= Solution
The materials interpretation is a cooling-rate gradient in a near-eutectoid <steel>. Before transformation, the parent <thermodynamic phase> is <austenite>. A small section can cool rapidly enough to suppress diffusional decomposition and form <martensite>, a supersaturated, highly strained transformation product. A much thicker interior cools more slowly and can transform diffusively to <pearlite>: alternating <ferrite> and <cementite> lamellae grouped into colonies. Bainite or mixed products are also possible for intermediate histories.
The likely contrast is therefore \b[a martensite-rich rapidly cooled region and a more pearlitic slowly cooled interior]. Brittle fracture is promoted by the highly strained martensitic state, residual <stresses> and local <stress> concentration. A larger intact interior need not share the same microstructure or local loading. The observations are consistent with this contrast, not a measurement proving pure <thermodynamic phases> everywhere.
The sketches below are generic <steel microstructures>, rather than a component design or fabrication prescription. No cooling curve, dimensions or transformation diagram is provided, so exact <thermodynamic phase> fractions, plate sizes and a universally pearlitic centre cannot be inferred.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2015/paper-1-steel-microstructures.png]
{title=Original qualitative steel microstructure sketches: lamellar pearlite, a strained martensitic plate structure and carbide precipitation within a transformed matrix}
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