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Radiation-enhanced phase separation in amorphous Fe40Ni40P20
Acta metall. Vol. 31, No. 11, pp. 2961-2967, 1989 Printed in Great Britain. All rights reserved
OOOI-6160/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc
RADIATION-ENHANCED PHASE SEPARATION AMORPHOUS Fe,Ni,P,, R. GERLING,
F. P. SCHIMANSKY
GKSS-Forschungszentrum,
and
IN
R. WAGNER
D-2054 Geesthacht, F.R.G.
(Received 22 March 1989)
exposed to neutron-irradiation and subsequently Abstract-Specimens of amorphous Fe,Ni,P,, annealed at different temperatures are shown to undergo a similar phase separation into amorphous P-enriched and P-depleted regions as occurs in specimens annealed without prior irradiation. Whilst the radius (N 3.2 nm) of the P-rich regions is independent of whether the specimen has been irradiated or not, the onset of phase separation occurs for irradiated samples at lower temperatures; under identical annealing conditions the volume fraction of P-rich clusters is much larger in irradiated FeNiP than in un-irradiated material. The faster phase separation kinetics are a consequence of the irradiation-induced excess volume which allows for an increased mobility of individual atoms. R&m&Des Bchantillons de Fe,Ni,P, amorphe soumis $ une irradiation par neutrons, puis recuits 1 diffkrentes tempkratures, subissent une separation des phases en zones amorphes enrichies ou appauvries en phosphore, semblable $ celle qui se produit dans les tchantillons recuits sans irradition prbalable. Alors que le rayon (_ 3,2 nm) des r&ions riches en phosphore est indtpendant du fait due l’ichantillon ait Ct& irradie ou non, la sCparation des phases dkmarre i des tempdratures infkrieures dans le cas des &chantillons irradiks; pour des conditions de recuit identiques la fraction volumique des amas riches en phosphore est bien plus grande dans FeNiP irradiC que dans le mat&au non irradit. Les cinCtiques plus rapides de sCparation des phases sont une consequence de l’augmentation de volume due g l’irradiation, qui permet une mobilitt accrue des atomes individuels. wurde mit Neutronen bestrahlt und anschlieDend bei verZusammenfaasurg-Amorphes Fe,Ni,P,, schiedenen Temperaturen ausgelagert. iihnlich wie bei unbestrahlten Proben tritt eine Entmischung in P-reiche und P-verarmte Gebiete ein. Wiihrend der Radius (-3.2 nm) der P-reichen Gebiete fiir n-bestrahlte und unbestrahlte Proben stets gleich ist, setzt die Phasenseparation in bestrahltem Material bei niedrigeren Temperaturen ein als in unbestrahlten Proben. Bei gleichen Auslagerungstemperaturen ist der Volumenanteil an P-reichen Gebieten grijl3er in n-bestrahltem FeNiP als in nicht bestrahltem Material. Diese verlnderte Entmischungskinetik ist eine Folge des strahlungsinduzierten OberschuDvolumens, das eine erhijhte Beweglichkeit einzelner Atome zuliiI3t.
1. INTRODUCTION During isochronal annealing (43 h) the onset of crysoccurs at 314°C. tallization of amorphous Fe,Ni,P,
However, prior to crystallization which may be seen as being the last step in the course of the structural relaxation, other relaxation processes cause changes in several properties of amorphous Fe,,Nid,,Pz,.
Once the specimens have lost their ductility as a consequence of the loss of free volume, they can regain their full ductility either by (i) properly chosen thermal treatments [2] or (ii) low dose (-2.10” nf/ cm*) neutron-irradiation [3]. Both methods (i) and (ii) were shown to lead to a decrease of the density of the
(i) At about 22O”C, excess free volume is annealed out [ 11. As a consequence, the density increases by N 0.1% and the alloy becomes brittle, Fig. 1. (ii) At about 304”C, phase separation occurs [l], with an associated slight increase of the density and further embrittlement, Fig. 1. This phase separation could be detected clearly by small angle neutron scattering [SANS] experiments. SANS analyses indicated the formation of P-enriched and P-depleted regions of average radius 34 nm in specimens aged between 285 and 304°C. Assuming the P-rich regions to have a composition of (Fe,Ni),,P,, their volume fraction is N 5%.
Anneollng
Temperature
I OCI
Fig. 1. The relative strain at fracture and the relative density of amorphous Fe,Ni,P,, as a function of the isochronal (43 h) annealing temperature.
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GERLING
et al.:
RADIATION-ENHANCED
glassy alloy via the formation of excess volume. This, in turn, causes the reductilization. At its best, the thermal treatment (i) gives rise to a density decrease of less than about -0.1%. This was found to be sufficient for reductilization of those specimens which had been thermally embrittled at lower temperatures where for kinetic reasons phase separation is not yet possible. In contrast, by neutron-irradiation (ii) as much as _ 1% excess volume can be generated. As a consequence, samples of Fe,Ni,,P,, embrittled at any temperature below the onset of crystallization can regain their fully ductile behaviour. In particular, even the embrittling effect of a phase separation can be compensated by subsequent generation of sufficient irradiation-induced excess volume. SANE&experiments on n-irradiated Fe,Ni,P*, showed the existence of irradiation-induced extended “defects” with a reduced atomic density [4]. The volume fraction of defects (r - 10 nm) increases with increasing n-fluence in the same way as the swelling does; thus these “defects” are seen to contain the irradiation-induced excess volume. Apart from its beneficial influence on the ductility, the irradiation-induced excess volume may have further consequences for irradiated samples. Recently Horvath et al. [5] reported a decrease in the diffusion coefficient of Fe and Fe-based amorphous alloys as structural relaxation proceeds. The reduced atomic mobility thus appears to be correlated with the loss of excess free volume during the relaxation process. According to these results, the generation of irradiation-induced excess volume ought to give rise to an enhanced mobility of the constituents of the amorphous alloy. These considerations are supported by the results of Averback and Hahn [6], who measured radiation-enhanced diffusion in amorphous NiZr after ion-bombardment. In the present paper it will be shown that irradiation of Fe,ONi,P,, prior to thermal annealing expands the temperature regime over which phase separation occurs to lower temperatures and leads to a significant increase in the volume fraction of the P-rich clusters with respect to un-irradiated specimens. Since the phase separation is based on the diffusion of individual atoms, these results demonstrate that irradiation-induced excess volume enhances diffusion in amorphous Fe,,Ni,P,,.
2. EXPERIMENTAL
The amorphous Fe,Ni,P,, ribbon was 11 mm wide, 25 pm thick and had a total length of about 70 m. For the isochronal (43 h) heat treatments the specimens were sealed into evacuated quartz tubes. For the irradiation experiments the specimens were sealed into Al-tubes and subsequently inserted into an in-core reactor position. The flux of fast neutrons position was (E > 0.1 MeV) in the chosen
PHASE SEPARATION
& = 2.2. 1On nf/cm2 s. The temperature irradiation never exceeded 70°C.
during
the
2.1. Small angle neutron scattering (SANS) The SANS experiments were performed at the GKSS-Forschungszentrum Geesthacht, F.R.G., employing neutrons with wave lengths of 1 = 0.5 nm and a resolution of AL/L = 0.2. The ribbon was cut into foils of 1 x 1 cm2; 180 foils (N 5 g) were then stacked-each with the same orientation-to form a single SANS sample. The samples were exposed to the n-beam with the plane of the foils perpendicular to the beam; the direction of easy magnetization was chosen parallel to the applied external magnetic field (max. 2.3 T). The sample could be cooled down to 30 K. Two position sensitive BF, counters were used to detect the intensities of the scattered neutrons as a function of the scattering vector 4R Q = - sin 0 (26: scattering angle). 1 The two counter tubes were aligned parallel (horizontal) and perpendicular (vertical) with respect to the applied magnetic field H. The vertical counter therefore records both nuclear plus magnetic scattering (Q I H), while the horizontal counter only collects the nuclear scattering (Q 11H). The Q range covered was: 6. 10m26 Q 6 2.5 nm-‘. The intensities were corrected for background and absorption; the data were desmeared with respect to the wave-length and geometrical effects. In some parts of the Q-range the SANS-intensities were calibrated using the known incoherent scattering of a vanadium standard. Within the frame-work of the two-phase model (particles embedded in a matrix) the nuclear and magnetic SANS intensities are given by d$ (Q),\$ = const. (Aq;!i )2.[E(Q)]2
(1)
with E(Q): the single particle form factor F(Q) in the case of a dilute system of identical particles. In other cases E(Q) includes the interparticle interferences and/or the averages of F,(Q); Au: the difference in either the nuclear or the magnetic scattering length density between particle and matrix with:
AT/_ = $
- + M
(3) P
b : average nuclear scattering length for the matrix (l?; or particle (P); V, p: average atomic volume for the matrix (M) or particle (P). pM,p: average magnetic scattering length; proportional to the average atomic magnetic moment of the matrix (M) or particle (P). From the variation of the SANS-intensity dz/dn with Q, the volume fraction F and the radius R of the scattering particles (inhomogeneities) can be calcu-
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GERLING et al.: RADIATION-ENHANCED PHASE SEPARATION lated, if the respective scattering lengths and atomic volumes of the matrix and particles are known. The radius R of the spherical particles assumed and their volume fraction F can be calculated from
a) Nuclear
Scatterlng
(4)
and
Im
b 1 Nuclear +Magnetic
Scattering
Q2g(QNQ
F.(l-F)= 2.2. Complementary
’
27~?.(Arj)~
’
(5)
methods
The state of embrittlement, in terms of the relative strain at fracture, (or), was evaluated from bending tests by bending strips of the alloy between parallel plates. The density (p) of as-quenched and annealed specimens was obtained by weighing foils (- 100 mg) in both air and tetrabromoethane. Thermal data such as crystallization temperature (Z’,) and the crystallization enthalpy (AH) were measured with a differential scanning calorimeter at heating rates of 20 K/min. X-ray diffraction patterns were recorded from both the dull and the shiny side using a wavelength of 1,, = 0.1791 nm. 3. THE SANS-INTENSITIES AFTER ANNEALING OF AS-QUENCHED AND OF PRE-IRRADIATED Fe,Ni,P, Three SANS-samples of as-quenched Fe,Ni,P, were prepared. Two of them were exposed to n-irradiation, one to a total fluence of 6.5.10” and the other to 5.2.1019nf/cm2; the third one served as non-irradiated reference sample. Subsequently all three specimens were investigated by means of SANS. The variation of the scattering intensities with the scattering vector Q are shown in Fig. 2. The SANSintensities from all samples were found to be identical 18
t
+ 1
Q
[ nm-‘1
Fig. 2. The SANS-intensity dE/dn as a function of the scattering vector Q for as-quenched and neutron-irradiated amorphous Fe,Ni,P,,.
I
I
10.'
100 0 Inm-‘1
Fig. 3. The small angle scattering intensity, dZ/dQ, as a function of the scattering vector Q for as-quenched Fe,Ni,P, and annealed for 43 h at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering. in the vertical and the horizontal counter; hence, none of the three samples gave rise to magnetic SANS.
Figure 2 therefore reveals the pure nuclear scattering intensity. As reported previously [4], the enhanced SANS-intensity from the irradiated samples originates from irradiation-induced defects with a reduced atomic density and average diameter of -20 nm. Subsequently the samples were annealed successively (for 43 h) at 245,285 and 304°C; after each heat treatment the specimens were again investigated by means of SANS. Annealing at 245°C produced no change in the SANS-intensities. In Figs 3, 4 and 5 only the SANS-intensities after annealing, at 285 and 304°C are shown together with the corresponding SANS curves for the non-heat treated samples as reference. Figure 3(a,b) display the SANS curves from pure nuclear (a) and from nuclear plus magnetic (b) scattering from the un-irradiated sample. For each sample it was verified that the applied magnetic field of 2.3 T is sufficiently strong for avoiding domain wall scattering or scattering from orientational fluctuations of the magnetization. Thus the horizontal counter records only the pure nuclear scattering intensity, while the vertical counter records both the nuclear and the magnetic scattering. The difference between the two recorded SANS-intensities then yields the pure magnetic SANS-intensity. Annealing of the un-irradiated sample at 285°C leaves its SANS-intensity unchanged with respect to the as-quenched state [Fig. 3(a,b)]. Only annealing at 304°C leads to an increase of the scattering intensity. Both the nuclear and the magnetic SANS curves show relative maxima at Q - 0.5 nrn-‘, though being much more pronounced in the magnetic scattering [Fig. 3(b)].
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GERLING et al.: al Nuclear
RADIATION-ENHANCED
PHASE SEPARATION ‘]I
Sea ttering
Nuclear
Scattering
Vv
R I
I
10-l
0 1nm-‘I
I
100
I
I
10-l
I
0 I nm-‘I
10°
Fig. 4. The variation of the SANS-intensity dZ/dG with the scattering vector Q for n-irradiated (4 t = 6.5.10” nf/cm2) Fe,Ni,P,, and after annealing (43 h) at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering.
Fig. 5. The variation of the SANS-intensity dZ/dG with the scattering vector Q for n-irradiated (4 t = 5.2. lOI nfjcm’) Fe,Ni,P, and annealed for 43 h at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering.
The SANS curves of the sample pre-irradiated to 6.5.10” nf/cm* (“medium dose sample”) and subsequently annealed are shown in Fig. 4(a,b). After annealing at 285°C the nuclear plus magnetic scattering [Fig. 4(b)] is slightly increased with a maximum at about 0.5 nm-‘. The nuclear scattering remains almost unchanged. After annealing at 304°C strong increases in scattered intensity are recorded in both counter tubes. As for the un-irradiated sample, the SANS curve display a peak at Q N 0.5 nm-‘; again the magnetic scattering is much stronger than the nuclear one. The SANS-intensities of the sample pre-irradiated to 5.2. lOI nfjcm’ (“high dose sample”) and subsequently annealed are shown in Fig. 5(a,b). Although the n-fluence of this sample was about eight times higher than for the medium dose sample, the SANSintensities are only slightly higher than the corresponding ones from the medium dose sample. From these results it may be concluded that decomposition of as-quenched Fe,Ni,P,, which gives rise to SANS-intensities, is confined to temperatures above 285°C. In contrast, decomposition of the pre-irradiated material occurs at significantly lower temperatures, i.e. above 245°C.
ment. Thus the enhancement
4. PHASE SEPARATION IN PRE-IRRADIATED AND ANNEALED Fe,Ni,P, Complementary investigations using thermal analysis, X-ray diffraction, transmission electron microscopy and SANS on Fe,Ni,,P,, annealed at 304°C (43 h) have shown no evidence for the formation of crystallites during this particular heat treat-
of the SANS-intensity observed after annealing could unequivocally be attributed to a phase separation into P-enriched clusters and the amorphous P-depleted matrix [l]. For the present study it has been proved by DSC (Fig. 6) and X-ray diffraction (from both the shiny and the dull side of the amorphous ribbon) that the preceding intermediate anneals at 245 and 285°C did not cause any crystallites to form either during the final anneal at 304°C. Thus the enhanced SANS-intensity observed after successive anneals at 245, 285 and 304°C [Fig. 3(a,b)] also stems exclusively from a phase separation in the glassy alloy. As shown in Figs 4 and 5, annealing of the pre-irradiated samples leads to a significant enhancement of the SANS-intensity. The shapes of the SANS curves are similar to the corresponding curves for the as-quenched and annealed sample, Fig. 3; hence, most likely phase separation also occurs in the preirradiated samples during annealing. A priori, however, the formation of crystallites or enhanced density fluctuations may not be ruled out as the origin of the enhanced SANS-intensity. In the following, it will be shown that the enhancement of the SANS-intensity for the medium dose sample is, in fact, due to a phase separation (the same arguments hold for the high dose specimen). Thermal analysis of the medium dose sample after annealing at 304°C (Fig. 6) yields the crystallization enthalpy to be exactly the same as for the un-irradiated sample. The presence of crystallites in the pre-irradiated sample can therefore be ruled out. Moreover, X-ray diffraction also confirmed the sample to be fully amorphous. On the other hand, from
GERLING et al.: RADIATION-ENHANCED PHASE SEPARATION
B
a f :
0.9
I
2& Anneahng
Temperature
I;;;+’ l2L5+285+) 3OL I OC 1
Fig. 6. The variation of the relative density and the crystallization enthalpy of as-quenched and n-irradiated (4 .f = 6.5. IO’*nf/cm2)Fe,Ni,P,, with annealing temperature. the nuclear and the magnetic SANS-intensities, the volume fraction of crystallites can independently be calculated. We therefore assumed the presence of y-(Fe,Ni)-crystallites, which are known to be the first crystalline phase which occurs upon thermal crystallization [l]. However, the volume fraction calculated either from the nuclear or the magnetic scattering varies by a factor of 4, which is far beyond the experimental uncertainty and thus also allows the presence of crystallites to be safely ruled out. As has been reported previously [4], neutronirradiation generates extended density fluctuations. It is thus conceivable that the defects experience coarsening during the subsequent heat treatments. However, as can be inferred from Fig. 6, the irradiation-induced excess volume anneals out during the heat treatments, and after successive annealing at 245, 285 and 304°C the densities of both the asquenched and the pre-irradiated sample are identical. Moreover, while the irradiation-induced density fluctuations only gave rise to a nuclear SANS-intensity [4], annealing leads to a strong magnetic scattering. This can not be interpretated in terms of a coarsening of the density fluctuations, but rather indicates the formation of a different type of heterogeneity. Following these considerations, the enhanced SANS-intensity observed after annealing of the preirradiated samples is due to an irradiation-enhanced phase separation of the amorphous matrix into Penriched and P-lean regions. 5. SPATIAL EXTENSION OF THE P-ENRICHED CLUSTERS For the following quantitative analysis of the scattering centers formed during annealing, surface scattering has to be taken into account adequately. In fact, as has been shown by Rodmaq et nl. [7], surface scattering can be an important contribution to the SANS-intensity. Similarly, the SANS-intensity from the virgin as-quenched Fe,,Ni,,P,, (Fig. 3) is seen to
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stem markedly from surface scattering. Currently it can not be specified which part is due to surface scattering and which part is caused by any inherent heterogeneities within the amorphous bulk. We are, however, not concerned with the scattering centers already present in the as-quenched state or after the irradiation, but rather in the scattering centers formed during annealing. Therefore, the SANS-intensity from the original as-quenched or pre-irradiated state is always subtracted from that of the corresponding annealed state. (This procedure assumes that the surface scattering is unchanged during annealing or irradiation. This in fact is the case as has been discussed in detail in Refs [l] and [4].) The radius of the scattering centers which are assumed to be spherical, can be calculated from the integrated SANS-intensity according to equation (4). Since the measurements are confined to 6. lo-’ nm-’ 6 Q 6 2.5 nm-‘, an extrapolation beyond these limits appears to be necessary. (i) For each of the three samples the scattering intensities from the two corresponding annealed and un-annealed states become almost identical for Q beyond 2.5 nm-’ (Figs 3, 4 and 5). Hence, upon subtraction of the two corresponding scattering curves, dZ/dn becomes infinitely small in the range 2.5 nm-’ 6 Q 6 cc, and the contribution of the integral intensity is negligibly small. (ii) At Q = 6.10-* nm-’ subtraction of the two corresponding scattering intensities yields finite values for dZjdI2 (Q). For 0 6 Q 6 6.10-*nm-‘, dZ/dn (Q) is taken from a linear extrapolation. However, in this range, Q is sufficiently small that Q .d.Zjdn or Q’.dZ/dR only make minor contributions to the full integral. Thus the error introduced by the linear approximation of dZ/dn (Q) is also rather small. After annealing the pre-irradiated samples at 285°C an enhanced magnetic scattering is recorded, while the nuclear scattering remains nearly unchanged, cf Figs 4 and 5. This is in contrast to the un-irradiated sample where the phase separation gives rise to a magnetic and a nuclear scattering (Fig. 3). This apparent discrepancy, however, is readily explained by the reduction of the irradiationinduced excess volume which occurs concomitantly with the phase separation during annealing the preirradiated specimens. As is shown in Fig. 6, at 285°C a considerable amount of irradiation-induced excess volume is already annealed out. Since this excess volume has caused an enhanced nuclear SANSintensity, its annihilation must lead to a reduced nuclear SANS-intensity. On the other hand, as a consequence of the onset of phase separation, the nuclear scattering is increased. The result of these concurrently occuring processes is an apparently unchanged nuclear SANS-intensity. The irradiationinduced defects did not give rise to a magnetic
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GERLING el al.:
RADIATION-ENHANCED PHASE SEPARATION
Table 1. Volume fraction F and mean radius R of amorphous (Fe,Ni),,P,, clusters for as-quenched and for two differently pre-irradiated Fe,Ni,P,, samples after annealing at 285 and 304°C. The indices msg. and nucl. indicate whether F and R were calculated from magnetic or from nuclear SANS Specimen treatment prior to annealing As-quenched 6.5, IO’* nf/cm* 5.2, lOI nf/cm2 As-quenched 6.5. 10” nf/cm* 5.2. lOI nf/cm*
Annealing conditions
F““Cl (%)
285°C
304°C
SANS-intensity. The annihilation of these defects upon thermal annealing therefore has no influence on the magnetic scattering intensity. This enables us to calculate the average radius of the scattering centers from the magnetic SANS-intensity which is obtained as the difference of the intensities recorded in the vertical and the horizontal counters. The results are shown in Table 1. The average radius of the scattering centers is 3.2 + 0.5 nm. Surprisingly, within the error bars it is about the same for all samples regardless of whether they have been pre-irradiated or not, and regardless of the chosen annealing temperature (285 or 304°C). These results may be taken as further evidence that the scattering centers detected after annealing in both the as-quenched and the preirradiated sample are the same, namely P-enriched regions within a P-depleted matrix. The question remains open why the P-rich clusters have about identical sizes regardless of the conditions under which they have formed.
I 23-25.5 27.5-31
(i) One of the crystalline phases which appears on thermal crystallization of amorphous Fe,Ni,P,, is tetragonal (Fe,Ni),P. (ii) Employing analytical field ion microscopy, Piller and Haasen [8] detected B-concentration fluctuations in as-quenched Fe,,Ni,B,, . The B-rich regions, the volume fraction of which increased upon thermal annealing, had compositions close to (Fe,Ni),,B,,. Even if (Fe,Ni),,P,, were not to represent the ‘true’ composition of the P-rich clusters, the relative volume fractions remain still valid as long as the “true”
5-l I.5
3.6 3.2 3.1 3.0 3.1
composition does not vary from sample to sample. For the evaluation of the volume fraction of these P-rich clusters the following parameters were used: ??
??
??
6. VOLUME FRACTION OF THE P-RICH CLUSTERS The volume fractions of the scattering centers can be calculated by means of equation (5). Apart from Q and dC/dS2 (Q) which are obtained from the experimental scattering curves, A? must be known for the application of equation (5). This requires the composition of the scattering centers to be known. For this purpose the P-rich clusters in both unirradiated and pre-irradiated samples were assumed to have the same composition of (Fe,Ni),,PzJ. This particular composition is rationalized with the following results:
(%) No decomposition I .5S3 2.66
43 h at
43 h at
Fmag
??
The mean coherent nuclear scattering lengths b [Fe@Ni,,P,,] and b [(Fe,Ni),,P,,] were calculated from the known values for pure Fe, Ni and P, and weighted with the particular concentration of each element. The average atomic volumes V [Fe,Ni,P,] for un-irradiated and pre-irradiated samples were obtained from density measurements; I’ [(Fe,Ni),,P,,] was extrapolated from the density of Fe40Ni40P20by considering the variation of the densities of both Fe-P [9] and Ni-P [lo] with P-concentration. The magnetic scattering length p [Fe,Ni,,P2,] was obtained by measuring the spontaneous magnetization; p [(Fe,Ni)7SP25] was evaluated from data given in the literature [1 11. However, since the data presented in [1 1] do not cover phosphorous concentrations up to 25%, p [(Fe,Ni),,P,,] had to be extrapolated from the literature data and therefore appears to be the most uncertain one. For each of the above parameters, the respective depletion of the matrix in phosphorous is taken into account.
After annealing at 285°C phase separation is found to occur in the pre-irradiated samples. Because there is no nuclear SANS-intensity, the volume fraction was calculated from the magnetic scattering as given by the difference of the intensities recorded in the vertical and the horizontal counters. Owing to the large volume fractions of P-rich clusters generated in both pre-irradiated samples during annealing at 304°C the amorphous matrices are markedly depleted in phosphorous. For the Pdepleted matrices the magnetic scattering lengths cannot be evaluated reliably (see above), so calculations of the volume fraction on the basis of magnetic scattering were not made. Therefore, for the preirradiated samples annealed at 304°C the cluster volume fractions have been evaluated exclusively from the nuclear scattering intensities. Only for the un-irradiated sample annealed at 304°C was the volume fraction of the (Fe,Ni),,P,,clusters calculated from both nuclear and magnetic
GERLING ef al.: RADIATION-ENHANCED PHASE SEPARATION scattering. The results are summarized in Table 1. The upper and the lower limits for Fmag.reflect the uncertainty in the calculation of the average magnetic scattering length. The given ranges for F,,,,, of the pre-irradiated samples result from employing the SANS curve from either the as-quenched material or the pre-irradiated (non-heat-treated) one as reference. As outlined before, the cluster volume fraction Fin pre-irradiated samples was derived from magnetic SANS after annealing at 285°C and from nuclear SANS after annealing at 304°C. For the un-irradiated and aged (304°C) sample F could be derived from both magnetic and nuclear SANS. Both values are in fair agreement, thus confirming that reliable values of the cluster volume fraction can be derived from either scattering. The results quoted in Table 1 show phase separation to commence in pre-irradiated samples at lower temperatures than in un-irradiated specimens. Furthermore, after identical annealing treatments, the volume fraction of P-rich clusters in pre-irradiated samples exceeds that in un-irradiated material by far. On the other hand, after annealing at 285°C the volume fraction of P-rich clusters in the high dose (5.2. 10’9nf/cm2) sample is only twice that in the medium dose (6.5.10” nf/cm*) specimen; after annealing at 304°C the volume fractions differ by only = 25%. Since the n-fluences of the two pre-irradiated samples differ by a factor of eight, the decomposition kinetics are evidently not simply correlated with the damage level. The enhancement of phase separation in irradiated samples rather must be seen to be related to the amount of irradiation induced excess volume. In fact, the swelling of the irradiated samples was found not to vary in proportion to the damage level, but merely to increase by a factor of two, as does the cluster volume fraction, if the neutron fluence is increased from the medium to the high dose [4]. As is shown in Fig. 6 during annealing between 245 and 304°C the density of the pre-irradiated material is smaller (-0.25% for the medium-dose sample) than that of the as-quenched material. Thus, unlike the as-quenched material where most of the quenched-in excess free volume is already annealed out during structural relaxation below 245°C (cf. Fig. l), a significant amount of irradiation-induced excess volume is still contained in the amorphous structure of the pre-irradiated specimens during annealing at T 6 304°C. Obviously like the excess free volume prior to structural relaxation, the irradiation-
2961
induced excess volume enhances the mobility of the atomic species in the amorphous alloy. Unlike the excess free volume retained from quenching, however, the irradiation-induced excess volume is still available at elevated temperatures when phase separation occurs, thus accelerating the decomposition of amorphous FeMNi,,P,, into P-rich clusters and a P-lean amorphous matrix. 7. CONCLUSIONS 1. Phase separation occurs in both as-quenched and neutron-irradiated amorphous Fe@Ni,P,, upon thermal annealing below the crystallization temperature. 2. The phase separation leads to the formation of P-enriched clusters within a P-depleted matrix. 3. The radius of the P-rich clusters is always 3.2 nm regardless of the chosen annealing temperature and of whether the specimen has been preirradiated or not. 4. In pre-irradiated Fe,,Ni,P,, the onset of the phase separation occurs at lower temperatures than in un-irradiated material. 5. Under identical annealing conditions the volume fraction of P-rich clusters in pre-irradiated is much larger than in un-irradiated material. 6. The faster phase separation kinetics are attributed to the irradiation-induced excess volume which is present up to the onset of crystallization, and allows for an enhanced mobility of individual atoms. REFERENCES 1. R. Gerling, F. P. Schimansky and R. Wagner, Acta metall. 36, 575 (1988). 2. R. Gerling, F. P. Schimansky and R. Wagner, Scripra metall. 22, 1291 (1988). 3. R. Gerling, F. P. Schimansky and R. Wagner, Accu metall. 35, 1001 (1987). 4. F. P. Schimansky, R. Gerling and R. Wagner, Mafer. Sci. Engng 9l, 173 (1988). 5. J. Horvath, J. Ott, K. Pfahler and W. Ulfert, Muter. Sci. Engng 97, 409 (1988).
6. R. S. Averback and H. Hahn, Phys. Rev. B 37, 10383 (1988).
7. B. Rodmaq,
Ph. Mangin and A. Chamberod, J. Physique C8 46, 499 (1985). 8. J. Piller and P. Haasen, Actu metall. 30, 1 (1982). 9. J. Logan, AEC Res. Development Rep. No. 57, Contract No. AT-(04-3)-822 (1974). 10. G. S. Cargill III, J. appl. Phys. 41, 12 (1970). 11. J. J. Becker, F. E. Luborsky and J. L. Walter, I.E.E.E. Trans. Mugn 13, 988 (1977).
OOOI-6160/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc
RADIATION-ENHANCED PHASE SEPARATION AMORPHOUS Fe,Ni,P,, R. GERLING,
F. P. SCHIMANSKY
GKSS-Forschungszentrum,
and
IN
R. WAGNER
D-2054 Geesthacht, F.R.G.
(Received 22 March 1989)
exposed to neutron-irradiation and subsequently Abstract-Specimens of amorphous Fe,Ni,P,, annealed at different temperatures are shown to undergo a similar phase separation into amorphous P-enriched and P-depleted regions as occurs in specimens annealed without prior irradiation. Whilst the radius (N 3.2 nm) of the P-rich regions is independent of whether the specimen has been irradiated or not, the onset of phase separation occurs for irradiated samples at lower temperatures; under identical annealing conditions the volume fraction of P-rich clusters is much larger in irradiated FeNiP than in un-irradiated material. The faster phase separation kinetics are a consequence of the irradiation-induced excess volume which allows for an increased mobility of individual atoms. R&m&Des Bchantillons de Fe,Ni,P, amorphe soumis $ une irradiation par neutrons, puis recuits 1 diffkrentes tempkratures, subissent une separation des phases en zones amorphes enrichies ou appauvries en phosphore, semblable $ celle qui se produit dans les tchantillons recuits sans irradition prbalable. Alors que le rayon (_ 3,2 nm) des r&ions riches en phosphore est indtpendant du fait due l’ichantillon ait Ct& irradie ou non, la sCparation des phases dkmarre i des tempdratures infkrieures dans le cas des &chantillons irradiks; pour des conditions de recuit identiques la fraction volumique des amas riches en phosphore est bien plus grande dans FeNiP irradiC que dans le mat&au non irradit. Les cinCtiques plus rapides de sCparation des phases sont une consequence de l’augmentation de volume due g l’irradiation, qui permet une mobilitt accrue des atomes individuels. wurde mit Neutronen bestrahlt und anschlieDend bei verZusammenfaasurg-Amorphes Fe,Ni,P,, schiedenen Temperaturen ausgelagert. iihnlich wie bei unbestrahlten Proben tritt eine Entmischung in P-reiche und P-verarmte Gebiete ein. Wiihrend der Radius (-3.2 nm) der P-reichen Gebiete fiir n-bestrahlte und unbestrahlte Proben stets gleich ist, setzt die Phasenseparation in bestrahltem Material bei niedrigeren Temperaturen ein als in unbestrahlten Proben. Bei gleichen Auslagerungstemperaturen ist der Volumenanteil an P-reichen Gebieten grijl3er in n-bestrahltem FeNiP als in nicht bestrahltem Material. Diese verlnderte Entmischungskinetik ist eine Folge des strahlungsinduzierten OberschuDvolumens, das eine erhijhte Beweglichkeit einzelner Atome zuliiI3t.
1. INTRODUCTION During isochronal annealing (43 h) the onset of crysoccurs at 314°C. tallization of amorphous Fe,Ni,P,
However, prior to crystallization which may be seen as being the last step in the course of the structural relaxation, other relaxation processes cause changes in several properties of amorphous Fe,,Nid,,Pz,.
Once the specimens have lost their ductility as a consequence of the loss of free volume, they can regain their full ductility either by (i) properly chosen thermal treatments [2] or (ii) low dose (-2.10” nf/ cm*) neutron-irradiation [3]. Both methods (i) and (ii) were shown to lead to a decrease of the density of the
(i) At about 22O”C, excess free volume is annealed out [ 11. As a consequence, the density increases by N 0.1% and the alloy becomes brittle, Fig. 1. (ii) At about 304”C, phase separation occurs [l], with an associated slight increase of the density and further embrittlement, Fig. 1. This phase separation could be detected clearly by small angle neutron scattering [SANS] experiments. SANS analyses indicated the formation of P-enriched and P-depleted regions of average radius 34 nm in specimens aged between 285 and 304°C. Assuming the P-rich regions to have a composition of (Fe,Ni),,P,, their volume fraction is N 5%.
Anneollng
Temperature
I OCI
Fig. 1. The relative strain at fracture and the relative density of amorphous Fe,Ni,P,, as a function of the isochronal (43 h) annealing temperature.
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et al.:
RADIATION-ENHANCED
glassy alloy via the formation of excess volume. This, in turn, causes the reductilization. At its best, the thermal treatment (i) gives rise to a density decrease of less than about -0.1%. This was found to be sufficient for reductilization of those specimens which had been thermally embrittled at lower temperatures where for kinetic reasons phase separation is not yet possible. In contrast, by neutron-irradiation (ii) as much as _ 1% excess volume can be generated. As a consequence, samples of Fe,Ni,,P,, embrittled at any temperature below the onset of crystallization can regain their fully ductile behaviour. In particular, even the embrittling effect of a phase separation can be compensated by subsequent generation of sufficient irradiation-induced excess volume. SANE&experiments on n-irradiated Fe,Ni,P*, showed the existence of irradiation-induced extended “defects” with a reduced atomic density [4]. The volume fraction of defects (r - 10 nm) increases with increasing n-fluence in the same way as the swelling does; thus these “defects” are seen to contain the irradiation-induced excess volume. Apart from its beneficial influence on the ductility, the irradiation-induced excess volume may have further consequences for irradiated samples. Recently Horvath et al. [5] reported a decrease in the diffusion coefficient of Fe and Fe-based amorphous alloys as structural relaxation proceeds. The reduced atomic mobility thus appears to be correlated with the loss of excess free volume during the relaxation process. According to these results, the generation of irradiation-induced excess volume ought to give rise to an enhanced mobility of the constituents of the amorphous alloy. These considerations are supported by the results of Averback and Hahn [6], who measured radiation-enhanced diffusion in amorphous NiZr after ion-bombardment. In the present paper it will be shown that irradiation of Fe,ONi,P,, prior to thermal annealing expands the temperature regime over which phase separation occurs to lower temperatures and leads to a significant increase in the volume fraction of the P-rich clusters with respect to un-irradiated specimens. Since the phase separation is based on the diffusion of individual atoms, these results demonstrate that irradiation-induced excess volume enhances diffusion in amorphous Fe,,Ni,P,,.
2. EXPERIMENTAL
The amorphous Fe,Ni,P,, ribbon was 11 mm wide, 25 pm thick and had a total length of about 70 m. For the isochronal (43 h) heat treatments the specimens were sealed into evacuated quartz tubes. For the irradiation experiments the specimens were sealed into Al-tubes and subsequently inserted into an in-core reactor position. The flux of fast neutrons position was (E > 0.1 MeV) in the chosen
PHASE SEPARATION
& = 2.2. 1On nf/cm2 s. The temperature irradiation never exceeded 70°C.
during
the
2.1. Small angle neutron scattering (SANS) The SANS experiments were performed at the GKSS-Forschungszentrum Geesthacht, F.R.G., employing neutrons with wave lengths of 1 = 0.5 nm and a resolution of AL/L = 0.2. The ribbon was cut into foils of 1 x 1 cm2; 180 foils (N 5 g) were then stacked-each with the same orientation-to form a single SANS sample. The samples were exposed to the n-beam with the plane of the foils perpendicular to the beam; the direction of easy magnetization was chosen parallel to the applied external magnetic field (max. 2.3 T). The sample could be cooled down to 30 K. Two position sensitive BF, counters were used to detect the intensities of the scattered neutrons as a function of the scattering vector 4R Q = - sin 0 (26: scattering angle). 1 The two counter tubes were aligned parallel (horizontal) and perpendicular (vertical) with respect to the applied magnetic field H. The vertical counter therefore records both nuclear plus magnetic scattering (Q I H), while the horizontal counter only collects the nuclear scattering (Q 11H). The Q range covered was: 6. 10m26 Q 6 2.5 nm-‘. The intensities were corrected for background and absorption; the data were desmeared with respect to the wave-length and geometrical effects. In some parts of the Q-range the SANS-intensities were calibrated using the known incoherent scattering of a vanadium standard. Within the frame-work of the two-phase model (particles embedded in a matrix) the nuclear and magnetic SANS intensities are given by d$ (Q),\$ = const. (Aq;!i )2.[E(Q)]2
(1)
with E(Q): the single particle form factor F(Q) in the case of a dilute system of identical particles. In other cases E(Q) includes the interparticle interferences and/or the averages of F,(Q); Au: the difference in either the nuclear or the magnetic scattering length density between particle and matrix with:
AT/_ = $
- + M
(3) P
b : average nuclear scattering length for the matrix (l?; or particle (P); V, p: average atomic volume for the matrix (M) or particle (P). pM,p: average magnetic scattering length; proportional to the average atomic magnetic moment of the matrix (M) or particle (P). From the variation of the SANS-intensity dz/dn with Q, the volume fraction F and the radius R of the scattering particles (inhomogeneities) can be calcu-
2963
GERLING et al.: RADIATION-ENHANCED PHASE SEPARATION lated, if the respective scattering lengths and atomic volumes of the matrix and particles are known. The radius R of the spherical particles assumed and their volume fraction F can be calculated from
a) Nuclear
Scatterlng
(4)
and
Im
b 1 Nuclear +Magnetic
Scattering
Q2g(QNQ
F.(l-F)= 2.2. Complementary
’
27~?.(Arj)~
’
(5)
methods
The state of embrittlement, in terms of the relative strain at fracture, (or), was evaluated from bending tests by bending strips of the alloy between parallel plates. The density (p) of as-quenched and annealed specimens was obtained by weighing foils (- 100 mg) in both air and tetrabromoethane. Thermal data such as crystallization temperature (Z’,) and the crystallization enthalpy (AH) were measured with a differential scanning calorimeter at heating rates of 20 K/min. X-ray diffraction patterns were recorded from both the dull and the shiny side using a wavelength of 1,, = 0.1791 nm. 3. THE SANS-INTENSITIES AFTER ANNEALING OF AS-QUENCHED AND OF PRE-IRRADIATED Fe,Ni,P, Three SANS-samples of as-quenched Fe,Ni,P, were prepared. Two of them were exposed to n-irradiation, one to a total fluence of 6.5.10” and the other to 5.2.1019nf/cm2; the third one served as non-irradiated reference sample. Subsequently all three specimens were investigated by means of SANS. The variation of the scattering intensities with the scattering vector Q are shown in Fig. 2. The SANSintensities from all samples were found to be identical 18
t
+ 1
Q
[ nm-‘1
Fig. 2. The SANS-intensity dE/dn as a function of the scattering vector Q for as-quenched and neutron-irradiated amorphous Fe,Ni,P,,.
I
I
10.'
100 0 Inm-‘1
Fig. 3. The small angle scattering intensity, dZ/dQ, as a function of the scattering vector Q for as-quenched Fe,Ni,P, and annealed for 43 h at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering. in the vertical and the horizontal counter; hence, none of the three samples gave rise to magnetic SANS.
Figure 2 therefore reveals the pure nuclear scattering intensity. As reported previously [4], the enhanced SANS-intensity from the irradiated samples originates from irradiation-induced defects with a reduced atomic density and average diameter of -20 nm. Subsequently the samples were annealed successively (for 43 h) at 245,285 and 304°C; after each heat treatment the specimens were again investigated by means of SANS. Annealing at 245°C produced no change in the SANS-intensities. In Figs 3, 4 and 5 only the SANS-intensities after annealing, at 285 and 304°C are shown together with the corresponding SANS curves for the non-heat treated samples as reference. Figure 3(a,b) display the SANS curves from pure nuclear (a) and from nuclear plus magnetic (b) scattering from the un-irradiated sample. For each sample it was verified that the applied magnetic field of 2.3 T is sufficiently strong for avoiding domain wall scattering or scattering from orientational fluctuations of the magnetization. Thus the horizontal counter records only the pure nuclear scattering intensity, while the vertical counter records both the nuclear and the magnetic scattering. The difference between the two recorded SANS-intensities then yields the pure magnetic SANS-intensity. Annealing of the un-irradiated sample at 285°C leaves its SANS-intensity unchanged with respect to the as-quenched state [Fig. 3(a,b)]. Only annealing at 304°C leads to an increase of the scattering intensity. Both the nuclear and the magnetic SANS curves show relative maxima at Q - 0.5 nrn-‘, though being much more pronounced in the magnetic scattering [Fig. 3(b)].
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GERLING et al.: al Nuclear
RADIATION-ENHANCED
PHASE SEPARATION ‘]I
Sea ttering
Nuclear
Scattering
Vv
R I
I
10-l
0 1nm-‘I
I
100
I
I
10-l
I
0 I nm-‘I
10°
Fig. 4. The variation of the SANS-intensity dZ/dG with the scattering vector Q for n-irradiated (4 t = 6.5.10” nf/cm2) Fe,Ni,P,, and after annealing (43 h) at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering.
Fig. 5. The variation of the SANS-intensity dZ/dG with the scattering vector Q for n-irradiated (4 t = 5.2. lOI nfjcm’) Fe,Ni,P, and annealed for 43 h at 285 and 304°C. (a) Nuclear scattering only; (b) nuclear + magnetic scattering.
The SANS curves of the sample pre-irradiated to 6.5.10” nf/cm* (“medium dose sample”) and subsequently annealed are shown in Fig. 4(a,b). After annealing at 285°C the nuclear plus magnetic scattering [Fig. 4(b)] is slightly increased with a maximum at about 0.5 nm-‘. The nuclear scattering remains almost unchanged. After annealing at 304°C strong increases in scattered intensity are recorded in both counter tubes. As for the un-irradiated sample, the SANS curve display a peak at Q N 0.5 nm-‘; again the magnetic scattering is much stronger than the nuclear one. The SANS-intensities of the sample pre-irradiated to 5.2. lOI nfjcm’ (“high dose sample”) and subsequently annealed are shown in Fig. 5(a,b). Although the n-fluence of this sample was about eight times higher than for the medium dose sample, the SANSintensities are only slightly higher than the corresponding ones from the medium dose sample. From these results it may be concluded that decomposition of as-quenched Fe,Ni,P,, which gives rise to SANS-intensities, is confined to temperatures above 285°C. In contrast, decomposition of the pre-irradiated material occurs at significantly lower temperatures, i.e. above 245°C.
ment. Thus the enhancement
4. PHASE SEPARATION IN PRE-IRRADIATED AND ANNEALED Fe,Ni,P, Complementary investigations using thermal analysis, X-ray diffraction, transmission electron microscopy and SANS on Fe,Ni,,P,, annealed at 304°C (43 h) have shown no evidence for the formation of crystallites during this particular heat treat-
of the SANS-intensity observed after annealing could unequivocally be attributed to a phase separation into P-enriched clusters and the amorphous P-depleted matrix [l]. For the present study it has been proved by DSC (Fig. 6) and X-ray diffraction (from both the shiny and the dull side of the amorphous ribbon) that the preceding intermediate anneals at 245 and 285°C did not cause any crystallites to form either during the final anneal at 304°C. Thus the enhanced SANS-intensity observed after successive anneals at 245, 285 and 304°C [Fig. 3(a,b)] also stems exclusively from a phase separation in the glassy alloy. As shown in Figs 4 and 5, annealing of the pre-irradiated samples leads to a significant enhancement of the SANS-intensity. The shapes of the SANS curves are similar to the corresponding curves for the as-quenched and annealed sample, Fig. 3; hence, most likely phase separation also occurs in the preirradiated samples during annealing. A priori, however, the formation of crystallites or enhanced density fluctuations may not be ruled out as the origin of the enhanced SANS-intensity. In the following, it will be shown that the enhancement of the SANS-intensity for the medium dose sample is, in fact, due to a phase separation (the same arguments hold for the high dose specimen). Thermal analysis of the medium dose sample after annealing at 304°C (Fig. 6) yields the crystallization enthalpy to be exactly the same as for the un-irradiated sample. The presence of crystallites in the pre-irradiated sample can therefore be ruled out. Moreover, X-ray diffraction also confirmed the sample to be fully amorphous. On the other hand, from
GERLING et al.: RADIATION-ENHANCED PHASE SEPARATION
B
a f :
0.9
I
2& Anneahng
Temperature
I;;;+’ l2L5+285+) 3OL I OC 1
Fig. 6. The variation of the relative density and the crystallization enthalpy of as-quenched and n-irradiated (4 .f = 6.5. IO’*nf/cm2)Fe,Ni,P,, with annealing temperature. the nuclear and the magnetic SANS-intensities, the volume fraction of crystallites can independently be calculated. We therefore assumed the presence of y-(Fe,Ni)-crystallites, which are known to be the first crystalline phase which occurs upon thermal crystallization [l]. However, the volume fraction calculated either from the nuclear or the magnetic scattering varies by a factor of 4, which is far beyond the experimental uncertainty and thus also allows the presence of crystallites to be safely ruled out. As has been reported previously [4], neutronirradiation generates extended density fluctuations. It is thus conceivable that the defects experience coarsening during the subsequent heat treatments. However, as can be inferred from Fig. 6, the irradiation-induced excess volume anneals out during the heat treatments, and after successive annealing at 245, 285 and 304°C the densities of both the asquenched and the pre-irradiated sample are identical. Moreover, while the irradiation-induced density fluctuations only gave rise to a nuclear SANS-intensity [4], annealing leads to a strong magnetic scattering. This can not be interpretated in terms of a coarsening of the density fluctuations, but rather indicates the formation of a different type of heterogeneity. Following these considerations, the enhanced SANS-intensity observed after annealing of the preirradiated samples is due to an irradiation-enhanced phase separation of the amorphous matrix into Penriched and P-lean regions. 5. SPATIAL EXTENSION OF THE P-ENRICHED CLUSTERS For the following quantitative analysis of the scattering centers formed during annealing, surface scattering has to be taken into account adequately. In fact, as has been shown by Rodmaq et nl. [7], surface scattering can be an important contribution to the SANS-intensity. Similarly, the SANS-intensity from the virgin as-quenched Fe,,Ni,,P,, (Fig. 3) is seen to
2965
stem markedly from surface scattering. Currently it can not be specified which part is due to surface scattering and which part is caused by any inherent heterogeneities within the amorphous bulk. We are, however, not concerned with the scattering centers already present in the as-quenched state or after the irradiation, but rather in the scattering centers formed during annealing. Therefore, the SANS-intensity from the original as-quenched or pre-irradiated state is always subtracted from that of the corresponding annealed state. (This procedure assumes that the surface scattering is unchanged during annealing or irradiation. This in fact is the case as has been discussed in detail in Refs [l] and [4].) The radius of the scattering centers which are assumed to be spherical, can be calculated from the integrated SANS-intensity according to equation (4). Since the measurements are confined to 6. lo-’ nm-’ 6 Q 6 2.5 nm-‘, an extrapolation beyond these limits appears to be necessary. (i) For each of the three samples the scattering intensities from the two corresponding annealed and un-annealed states become almost identical for Q beyond 2.5 nm-’ (Figs 3, 4 and 5). Hence, upon subtraction of the two corresponding scattering curves, dZ/dn becomes infinitely small in the range 2.5 nm-’ 6 Q 6 cc, and the contribution of the integral intensity is negligibly small. (ii) At Q = 6.10-* nm-’ subtraction of the two corresponding scattering intensities yields finite values for dZjdI2 (Q). For 0 6 Q 6 6.10-*nm-‘, dZ/dn (Q) is taken from a linear extrapolation. However, in this range, Q is sufficiently small that Q .d.Zjdn or Q’.dZ/dR only make minor contributions to the full integral. Thus the error introduced by the linear approximation of dZ/dn (Q) is also rather small. After annealing the pre-irradiated samples at 285°C an enhanced magnetic scattering is recorded, while the nuclear scattering remains nearly unchanged, cf Figs 4 and 5. This is in contrast to the un-irradiated sample where the phase separation gives rise to a magnetic and a nuclear scattering (Fig. 3). This apparent discrepancy, however, is readily explained by the reduction of the irradiationinduced excess volume which occurs concomitantly with the phase separation during annealing the preirradiated specimens. As is shown in Fig. 6, at 285°C a considerable amount of irradiation-induced excess volume is already annealed out. Since this excess volume has caused an enhanced nuclear SANSintensity, its annihilation must lead to a reduced nuclear SANS-intensity. On the other hand, as a consequence of the onset of phase separation, the nuclear scattering is increased. The result of these concurrently occuring processes is an apparently unchanged nuclear SANS-intensity. The irradiationinduced defects did not give rise to a magnetic
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GERLING el al.:
RADIATION-ENHANCED PHASE SEPARATION
Table 1. Volume fraction F and mean radius R of amorphous (Fe,Ni),,P,, clusters for as-quenched and for two differently pre-irradiated Fe,Ni,P,, samples after annealing at 285 and 304°C. The indices msg. and nucl. indicate whether F and R were calculated from magnetic or from nuclear SANS Specimen treatment prior to annealing As-quenched 6.5, IO’* nf/cm* 5.2, lOI nf/cm2 As-quenched 6.5. 10” nf/cm* 5.2. lOI nf/cm*
Annealing conditions
F““Cl (%)
285°C
304°C
SANS-intensity. The annihilation of these defects upon thermal annealing therefore has no influence on the magnetic scattering intensity. This enables us to calculate the average radius of the scattering centers from the magnetic SANS-intensity which is obtained as the difference of the intensities recorded in the vertical and the horizontal counters. The results are shown in Table 1. The average radius of the scattering centers is 3.2 + 0.5 nm. Surprisingly, within the error bars it is about the same for all samples regardless of whether they have been pre-irradiated or not, and regardless of the chosen annealing temperature (285 or 304°C). These results may be taken as further evidence that the scattering centers detected after annealing in both the as-quenched and the preirradiated sample are the same, namely P-enriched regions within a P-depleted matrix. The question remains open why the P-rich clusters have about identical sizes regardless of the conditions under which they have formed.
I 23-25.5 27.5-31
(i) One of the crystalline phases which appears on thermal crystallization of amorphous Fe,Ni,P,, is tetragonal (Fe,Ni),P. (ii) Employing analytical field ion microscopy, Piller and Haasen [8] detected B-concentration fluctuations in as-quenched Fe,,Ni,B,, . The B-rich regions, the volume fraction of which increased upon thermal annealing, had compositions close to (Fe,Ni),,B,,. Even if (Fe,Ni),,P,, were not to represent the ‘true’ composition of the P-rich clusters, the relative volume fractions remain still valid as long as the “true”
5-l I.5
3.6 3.2 3.1 3.0 3.1
composition does not vary from sample to sample. For the evaluation of the volume fraction of these P-rich clusters the following parameters were used: ??
??
??
6. VOLUME FRACTION OF THE P-RICH CLUSTERS The volume fractions of the scattering centers can be calculated by means of equation (5). Apart from Q and dC/dS2 (Q) which are obtained from the experimental scattering curves, A? must be known for the application of equation (5). This requires the composition of the scattering centers to be known. For this purpose the P-rich clusters in both unirradiated and pre-irradiated samples were assumed to have the same composition of (Fe,Ni),,PzJ. This particular composition is rationalized with the following results:
(%) No decomposition I .5S3 2.66
43 h at
43 h at
Fmag
??
The mean coherent nuclear scattering lengths b [Fe@Ni,,P,,] and b [(Fe,Ni),,P,,] were calculated from the known values for pure Fe, Ni and P, and weighted with the particular concentration of each element. The average atomic volumes V [Fe,Ni,P,] for un-irradiated and pre-irradiated samples were obtained from density measurements; I’ [(Fe,Ni),,P,,] was extrapolated from the density of Fe40Ni40P20by considering the variation of the densities of both Fe-P [9] and Ni-P [lo] with P-concentration. The magnetic scattering length p [Fe,Ni,,P2,] was obtained by measuring the spontaneous magnetization; p [(Fe,Ni)7SP25] was evaluated from data given in the literature [1 11. However, since the data presented in [1 1] do not cover phosphorous concentrations up to 25%, p [(Fe,Ni),,P,,] had to be extrapolated from the literature data and therefore appears to be the most uncertain one. For each of the above parameters, the respective depletion of the matrix in phosphorous is taken into account.
After annealing at 285°C phase separation is found to occur in the pre-irradiated samples. Because there is no nuclear SANS-intensity, the volume fraction was calculated from the magnetic scattering as given by the difference of the intensities recorded in the vertical and the horizontal counters. Owing to the large volume fractions of P-rich clusters generated in both pre-irradiated samples during annealing at 304°C the amorphous matrices are markedly depleted in phosphorous. For the Pdepleted matrices the magnetic scattering lengths cannot be evaluated reliably (see above), so calculations of the volume fraction on the basis of magnetic scattering were not made. Therefore, for the preirradiated samples annealed at 304°C the cluster volume fractions have been evaluated exclusively from the nuclear scattering intensities. Only for the un-irradiated sample annealed at 304°C was the volume fraction of the (Fe,Ni),,P,,clusters calculated from both nuclear and magnetic
GERLING ef al.: RADIATION-ENHANCED PHASE SEPARATION scattering. The results are summarized in Table 1. The upper and the lower limits for Fmag.reflect the uncertainty in the calculation of the average magnetic scattering length. The given ranges for F,,,,, of the pre-irradiated samples result from employing the SANS curve from either the as-quenched material or the pre-irradiated (non-heat-treated) one as reference. As outlined before, the cluster volume fraction Fin pre-irradiated samples was derived from magnetic SANS after annealing at 285°C and from nuclear SANS after annealing at 304°C. For the un-irradiated and aged (304°C) sample F could be derived from both magnetic and nuclear SANS. Both values are in fair agreement, thus confirming that reliable values of the cluster volume fraction can be derived from either scattering. The results quoted in Table 1 show phase separation to commence in pre-irradiated samples at lower temperatures than in un-irradiated specimens. Furthermore, after identical annealing treatments, the volume fraction of P-rich clusters in pre-irradiated samples exceeds that in un-irradiated material by far. On the other hand, after annealing at 285°C the volume fraction of P-rich clusters in the high dose (5.2. 10’9nf/cm2) sample is only twice that in the medium dose (6.5.10” nf/cm*) specimen; after annealing at 304°C the volume fractions differ by only = 25%. Since the n-fluences of the two pre-irradiated samples differ by a factor of eight, the decomposition kinetics are evidently not simply correlated with the damage level. The enhancement of phase separation in irradiated samples rather must be seen to be related to the amount of irradiation induced excess volume. In fact, the swelling of the irradiated samples was found not to vary in proportion to the damage level, but merely to increase by a factor of two, as does the cluster volume fraction, if the neutron fluence is increased from the medium to the high dose [4]. As is shown in Fig. 6 during annealing between 245 and 304°C the density of the pre-irradiated material is smaller (-0.25% for the medium-dose sample) than that of the as-quenched material. Thus, unlike the as-quenched material where most of the quenched-in excess free volume is already annealed out during structural relaxation below 245°C (cf. Fig. l), a significant amount of irradiation-induced excess volume is still contained in the amorphous structure of the pre-irradiated specimens during annealing at T 6 304°C. Obviously like the excess free volume prior to structural relaxation, the irradiation-
2961
induced excess volume enhances the mobility of the atomic species in the amorphous alloy. Unlike the excess free volume retained from quenching, however, the irradiation-induced excess volume is still available at elevated temperatures when phase separation occurs, thus accelerating the decomposition of amorphous FeMNi,,P,, into P-rich clusters and a P-lean amorphous matrix. 7. CONCLUSIONS 1. Phase separation occurs in both as-quenched and neutron-irradiated amorphous Fe@Ni,P,, upon thermal annealing below the crystallization temperature. 2. The phase separation leads to the formation of P-enriched clusters within a P-depleted matrix. 3. The radius of the P-rich clusters is always 3.2 nm regardless of the chosen annealing temperature and of whether the specimen has been preirradiated or not. 4. In pre-irradiated Fe,,Ni,P,, the onset of the phase separation occurs at lower temperatures than in un-irradiated material. 5. Under identical annealing conditions the volume fraction of P-rich clusters in pre-irradiated is much larger than in un-irradiated material. 6. The faster phase separation kinetics are attributed to the irradiation-induced excess volume which is present up to the onset of crystallization, and allows for an enhanced mobility of individual atoms. REFERENCES 1. R. Gerling, F. P. Schimansky and R. Wagner, Acta metall. 36, 575 (1988). 2. R. Gerling, F. P. Schimansky and R. Wagner, Scripra metall. 22, 1291 (1988). 3. R. Gerling, F. P. Schimansky and R. Wagner, Accu metall. 35, 1001 (1987). 4. F. P. Schimansky, R. Gerling and R. Wagner, Mafer. Sci. Engng 9l, 173 (1988). 5. J. Horvath, J. Ott, K. Pfahler and W. Ulfert, Muter. Sci. Engng 97, 409 (1988).
6. R. S. Averback and H. Hahn, Phys. Rev. B 37, 10383 (1988).
7. B. Rodmaq,
Ph. Mangin and A. Chamberod, J. Physique C8 46, 499 (1985). 8. J. Piller and P. Haasen, Actu metall. 30, 1 (1982). 9. J. Logan, AEC Res. Development Rep. No. 57, Contract No. AT-(04-3)-822 (1974). 10. G. S. Cargill III, J. appl. Phys. 41, 12 (1970). 11. J. J. Becker, F. E. Luborsky and J. L. Walter, I.E.E.E. Trans. Mugn 13, 988 (1977).