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FIM-atom probe analysis of thin nitride platelets in Fe-3 at.% Mo
Sc~Jptt~ MI!TALLURGICA
Vol. 5, pp. 8 6 5 - 8 7 0 , 1971 P r i n t e d in the U n i t e d States
Pergamon
Press,
Inc
FIM-ATOM PROBE ANALYSIS OF THIN NITRIDE PLATELETS IN Fe-3 at.~ Mo S. S. Brenner and S. R. Goodman
U. S. Steel Fundamental Research Laboratory
Monroeville, Pennsylvania
(Received August
13,
15146
1971)
While nltridlng has been a well known process for hardening the surfaces of steel for many year%
only recently have attempts been made to clarify the morphology and
composition of the precipitated nitride phases.(l,2,3,4,5)
At low nltriding temperatures
the precipitate commonly consists of a dense mass of platelets which are often too numerous and too thin to be resolved in the electron microscope.
The platelets give rise to
diffraction streaking and have been compared to Guinier-Preston zones found in facecentered-cubic alloys.
They are believed(5) %o undergo several phase transitions before the
equilibrium composition is attained.
Chemical analyses of the extracted precipitate have
been made,(4) but an in-situ analysis has been beyond the scope of conventional analytical[ tools.
With the development of the FIM-A%om Probe,(6,7) it is now possible to resolve the
precipitate and to determine directly its composition by an atom-by-atom analysis.
This
paper which demonstrates this capability, reports some initial results of a study of the mechanism of nitride formation. The Fe-3 at.~ Mo alloy was drawn to 0.i mm dia. wire and was heated for 14 days at 480 C in an ii~ NH3/H 2 atmosphere.
From the observed kinetics of nitrogen uptake of a
0.25 n~n thick sheet of the alloy, it was concluded that the wires were fully nitrided. hardness of the nitrlded alloys was 775 DPH measured with a i00 gm load.
a,
b.
FIG. i a.
b.
Electron transmission micrograph of nltrided alloy. Neon ion image of same alloy. Lines of bright atoms delineate the edges of the nitride platelets. 865
The
866
FIM-ATOM PROBE OF THIN NITRIDE PLATELETS
Vol.
5,
No.10
Figure i shows a transmission electron micrograph and an FIMmicrograph of the nitrided alloy.
While the HM micrograph indicates the presence of precipitate~ it can only
be clearly resolved in the FIM. The rows of bright atoms are the edges of the nitride 0
platelets estimated to be 2 to 3 atom layers thick (3-5 A).
The FIM micrograph was obtained
at 80 K at a field about 10% below the best image field for the matrix; under these conditions the platelets are seen best.
At lower temperatures
(30 K) and higher fields at which
the matrix is imaged more clearly, the image of the atoms in the platelets is considerably smaller giving a better indication of their thickness. From the ion micrographs it can be clearly deduced that the platelets lie on [lO0]a matrix planes~ as suggested previously.(5)
Since the axis of the specimen (Fig. ib) is
parallel to the EllO] direction, three sets of {lO0~a planes intersect the tip surface--two at 45° to the specimen axis and one parallel to it.
In Fig. 2 the stereographically
F-~G. 2
Orientation analysis of platelets. Curved lines are the traces of [lO0]a matrix planes. Nitride platelets are parallel to these planes. projected traces of some of these planes are superimposed on the properly scaled ion-image of the alloy. Each nitride disk is observed to be parallel to one of the sets of [i00]~ planes. As the alloy is field-evaporated~ the traces of the edges of the 45 ° oriented platelets move toward their respective poles while those of the parallel oriented platelets remain stationary, as is expected. The dimensions, shape and number density of the platelets can be determined with reasonable precision from the calculated image magnification and the measured depth they extend into the bulk of the specimen.
The platelets are circular or elliptical as long as
there is no impingement by other platelets.
Their diameters range from 50 to 150 I.
Their
number density varies widely from specimen to specimen; in the specimen shown in Fig. i~ the platelet density is in excess of lol8/cm 3. The atom probe, a combination of a field-ion microscope and a time-of-flight mass spectrometer, has been described elsewhere.(8)
To avoid sampling the matrix 9 it is necessary
to reduce the diameter of the area of analysis to a size comparable to the platelet thickness. This was accomplished by mounting the imaging assembly on a drive shaft that can change the
Vol.
5, No.
I0
FIM-ATOM
PROBE
OF T H I N
NITRIDE
PLATELETS
867
tip-to-screen distance, thereby making it possible to vary continuously the image magnification and hence the diameter of the specimen area projected at the probe hole; at the greatest magnification the diameter of the area probed was usually less than 4 A.
To
analyze a platelet, its image was positioned over the probe hole in the screen (Fig. 3) and single voltage pulses were applied to field-evaporate the specimen.
Only those atoms imaged
over the probe hole can reach the ion detector producing the tin~ng signals, which are recorded on a Tektronix Type 549 storage oscilloscope. SCREEN
S ~PEM ClEN
~. J~l . . . .
9ETECTOR
FIG. 3 Principle of operation of atom probe. Specimen is manipulated until image of precipitate is over probe hole in screen. Only atoms imaged over hole can reach detector when specimen is field-evaporated. Figure 4 shows the atom probe spectrum from a platelet that was oriented parallel to the wire axis and that could be analyzed most easily because its intersection with the surface remained stationary.
The data were obtained at a constant tip potential (i.e.
15
20
25
30 35 4O TI~E OF FLIGHT (Ms)
45
FIG. 4 Atom probe spectrum of n i t r i d e p l a t e l e t . indicate position of probe hole.
Arrows
blunting of the tip was negligible) which increases the accurcy of the analysis because it is unnecessary to normalize the data to an average potential.
Within the reading accuracy of
the oscilloscope (25 to 50 ns), 146 of the 149 signals recorded can be attributed to nitrogen, iron, molybdenum and molybdenum nitride (MoN).
Three signals were from neon atoms from the
residual imaging gas (5 x 10 -8 torr) that were field-adsorbed on the tip surface.(9) ca].:.~lated M/n ranges of the different isotopes are indicated by the brackets. between the calculated and the experimental values is good.
The
The fit
The composition of the platelet
Vol.
5, No.
I0
FIM-ATOM
PROBE
OF T H I N
NITRIDE
PLATELETS
40 as.= m
i
TIME OF FLIGHT l~sl
FIG. 5 Atom probe spectrum of "massive" precipitate indicating a composition of 54 at.% Mo, 5% Fe and 41% N. Mo3N 2 with some of the molybdenum replaced by iron.
A discussion of the imaging
characteristics and structural features of this phase will be given in a future paper.
Acknowledgment The nitriding of the wires and the nitrogen isotherm measurements were kindly performed by R. M. Lytle.
We are grateful to Dr. J. T. McKinney for his invaluable
assistance particularly with the trace analysis and for the many helpful discussions. We also thank Dr. H. H. Podgumski for his advice and suggestions regarding the nitriding. References
i.
A. S. Keh and H. A. Wriedt, Trans. AIME 224, 560 (1962).
2.
K. F. Hale and D. McLean, J. Iron Steel Inst. 201, 337 (1963).
3.
H. H. Podgurski and H. E. Knechtel, Trans. AIME 245. 1595 (1969).
4.
H. E. Knechtel, Ph.D. Thesis~ Univ. of Pittsburgh~ 1970.
5.
D. L. Speirs, W. Roberts, P. Grieveson and K. H. Jack~ Proc. 2nd. Int. Conf. on Mechanism of Strengthening 9 September 1970.
6.
E. W. M~ller, I. A. Panitz and S. B. McLane, Rev. Sci. Insts. ~
7.
S. S. Brenner and J. T. McKinney, Appl. Phys. Letters ~
8.
S. S. Brenner and J. T. McKinney, Surf. Sci. ~
9.
E. W. Muller, Quart. Rev. ~
177 (1969).
i0. H. H. Podgurski, private communication.
83 (1968).
29 (1968).
83 (1970).
869
Vol. 5, pp. 8 6 5 - 8 7 0 , 1971 P r i n t e d in the U n i t e d States
Pergamon
Press,
Inc
FIM-ATOM PROBE ANALYSIS OF THIN NITRIDE PLATELETS IN Fe-3 at.~ Mo S. S. Brenner and S. R. Goodman
U. S. Steel Fundamental Research Laboratory
Monroeville, Pennsylvania
(Received August
13,
15146
1971)
While nltridlng has been a well known process for hardening the surfaces of steel for many year%
only recently have attempts been made to clarify the morphology and
composition of the precipitated nitride phases.(l,2,3,4,5)
At low nltriding temperatures
the precipitate commonly consists of a dense mass of platelets which are often too numerous and too thin to be resolved in the electron microscope.
The platelets give rise to
diffraction streaking and have been compared to Guinier-Preston zones found in facecentered-cubic alloys.
They are believed(5) %o undergo several phase transitions before the
equilibrium composition is attained.
Chemical analyses of the extracted precipitate have
been made,(4) but an in-situ analysis has been beyond the scope of conventional analytical[ tools.
With the development of the FIM-A%om Probe,(6,7) it is now possible to resolve the
precipitate and to determine directly its composition by an atom-by-atom analysis.
This
paper which demonstrates this capability, reports some initial results of a study of the mechanism of nitride formation. The Fe-3 at.~ Mo alloy was drawn to 0.i mm dia. wire and was heated for 14 days at 480 C in an ii~ NH3/H 2 atmosphere.
From the observed kinetics of nitrogen uptake of a
0.25 n~n thick sheet of the alloy, it was concluded that the wires were fully nitrided. hardness of the nitrlded alloys was 775 DPH measured with a i00 gm load.
a,
b.
FIG. i a.
b.
Electron transmission micrograph of nltrided alloy. Neon ion image of same alloy. Lines of bright atoms delineate the edges of the nitride platelets. 865
The
866
FIM-ATOM PROBE OF THIN NITRIDE PLATELETS
Vol.
5,
No.10
Figure i shows a transmission electron micrograph and an FIMmicrograph of the nitrided alloy.
While the HM micrograph indicates the presence of precipitate~ it can only
be clearly resolved in the FIM. The rows of bright atoms are the edges of the nitride 0
platelets estimated to be 2 to 3 atom layers thick (3-5 A).
The FIM micrograph was obtained
at 80 K at a field about 10% below the best image field for the matrix; under these conditions the platelets are seen best.
At lower temperatures
(30 K) and higher fields at which
the matrix is imaged more clearly, the image of the atoms in the platelets is considerably smaller giving a better indication of their thickness. From the ion micrographs it can be clearly deduced that the platelets lie on [lO0]a matrix planes~ as suggested previously.(5)
Since the axis of the specimen (Fig. ib) is
parallel to the EllO] direction, three sets of {lO0~a planes intersect the tip surface--two at 45° to the specimen axis and one parallel to it.
In Fig. 2 the stereographically
F-~G. 2
Orientation analysis of platelets. Curved lines are the traces of [lO0]a matrix planes. Nitride platelets are parallel to these planes. projected traces of some of these planes are superimposed on the properly scaled ion-image of the alloy. Each nitride disk is observed to be parallel to one of the sets of [i00]~ planes. As the alloy is field-evaporated~ the traces of the edges of the 45 ° oriented platelets move toward their respective poles while those of the parallel oriented platelets remain stationary, as is expected. The dimensions, shape and number density of the platelets can be determined with reasonable precision from the calculated image magnification and the measured depth they extend into the bulk of the specimen.
The platelets are circular or elliptical as long as
there is no impingement by other platelets.
Their diameters range from 50 to 150 I.
Their
number density varies widely from specimen to specimen; in the specimen shown in Fig. i~ the platelet density is in excess of lol8/cm 3. The atom probe, a combination of a field-ion microscope and a time-of-flight mass spectrometer, has been described elsewhere.(8)
To avoid sampling the matrix 9 it is necessary
to reduce the diameter of the area of analysis to a size comparable to the platelet thickness. This was accomplished by mounting the imaging assembly on a drive shaft that can change the
Vol.
5, No.
I0
FIM-ATOM
PROBE
OF T H I N
NITRIDE
PLATELETS
867
tip-to-screen distance, thereby making it possible to vary continuously the image magnification and hence the diameter of the specimen area projected at the probe hole; at the greatest magnification the diameter of the area probed was usually less than 4 A.
To
analyze a platelet, its image was positioned over the probe hole in the screen (Fig. 3) and single voltage pulses were applied to field-evaporate the specimen.
Only those atoms imaged
over the probe hole can reach the ion detector producing the tin~ng signals, which are recorded on a Tektronix Type 549 storage oscilloscope. SCREEN
S ~PEM ClEN
~. J~l . . . .
9ETECTOR
FIG. 3 Principle of operation of atom probe. Specimen is manipulated until image of precipitate is over probe hole in screen. Only atoms imaged over hole can reach detector when specimen is field-evaporated. Figure 4 shows the atom probe spectrum from a platelet that was oriented parallel to the wire axis and that could be analyzed most easily because its intersection with the surface remained stationary.
The data were obtained at a constant tip potential (i.e.
15
20
25
30 35 4O TI~E OF FLIGHT (Ms)
45
FIG. 4 Atom probe spectrum of n i t r i d e p l a t e l e t . indicate position of probe hole.
Arrows
blunting of the tip was negligible) which increases the accurcy of the analysis because it is unnecessary to normalize the data to an average potential.
Within the reading accuracy of
the oscilloscope (25 to 50 ns), 146 of the 149 signals recorded can be attributed to nitrogen, iron, molybdenum and molybdenum nitride (MoN).
Three signals were from neon atoms from the
residual imaging gas (5 x 10 -8 torr) that were field-adsorbed on the tip surface.(9) ca].:.~lated M/n ranges of the different isotopes are indicated by the brackets. between the calculated and the experimental values is good.
The
The fit
The composition of the platelet
Vol.
5, No.
I0
FIM-ATOM
PROBE
OF T H I N
NITRIDE
PLATELETS
40 as.= m
i
TIME OF FLIGHT l~sl
FIG. 5 Atom probe spectrum of "massive" precipitate indicating a composition of 54 at.% Mo, 5% Fe and 41% N. Mo3N 2 with some of the molybdenum replaced by iron.
A discussion of the imaging
characteristics and structural features of this phase will be given in a future paper.
Acknowledgment The nitriding of the wires and the nitrogen isotherm measurements were kindly performed by R. M. Lytle.
We are grateful to Dr. J. T. McKinney for his invaluable
assistance particularly with the trace analysis and for the many helpful discussions. We also thank Dr. H. H. Podgumski for his advice and suggestions regarding the nitriding. References
i.
A. S. Keh and H. A. Wriedt, Trans. AIME 224, 560 (1962).
2.
K. F. Hale and D. McLean, J. Iron Steel Inst. 201, 337 (1963).
3.
H. H. Podgurski and H. E. Knechtel, Trans. AIME 245. 1595 (1969).
4.
H. E. Knechtel, Ph.D. Thesis~ Univ. of Pittsburgh~ 1970.
5.
D. L. Speirs, W. Roberts, P. Grieveson and K. H. Jack~ Proc. 2nd. Int. Conf. on Mechanism of Strengthening 9 September 1970.
6.
E. W. M~ller, I. A. Panitz and S. B. McLane, Rev. Sci. Insts. ~
7.
S. S. Brenner and J. T. McKinney, Appl. Phys. Letters ~
8.
S. S. Brenner and J. T. McKinney, Surf. Sci. ~
9.
E. W. Muller, Quart. Rev. ~
177 (1969).
i0. H. H. Podgurski, private communication.
83 (1968).
29 (1968).
83 (1970).
869