Materials Science notes - Some simple terms, definitions...

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Some simple terms, definitions and examples about crystal structures: 21.10.2009 Alloy: A metallic substance that is composed of two or more elements. Most of the industrial alloys consist of solid solution that can be interstitial or substantial solid solutions. However, alloys can also contain compounds as in steel. Steel is an Fe-C alloy and can contain up to 2% C in it. Some of the carbon atoms forms an interstitial solid solution with iron atoms. Some of them forms a compound called as Fe3C (cementite). So, what is the difference between a solid solution or a compound? In a solid solution (substantial or interstitial), there is no single ratio for the elements. They can form a solid solution in various amounts. On the other hand, in a compound, elements have a certain stoichiometric ratio. For example: Ferrite is a solid solution that is formed by incorporating of C atoms in bcc Fe unit cell. In ferrite C atoms can be present up to 0.025 % at 723 ° C. Austenite, another solid solution, that is formed by incorporating of C atoms in fcc Fe unit cell. In austenite, C atoms can be present up to 2% at 1148 ° C. However, cementite is a compound that is formed between C and Fe atoms. But there is a certain stoichiometric ratio between the atoms. The weight ratio of C in cementite 6.67% Since there is no certain ratio for the elements involved in a solid solution. We generally express the amount of an element in a solid solution as weight ratio (wt %) or atomic ratio (at.%). What is the difference between them? Weight ratio uses the weight of the atoms in a sold solution or in more general term in an alloy. For example, In the weight percentage concept, assume that you have 1000 kg of alloy. This alloy consists of 5% element A and 95 % element B. It means that you have 0.05 x 1000 = 50 kg element A in your alloy, and 950 kg element B. In the atomic weight concept, the number of atoms is considered not their weights. For example, assume that you have an alloy. That alloy contains A and B atoms. Atomic ratio (or atomic percentage) of A atoms is 10% and Atomic ratio of B atoms is 90%. It means that 10% of all atoms present in your alloy are A atoms and 90% of all atoms are B atoms. Weight ratio (or weight percentage) is designated w/o
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This note was uploaded on 01/10/2011 for the course CHEM 100 taught by Professor Kara during the Spring '10 term at Baptist University of the Americas.

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Materials Science notes - Some simple terms, definitions...

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