There are two types of intermetallic compounds which are often encountered in

There are two types of intermetallic compounds which

This preview shows page 64 - 67 out of 79 pages.

There are two types of intermetallic compounds which are often encountered in metallurgy. a) Electron compounds These compounds are of definite chemical crystal structure and arise if the two alloying metals are of different crystal structure, valency, and if one of these metals is electro- positive with the other being electro- negative, an example of this type of electron compound being an alloy of the elements Magnesium and Tin which combine to form an intermetallic compound Mg 2 Sn. The composition of the compound is fixed and consists of two atoms of Magnesium combining with one atom of Tin. Metallic compounds form a crystal lattice with the atoms of the alloying metals taking up specific positions within the lattice. These compounds are usually hard and brittle. b) Interstitial compounds Interstitial compounds, as the name suggests form between metals, or metals and non-metallic elements, with atom sizes very similar to those that form interstitial solid solution. One set of atoms fit into the spaces, or interstices, between the larger atoms.
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~ Page 65 of 79 ~ Iron Carbide (Fe 3 C) or cementite which is important in the study of Iron-Carbon diagrams is an example of an interstitial compound. As the chemical symbol for Cementite is Fe 3 C, we know that cementite is an interstitial compound containing 3 iron atoms for every 1 atom of carbon. The illustration here shows the carbon atom (in red) fitting in between the larger iron atoms thus forming a lattice structure which will form the intermetallic compound, Cementite. EXERCISE 1. Discuss the main advantages and limitations of metal alloys compared with pure metals. 2. Describe what is meant by an ‘intermetallic compound’ and how its properties are likely to vary from those of a solid solution. 3. Explain the difference between substitutional and interstitial solid solutions. 4. Briefly explain what is meant by the following terms: (a) solvent (b) solute (c) solution (d) saturated. 5. Describe the difference between a pure metal and an ‘alloy’.
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~ Page 66 of 79 ~ COOLING CURVES Most substances can exist as gases, liquids and solids, depending upon their temperature. Water is one such substance, which can exist as a gas or vapour (steam) if it is sufficiently hot, as a liquid (water), and as a solid (ice) if sufficiently cold. If water is raised to its boiling point and allowed to cool slowly, the change in temperature with time can be plotted as a graph as shown in Fig. 1. Such a graph is called a cooling curve. It can be seen from the graph that where a change of state occurs (such as liquid water to solid ice) there is a short pause in the cooling process. This pause is referred to as an arrest point and is the result of the water giving up latent heat energy as it changes into ice.
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