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Fig. 18.2.
carbides. 1000~.
carbides. Figure 18.4 shows the correspondence
between the morphologies of the extracted precipitate
and that indicated by the autoradiograph.
A selected area diffraction pattern and an electron
probe microanalyzer trace are also included.
This type of precipitate has not yet been identified,
but the diffraction pattern does not correspond
to MbC. A comparison of the microprobe
results with standards shows that the noncarbon
constituents are about 90% Mo and 10% Cr. However,
some M,C carbides are also present after
high-temperature anneals and exist in several
morphologies.
We are concluding our investigation of precipitation
in standard Hastelloy N and will be evaluating
the information generated and attempting
to relate our observations to observed changes in
mechanical properties.
Because of the attractive in-reactor properties
of the titanium-modified Hastelloy N, we are expanding
our studies to include this material. Optical
metallography shows the alloy to be quitc
free of precipitate. Initial electron microscopy
studies of two experimental heats revealed that
220
Extraction Replica from Hastelloy N (Heat 5065)Aged 4 hr at 1600OF. All precipitates are M 6 C
a considerable amount of very thin, fine precip-
itate was present in the grain boundaries. This
was rarely detected in thin-foil transmission mi-
croscopy but was readily seen by the extractim
replica technique.
Selected area electron diffraction studies on
the fine grain-boundary precipitates have revealed
at least two phases, which exist in many mor-
phologies. Although these precipitates have not
been positively identified, indications are quite
good that Ti,O is one phase and that it is present
in conjunction with the second type. The other
phase has a face-centered cubic structure with a
lattice parameter of about 4.27 A and may exist
independently of the former. Considering chemical
composition, TIC seems to be the most likely com-
position of the second phase. However, the re-
ported lattice parameter for Tic is 4.33 A, com-
pared with our measured value of 4.27 A. TiN and
TiB have lattice parameters more closely related
to the measured value; however, chemical analyses
of the heats involved show the boron and nitrogen
contents to be extremely low.