A popular metastable omega phase in body-centered cubic steels

Microstructural Evolution and Carbides in Quenched Ultra-low Carbon (Fe–C) Alloys

“…24,30) Then, it is about 0.53 Å for a bcc = 2.85 Å. The smallest radius of the octahedral interstitial site in ω-Fe structure is almost equal to that in γ-Fe, much larger than that in α-Fe.…”

“…A newly found metastable ω-Fe phase may answer this question. [24][25][26] At the M s -M f temperature interval, the carbon atoms can incorporate into the ω-Fe phase, which has a primitive hexagonal structure and special orientation relationships with α-Fe.…”

“…7(i). [24][25][26] The corresponding dark field images (Figs. 7(c) and 7(d)) obtained using the ω-Fe diffraction spots clearly show that the fine ω-Fe particles are at the twinning boundaries.…”

A Simple Method for Observing <i>ω</i>-Fe Electron Diffraction Spots from <112>_<i>α</i>-Fe Directions of Quenched Fe–C Twinned Martensite

“…15,17) In twinned martensite, the presumed double electron diffraction spots at 1/3{112} and 2/3{112} are always observed together with the twined structure during transmission electron microscopy (TEM) observations. [6][7][8][9][10] The nature of these spots at 1/3{112} and 2/3{112} have been recently explained based on a new metastable hexagonal ω -Fe phase (AB2-type, space group: p6/mmm, 3 atoms in one unit cell), which is distributed at the twin boundaries and has the crystallographic relationship with α-Fe: [18][19][20][21] Theoretical calculation results have also predicted the existence of the metastable ω -Fe phase and its relationship with the twin boundary (the twin boundary structure and the ω -Fe phase can stabilize each other energetically). [21][22][23][24][25] The ω -Fe phase has a particle-like morphology, the size is about 1-3 nm and the size distribution is very narrow.…”

“…The following factors may be responsible for this recognition of the ω -Fe phase: (1) It is assumed that the twinning structure are commonly based on the bcc twin forms from bcc structure and fcc (face-centered cubic) twins form from fcc structure, while in fact the twinned martensite is formed directly from fcc austenite in quenched Fe-C alloys. (2) As explained earlier, 15,18,20) most of the ω -Fe diffraction spots are overlapped with bcc matrix and bcc twin, as well as the double diffraction spots, but were considered to be only double diffraction spots. (3) The size of the ω -Fe crystallites is too small to be resolved with clarity.…”