Scripta
METALLURGICA
Vol. Printed
8, pp. 15-22, in the U n i t e d
1974 States
Pergamon
Press,
Inc.
MOSSBAUER STUDY OF PRECIPITATION IN Fe-Ni-Mo AND Fe-Ni-Co-Mo MARAGING ALLOYS G E N I N , G. LE CAER de M4tallurgie , Ecole des Mines 54000 Nancy , France. Ph. MAITREPIERRE and B.J. THOMAS Groupe M@tallurgie Physique , Institut de Recherches de la Sid@rnrgie FranQaise , 78104 Saint-Germain-en-Laye,France. J.M.
Laboratoire
Received
October
25,
1973)
MSssbauer spectroscopy is potentially a very valuable technique for the study of precipitation phenomena in iron-base alloys since
~TFe is one of the few natu-
rally occuring isotopes which give rise to a large MSssbauer resonance
(1,2). The
technique has been notably applied to the study of precipitation in iron rich Fe-Mo alloys by Marcus, quenched alloys,
Schwartz and Fine
(3,4). These authors observed that, in the as-
the six peaks of the iron spectrum were broadened into six envelopes
which were analysed as a superposition of three sets of peaks one set each for iron atoms with 0,1 and 2 molybdenum atoms amongst their nearest neighbours.
During aging
the form of the peaks was observed to change progressively with aging time. This behaviour corresponds to modifications in the relative intensities of the three subspectra and was attributed to a redistribution of the Mo atoms in the alloy.
Simulta-
neously a new peak appeared in the center of the spectrum due to the precipitation of a paramagnetic phase containing a fraction of the iron atoms. The isomer shift associated with this peak was shown to be compatible with the isomer shifts of Fe Mo intermetallic compounds studied by the same authors (3). The interpretations x y were largely supported by the results obtained using other techniques on these and similar alloys (3, 4, 5). MSssbauer spectroscopy has recently been applied to ternary Fe-Ni-Mo and quaternary Fe-Ni-Co-Mo alloys (6, 7, 8).
The results obtained are quali-
tatively similar to those described above for Fe-Mo alloys.
In the present work MSssbauer spectroscopy has been used to study the aging behaviour of an Fe-12.2 at % Ni - 6.2 at % Mo alloy and an Fe-17.3 at % Ni-8.4 at % Co-2.9 at % Mo alloy for which a considerable amount of information relative to the precipitation processes had already been obtained by classical techniques electron microscopy,
X-ray diffraction
(hardness,
(9, 10, 11). The MSssbauer experiments were
performed at room temperature on electrolytically thinned 20 ~ - thick foils using an ELRON transmission MSssbauer spectrometer and a 57Co source in a palladium matrix. 15
16
MOSSBAUER
STUDY
OF
PRECIPITATION
IN M A R A G I N G
ALLOYS
Vol.
8, No.
Fe-No-Mo alloy. The first
set of M S s s b a u e r
6.2 Mo alloy a s - q u e n c h e d at 450°C.
spectra was obtained from samples of the F e - 1 2 . 2 N i -
and q u e n c h e d
This a g i n g t e m p e r a t u r e
observations
and aged for 2, 8, 16, 30, 64 and 122 hours
was chosen because previous
(11) has shown that a single p r e c i p i t a t i o n
times up to 200 hours at 450°C. of the m a r t e n s i t e logy. R e v e r s i o n
The p r e c i p i t a t e s
process was active for a g i n g
nucleate
rapidly
and their growth leads to a characteristic
austenite
is not observed before
electron m i c r o s c o p y
on the d i s l o c a t i o n s
" ribbon-like
" morpho-
200 hours aging.
0.12
0.10 0.06 0.0~
0.05 0.O4
'¢ o,04 0.02
0.02
O.Ob
3 7
(~
5
4
~
2
".
oF_.~-~-~-4-~-2
i
~~'~
VELOCITY(mm,'=)
~'4
~
6
-a-
-b-
0.12
0.10
O.O8
Fi~. I - R o o m - t e m p e r a t u r e M S s s b a u e r spectra of the Fe-12.2 Ni-6.2 Mo (at %) alloy. - a - As-quenched - b - Aged 30 hours at 450°C - c - Aged 122 hours at 450°C.
z I-
i ,,¢
r
o.o4
+
o e,
,
,~,
,~
,_, _1 2 1
,
0
1
,
2
,
VELOCITy(minis) -C-
Three
selected
from the u n a g e d
spectra are shown in Fig.
sample
ding to 3 distinct
(Fig.
fields are r e s p e c t i v e l y
I. The fine structure
I a) can be d e c o m p o s e d
environments
of the Fe atoms
into 3 basic
(labelled
of the spectrum
spectra correspon-
I, 2 and 3)- The hyperfine
H I = 337 ~ 0.5 k O e , H 2 = 293 ± 2 k O a a n d H 3 = 250 ~ 5 kO e
F o l l o w i n g the i n t e r p r e t a t i o n
of Marcus and A1.
atoms which have no i n t e r a c t i o n
with Mo atoms.
-~
VELOCITY(r.,,./=)
(3,4), peak I can be a s s o c i a t e d This means
to
that these atoms have no
1
Vol.
8, No.
1
Mo n e i g h b o r s
MOSSBAUER
in the first
Peaks 2 and 3 c o r r e s p o n d determining
STUDY
OF
two shells
(12) or p o s s i b l y
the areas u n d e r the various peaks,
obtained from various models It was shown
(11) that,
to the p r e c i p i t a t i o n
tent. A f t e r phase.
The i n t e n s i t i e s
peak is c l e a r l y apparent
Measurement
(Fig.
(Fig.
compounds
In order to study the k i n e t i c s
where X is the fraction
of the p a r a m a g n e t i c
can be
Fe con-
peak shows that
The isomer shift
of the p r e c i p i t a t i o n
process
of Fe atoms in the p r e c i p i t a t e
experiment
on a sample
of Xp with time is p l o t t e d
Q9
phase,
of the precipifor Fe Mo x y
through the variawe define Xp = X/X F
after a given aging time and
process.
A value
of X F
~
0.122 was
in which the p r e c i p i t a t i o n in Fig.
,
was com-
2.
r
~
,
,
r
I
i
.#
Q8
o7
i
I
// /
"1
/'
X
/
/ s
J
o~
ts
o,3
/
I
/
/
/
/
o
/
X
0,2
sharply and a paraThis decrease
with the isomer shift m e a s u r e d
at the end of the p r e c i p i t a t i o n
The v a r i a t i o n
of the various
phase with an a p p r e c i a b l e
of Fe atoms in the p r e c i p i t a t e
obtained from a separate plete.
of Mo
(3) (13).
tion of the c o n c e n t r a t i o n
X F the f r a c t i o n
sites.
Ic) most of the Mo is in the p r e c i p i t a t e
intensity
~ - 0.28 mm/s w h i c h is compatible
intermetallic
lattice
intensity
of peaks 2 and 3 decrease
of a M o - c o n t a i n i n g
of the relative
of
to those
a random d i s t r i b u t i o n
Ib) the relative
11.8 % of the Fe atoms are in the precipitate. tate is
on the C.C.
sample,
By
the p r o p o r t i o n
can be compared
in the center of the spectrum.
122 h o u r s a g i n g at 450°C
(5).
spectra.
A f t e r a g i n g at 450°C for 30 hours peaks changes markedly.
17
richer Mo environments.
one can calculate
of Mo atoms d i s t r i b u t i o n s
even in the a s - q u e n c h e d
ALLOYS
the first three shells
I, 2 and 3. The results
atoms cannot account for the observed
attributed
IN M A R A G I N G
to Fe atoms with p r o g r e s s i v e l y
Fe atoms with the e n v i r o n m e n t s
magnetic
PRECIPITATION
,," 01
,/
0,1 !
o
Fig.2 -
i
lo
i
i
10o
Aging time {hours)
A l l o y Fe-12.2 Ni - 6.2 Mo(at %). V a r i a t i o n of X_(o) and XTT (e) • with a g i n g t±me at 450°C~
I0
~)o
Aging time (hours)
Fig. ~ - A l l o y Fe-12.2 Ni-6.2 Mo (at %). I V a r i a t i o n of Log ( ~ ) with o I-X a g i n g time at 450 C. P
9
18
MOSSBAUER
STUDY
OF
PRECIPITATION
IN M A R A G I N G
ALLOYS
The variation with aging time of the hyperfine field without Mo near neighbors
Vol.
8, No.
1
(h.f.) H I (Fe atoms
) was also followed by computing the ratio ~ = ( H I - H I O ) / H I F ,
were H10 is the hyperfine
field in the as-quenched sample ( ~ 337 kOe), H I the h.f. F field after a given aging time and H I the h.f. field after complete precipitation ( ~ 342 kOe).
The variation
of X H with time is
clearly seen that the h.f. field increase
the Fe atoms have gone into the precipitate
fraction of
phase. This shows that Mo atoms cluster
during the early stages of the aging process, 1 In figure 3, Log - - ~
also plotted in Fig. 2. It can be
starts before any noticeable
as suggested by various authors
(5) (7).
is plotted versus aging time on a Log scale. The short-
comings of this type of analysis are well known, but the fact that only one precipitation reaction occurs at 450°C up to 122 hours aging justifies
its use in the pre-
sent context. A straight line of slope n ~ 1 is thereby obtained, ponent of time in the Johnson-Mehl
formula Xp = 1 - exp(-ktn).
in good agreement with theoretical
predictions
ders of precipitate
Such a value of n is
(14) for the thickening of long cylin-
phase. This is indeed the case beyond 8 hours aging at 450°C,
since electron microscopy red by precipitates
where n is the ex-
observations
have shown that all the dislocations
are cove-
at that stage of aging (11).
In the publication
already quoted
followed by measurements
kinetics at 450°C were
(11), the precipitation
of the matrix lattice parameter.
/
Fig. 4 - Alloy Fe-12.2 Ni-6.2 Mo (at %). Comparison of MSssbauer spectroscopy results with lattice parameter measurements
(Xp)
(Xa).
Q5
I
~
I
I
J
i
i
i
i
Xo a
In Figure 4, Xp
is plotted versus X a , defined as Xa
the lattice parameter in the as-quenched
state,
a the lattice
-
a
a° _ aF o
, where a ° is parameter
after a given aging time at 450°C and a F the lattice parameter after complete precipitation.
The good agreement between the two methods is clearly apparent.
Vol.
8,
No.
1
M'OSSBAUER STUDY OF P R E C I P I T A T I O N
IN MARAGING ALLOYS
19
Fe-Ni-Co-Mo alloy. A limited number of MSssbauer experiments were performed on a quaternary Fe-17.3 Ni-8.4 Co-2.9 Mo as-quenched and aged at 350°C. It has been previously shown that on aging this type of alloy at temperatures below about 450°C,
the precipitates
are not nucleated on dislocations but instead are randomly distributed throughout the matrix
(9, 10, 15, 16). These precipitates are small spheroidal particles 20-30
in diameter with an ordered structure derived from the BCC lattice. The superlattice has lower symmetry than the parent lattice and appears in 4 distinct orientations relative to the matrix
(9, 10, 15). Similar precipitates are also observed in certain
ternary Fe-Ni-Mo alloys
(10). Yedneral and Perkas
(15) have shown that the super!at-
tice diffraction spots can be accounted for by segregation of molybdenum atoms onto every third plane of a given (111],i family. The aim of the present work was to try to obtain some information concerning the arrangement
of the Fe atoms in the super-
lattice.
0,I0 0,I0
o
tl
tI
t~
! o.o~; 0,02 -8
7
6
-5
-4
-3
2
-1
I 0
1
2
3
4
5
6
7
8
5 6 7 8
VELOCITY(mmll)
VEL~ITY(mmls)
-aFig.
-b-
5 - Room-temperature spectra of the Fe-17.3 Ni-8.4 Co-2.9 Mo (at % ) - a - As-quenched. - b - Aged 1024 hours at 350°C.
The spectrum obtained in the as-quenched martensitic main Fe environments
state (Fig.5a) shows two
(there might be a very small peak corresponding to a third type
of environment but its intensity is too weak to be included in the analysis).
The
h.f. fields were respectively H I ~ 347 kO e and H 2 ~ 303 kO e. After aging for 1024 hours at 350°C
(an advanced stage of homogeneous precipitation)
peak 2 has all but
vanished and the spectrum corresponds to that of a nearly molybdenum-free matrix with H I ~
355 kO e (Fig. 5b). Moreover there is no evidence of a paramagnetJc
peak
in the center of the spectrum which shows that the precipitate contains very few iron atoms, but does not preclude the presence of nickel and/or cobalt atoms.
Discussion and conclusions. The results of the MSssbauer study of precipitation at 450°C in the Fe-12.2Ni-6.2
20
MOSSBAUER
STUDY
OF P R E C I P I T A T I O N
Mo alloy are complementary
IN M A R A G I N G
ALLOYS
Vol.
to the results obtained by electron microscopy
8, No.
and
lattice parameter measurements.
Firstly,
the precipitation
wholly compatible
Secondly,
kinetics
(n ~ I
with the observed "ribbon-like"
by combining an analysis
in the Johnson-Mehl morphology
of the MSssbauer
equation)
spectrum obtained from the
specimen aged for 122 hours with the lattice parameter measurements specimen,
it is possible
precipitate
to make an estimate
at the end of the precipitation
of the composition reaction.
are
of the particules.
from the same
of the matrix and the
The results which are presented
in the table below were obtained in the following way
:
TABLE I Composition of precipitate and matrix at the end of precipitation (for 100 atoms of alloy).
Alloy
Fe
Ni
Mo
81.6
12.2
6.2
9.7
0.7
5.2
71.9
11.5
Precipitate Matrix
The number of Fe atoms in the precipitate intensity
of the paramagnetic
was calculated from the relative
peak as described earlier.
number of iron atoms was obtained with corresponds of the alloy). the MSssbauer matrix,
This gives by difference
A value of 11.8 % of the
to 9.7 atoms of Fe (for 100 atoms
71.9 Fe atoms remaining in the matrix.
spectrum shows that there are very few molybdenum
the ratio of nickel
I
to iron in the matrix can be estimated by comparing
h.f. field of peak I (H I = 342 k0 e) with recently determined values in binary Fe-Ni alloys
ponds to 11.5 nickel atoms in the matrix.
analysis.
As pointed out by Marcus and al.
in the matrix.
These calculations of the precipitation
~
I
(4) when the molybdenum
concen-
concerning
However by comparing the lattice parameter measurements
as a function
mated that approximately
contain
has already been shown by
spectroscopy yields little information
of the matrix at the end of precipitation lattice parameters
of the h.f. field
It follows the precipitates
of nickel in the precipitates
tration is very small MSssbauer the molybdenum
the
(17). The value of Ni/Fe thus obtained was 0.16 which corres-
nickel atom. The presence chemical
Since
atoms remalng in the
(2.874 ~) with previous measurements
of nickel and molybdenum
contents
of
(11) it was esti-
I Mo atom remains in the martensite.
suggest that the composition
process approximates
of the precipitate
to the formula
at the end
(Fe,Ni)2Mo rather than to
1
Vol.
8, No.
the formula
1
M O S S B A U E RSTUDY OF PRECIPITATION IN MARAGING ALLOYS
(FsNi)Mo which was obtained by chemical
microanalysis
on extracted precipitates
It should again be emphasised ful information
concerning
21
analysis and electron probe
(11).
that M8ssbauer
the kinetics
spectroscopy not only yields use-
of the actual precipitation
process
( varia-
tion of Xp with aging time ) but also gives some insight into the simultaneous
rear-
rangement
of
of the atoms in the matrix solid solutions as shown by the variation
the h.f. field.
The MSssbauer
experiments
clearly that the homogeneous
(and in certain Fe-Ni-Mo alloys to molybdenum
performed
on the quaternary Fe-Ni-Co-Mo
precipitation (10))
alloy show
observed at low temperature
corresponds
rich zones which are virtually
to the segregation
in Fe-Ni-Co-Mo of Mo atoms in-
iron free. This confirms again the gene-
ral tendancy of Mo to cluster in these Fe-Ni and Fe-Ni~Co matrices.
In terms of the
model proposed by Yedneral and Perkas these results imply that the formation of the ordered structure third (111)~
involves not only the segregation
plane but also the replacement
within the precipitates
It would be quite interesting
is limited to molybdenum
of the V and VI groups
atoms onto every
of iron atoms on the other (111)~ planes
by nickel and/or cobalt atoms.
to confirm whether this behaviour to other elements
of molybdenum
or whether it also applies
such as Nb, W, Ta, as might be expected.
Acknowledgments. The authors wish to thank Mr. J. BOURGEOT
for his continued aid in this research.
References. I.
H. FRAUENFELDER,
2.
N.M. GREENWOOD and T.C. CIBB, MSssbauer London (1971).
3.
H.L. MARCUS,
4.
H.L. MARCUS and L.H.
5.
T. ERICSSON,
S. NOURIKIS
6.
H.L. MARCUS,
L.H. SCHWARTZ and M.E. FINE, Trans. ASM, 59, 468 (1966)..
7.
P.L. GRUZIN, Y.L. RODIONOV, Y.D. ZHAROV, V.S. MKRTCHYAN, M.D. PERKAS, Soviet Phys. Doklady, 17, 64 (July 1972).
8.
P.L. GRUZIN,
The MSssbauer
M.E. FINE and L.H. SCHWARTZ,
effect. W.A. Benjamin,
SCHWARTZ,
Science, ~ , 901
and Y.A.LI,
2 b 475 (1972). 9.
B.J. THOMAS,
M4m.
Sci. Rev. M4t.,
58, 4750 (1967).
162, 259 (1967).
J. Mat.
V.S. MKRTCHYAN
(1962).
Champan et Hall Ltd.,
J. Appl. Phys.,
Phys. Rev.,
and J.B. COHEN,
Y.L. RODIONOV,
spectroscopy.
Inc. New-York
68, n ° 3, 212 (1971).
(1970).
A.F. YEDNERAL
and
Soviet Phys. Deklady,
22
M O S S B A U E R STUDY OF P R E C I P I T A T I O N
IN M A R A G I N G ALLOYS
Vol.
8, No.
10.
J. BOURGEOT, Ph. MAITREPIERRE, J. MANENC and B.J. THOMAS, in Electron Microscopy and Structure of Materials, G. THOMAS and al., Ed. p. 818 (Berkeley 1971).
11.
J. BOURGEOT, Ph. MAITREPIERRE, J. MANENC and B.J. THOMAS, M@m. Sci. Rev. M~t., 70, n ° 2, 125 (1973).
12.
VINCZE and I.A. CAMPBELL,
13.
R. COURRIER, Th~se de Doctorat, University of Nancy (1972).
14.
J.W. CHRISTIAN, The theory of transformation in metals and alloys. Pergamon Press, Oxford (1965).
15.
A.F. YEDNERAL and M.D. PERKAS, Phys. Met. and Metallography, 33, n° 2, 89 (]973).
16.
G. MAEDER and C. SERVANT, 16~me Colloque de M~tallurgie, Saclay (France), June 1973.
17.
G. LE CAER and R. COURRIER,
J. Phys. F. : Metal Phys., ~,647 (1973).
Unpublished results.
1