Why Are Aluminum Alloys Stronger than Pure Aluminum?

Aluminum is one of the most widely used metals in modern society. The density of aluminum (2.7 g/cm3) is only about 1/3 of the density of steel, and it is not easy to corrode. In most cases, we do not use pure aluminum, but aluminum alloys composed of other metals mixed together in a certain proportion. For example, if aluminum is mixed with copper, it becomes an aluminum-copper alloy; if it is mixed with magnesium, it becomes an aluminum-magnesium alloy. The “mixing together” mentioned here is not beating the two metals into one piece like blacksmithing, or crushing them all and mixing them together, but melting them at high temperature to form an alloy that is uniformly mixed at the atomic scale.

The most significant feature of the alloy is that it has some physical properties that the original pure metals did not have. Take aluminum alloy as an example, its hardness is much greater than that of pure aluminum. We know that pure aluminum is very soft and very easy to bend or deform. This is of course a great advantage in some cases, but it is not a good idea if you want to use it to build aircraft and ships; But aluminum alloys perfectly solve this problem. In 1906, German metallurgist Alfred Wilm first added a little copper and a small amount of magnesium to aluminum to make a hard aluminum alloy, which was applied to aircraft manufacturing during World War I.

The main reason why aluminum alloy is harder than pure aluminum is due to its modified structure. The structure of alloys is much more complex than that of pure metals. Because the atomic arrangement in alloys and simple substances is very different, the formation of alloys often improves the properties of simple elements. For example, the strength of steel is greater than that of its main constituent element, iron. The main elements added to aluminum alloys are magnesium, manganese, copper, zinc, and silicon. After they are heat treated with aluminum at a certain temperature, they are quickly cooled. After a period of time, the supersaturated solid solution produced will gradually precipitate metal compounds. At this time The strength of the resulting aluminum alloy will be significantly improved (this phenomenon is called “age hardening”). High-strength aluminum alloys are now widely used in various fields of industry. Alloys are substances with metallic properties that are synthesized by two or more metal elements (or metal and non-metal elements) by a certain method. According to the number of constituent elements, it can be divided into binary alloys, ternary alloys and multi-element alloys. The constituent elements of most alloys are metals, such as brass is an alloy of copper and zinc, bronze is an alloy of tin and copper; but there are also non-metallic elements in some alloys, for example, steel is composed of iron and carbon composed of alloys.

Aluminum alloys made cookware

According to the interaction between the constituent elements in the alloy, the alloy can generally be divided into three structural types: metal solid solution formed by mutual dissolution; metal compound formed by chemical interaction with each other; mechanical mixture without chemical interaction.

Atoms of one element (metal or nonmetal) dissolve into the crystal of another metal element to form a uniform solid solution, and the resulting alloy is called “metal solid solution”. The metal solid solution is a uniform liquid phase in the liquid state, and after it turns into a solid state, it still maintains the uniformity of the organizational structure and can maintain the original lattice type of the solvent element. Since the diameters of the solute atoms and the solvent atoms cannot be exactly the same, when the solvent atoms dissolve into the solute atoms, the original lattice points will be distorted, and they can hinder the deformation of the material caused by external forces, thus increasing the strength of the solid solution , while its ductility and conductivity will decrease. This phenomenon of increasing the deformation resistance, strength and hardness of metal materials by dissolving solute elements to form solid solutions is called “solid solution strengthening”, which is one of the important ways to strengthen metal materials.

When the number of solute atoms added to the alloy exceeds the solubility of the solvent metal, in addition to the formation of a solid solution, a new phase will also appear, the most common being the formation of a metal compound. Metal compounds generally have complex crystal structures, high melting points, high hardness, and high brittleness. When metal compounds appear in the alloy, the strength, hardness and wear resistance of the alloy increase, while the plasticity and toughness decrease.

To sum up, the performance of the alloy is determined by its structure, and different types of alloys have different performance characteristics.