Now please proceed to the first reading assignment (shown below) from your e-book. After you have completed that reading please return to this page and continue the web reading.
Read pp 99-120 (Ch. 5) in Introduction to Materials ebook
Now you should be able to distinguish between single-crystal and polycrystalline materials. If you cannot draw unit cells for face-centered cubic, body-centered cubic, and hexagonal close-packed crystal structures, you should review those.
Metals routinely form crystals. However, sometimes metal is formed from many grains rather than a single crystal. A grain is a region of single crystallinity and material with many grains is a material with many crystals (grains) that are misaligned to each other. This would be termed a polycrystalline material.
Many materials, e.g. iron, titanium, and carbon, possess two or more distinct crystal structures, which is referred to as allotropy or polymorphism. We have discussed metals as though they form perfect crystals, but it turns out that in real life a perfect crystal is not possible.
In the next section, we will introduce crystal imperfections, which in many cases lead to desirable materials properties.