Conversely mixtures can be created by mechanical means alone but a compound can

Conversely mixtures can be created by mechanical

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Conversely, mixtures can be created by mechanical means alone, but a compound can only be created (either from elements or from other compounds, or a combination of the two) by a chemical reaction. Some mixtures are so intimately combined that they have some properties similar to compounds and may easily be mistaken for compounds. One example is alloys . Alloys are made mechanically, most commonly by heating the constituent metals to a liquid state, mixing them thoroughly, and then cooling the mixture quickly so that the constituents are trapped in the base metal. Other examples of compound-like mixtures include intermetallic compounds and solutions of alkali metals in a liquid form of ammonia. Phase Phase is a uniform part of an alloy, having a certain chemical composition, texture and structure throughout, and which is separated from other alloy constituents or phases by a phase boundary, or clearly defined surfaces. Alloy structures may be single phase or multi-phase. An alloy phase may be in form of valence compound (substance formed from two or more elements), with a fixed ratio determining the composition) or in form of a mixture such as a solid solution. TYPES OF ALLOYS Alloys are of four (4) types: o Solid Solutions o Eutectics o Combination o Intermetallic Compounds 1. SOLID SOLUTIONS Solid solution is a single phase, where two or more substances are completely soluble in each other in the solid state/phase, meaning that atoms or molecules of the two substances take up positions in a common crystal lattice forming a single phase.
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~ Page 62 of 79 ~ Depending on the ratio of the solvent (matrix) metal atom size and solute element atom size, two types of solid solutions may be formed, substitution or interstitial. Solid solutions may be of either the substitutional or interstitial type. Solid solution formation usually causes increase of electrical resistance and mechanical strength and decrease of plasticity of the alloy. Atoms in interstitial or substitutional solid solution cause strain to be developed in the parent lattice. As there is an upper limit to the amount of strain that can be tolerated in a crystal lattice, it follows that there will be some restrictions to solid solution formation. The nature of solid solution in metal systems was extensively studied by Hume-Rothery and his work is summarised in the following rules: (a) Relative size If the sizes of the atoms of two metals do not differ by more than 14%, conditions are favourable for the formation of substitutional solid solutions. If the relative sizes of atoms differ by more than 14%, solid solution formation, if it occurs at all, will be extremely limited. Interstitial solid solutions may be formed if the atoms of the solute element are very small in comparison with those of the solvent metal.
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