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Hyperfine field distributions and crystallisation processes in


									J. P h ) s . F: Met. P h l s . 15 (1985) 2289-2295. Printed   in   Great Britain

Hyperfine field distributions and crystallisation processes in
amorphous Fe&rzBl&          powder prepared by the
comminution of melt-spun riibons

                    R A Dunlap and K Din1
                   Department of Phlsics. Dalhousie UniLersit!. Halifax. NoLa Scotia. Canada B3H !J5

                   Received 3 September 1984. in final form 3 Mal I Y 8 5

                   Abstract. Fully amorphous Fe78Cr2B12Si8    powder has been prepared b> grinding h > d r o g e n ~
                   embrittled melt-spun ribbons. Mossbauer effect and differential thermal analysis m e a s u r e
                   ments have been performed on powders as a function of grinding time and also as a function
                   of subsequent annealing temperature. The results suggest that grinding produces changes
                   in the short-range order which can be remoLed. in part. by annealing.

1. Introduction

While amorphous Fe-based alloys have a number of potential commercial applications
based on their superior magnetic and structural properties, these uses are limited to a large
extent by the physical dimensions of foils and ribbons which can be produced. In order to
broaden the range of applications of these alloys the production of amorphous powder is of
interest. For a recent review of research in amorphous powders see Miller (1983). The
techniques for the production of amorphous powder can be divided into two categories: ( 1 )
those which produce powder directly from the melt and (2) those which produce powder
by the comminution of amorphous ribbons. The latter method has the advantage that the
quench rate is independent of the particle size of the powder. However. it has the
disadvantage that most Fe-based amorphous alloys are particularly hard and have high
tensile strength. Thus the comminution process is difficult unless the ribbons are somehow
embrittled. Investigations in the past have therefore concentrated primarily on powders
produced by the first method. In this work we report on an investigation of hyperfine field
distributions and crystallisation processes in amorphous Fe,sCr,B,2Si, powder prepared
by the comminution of hydrogen-embrittled melt-spun ribbons. We recently reported on
the details of the crystallisation processes (Dunlap et a1 1985) of as-quenched Fe-Cr-B-Si

2. Experimental methods

Ribbons of amorphous Fe,,Cr,B,,Si, approximately 1 mm wide by 1 5 p m thick were
prepared by rapid quenching from the melt onto the surface of a single C u roller. Ribbons
were embrittled by electrolytic hydrogenation in a 0.2 M solution of H,SO, containing
10 PPM NaAsO,. The charging time was 15 minutes at a current density of about

0305-4608/85/112289 + 07s02.25                     0 1985 The Institute of Physics                         2289
2290           R A Dunlap and K Dini

100 A m-2. Powder was produced by grinding the ribbons immediately after
hydrogenation with an alumina mortar and pestle mounted in a Fisher model 155 grinder.
After grinding, the amorphous Fe-Cr-B-Si         powder was washed in acetone and
magnetically separated to remove any AI,O,. Differential thermal analysis (DTA),
Mossbauer effect and x-ray diffraction measurements were performed according to the
conventional procedures described previously (Dini et a1 1984). All DTA measurements
were made for samples of 25 mg. Mossbauer effect spectra were fitted using the Fourier
expansion method of Window (1 97 I). The details of the choice of proper fitting parameters
in this method have been discussed recently by Dunlap and Stroink (1984a). Scanning
electron micrographs (SEM) were obtained on a JEOL model JX-35 electron microscope.

3. Results

A SEM of Fe,,Cr,B,,Si, which was ground for 20 minutes is shown in figure 1. We observe
that the particles are of irregular shapes and cover a range of sizes from about 1 pm to
 IOpm. A sample ground for two hours showed particles of similar shapes and a similar
range of sizes although smaller by about a factor of three.
     Mossbauer effect spectra and hyperfine field distributions, P ( H ) , shown in figure 2
illustrate the effects of hydrogenation and grinding. The P ( H ) for the as-quenched alloy
shows a slight shoulder on the low-field side. This is consistent with previous studies of
P ( H ) in Fe-based amorphous alloys which contain group VIA transition metals (Dunlap
 1984, Dunlap and Stroink 1984b, Chien 198 I). The absorption of hydrogen has little effect
on P ( H ) although there is some change in the relative intensity of the second and fifth
absorption lines, indicating a realignment of the ferromagnetic domains. This same
behaviour has been reported for similar Fe-based amorphous alloys by Coey et a1 (1982).
Grinding produces two effects that increase in magnitude with increased grinding time, as
observed in figure 1. These are (i) an increase in the low-field component as well as a zero-
field component in P ( H ) and (ii) an increase in the asymmetry of the spectrum. We should
point out that our fitting routine does not take into account correlations between the isomer
shift and the hyperfine field. It is these correlations that are frequently responsible for the
asymmetry of the Mossbauer spectrum (Chien and Chen 1979).

               Figure 1.   SEM   of Fe78Cr2B,2Sie powder prepared by grinding for 20 minutes.
              A tnorphous Fe,,Cr,B,,Si,         po\c,der                                                 229 1

E                                                                                                   L.

Y                                                                                                   s


                                           15    0           100          2 00          300

                  Velocity imm     5.')                                H ikOei

              Figure 2. R o o t n ~ t m y " u r e Mbhsbauer spectrum 01' Fe,,Cr2BI2Si, ( a ) as~qtlenched.(11)
                                  Ii! drogenated and ground 20 minutes and ( d ) ti! dropenated and ground
              h! iirogei1ntt.d. ( c i
              t u I) hour\.

    The effects of annealing hydrogenated and ground Fe,,Cr,B,,Si, are illustrated in
figure 3. We observe that annealing at 3 7 5 K has essentially no effect on P ( H ) . According
to recent DTA measurements by R A Dunlap and J E Ball (1984. unpublished). annealing
at this temperature should be sufficient to cause any residual hydrogen to diffuse out of this

                  Veiocity ( m m i ' ~                               i
                                                                     t (kOe1
               Figure 3. Room tcmpcrature MZjssbauer spectra illustrating t h e effects of lo\\ temperature
               annealing: Io) ground 20 minutes and annealed at 375 K for 2 4 hours ;and ( b ) ground and
               annealed at 5 2 5 K for 24 hour\.
2292            R A Durilap arid K Dirii




                       1   1    1    1   1   1    1      1   1   1   1   1
                               700                    800                    90

                                                 T (K)
                Figure 4. U I \ ,can5 of F e - 8 C r 2 B i 2 S ~( 8) as quenched. ( b ) hbdrogenated and ground 20
                                                               ( I
                minute\. ( c ) hldrogenated and ground t u 0 hours ( d ) ground t w o hours and annealed at
                5 2 5 K for 2 1 hours

alloy. The figure shows. however, that annealing for 24 hours at 5 2 5 K is sufficient to
change the P ( H ) nearly back to its form before grinding.
     The effects of grinding and annealing on the shape of the crystallisation peak in
the DTA measurements are illustrated in figure 4. The as-quenched alloy shows three
exothermic peaks in the DTA. Dunlap et ai (1985) have recently reported x-ray diffraction
measurements which show that, in order of increasing temperature, the three peaks
correspond to the formation of a-FeCr. a - F e (with C r and Si) and Fe,B. It is seen here that
hydrogen absorption has no effect on the temperature or shape of the crystallisation peaks
in this alloy. Grinding has no appreciable effect on the temperature of the crystallisation or
on the general form of the DTA peaks. However, it does cause significant broadening of the
exotherms. As illustrated in the figure, annealing at 525 K causes some narrowing of the
peaks. although even after this heat treatment they remain broader than for the as-
quenched alloy.
     X-ray diffraction scans of as-quenched, hydrogenated and ground samples all showed
the alloys to be fully amorphous. No measurable shift in the locations of the diffraction
peaks for the amorphous structure was observed, indicating that there was no measurable
change in the \olume of the sample either on hydrogenation or on grinding.

4. Discussion

The effects of deformation on the physical properties of metallic glasses have been
investigated both experimentally and theoretically by several authors.
    From an investigation of Ni-P and Fe-Ni-B using transmission electron microscopy.
Dono\ an and Stobbs ( 1 9 8 I ) suggested that plastic deformation resulted in the formation of
shear bands. The formation of shear bands is supported by magnetisation measurements
                 A inorphous Fe,, Cr,B ,,Si8 powder.                                     2293

which show a significant increase in the coercive force, H,. in ferromagnetic amorphous
alloys which have undergone cold working (Gibbs and Evetts 1980, Luborsky et a / 1976).
It is commonly believed that the increase in H , results from the pinning of domain walls by
shear bands. In addition. there is evidence that there are short-range topological changes
caused by plastic deformation. Several authors have suggested that this is manifested by
regions of excess free volume (see, e.g.. Guoan et a1 1984, Donovan and Stobbs 1981.
Spaepen 1977. Argon 1979).
     While previous studies have concentrated on the properties of cold-worked samples,
the present l\,ork has investigated samples which have been embrittled with hydrogen prior
to deformation. It is not clear how the deformation in these alloys relates to that in
quenched alloys. As we shall see below. however, the results obtained here are consistent
with previous observations of deformed as-quenched alloys.
     We have observed three changes in Fe,,cr,B,,Si, as a result of grinding: namely. (i)
growth of a low-field component in P ( H ) , (ii) increased asymmetry of the Mossbauer
spectra and (iii) broadening of the exothermic crystallisation peaks in the DTA. These are
discus sed below.

4.1 . Hyperfine field distributions
The growth of the low-field component in the P ( H ) caused by grinding clearly indicates
that the local environment of some of the F e atoms is changing. The presence of a low-field
peak in P ( H ) is commonly reported for amorphous Fe-based alloys which contain C r (or
MO) (Chien and Hasegawa 1978. Whittle et a1 1982. Dunlap 1984). Chien (1981) has
suggested that the high- and low-field components result from F e with different numbers of
C r (or MO) near neighbours. This is consistent with the fact that the concentration of C r
(or MO) in (Fe, Cr)-metalloid (or (Fe. MO)-metalloid) alloys is known to significantly
change the most probable Fe hyperfine field (Boliang et a1 1984, Dunlap and Stroink
     As figure 2 shows. there is some change in the peak at about 100 kOe in the P ( H )
which results from grinding. The predominant change. however. as seen in the P ( H ) and
also in the shape of the spectra themselves. is the growth of a component near zero field. It
is possible that this is caused by Fe atoms which are in regions of enhanced free volume
caused by the grinding. The existence of such regions is consistent with previous results on
plastically deformed alloys (e.g.. Guoan et a1 1984). This would suggest that, although the
alloy was embrittled with hydrogen. the grinding resulted in plastic deformation. This is
also suggested by the shape of the particles shown in the S E M in figure 1.
     Annealing at 375 K has little effect on the hyperfine field distribution of the ground
sample. Annealing at 525 K for 24 hours, however, significantly reduces the intensity of
the zero-field component in the P ( H ) . This is obvious a s well in figure 3 in the structure
of the central portion of the Mossbauer spectrum. This temperature is consistent with
the findings of Luborsky et a1 (1976) and Guoan et a1 (1984) on Metglas 2826
                   Luborsky e f a1 (1976) observed a recovery of low coercive force in cold-
rolled samples which have been annealed at temperatures above             520 K. Guoan er a /
( I 984) observed a complete recovery of the as-quenched structure a s observed by x-ray
diffraction measurements in a cold-rolled sample which was annealed at 573 K.

4 . 2 . A s ~ ~ i n i n eofr the Mossbauer spectra
                         t ~~
The asymmetry of Mossbauer spectra of amorphous alloys is generally attributed to the
2294              R A Diriilap aiid K Diiii

existence of a correlation between the isomer shift, 6.and the hyperfine field, H , for each
Fe site. To first order Chien and Chen ( 1 979) have related these quantities as

     In the spectra of figure 2 we observe two effects caused by quenching which are related
to the symmetry of the spectra: (i) a slight increase in the symmetry of the outer
Mossbauer lines and (ii) a significant change in the inner lines. The former effect may result
from slight changes in the local environment of the high-field site. which alters the
correlation of the isomer shift and the hyperfine field. The predominant change in
asymmetry caused by grinding occurs in the central portion of the spectrum. Either a large
change in the relation given by equation ( I ) occurs here or. more likely. the emerging zero-
field component results from Fe sites which have a very different average value of 6. A
recovery of the symmetry of the central portion of the Mossbauer spectrum on annealing is
seen clearly in figure 3.

4.3. Ci;~~stullisatioii
Although the broadening of the DTA crystallisation peaks may be related to the changes in
the Mossbauer spectra. it is more likely that they result from the actual change in the
particle size. Walter et ai (1983) have observed that the crystallisation temperature of
amorphous Fe-B-Si powder prepared by spark erosion is a function of particle size.
Particles with diameters of the order of tens of micrometres are small enough to make a
measurable difference in the crystallisation temperature. It is not clear how the structure of
spark-eroded powder relates to that of comminuted ribbons. However, if the crystallisation
temperature is a function of particle size, then certainly the distribution of particle sizes in
the comminuted ribbon as illustrated in figure 1 would give rise to a broadening of the
crystallisation exotherm. We observe in figure 4 that the annealing procedure which
affected a nearly complete recovery of the as-quenched Mossbauer spectrum has only a
small effect on the shape of the DTA exotherm. This is consistent with the above suggestion.

4 . 4 . Coiiclrrsiori
In conclusion, we observe that the comminution of melt-spun ribbons which have been
embrittled with hydrogen is an efficient method of preparing fully amorphous powder with
particle diameters of the order of micrometres. Mossbauer measurements show that
grinding promotes the growth of a zero-field peak in the hyperfine field distribution. This
field component is essentially removed on annealing at 5 2 5 K. Finally. grinding causes
a broadening of the crystallisation exotherm observed in DTA measurements. This
broadening is affected only slightly by annealing.


The authors are grateful to E Dyer and C Collins of the Atlantic Research Laboratory of
the National Research Council of Canada for providing the S E M and to J E Ball and
B Fullerton for technical assistance. This work was supported by grants from the Natural
Sciences and Engineering Council of Canada and the Faculty of Graduate Studies.
Dalhousie University.


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