The atoms in solid metals are arranged in an ordered and repeating lattice pattern called the crystal structure. A crystalline material consists of a regular array of atoms that is repeated over a long distance compared with the atomic size. A simple analogy is the stacking of oranges in a grocery store, with each orange representing a single atom and each layer of oranges being a lattice plane (Fig. 4.2). The arrangement pattern of the atoms is defined by the unit cell of the crystal. The unit cell is the basic building block, having the smallest repeatable structure of the crystal, and it contains a full description of the lattice structure. A crystalline material is constructed of many unit cells joined face-to-face in a three-dimensional repeating structure.
4.2 Stacking of atoms within a crystalline material has similarities with the stacking of fruit.
Most metals at room temperature are found in one of three crystalline patterns: body centred cubic (bcc), face centred cubic (fcc) or hexagonal close packed (hcp). The unit cell structures of these crystals are shown in Fig. 4.3. For the main types of metals used in aircraft structures and engines, aluminium and nickel have a fcc structure; magnesium is hcp; and titanium is bcc (called α-Ti) or hcp (βα-Ti) depending on its alloy composition and heat treatment.
4.3 Unit cells for body centered cubic, face centered cubic, and hexagonal close packed crystals (a = width of unit cell; b = height of unit cell).
The atoms in crystals are packed close together and have a large number of nearest neighbour atoms (usually 8–12). The distance between the atoms within a unit cell is determined by the type of metal and its crystal structure. For example, the spacing between aluminium atoms is a/b = 0.40 nm, between magnesium atoms is a = 0.32 nm and b = 0.52 nm, and between titanium atoms (hcp) is a = 0.30 nm and b = 0.47 nm, where a is the width of the unit cell and b is the height of the unit cell. The tight packing is one reason why metals have high densities and high elastic stiffness compared with other solid materials such as polymers.
The atoms within the lattice crystal are connected by metallic bonding. Rather than the electrons being bound to the nucleus of a specific atom, the electrons are ‘free’ to move around the positively charged metal ions of the lattice. The electrons, which are called delocalised or conduction electrons, divide their density equally over all the atoms. For this reason, metal crystals are visualised as an array of positive ions in a sea of electrons, as shown in Fig. 4.4. The crystal structure is held together by the strong forces of attraction between the positive nuclei and delocalised electrons. Metallic bonding influences many of the properties of metals, such as their elastic modulus and electrical conductivity.
4.4 Representation of metallic bonding. Single metal atoms have their electrons bound to the nucleus. In a metal, the electrons are delocalised and shared between the atoms.
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