It is generally recognized that in any well behaved metal fatigue failure, the following sequence of events occurs.  The application of stress causes the movement of defects already present in the specimen, besides producing new defects. These defects concentrate in the regions of stress concentration. This defect concentration, which precedes crack nucleation and its eventual growth, actually amounts to environmental changes for Mossbauer atoms located in these regions. Changes in the atomic composition of the Mossbauer absorber atom environment are sensed by examining changes in the sensitive parameters of the Mossbauer spectra. A preliminary study of changes in the isomer shift and line width of commercial nonmagnetic steel specimen is reported

Singh, J. J.

English

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MOSSBAUER EFFECT AND ITS APPLICATIONS 
I N  MATERIALS RESEARCH 
Jag J.  Singh 
Langley Research Center 
Hampton, Virginia 
Presented at 
Symposium on Welding, Bonding, and Fastening 
Williamsburg, Virginia 
May 30 - June 1,  1972 
. .  
https://ntrs.nasa.gov/search.jsp?R=19740022839 2020-03-17T15:11:42+00:00Z
MDSSBAUER EFFECT ANI) ITS APPLICATIONS 
IN MATERIALS RESEARCH 
Jag J. Stngh 
Inngley Berearch Center 
INTROWCTION 
Mossbauer effect spectroscopic measurements provide data for a 
selected atam and its environments in a given matrix. The Mossbauer 
spectrum of a given atom, in a certain chemical environment, is unique 
and its characteristic parameters can be measured with 8 great degree of 
accuracy. The presence of any chemical impuritier, or changes in the 
atomic composition of its neighbsrhood, will cauoe ch8nger in the Mooobauer 
parameters. The measurement of there changes can serve as the basir for 
analysis of matrix variations. 
factors that can affect Mosrb8uer parameters.) 
(See Figure 1 for a s u m r y  of variour 
It is generally recognized that in any well-behaved metal fatigue 
failure, the following sequence of events occurs: The application of 
stress (cyclic or static) cause0 the movement of defects already present 
in the specimen, besides producing new defects. These defects concentrate 
in the regions of stress concentration. (In otherwise unflawed materials, 
fatigue cracks always nucleate in these regions. ) Thir defect Concentration, 
which precedes crack nucleation 8nd it8 eventual growth, actually amounts 
to environmental changes for Mossbauer atom located in these regions. - 
These changes in the atomic composition of the Horsbauer absorber atom 
environment can be rensed by examining ch8nges in the sensitive parameters 
2 
of the Mossbauer spectra. 
changes in the isomer shift and line width of commercial nonmagnetic 
steel specimen. The results of this study will be discussed in this 
paper. 
We have completed a preliminary study of 
EXPERIMENTAL PROCEDURE 
(a) Mossbauer Spectrometer. This study first involved the 
development of an efficient backscattering technique of Mossbauer 
spectroscopy, since the existing techniques (Refs. 1 and 2), shown in 
Figures 2 and 3, are either inefficient or require a specialized 
specimen geometry. The essential element of backscattering spectroscopy 
is the radiation detector, An annular detector, which would permit the 
collimated incident radiation to pass through to strike the test 
specimen located immediately behind it appears to meet the requirements 
of high detector-specimen solid angle as well as of in situ measurements. 
Such a detector has been developed and is shown in Figure 4. It consists 
of a 2'' x 2'' x 0.6" NaI(TA) crystal, with a 0.6" diameter central hole, 
coupled to two matched photomultipliers. 
electronic circuit for equalizing the gains and the time constants of 
the two multiplier channels is shown in Figure 5. 
shows the rest of the circuit - including the Mossbauer source motion 
A schematic diagram of the 
This figure also 
transducer and the multichannel analyzer. 
The incident beam has to be well-collimated in order to minimize 
the dispersion in the source-scatterer relative velocity and also to 
3 
avoid striking the radiation detector before striking the specimen 
(scatterer). A avgnified view of the source collimator and the test 
specimen is shown in Figure 6. Using the dimensions given in this 
figure, it is easily seen that the collimator assembly keeps the incident 
'photon beam divergence at the scatterer to an angle less than 2 . 5 O .  
. This low divergence reduces the source velocity dispersion at the 
scatterer to lcsr than 0.1 percent. 
The annular radiation counter, in combination with a stable electro- 
mechanical transducer and drive system producing motion of the source 
with constant acceleration through a range of,velocities, can be used 
to study Mossbauer spectra in both the transmission and scattering 
geometries. Figures 7 and 8 show the transmission and the backscattering 
2 spectra, respectively, obtained with a 0.8 mg/cm , Fe57-enriched iron 
target. Figure 9 shows a typical backocattered Mossbauer spectrum 
obtained with a 0.030-inch thick nonmagnetic (55-347) steel scatterer. 
The Mossbauer source in all these case8 was a 10-mc Co source in 57 
platinum matrix. The absorber/scatterer in each case was positioned to 
subtend an angle of n steradians at the detector. 
it is clear' that the present detector configuration is C8Qabh of 
From Figures 7 and 8, 
producing equally good transmiasion and backscattering Mossbauer spectra. 
Figure 9 clearly shows that the present syetem is uaable with scatterers 
of any arbitrary thickness (Ref. 3 ) ,  
(b) Metal 'Fatigue Damage Study. As indicated in the introduction, 
when a metal specimen is progressively fatigued, dislocations start 
v 
4 
p i l ing  up i n  the regions of stress concentration, I n  the absence of 
stress raisers, these dis locat ions w i l l  be randomly d is t r ibu ted  throughout 
the test  specimen and w i l l  not have a s igni f icant  effect i n  modifying 
the absorber atom surroundings. Following these arguments, aus ten i te  
steel specimen (SS-302, SS-321, and SS-347) with stress raisers were 
prepared i n  two d i f f e ren t  geometrical configurations shown i n  Figure 10. 
Experience has shown tha t  test specimen with these configurations develop 
cracks a t  the notches or the sharp c u t s ,  presumably due t o  dis locat ion 
concentration there. These specimen were subjected t o  tension-tension 
2 2 cycl ic  loading i n  the range 2 kgm/rw + 2 0  kgm/mm ak the rate of 1000 
cpm and the backscattered spectra  from them(*)measured a t  d i f f e ren t  
leve ls  of fa t igue cycles, 
isomer s h i f t ,  peak width, and the Mossbauer resonant f rac t ion  were 
These spectra  were analyzed and the values of 
computed-using a program described i n  Ref. 4, Typical r e s u l t s  showing 
the dependence of two selected Mossbauer parameters on the number of 
fa t igue cycles i n  (83-347) specimen are shown in  Figure 11. 
DISCUSSION 
The defect /dis locat ion p i l e  up i n  the regions of stress concentration 
i n  the test  specimen leads t o  the creat ion of several  nonequivalent 
Mossbauer absorber atom sites i n  the area under observation. I n  the 
("'The spectrawere obtained from the regions where stress raisers were 
located, namely, the  edges of the notches o r  the s ides  adjacent t o  the 
eloxed cut .  
t 
6454 i 
5 
case of point defects, it can be argued (Ref. 5) that the net result is 
the superposition of several modified Mossbauer frequencies, W' : 
- undisturbed Mossbauer frequency 
= concentration of defects of type m at 
.I frequency shift caused by the defects of type m at 4 
u)O 
where 
a Am 
u)hn 
where urn stands for the deformation tensor relating 
the frequency change and the deformation potential, 
ij 
This statistical summation should result in widening the Mossbauer peak 
1 
i and also changing the isomer shift. The effects of point defects and 
dislocations on line widths - in limiting cases - are expected to be of 
the following form (Ref. 5): 
1 I for Ln Nb *1 
0 2  for Nd R e l  J 
where Nd - linear dislocation density 
Ni = dislocation loop density 
R - characteristic dimension of the dislocation loop. and 
6 
Thus, one expects the Mossbauer parameters to continue to change with 
the fatigue cycles tiaa nucleus of the crack has been formed. 
change is expected to be of the "saturation" type since the later 
This 
dislocation development will have reduced effect in creating additional 
nonequivalent sites or lattice distortion. The "saturation" change can 
be related to the critical stage of the specimen when crack nucleation 
is imminent. 
An examination of Figure 11 shows that the peak width shows a 
distinct upward trend with the increasing number of fatigue cycles 
whereas the isomer shift is barely affected. 
to indicate that quadrupole splitting is playing a role. 
This MY be interpreted 
However, 
the specimen were commercial, polycrystalline steels of ill-defined 
history and detailed measurements on single crystal specimen should be 
nude before any firm conclusions can be drawn (Ref. 6). 
however, that the changes in the Mossbauer parameters with fatigue 
are small and the use of Wlal as the Mossbauer source may be more 
appropriate. (This stems from the long half life, Ref. 7, of Ta . 
(6.25 keV) state, leading to a natural line width of 3.2 x loo3 mdsec 
compared with 0.197 mm/sec for Pe57.) 
into the material under study will have a tendency to go to the grain 
boundaries where dislocations also tend to concentrate. 
It does appear, 
181 
which can be diffused 
7 
REFERENCES 
1, Mo86buer Effect Methodology, Vol. I, edited by I. J. Gruve-n. 
Published by Plenum Press, 1965. 
I 2. A. R, Swanson and J. J. Spijkerutan: 3. Appl. Phys. 41, 3155, 1970. 
3. J, J. Singh: Nucl. Instr. & Meth. (to be published) and NASA 
TN 0.6819, 1972. 
4. L. M. Howser, J. J. Singh, and R. E. Smith, Jr.: USA TM X-2522, 
1972. 
5. L. B. KVa8hnina and M. A. Krivoglaz: Fiz. Metal. Metrlloved, 23, 
1967. 
7. Tables of Isotope8 (6th Edition), edited by C. M. Ledeter, J, M. 
Hollander, and I. Patlman. Publirhed by John Wiley and Sons (1967). 
. 
Applications Of Mossbauet Spectroscopy To Physical 
Metallurgy Are Based On The Following Factors: 
( 1)  NEIGHBORING ATOM INTERACTIONS. 
(2) LOCAL CUBIC SYMMETRY 
(3) ELECTROSTATIC (QUADRUPOLE) INTERACTIONS. 
(4) LATTICE DISTORTION (POLYCRYSTALLINE ALLOYS) 
CHANGES IN THESE FACTORS, RESULTING FROM 
t DISLOCATION MOVEMENTS AND PILE UP, AFFECT 
THE INTERNAL MAGNETIC FIELD, THE ISOMER 
SHIFT, THE ELECTRIC FIELD GRADIENT AND THE 
LATTICE PARAMETERS AT THE ABSORBER ATOM 
SITES. THESE CHANGES MAY BE MORE 
PRONOUNCED IN MATERIALS WITH LOW STACKING 
FAAT ENERGIES. ’ 
t 
F I G U R E ’  1 
Argon - 10% Methane 
Counter Gas 
) 80005" Iron Foil Window 
Figure -2. Schematic Arrangement for Scattering Geometry 
(C - Counting) in Mossbauer Spectroscopy. 
FIGURE 2 
e 
L 
Drive b i t  
Figure - 3. . Schematic Arrangement for Scattering 
Geometry (Photon Counting) in Mossbauer 
Spectroscopy. 
F I G U R E  3 
. 
Focusing Voltage Hv Focusing Volt age 
(Adjust) (Stabilized) (Adjust) 
(dual beam scope) X t  I Pre.Amp.1 X2 (Dual Beam 
- Scope 
Scope) 
lAmplifierl-- Pulse Height Analyzer 
General Circuit fo r  Equalizing Photomultiplier Gains 
and Pulse Arrival Times. 
I Pre-Amp. 1 
(Mossbauer Spectrometer). 
Schematic Diagram of  the Signal Conditioning Circuit  
for Mossbauer Spectrometer. 
F IGURE 5 
653K 
i 
i 
(N3H1133dS lS31) 
I 
r i  I 
I 0 0 s  I 
Y 8 3  I 
I 0 m 
I 
v) 
W z 
e 
0 
LL 
a. 
3 
I- 
w 
v, 
I 
(D 
N 
W 
0 
- 3  
v, 
a 
‘ 0  
0 
W 
ar 
a 
a 
LL. 
- 
. .  
. ... 
c 
\ 
\ 
. . ... 
N 
t i
c 
0 
.8 
c I . .: . . . -  
_ _  1 
. _  / 
. . . .  
4.. . .  
\: / 
. . .  . . .  _. . ..-~. .......... AI_ . ...- .. .-- . ... . .  -. . - . . . . .  _ _  _- __.__*A _ _  . :_ T -. . - 
m 
0 
X 
-
.... 
. .  
I 
. . .  
9 .  ' . 
. . .  
. .  . .  . .  . . .  ._ . 
' . '. .. .. . . .  . . . .  
. . .  // . . .  
. . . .  . . .  ... . . .  . . . .  . . . .  . .* . . .  . . . . .  . . .  c 
.. . f  . . . . . .  . . . . . .  ... 
. .  * -/: . 
cn. 
W 
cI1 
3 
-.- 
LL, 
0.25" Diameter 
Semicircular 
Notches 
A 
12.0" 
I" t- 
0.007"X 0.015" 
Eloxed Cut 
Specimen Thickness = 0.030'' 
Geometrical Configurations of Steel Specimen used 
in the Study of Fatigue-induced changes ifd their 
Nossbauer Spectra 
F I G U R E  10 
n 
' 8  < 
E 
+< E 
U 
r, 
0.50 
0.40 
0.30 
( 
a = W 0.20 
8 
Q O  
(0.0159 * O.OOX)mm/sec (+I d r  dN -= I 06 c yc I es 
Number of Fatigue Cycles, N 
0.30 
0.20 
OJO 
(0.0048 * 0.0035 )rnm/sec 
106 cycles 
(-1 
I '  I I 
0 I 2 3 i (X IO6)  01 
Number of Fatigue Cycles, N 
\ 
Dependence of Mossbauer Peak Width (I?) and 
Jsomer Shift (1.S.) on Cyclic Stress in SS-347 
Steel Specimen. - *  
F IGURE 11 
(659*.ie 


Mossbauer effect and its applications in materials research

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