trix, Dijkstra assumed that there was no definite relationship between them, and that the first phase was not an intermedi- ate in the transition to the final state. This does not appear to be a convincing argument, and Dijkstra himself noted that the Fe4
N plates seemed to originate from the nuclei inside or very close to the plates of the unknown phase. One year later, in November 1957, Booker et al.18
confirming Jack’s discovery of α’’-Fe16 and Wert.51
asserting that these were the precipitates observed by Di- jkstra20
N2
published a paper precipitates and
At least three papers have claimed that
the growth of these nitrides in the Fe-N system is diffusion controlled.19,51,52
present in samples after 43 months of room temperature ag- ing.27
Phillips observed at 45,000X that both Fe16 ity that nitriding alloying elements might stabilize Fe16 N2
they are small enough that a transmission electron micro- scope (TEM) is required for imaging. Some of the better TEM images of precipitates were published by Hale22 Phillips.27
room temperature.26 Precipitates of Fe16
N2 and
Some of the images by Hale show with clarity that the precipitates were either face-on or edge-on to the direction of observation. They thus appear to be at 90 degrees to one another, indi- cating possible cubic growth directions relative to the matrix. In a situation such as extreme super- saturation the precipitates can grow thousands of times larger and become structured as disk pre- cipitates.26
coherent with the ferrite matrix along {100}α The Fe4
The Fe16 N2
precipitates lie on and are .18,22
N precipitates, on the other hand, are only semi-coherent with the ferrite in steel and form single-crystal plate-shaped precipitates with their {112} planes parallel to the plate faces.53
The set
of 12 habit planes is accepted to be the {210}α family.53
the relationship of {112}{210}α
Thus, the matrix and precipitate have .18,19,53
The de-
termination of this family as the habit planes for the precipitates is somewhat unusual because the plane is not of dense atomic packing, nor does it contain the matrix close-packed direction. Stud- ies have shown, however, that the atomic arrange- ments and spacing between the {210}α Fe4N
that the concentration of solutes other than carbon and nitro- gen were not determined and may have prolonged the time the Fe16
N2
was present. Jack briefly discussed the possibil- N2
at have a rosette-type appearance when N2 and Fe4
This period is much longer than would be expected for the metastable Fe16
to be present. Phillips commented N were
planes of {049}α
.This claim has not been refuted or supported in any further literature. In fact, it appears to have been simply ignored. Table 1 summarizes crystallographic data for ferrite and nitride precipitates in steel that are also suspected to be precipitated in aging cast irons (at room temperature).
tate nucleates directly on dislocations, and there are no intermediate phases. At this time, there is no reason to as- sume the iron-nitrides in cast iron have a different crystal structure or orientation relative to the ferrite than do the iron-nitrides in steel. The coherency of the precipitates, and thus their interaction with dislocations, may be rele- vant for understanding of how aging in cast irons is linked to changes in machinability.
of nitrogen GP zones homogenously nucleating first. It is perhaps difficult to conceive of a homogenous nucleation process in a material as heterogeneous as gray iron, but the presence of the GP zones may be a moot point since such zones have yet to be connected to a physical mani- festation during cast iron aging. The reviewed knowledge from steel research and the combination of kinetics and mechanical property measurements in GCI indicate that above about 200°C (392°F) the equilibrium Fe4
With the information concerning nitride precipitation in steel, a reasonable model can be proposed for the pre- cipitation in cast irons. Below 200°C (392°F), the follow- ing process is likely: Fe16
N2 → Fe4 N, with the possibly N precipi-
Table 1. Crystallographic data for ferrite and nitride precipitates in steel at room temperature
and {112}
planes are similar when considered in all three dimensions, and all other matches explored would be distinctly different in atomic spacing.53 The similarity of atomic spacing between {210}α {112}Fe4N
and
planes may explain the choice of habit planes for the nitrides. In 1987, Dahmen et al.19 claimed that Fe4
International Journal of Metalcasting/Spring 10 N has four variants with habit
Figure 10. Experimentally measured changes in average UTS from age strengthening as a function of thermodynamically modeled iron- nitride content at room temperature.14 fit (i.e. R2 precipitate.
=0.96), showing that Fe4
Linear correlation is a good N is an excellent suspect for the
51
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