Formation of θ-Fe₃C Cementite via θ′-Fe₃C (ω-Fe₃C) in Fe–C Alloys

Abstract

Cementite (θ-Fe3C), as a well-known hard-phase particle, makes carbon steel strong and hard. As for the carbide formed from the Fe–C martensite structure, θ-Fe3C has been traditionally believed to precipitate from the martensite via a classical nucleation and grain growth mechanism. However, recent experimental results have revealed that the ω-Fe3C fine carbide particles in the twin-boundary region of twinned Fe–C martensite are a potential precursor of θ-Fe3C carbides. These ω-Fe3C fine particles can transform into θ′-Fe3C carbide particles via a particle-coarsening process without involving any atomic movement. Interestingly, the metastable θ′-Fe3C carbide has a similar crystal structure to that of θ-Fe3C, and both have the same amount of iron and carbon atoms (12Fe + 4C) in their unit cells. Thus, a θ′-Fe3C (ω-Fe3C) → θ-Fe3C transformation path has been proposed with the transformation mechanism investigated crystallographically. Transmission electron microscopy observations on the quenched high carbon Fe–C binary alloys have confirmed that a large θ-Fe3C particle is actually composed of a great number of ultrafine θ-Fe3C grains with almost the same crystal orientation, or the coarsening of a θ-Fe3C particle can be attributed to the aggregation of numerous ultrafine θ-Fe3C grains, which are transformed from ω-Fe3C via the path ω-Fe3C → ω′-Fe3C → θ′-Fe3C → θ-Fe3C.