Motivating question/phenomenon

In Figure. 20 we show, schematically, a morphology of precipitates known as the Widmanstätten ferrite lath morphology; there are two austenite (fcc phase of a solid solution of iron and carbon) grains, namely $\gamma_1$ and $\gamma_2$; the $\alpha$ (ferrite - a bcc solid solution of iron and carbon) phase nucleates at the grain boundary that separates the two $\gamma$ grains; it grows into one of the $\gamma$ phases as a lath. The schematic shows both the side view and the top view of these ferrite laths. What is the reason for the formation of Widmanstätten morphology?

Figure 20: A schematic (side and top) view of the Widmanstätten ferrite lath morphology.

[scale=0.4]Figures/WidmanstattenLaths.pdf

In Figure. 21, we show a particular morphology of preciptiate seen at the interface between two grains of another phase (say, $\alpha$); this morphology is known as grain-boundary allotriomorph. What is the reason for the formation of such allotriomorphs?

Figure 21: A schematic view of grain-boundary allotriomorph.

[scale=0.4]Figures/GBAllotriomorph.pdf