An alloying element is a chemical element deliberately included in an alloy to control its properties or processing behaviour. In steel, carbon changes the phases and transformations available during heat treatment; other additions can modify corrosion, hardenability or high-temperature behaviour.
The effect depends on how an element is incorporated. Atoms dissolved in a solid solution can hinder dislocation motion, while an element concentrated in a secondary phase can affect strength through the size and distribution of particles. Thus a bulk composition does not uniquely determine mechanical strength: processing controls the microstructure through which the composition acts.[ref]
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