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

Cementite (theta-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, theta-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 omega-Fe3C fine carbide particles in the twin-boundary region of twinned Fe-C martensite are a potential precursor of theta-Fe3C carbides. These omega-Fe3C fine particles can transform into theta'-Fe3C carbide particles via a particle-coarsening process without involving any atomic movement. Interestingly, the metastable theta'-Fe3C carbide has a similar crystal structure to that of theta-Fe3C, and both have the same amount of iron and carbon atoms (12Fe + 4C) in their unit cells. Thus, a theta'-Fe3C (omega-Fe3C) -> theta-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 theta-Fe3C particle is actually composed of a great number of ultrafine theta-Fe3C grains with almost the same crystal orientation, or the coarsening of a theta-Fe3C particle can be attributed to the aggregation of numerous ultrafine theta-Fe3C grains, which are transformed from omega-Fe3C via the path omega-Fe3C -> omega'-Fe3C -> theta'-Fe3C -> theta-Fe3C.

Automated summary

The cementite θ-Fe3C in carbon steel contributes to its strength and hardness. Recent experiments have shown that the fine ω-Fe3C particles can transform into θ'-Fe3C and then into θ-Fe3C, suggesting a new transformation mechanism.