Eventually, the interdendriticspaces between the dendrite arms crystallize to yield a more regular crystal. Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.ĭuring freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other. For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines. The figure to the right shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube. The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching.
In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages. Slow cooling generally results in larger grains which will have lower strength, hardness and ductility. Rapid cooling generally results in more nucleation points and smaller grains (a fine grain structure). The spangles that are seen on newly galvanized metals are grains. Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye.
The atoms between the grains (at the grain boundaries) have no crystalline structure and are said to be disordered. The interface formed between grains is called a grain boundary. A grain is merely a crystal without smooth faces because its growth was impeded by contact with another grain or a boundary surface. In engineering materials, a crystal is usually referred to as a grain. The crystals increase in size by the progressive addition of atoms and grow until they impinge upon adjacent growing crystal.Ī) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope. The final sizes of the individual crystals depend on the number of nucleation points.
At the solidification temperature, atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals. The moment a crystal begins to grow is know as nucleation and the point where it occurs is the nucleation point. Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline (more than one crystal) solid forms. The expense of producing single crystal materials is only justified for special applications, such as turbine engine blades, solar cells, and piezoelectric materials. In engineering materials, single crystals are produced only under carefully controlled conditions. The crystallization of a large amount of material from a single point of nucleation results in a single crystal.
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