A Simple Method for Observing &lt;i&gt;ω&lt;/i&gt;-Fe Electron Diffraction Spots from &lt;112&gt;&lt;sub&gt;&lt;i&gt;α&lt;/i&gt;-Fe&lt;/sub&gt; Directions of Quenched Fe–C Twinned Martensite

Xiang

Chen

et al. 2020

Sci Rep

A transition of ω-fe 3 c → ω′fe 3 c → θ′-fe 3 c in fe-c martensite carbon steel is strong primarily because of carbides with the most well-known one being θ-fe 3 c type cementite. However, the formation mechanism of cementite remains unclear. in this study, a new metastable carbide formation mechanism was proposed as ω-fe 3 c → ω′-fe 3 c → θ′-fe 3 c based on the transmission electron microscopy (teM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal ω-fe 3 C fine particles are distributed in the martensite twinning boundary alone, while two metastable carbides (ω′ and θ′) coexist in the quenched pearlite. these two carbides both possess orthorhombic crystal structure with different lattice parameters (a θ′ = a ω′ = a ω = 2 a αfe = 4.033 Å, b θ′ = 2 × b ω′ = 2 × c ω = 3a α-fe = 4.94 Å, and c θ′ = c ω′ = 3a ω = 6.986 Å for a α-fe = 2.852 Å). the θ′ unit cell can be constructed simply by merging two ω′ unit cells together along its b ω′ axis. thus, the θ′ unit cell contains 12 Fe atoms and 4 C atoms, which in turn matches the composition and atomic number of the θ-fe 3 c cementite unit cell. the proposed theory in combination with experimental results gives a new insight into the carbide formation mechanism in fe-c martensite.

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

A transition of ω-Fe3C → ω′-Fe3C → θ′-Fe3C in Fe-C martensite

Xiang

Chen

et al. 2020

Sci Rep

“…This is because the twinning boundary is only one of the {112} planes (not every 112 plane can be the twinning plane) in a BCC-structured martensite. No twin diffraction contrast can be observed if the twinning plane is inclined significantly to the incident electron beam 21 , 22 . Notably, the twinning plane or boundary of the short twin is normally inclined to the lath boundary, in contrast to the long twin whose twinning plane is parallel to the lath boundary.…”

Section: Resultsmentioning

confidence: 99%

“…The selected-area electron diffraction (SAED) pattern (Fig. 2(b) ) shows a typical BCC {112}〈111〉-type twin structure with extra spots at 1/3( 12) and 2/3( 12), which are the diffraction spots from a metastable hexagonal ω-Fe phase 21 – 27 . Both the matrix crystal and twin crystal are thin with a thickness of several nanometers or tens of nanometers, and the twin boundaries seem to be straight or flat.…”

Section: Resultsmentioning

confidence: 99%

Lath formation mechanisms and twinning as lath martensite substructures in an ultra low-carbon iron alloy

Guo

Imura

et al. 2018

Sci Rep

Lath martensite is the dominant microstructural feature in quenched low-carbon Fe-C alloys. Its formation mechanism is not clear, despite extensive research. The microstructure of an Fe-0.05 C (wt.%) alloy water-quenched at various austenitizing temperatures has been investigated using transmission electron microscopy and a novel lath formation mechanism has been proposed. Body-centered cubic {112}〈111〉-type twin can be retained inside laths in the samples quenched at temperatures from 1050 °C to 1200 °C. The formation mechanism of laths with a twin substructure has been explained based on the twin structure as an initial product of martensitic transformation. A detailed detwinning mechanism in the auto-tempering process has also been discussed, because auto-tempering is inevitable during the quenching of low-carbon Fe-C alloys. The driving force for the detwinning is the instability of ω-Fe(C) particles, which are located only at the twinning boundary region. The twin boundary can move through the ω ↔ bcc transition in which the ω phase region represents the twin boundary.

“…Recent experimental investigation carried out on transmission electron microscopy (TEM) has revealed that metastable ω-Fe 3 C fine particles exist at the twin boundaries in the twinned martensite structure. − These twin-boundary ω-Fe 3 C particles have been observed to transform into cementite particles during in situ TEM heating; and no carbide is present in the twin-boundary-free region in the twinned martensite structure. , Earlier in situ TEM investigations also suggested that the cementite particles prefer to form at the twin boundaries or grain boundaries . These results suggest that the ω-Fe 3 C metastable phase is a potential precursor of θ-Fe 3 C in the Fe–C martensite structure.…”

Section: Introductionmentioning

confidence: 99%

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

Crystal Growth & Design

Chen

Xiang

2021

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.