Abstract
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.
Highlights
Martensite transformation mechanisms have long been discussed owing to their important role in understanding the various microstructures formed in steels, in the quenched state of Fe-C alloys with different levels of carbon
The twin structure is common in other quenched low-carbon martensites such as Fe-Ni-C12–14; the twin formation mechanism (BCC {112} 〈111〉-type twin formed in austenite with face-centered cubic (FCC) structure) is still unclear, the related research activity started in the early 20th century[15,16,17]
In order to confirm whether a twin can be observed in ultra-low-carbon alloys, the microstructure of an Fe-0.05 C alloy subjected to water-quenching at temperatures from 1050 °C to 1350 °C has been examined using transmission electron microscopy (TEM)
Summary
Martensite transformation mechanisms have long been discussed owing to their important role in understanding the various microstructures formed in steels, in the quenched state of Fe-C alloys with different levels of carbon. A large, plate-like form of quenched martensite with a body-centered cubic (BCC) {112}〈111〉-type twin structure as its substructure is formed in high-carbon steels or alloys with a low Ms (martensite start temperature), whereas the martensite in low-carbon steels, normally called lath martensite, has been suggested to have a dislocation structure as its lath substructure after quenching These different substructures have resulted in the suggestion of various kinds of transition mechanisms from austenite (γ-Fe) to ferrite (α-Fe)[1,2,3,4,5,6,7,8,9,10]. A new lath formation mechanism has been discussed at atomic scale for easy understanding
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