There are three types of interstitial sites: trigonal, tetragonal and octahedral. Three coordinate trigonal intersticial sites appear within a layer, the other two – between layers.

It turns out that face-centered cubic and hexagonal close-packed crystal structures pack atoms equally tightly. Some metals with hexagonal close-packed crystal structures include cobalt, cadmium, zinc, and the α phase of titanium.

The term “closest packed structures” refers to the most tightly packed or space-efficient composition of crystal structures (lattices). To maximize the efficiency of packing and minimize the volume of unfilled space, the spheres must be arranged as close as possible to each other.

Crystal Structure: Closest Packing

• The most efficient conformation atomic spheres can take within a unit cell is known as the closest packing configuration.
• Densely packed atomic spheres exist in two modes: hexagonal closest packing (HCP) and cubic closest packing (CCP).

Cubic Close Packing. Face Centered Cubic Cell. Closest packed means that the atoms are packed together as closely as possible. The FCC unit cell is actually made of four cubic close packed layers (click to show the unit cell with layers). The first layer of atoms pack together as close as possible.

Body-centered cubic (abbreviated cI or bcc) Face-centered cubic (abbreviated cF or fcc, and alternatively called cubic close-packed or ccp)

The face-centered cubic structure has an atom at all 8 corner positions, and at the center of all 6 faces. The body-centered cubic structure has an atom at all 8 corner positions, and another one at the center of the cube….

As a result of the quenching, the face-centered cubic austenite transforms to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. The shear deformations that result produce a large number of dislocations, which is a primary strengthening mechanism of steels.

Ferrite. Ferrite (α), is the crystal arrangement for pure iron. This form exists as part of the structure in most steels and can usefully absorb carbides of iron and other metals by diffusion in the solid state. Ferrite takes a body centred cubic (bcc) form and is soft and ductile.

2 Crystal Structure and the Solubility of Carbon. Now ferrite and delta ferrite have a body-centered cubic (BCC) structure, as shown in Figure 13.13 and shown again for convenience in Figure 14.5. These two forms of iron can largely be regarded as the same phase, albeit separated by a temperature gap.

Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons.

4. What is the crystal structure of ϒ iron? Explanation: Pure iron exists in three allotropic phases of α iron, ϒ iron, and δ iron. α iron and δ iron appear as body-centered cubic, whereas ϒ iron is a face-centered cubic that is stable between 908oC and 1535oC.

A face-centered cubic crystal structure will exhibit more ductility (deform more readily under load before breaking) than a body-centered cubic structure. The bcc lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the bcc form.

Under equilibrium cooling conditions, liquid iron first solidifies with a body centred cubic (bcc) crystal structure at 1538 °C which then transforms to a face centred cubic (fcc) structure at 1394 °C; finally, this fcc solid transforms again into a bcc structure at 912 °C which is stable right up to room temperature …

Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure.

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite.

Steel is graded as a way of classification and is often categorized into four groups—Carbon, Alloy, Stainless, and Tool.

1. Carbon Steels only contain trace amounts of elements besides carbon and iron.
2. Alloy Steels contain alloying elements like nickel, copper, chromium, and/or aluminum.

Austenite has a cubic-close packed crystal structure, also referred to as a face-centred cubic structure with an atom at each corner and in the centre of each face of the unit cell. Ferrite has a body-centred cubic crystal structure and cementite has an orthorhombic unit cell containing four formula units of Fe3C.

Among the austenite stabilizers are nickel, carbon, manganese and nitrogen. The ferrite stabilizers are chromium, silicon, molyb- denum and columbium. It is the balance between the two types of alloying elements that controls the quantity of ferrite in the weld metal.

Austenitization means to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. The more open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel.

The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment.

Normalising is a heat treatment process that is used to make a metal more ductile and tough after it has been subjected to thermal or mechanical hardening processes. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.

Normalisation is mainly used on carbon and low alloyed steels to normalise the structure after forging, hot rolling or casting. The hardness obtained after normalising depends on the steel dimension analysis and the cooling speed used (approximately 100-250 HB).