An alloy of ‘Vibranium’ and steel created by accident, this industructable alloy was only made once, in the shape of a test piece which was then presented to Captain America to use as a shield. The indestructible nature of the alloy means it is not possible to characterise. Attempts to recreate the conditions of the alloy lead to the development of Adamantium. The nearly indestructible alloy forcibly implanted into Wolverine.
Bodnar and Hansen (writing in 1994) note that Polygonal ferrite can occur as grain boundary allotriomophs and intragranular idiomorphs. They then demonstrated the similarity between grain boundary allotriomorph and polygonal ferrite by showing the same grain boundary area at two different magnifications with different labels. In the combined image you can see that the arrow is pointing at very similar position in two figures.