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These aging studies will be expanded to include
longer aging times, alloys with varying titanium
and carbon levels, and complete metallographic
examination to ascertain metallurgical changes.
18.3. PHASE IDENTIFICATION STUDIES
IN HASTELLOY N
R. E. Gehlbach
The effects of thermornechanical treatments on
the mechanical properties of I-Iastelloy N indicated
the need for an electron microscope investigation
to understand the nature of the elevated-temperature
embrittlement problem. It was noticed that
the transition from transgranular to intergranular
failurs in the temperature range of the ductility
decrease indicated probable formation of a brittle
grain-boundary product - either a fine precipitate
or segregation of impurity elements to grain boundaries.
Preliminary examination of thinned foils by transmission
electron microscopy showed precipitation
occurring in the same temperature range as that of
the pronounced ductility change. 'rhus a detailed
study of precipitation in Hastelloy N was initiated
to identify the various types, morphologies, and
compositions of precipitates and to determine their
relationship to the mechanical properties.
Several complementary approaches are being
used for the purposes of precipitate identification.
Thin-foil transmission microscopy is employed for
observations of fine matrix precipitate, dislocation
structure, and, to a lesser extent, grain-boundary
precipitate. Due to the heterogeneous nature of
precipitation in Hastelloy N, sampling problems
are encountered with transmission rriicroscopy.
This technique is also limited in that the true
nature of the precipitates is not apparent and that
very thin grain-boundary films are frequently undetectable.
Extraction replication techniques overcome the
transmission limitations. Here, conventional
metallographic specimens are lightly etched to
remove the polished surface and to reveal precipitates
and are then coated with a layer of carbon.
Extraction replicas are obtained by heavy
electrolytic etching through the carbon film, which
dissolves the matrix, leaving the precipitates adhering
to the carbon substrate. This permits observation
of precipitates in the same distribution
219
as that existing in the bulk sample with the exception
that grain-boundary precipitates are no
longer supported by the matrix and collapse against
the substrate as shown in Fig. 18.2. This allows
direct observation of the grain-boundary precipitates
on essentially the surface of the original
grain boundary, providing inforiliation on true precipitate
morphology and permitting examination
of thin films.
Identification of precipitates is simplified with
extraction replicas, since interference from the
matrix is eliminated. Selected area electron diffraction
is being used for the identification of
crystal structure and the determination of lattice
parameters.
Semiquantitative cornpositional analysis of individual
particles is being performed using an
electron probe microanalyzer accessory on one
of the electron microscopes. Thus, by coordinating
various approaches available in electron microscopy,
precipitation processes may be studied
carefully.
Much of our effort has been concentrated on
standard Hastelloy N. The microstructure of this
material is characterized by stlingers of massive
M,C carbides, the metallic constituents being
primarily nickel. and molybdenum with some chromium.
The carbides are very rich in silicon compared
with the matrix. On aging or testing in the
temperature range 1112 to 1652OF, a fine grainboundary
precipitate forms (Fig. 18.2). This
precipitate is also M,C with the same lattice parameter
(approximately 11.0 A) as the large blocky
carbides. Work is in progress to attempt to detect
any compositional differences between the two
morphologies of M,C carbides. Initial microprobe
work using extraction replicas indicates that the
grain-boundary carbides are richer in silicon than
the large blocky type. All precipitates found in
Hastelloy N which has not been subjected to temperatures
in excess of 2372°F have been M,C
carbides.
When standard Hastelloy N is annealed at temperatures
above about 2372OF, the M,C begins to
transform to an intergranular lamellar product.
Autoradiography, using 14C as a tracer, shows
that this product is not a carbide and that the
carbon seems to be rejected (Fig. 18.3). The
blocky precipitates (Fig. 18.3) are seen to be
~ ........_..
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3H. E. McCoy, MSR Program Semiann. Progr. Rept.
Aug. 31, 1965, ORNL-3872, pp. 91-102.