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Alloys of Aluminium

Aluminium readily alloys with many other metals, and a large number of intermetallic compounds have been described. In general these compounds have little tendency to form continuous series of solid solutions with one another or with aluminium, and, accordingly, almost the only alloys of aluminium of engineering value are those which consist very largely of aluminium and those which contain but a few per cent, of the element.

Sodium and aluminium are not mutually soluble, beyond perhaps a very slight extent, and form no chemical compound.

Copper and aluminium alloy

A number of investigations of the system copper- aluminium have been made by the thermal method, but the results are somewhat contradictory. The compounds Al2Cu, AlCu, and AlCu3 have been stated by several observers to exist. The "liquidus" curve has six or seven branches, only one of which appears to correspond to the separation of a pure compound, viz. Al2Cu. The other branches refer to the separation of solid solutions. By a study of the electrical properties of these alloys, Broniewski has confirmed the existence of the compounds already mentioned, and in addition has indicated the existence of another compound, Al2Cu3.

Aluminium-copper alloys containing upwards of 10 per cent, of aluminium have a fine golden-yellow colour, do not tarnish, are practically non-corrodible by sea-water, and are largely used under the name of aluminium bronzes. The tensile strength reaches the value 39 tons per sq. inch for the alloy with 10 per cent, of aluminium; the ductility is at a maximum with 7 per cent, of aluminium. The light copper-aluminium alloys containing upwards of 4 per cent, of copper are also of great value; by rolling and drawing, the tensile strength of the 4 per cent, alloy can be raised to 20 tons per sq. inch (Carpenter and Edwards). A few per cent, of nickel are sometimes added to these alloys rich in aluminium, to obtain good rolling alloys.

Silver and aluminium alloy

Silver and aluminium alloy containing 4 per cent, of silver has been used in the construction of balance beams; the alloy with 33 per cent, has been employed for making tablespoons, etc. Two intermetallic compounds are known, AlAg3 and Al2Ag3. They form a continuous series of solid solutions with one another; the former gives an incomplete series of solid solutions with silver, and the latter an incomplete series with aluminium.

Gold and aluminium alloy

Five intermetallic compounds are known, Au4Al, Au8Al3 or Au5Al2, Au2Al, AuAl, and AuAl2. The compounds Au2Al and AuAl2 are stable at their melting-points, and the latter has a beautiful purple colour.

Magnesium and aluminium alloy

It is difficult to study this system by the thermal method. The "liquidus" appears to consist of three branches, the middle branch corresponding to the solid phase Al2Mg3 or Al3Mg4. This middle branch, however, is extremely flat. According to Broniewski, there are two intermetallic compounds, AlMg and Al2Mg3, which form with one another a continuous series of solid solutions. The former gives an incomplete series of solid solutions with aluminium, and the latter an incomplete series with magnesium.

Aluminium alloys containing 1-2 per cent, of magnesium and a little copper, nickel, and tin are largely used for construction purposes under the name of magnalium; the alloy duralumin contains 0.5 per cent, of magnesium, a few per cent, of copper, iron, and manganese, and over 90 per cent, of aluminium.

Zinc and aluminium alloy

It has been generally supposed that these metals form two series of solid solutions, the " liquidus " exhibiting a eutectic point, but Rosenhain and Archbutt have shown that there are three branches to the " liquidus," a compound Al2Zn3 separating along the middle branch, which covers the region from 5-17 per cent, of aluminium.

The alloys known as ziskon and zisium are zinc-aluminium alloys, and are used for parts of scientific instruments. The alloys containing upwards of 20 per cent, of zinc are valuable casting alloys, largely used in the motor industry. In actual practice, the amount of zinc seldom exceeds 10 per cent., and 2 per cent, of copper is added.

Other aluminium alloys

The zinc-copper-aluminium alloys containing 31-27 per cent, of zinc, 68-70 per cent, of copper, and 1-3 per cent, of aluminium, form strong alloys, known commercially as aluminium brasses.

Cadmium and aluminium alloys, when fused and mixed, form two conjugate phases, one of aluminium containing 2 or 3 per cent, of cadmium, the other of cadmium containing less than 1 per cent, of aluminium.

Mercury alloys forms, with sufficient aluminium, a brittle solid amalgam which readily oxidises and decomposes water, forming mercury, aluminium hydroxide, and hydrogen. Aluminium is superficially amalgamated when it is immersed in mercuric chloride solution, and when so treated it rapidly decomposes water. The reaction is inhibited to a considerable extent by the presence of a little copper as impurity in the metal.

Calcium and aluminium alloys have been studied, and the compound CaAl3 isolated.

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