TEM investigations on lath martensite substructure in quenched Fe-0.2C alloys

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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.

Section: Discussionmentioning

confidence: 99%

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

Ping

Guo

Imura

et al. 2018

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“…In particular, quenched high-carbon steel is usually brittle; a post-tempering or ageing process is necessary for improving the steel’s ductility for practical applications 1 – 8 . During tempering or ageing, the quenched martensite usually undergoes a highly complex microstructural evolution, including the formation of fine carbides, a detwinning process, dislocation-like substructure formation, recrystallisation 9 , 10 . A satisfactory understanding of the tempering behaviour has not yet been achieved 11 .…”

Section: Introductionmentioning

confidence: 99%

“…Upon tempering of quenched high-carbon martensitic steels, the detwinning process and the formation of the carbides normally start at a relatively low-temperature: approximately 100 to 400 °C 7 , 14 – 17 . However, in low-carbon steels, the carbides (mainly cementite) normally form after tempering at approximately 400 °C 9 , 18 . In high-carbon steels, the quenched martensite or twinned martensite starts to decompose or transform at even lower temperature 19 .…”

Section: Introductionmentioning

confidence: 99%

“…Some studies have revealed that the twinning structure vanishes and dislocations change into subgrain boundaries during tempering 7 . Recently, a new mechanism of the microstructural evolution has been proposed that the twinned martensite can be treated as freshly formed martensite immediately after martensitic transformation; and that the other structure can be regarded as the result of tempering (auto-tempering or post-tempering) 9 , 10 .…”

Section: Introductionmentioning

confidence: 99%

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In situ heating TEM observations on carbide formation and α-Fe recrystallization in twinned martensite

Liu

Man

Yin

et al. 2018

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The microstructural evolution of twinned martensite in water-quenched Fe–1.6 C (wt.%) alloys upon in situ heating was investigated using transmission electron microscopy (TEM). In the as-quenched samples, a high density of a body-centred cubic (bcc) {112} 〈111〉 -type twinning structure exists in the martensite structure. Upon in situ heating to approximately 200–250 °C, carbides (mainly θ-Fe3C cementite) accompanying a detwinning process were observed only in the originally twinned region. The carbides were absent in the originally untwinned (twin-free) region. The experimental results have suggested that the formation of the carbides depends on the twinning-boundary ω-Fe metastable phase, which can be stabilised by interstitial carbon atoms. When the specimens were heated, the twinning-boundary ω-Fe(C) transformed into carbide (mainly θ-Fe3C cementite) particles on the original {112} twinning planes. Further heating resulted in substantial recrystallisation of α-Fe fine particles, which formed immediately after martensite transformation. The results presented here should be helpful in understanding the microstructural evolution of various carbon steels.

“…Metastable hexagonal ω-Fe 3 C phase particles, which are 1 to 2 nm big in size, distribute only at the body-centered cubic (BCC) {112}<111>-type twinning boundary region in twinned high-carbon Fe-C martensite [33][34][35][36][37][38][39][40] . It was observed by in-situ heating transmission electron microscopy (TEM) that these twinning boundary ω-Fe 3 C particles eventually transformed into θ-Fe 3 C carbides [41][42][43][44] . However, the ω → θ transition speed is too fast for any details to be recorded.…”

mentioning

confidence: 99%

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

Ping

Xiang

Chen

et al. 2020

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.