Became on 95 % consist of gland. Therefore, a good start for understanding them is to study the nature of solid iron.. A small piece of iron is made up of millions of small crystals – grains. Typical grain size from 30 to 50 micrometers. The boundaries between crystals are called grain boundaries.. The grains, together with the boundaries between them, make up iron microstructure.
Let's take a bar of pure iron, eg, diameter 25 mm. Cut a disk out of it in the shape of a large coin. We will polish the surface of this disc, starting with the coarsest polish, and gradually we will reach the thinnest, until the plane of the sample looks like a mirror. Then we immerse this mirror disk approximately 20-30 seconds into the mix with 2-5 % nitric acid with methyl alcohol. This process is called etching., after which the mirror surface of our sample becomes a dull gray. If you look at this surface in an optical microscope at 100x magnification, then we find the picture, shown on the right in the figure 1 – That's what it is iron microstructure.
Selected areas microstructures , which are numbered from 1 to 5, called iron grains, and the boundaries between them (such as between the grains 4 and 5 and pointed to by the arrow) called grain boundaries. The average grain size is very small. At 100x magnification, this photograph is 200 μm shown by a double-edged arrow.
The average grain diameter in this sample is 125 μm (recall, what 1 μm = 1 micrometer = 0,001 mm). Micrometer (it is sometimes called the old-fashioned micron) rather small value. The thickness of aluminum foil and a human hair is approximately 50 μm. Although the grain size of our sample appears to be quite small, he is much more, than most industrial iron bars.
Iron crystal lattice
The main building blocks of solids, such as salt or ice, are molecules. Each molecule consists of two or more atoms, eg, sodium + chlorine (NaCl), like table salt and hydrogen + oxygen, like ice (H2O). In metals, However, these building blocks are individual metal atoms: iron atoms (Fe) in iron bar or copper (With) in copper wire. Every grain in the picture 1 is that, what is called a crystal. In crystal, which consists of atoms, all atoms are uniformly arranged in layers. As it shown on the picture 2, if you draw lines, which connect the centers of atoms, then three-dimensional rows of small cubes will fill the entire space, occupied by a single grain. This three-dimensional structure is called crystal lattice atoms.
Ferrite crystal lattice
In iron at room temperature, these cubes have atoms in each of the eight corners and one atom right in the center of the cube.. This crystal lattice called body-centered, and the geometric arrangement of atoms is called a body-centered lattice. Body-centered iron crystal lattice called ferrite. Another name for ferrite is alpha-iron or alpha-iron.
Iron microstructure: how grain boundaries are formed
The nature of the grain boundary is shown at the bottom of the figure. 2. This boundary is a surface, generally not flat, along which two grains with different orientations intersect. Plane A grain 4 is at a much steeper angle, than plane A of grain 3. If you turn the grain 4 clockwise to coincide with plane A of grain 3, then the grain boundary will disappear and the two grains will become one large grain.
Why microstructure is so clearly visible with a microscope?
To identify microstructures iron (or steel) pickled in acid, its atoms are chemically removed from the surface. The rate of removal of iron atoms during etching of the sample depends on the orientation of the crystal, with which it acts with acid. As each grain represents a different orientation, each grain is harvested at a different speed. for instance, plane, which form the faces of the body-centered cubes, poison much slower, than other crystal planes. Therefore, after a certain period of etching, small steps are formed along the grain boundaries, as shown in the picture 3. for instance, along the border of a rapidly etching grain, you can see a step to the surrounding grains. This rung usually diffuses light well and the edges appear as dark lines.. This is how the microstructure becomes visible when viewed through a microscope..
Source: John D. Verhoeven