A Mechanism For The Formation of Lower Bainite
A Mechanism For The Formation of Lower Bainite
A Mechanism For The Formation of Lower Bainite
A diffusional mechanism for the formation of lower bainite is proposed based primarily on
transmission electron microscopy (TEM) observations of isothermally reacted specimens of Fe-
C-2 pct Mn alloys. The mechanism involves the initial precipitation of a nearly carbide-free
ferrite "spine," followed by sympathetic nucleation of "secondary (ferrite) plates" which lie at
an angle to the initial "spine." Carbide precipitation subsequently occurs in austenite at
ferrite: austenite boundaries located in small gaps between the "secondary plates." An "an-
nealing" process then occurs in which the gaps are filled in by further growth of ferrite and
additional carbide precipitation; the annealing out of fenite: ferrite boundaries between impinged
"secondary plates" completes this process. This annealing stage contributes to the final ap-
pearance of lower bainite sheaves as monolithic plates containing embedded carbides. The pres-
ent mechanism accounts for the single variant of carbides oriented at an angle to the sheaf axis
repeatedly reported in lower bainite; it is also consistent with the previous observation of one
"rough" side and one "smooth" side of lower bainite "plates."
heavy arrow in each of these figures points out the ini- boundaries (partially) enclosing the gaps. More time is
tiating ferrite plate. thus available for carbide nucleation at the austenite: ferrite
Once the initiating ferrite plate of a sheaf, which may interface.
be termed its "spine," has appeared, other ferrite crystals The fourth and final stage in the development of a
are then sympathetically nucleated at it but often on only lower bainite sheaf is further ferrite growth around the
one of its two broad faces, as in Figure 2. These crys- carbides, and perhaps additional carbide precipitation,
tals, whose morphology often appears to approximate that until the small amount of austenite remaining in the gaps
of a thick plate, frequently develop at a marked angle to has been decomposed. If a significant proportion of Si
the spine. Figure 3(a) shows another example of this is present in the alloy, though, both the third and fourth
structure near the tip of a lower bainite sheaf. In steps of this sequence can be greatly inhibited.
Figure 3(b), the higher magnification employed permits The overall external shape of the sheaf will thus quickly
observation of serrations at the sides of the sheaf, de- lose its initial serrated appearance and will soon more
veloped where the individual "secondary plates" have nearly resemble that of a lenticular ferrite monocrystal
lengthened unequally, perhaps because they nucleated at
different times. Figure 4 illustrates this type of structure
in the 0.34 pct C alloy; gaps between adjacent "second-
ary plates" are evident (indicated by arrows in Figure 4).
The third step in this process is the precipitation of
carbides in the small gaps between "secondary plates"
(which may have already impinged along other portions
of their interphase boundaries). In the present alloys,
electron diffraction studies indicated that the carbide was
cementite. A typical example of the diffraction analysis
is shown in Figure 5. (Also note the ferrite spine
in Figure 5(a).*) These carbides were presumably nu-
*The appearance of a few carbides in the region of the spine shown
in Figure 5(a) is most likely due to a stereological effect in which the
carbides associated with sympathetically nucleated ferrite plates over-
lap the spine, either above or below the spine, within the T E M foil.
However, it is also possible that some carbide precipitation occurred
at austenite:ferrite boundaries of spines at boundary orientations where (a)
their growth was markedly slowed by a low density of growth ledges
and later resumed, or reinitiated, by sympathetic nucleation, t44,451
( i )
(- i )
(b)
C
I
"
!
I
'
I
!
' ) Fig. 2 - - A lower bainite sheaf formed in Fe-0.95 wt pct C-1.93 wt
pct Mn reacted at 250 ~ for 15,000 s: (a) bright-field T E M micro-
Fig. 1 - Schematic representation of an upper bainite "sheaf." graph and (b) corresponding dark-field micrograph.
shorter and thinner than the spine first formed. Length- shown in Figure 5 of Hehemann's tS] well-known review
ening rapidly, such spines would physically prevent fur- of the bainite reaction, reproduced here in Figure 9.
ther lengthening of large numbers of secondary plates The second complication in the morphology of ferrite
(formed earlier), as shown in Figure 8. Several ferrite in lower bainite sheaves to be noted is the strong ten-
"spines" can thus be discerned in the lower bainite sheaf dency for individual ferrite crystals within these sheaves
to form facets diagonally across their leading edge. This
feature is, of course, most readily observed at the lead-
ing tip of a sheaf, as illustrated in Figure 10; arrowheads
point out prominent examples of such facets. Ohmori
e t al. t48] also drew attention to facets of this type in their
sketches of bainite sheaves, emphasizing their potential
for serving as nucleation sites for bainitic carbides. Car-
bides growing "allotriomorphically" or as plates along