21-Mat-B6 Ceramic Materials · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A hypereutectoid steel (wt%C between 0.77 and roughly 2.0) cools from the single-phase austenite region and first crosses the $A_{cm}$ boundary — the solubility line on the high-carbon side of the eutectoid, above which austenite alone is stable and below which austenite coexists with proeutectoid cementite. The microstructure that eventually forms is a direct consequence of WHERE that proeutectoid cementite chooses to nucleate.
Heterogeneous nucleation is always energetically favoured over nucleation inside a defect-free grain interior, because forming a new cementite particle ON an existing austenite grain boundary replaces a portion of the relatively high-energy boundary with two lower-energy $\gamma/\text{Fe}_3\text{C}$ interfaces, lowering the net activation-energy barrier for nucleation. Grain boundaries therefore become the preferred nucleation SITES for the proeutectoid phase, exactly as they are for proeutectoid ferrite in a hypoeutectoid steel — the only difference between the two cases is which phase (ferrite vs. cementite) is thermodynamically favoured to precipitate, which is set purely by which side of the eutectoid composition the alloy sits on.
Once nucleated on a grain boundary, a cementite particle grows preferentially ALONG the boundary rather than into either grain interior, because the boundary itself offers a continuous, higher-diffusivity path for carbon to be collected from the surrounding austenite (grain-boundary diffusion is orders of magnitude faster than bulk lattice diffusion at these temperatures) and because the boundary plane is already a favourable, low-mobility-barrier growth front. As cooling continues slowly through the two-phase ($\gamma+\text{Fe}_3\text{C}$) region above 727°C, individual cementite nuclei impinge on and coalesce with their neighbours along the SAME boundary faster than they thicken into the grain interiors, producing a thin but essentially CONTINUOUS cementite film that outlines the entire former austenite grain boundary network by the time the steel reaches 727°C.
As the remaining austenite (now depleted of carbon by the proeutectoid cementite down to very close to the eutectoid composition, 0.77 wt%C) is cooled through 727°C, it transforms by the normal eutectoid reaction into pearlite (alternating ferrite/cementite lamellae) nucleating and growing inward from the grain-boundary network into each former-austenite grain interior. Because each original austenite grain transforms into its own pearlite colony (or several colonies), and the grain boundaries were already rimmed with the continuous proeutectoid cementite film BEFORE this transformation began, the finished microstructure shows each pearlite colony wrapped by that pre-existing, more-or-less continuous cementite boundary layer — distinct from, and formed BEFORE, the fine eutectoid cementite lamellae inside the pearlite itself.
