A hypoeutectoid alloy (carbon composition less than eutectoid). The first phase formed upon cooling from the austenite phase field is proeutectoid ferrite. Due to the lower solubility of carbon in ferrite, carbon is partitioned into the remaining austenite. At the eutectoid point the remaining carbon enriched austenite transforms to pearlite (a mixture of ferrite and cementite) which is the darker region of the micrograph. The proportion of pearlite is dependent upon the overall composition. The ferrite (light areas) is a good example of an allotriomorphic ferrite. This means that its shape does not reflect its internal crystalline symmetry as it nucleates on the austenite grain boundaries and hence follows the shape of the boundaries, the remaining austenite within the ferrite then transforms to pearlite, and is surrounded by the ferrite.
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