Jan 1, 1980 - can be studied by monitoring the time dependence of some structure sensitive property at different tem- peratures. ... point /T / ageing is used quite frequently for this. C purpose ... accuracy. Only indirect evidence is available for the exis- ... atoms involved in the rearrangement the CSRO would result in a ...
CoZZoque C8, suppl6ment au no 8, Tome 41, aoiit 1980, page C8-878
JOURNAL DE PHYSIQUE
STRUCTURAL R E L A X A T I O N I N FERROMAGNETIC M E T A L L I C GLASSES
** , I.W.
T. ~ e m & n ~ + A.S. , Schaafsma and F. van der ~ o u d e * ~
*** , H.A.
Donald
*** , B.
Davies
YY,Y I. Vincze
Fogarassy',
+ Central Research I n s t i t u t e for Physics, Budapest, Hungary.
**Solid State Physics Laboratory, MateriaZs Science Center, University o f Groningen, Melkweg 1 , 971 8 EP Groningen, The Netherlands. %**Department of Metallurgy, University of Sheffie ld, Grande Bretagne
.
Abstract.- The dependence of the Curie temperatures on heat treatment measured by DSC and ~assbauereffect in different metallic glasses [(Fe, N i ) B , Fe40Ni40P14Bb, F ~ N ~ C ~ P is B ] correlated to the change in the iron hyperfine field distribution. The results suggest the occurence of compositional short-range ordering in the structural relaxation. One of the most significant features of the glassy
be observed because it is within the experimental
state is its deviation from thermodynamic equili-
accuracy.
brium. It is clearly manifested in its relaxation
Only indirect evidence is available for the exis-
behaviour, i.e. the time dependence of different
tence of CSRO [7] which is a change In short range
physical properties below the glass temperature. In
correlation among the different atoms due to the
the rapid cooling process involved in the fabrica-
annealing. Depending on the number and kind of
tion of the metallic glasses the solidification in-
atoms involved in the rearrangement the CSRO would
dicates the loss of thermal equilibrium of the un-
result in a distortion of the shape of p(H)
dercooled melt. The quenched-in excess energy and
of the change in the number of iron atoms with dif-
volume can be reduced by annealing. Besides by di-
ferent local neighbourhoods. In the present paper
because
rect experiments [1,2] the details of the process
the flrst observation of this effect will be repor-
can be studied by monitoring the time dependence of
ted.
some structure sensitive property at different temperatures. The investigation of ferromagnetic Curie point /T / ageing is used quite frequently for this C purpose [3-81. In this paper the TC changes determined by Differential Scanning Calorimetry are correlated with the results of Mossbauer spectroscopy
l3
t
'A
in order to characterize the changes in compositio-
/ 590 K
nal and/or topological short range order on annea-
11
ling.
0
In principle, the iron hyperfine field distribution,
610 K
0
p(H)
can provide information on the two dominant
mechanisms of the structural relaxation, namely on
1
the decrease in excess free volume and on composi-
I
I
I
1
2
3
3
log t [ min]
tional short-range ordering (CSRO). The decrease in free volume is manifested in a sharpening of the
FIG. 1. The effect of isothermal annealing on the T
peaks of the pair distribution functions [1,2],
of amorphous Fe40Ni40P14B6.
in
an increase of the density [9] by about 0.5% which
The meaning of the clo-
sed symbols is explained in the text.
results in better defined atomic positions. The smaller scatter of the atomic positions would cause
The Curie point change of an Fe40Ni40P14B6
a narrowing (and perhaps some shift) of p(H)
2826)(T
but
C
as received = 510.5
+
(Metglas
1 K) is shown in fig.
its shape would remain unchanged. Unfortunately,
1. The main characteristics are in good agreement
the expected change is smaller than 1% which cannot
with the general trend of other investigations: upon annealing TC increases, but with increasing an-
*on leave from the Central Research Institute for Physics, Budapest.
nealing temperature the relaxation becomes faster
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808217
but results in a lower-Curiepoint. The closed sym-
According to Chen et al. [5] in general the change
bols indicate the reversibility of the TC changes.
in TC increases with decreasing metalloid content
The notation T1/T2 refers to a sample heat treated
and increasing number of the alloy constituents.
for approximately 1000 minutes at T1 followed by an
This trend agrees quite well with our expectation
appropriate annealing at T2. In the two examples,
based on the similarity of the local neighbourhoods
T
is 570 and 610 K, the fact that the T
changes
in the glass and in the relevant TM3B intermetallic
2 correspond to the T2 curves shows the reversible na-
compound [lo].
ture of the relaxation, in agreement with the re-
rounding is identical in the "stoichiometric"
In this model the chemical local sur-
sult of Egami [7].
TM75B25 glass and in the TM3B compound: in other
The annealing behaviour of amorphous (Fe 33Ni67)75B25 and (Fe25Ni75)80B20 can be investigated in a wider
words all the TM-atoms occupy the TM-sublattice-
temperature range due to the large difference be-
neighbours (Here, TM assumes the possibility of more
tween Curie point and crystallization temperature
than one kind of transition metal atoms). While,
(Tc
2
400 K, Tcr Q 700K). Besidesthe featuressimilar
sites, which have mainly metalloid atoms as nearest
chemically, the TM75B25 glass is a nearly ordered
to the previous case, one can observe an approxima-
phase, disorder is introduced when the B-content
tely linear increase of Tc with the logarithm of
is lowered. For example, in the off-stoichiometric
heat treatment time at lower temperatures. This de-
TMsOBZO glass part of the TM-atoms occupy metalloid
pendence may be characteristic for the slow relaxa-
sites, which results in an increased number of close
tion towards the metastable local energy minimum
contacts between the TM-atoms. This will enhance the
corresponding to the given temperature. Fig. 2 sum-
possibility of CSRO among the TM-atoms of different kinds. In the case when there is only one type of TM atom present, e.g. Fe80B20, we expect only a small AT C as it is observed [8]. The results of Fig. 2, namely that ATC is larger for the glass with 25 at.% B than for 20 at% B apparently contradict the above-discussed trend. However, the p(H)
distributions in Fig. 3 clearly show
FIG. 2. Comparison of (Fe,Ni)B glasses as a function of annealing temperature and time.
marizes the similarities and differences of TC ageing od these two Fe-Ni-B glasses with comparable TC but different metalloid content. It is apparent that the increase in TC is more pronounced in the glass containing 25 at% B. In all of the above cases the iron hyperfine field distribution remained
FIG. 3. Iron hyperfine field distributions for (Fe,
the same, no change in p(H) was observed for the
Ni)B glasses measured at 5 K.
different annealings. In part this is due to the mentioned insensitivity of p(H)
for the expected
small change in the topological short-range order
that this is not the case. The p(H)
of (Fe33Ni67)75
B25 is broader than that of (Fe25Ni75'80~20 which
and to the insensitivity of the iron hyperfine
indicates that the distribution of Fe atoms is bet-
fields to the Ni neighbours: the total change is
ter "ordered" in the latter case (most of the Fe
about 15% for the complete dubstitution of Fe by Ni.
atoms occupy a particular off-stoichiometric
JOURNAL DE PHYSIQUE
C8-880
site, while in the stoichiometric case we have ap-
positions of the maxima in the p(H)
proximately equal numbers of Fe atoms with 2B and
(fig. 4) are indicated. The separated lines 2 and 5
3B neighbours) [11]
. Since this
distributions
were obtained using Vincze's method [13].
(Fe25Ni75)80B20
glasg is quite "ordered" already in the as-received state no important compositional short-range orde-
se in the intensities of the low- and high-field
ring is expected during the structural relaxation.
components and the maximum-probability field increa-
Thus the anomalous ATC behaviour of (Fe,Ni)B glas-
ses (Fig. 5).
Neglecting the possible small contri-
ses gives an indirect evidence for the occurrence of CSRO. lore direct support for the presence of CSRO was obtained from the study of the Fe32Ni36Cr14P12B6
As received Annealed l2months 520K)
(MG
2826 A) alloy. In this case the Curie temperature
[T~ (as-rec.) = 270 K] is strongly reduced by the presence of the Cr atoms [3,12] which also have a strong influence on the Fe hyperfine fields. A long time (2 months) annealing at low temperature (520 K) results in a significant increase of TC (AT = C 45 K) and a definite change of the Mossbauer spectra (Fig. 4). These changes correspond to an increa-
FIG. 5. p ( H ) distributions for as-received and annealed MG 2826 A fFe32Ni36Cr14P12B6), resulting from a fit to the spectra shown in fig. 4. bution pf the small homogeneous decrease of the interatomic distances upon annealing %0.12% for this alloy [4] shape of p(H)
-
-
which is
the distortion in the
can be attributed t.o a change in the
CSRO. The number of Fe atoms with a smaller and larger number of Cr neighbours has increased due to this annealing. The kinetics of Curie-temperature ageing (Fig. 6) indicates that at least two processes are present (the slope changes at ATC % 25 K). According to Chen L3] the low temperature structural relaxation involves both topological and chemical short-range ordering. Although these processes overlap, the T kinetics of Fig. 6 strongly suggests C that above ATC 2 25 K the CSRO is considerably enhanced.
ACKNOWLEDGEMENTS.
-
We are grateful to Prof. A.J.
Dekker for stimulating discussions. This work forms part of the research program of the Foundation for Fundamental Research on Matter (FOY) with financial support from' the Netherlands Oeganization for the Advancement of Pure Research (ZWO).
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