaspoon, which will melt when used to stir a cup of hot tea.  * Warning: Notice the TOXIC Metals?

These low melting point alloys are regularly in use for more serious purposes, as for example, in automatic anti-fire sprinklers installed in the ceilings of buildings. Each jet of the water sprinkler system contains a piece of fusible alloy, so that if a fire occurs and the temperature rises sufficiently high, the alloy melts and the water is released through the jets of the sprinkler.



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