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

Carbon steel is strong primarily because of carbides with the most well-known one being θ-Fe3C type cementite. However, the formation mechanism of cementite remains unclear. In this study, a new metastable carbide formation mechanism was proposed as ω-Fe3C → ω'-Fe3C → θ'-Fe3C based on the transmission electron microscopy (TEM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal ω-Fe3C 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 (aθ' = aω' = aω = [Formula: see text]aα-Fe = 4.033 Å, bθ' = 2 × bω' = 2 × cω = [Formula: see text]aα-Fe = 4.94 Å, and cθ' = cω' = [Formula: see text]aω = 6.986 Å for aα-Fe = 2.852 Å). The θ' unit cell can be constructed simply by merging two ω' unit cells together along its bω' 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-Fe3C cementite unit cell. The proposed theory in combination with experimental results gives a new insight into the carbide formation mechanism in Fe-C martensite.

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

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.

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

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.

Microstructure of a newly developed gamma' strengthened Co-base superalloy.

The microstructure of a newly developed Co-base superalloy with enhanced high-temperature strength has been investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) techniques. It mainly consists of a typical gamma/gamma' (FCC/L1(2)) structure and a plate-shaped AB3-type (Ni,Co,Cr)3(Ti,Al) intermetallic compound with hexagonal structure (a approximately 5.1A and c approximately 12.5A). gamma' is formed with a bimodal distribution and fine gamma' has a cuboidal morphology. Cr and Co are enriched in the gamma phase, while Ti, Al and Ni are enriched in the gamma' phase. W and Mo are more or less uniformly distributed in both gamma and gamma'. Chemical composition analysis by 3DAP suggests that the plate-shaped phase has a higher Ti and lower Al content compared to that of gamma' phase, and the concentration of Ti, Co and Ni has a periodic variation along c-axis with a period of 12.5A in the plate-shaped phase.