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Rapid solidification and crystallization of a Zr-24at.%Fe alloy
Materials Science and Engineering, 73 (1985) 187-195
187
Rapid Solidification and Crystallization of a Zr-24at.%Fe Alloy G. K. DEY and S. BANERJEE
Physical Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085 (India) (Received May 30, 1984; in revised form November 1, 1984)
ABSTRACT
Rapidly solidified ribbons of Zr-24at.%Fe alloy produced in a melt-spinning apparatus have been found to contain a crystalline phase (which is base-centred orthorhombic with lattice parameters a = 3.320.7i, b = 1 0 . 9 8 0 . 4 and c = 8.800 A ) in an amorphous matrix. This phase is believed to be the Zr3Fe intermetallic c o m p o u n d which is produced through diffusionless solidification from a melt o f the same composition. The morphologies o f crystalline particles which are present as isolated crystals or as aggregates o f contiguous particles in the amorphous matrix are described and their possible mode o f formation discussed. The origin o f different types o f interface between these crystals and the amorphous ma trix is discussed in terms o f solute rejection. The phase formed after crystallization in the amorphous matrix was identified as the orthorhombic Zr3Fe phase. The morphology and distribution o f the crystals originating from the amorphous phase were examined and compared with those formed during solidification. The effects o f pre-existing crystals in the amorphous matrix during the crystallization process are discussed. The crystallization process was studied at two temperatures, -- 598 and 723 K. Crystallization at the lower temperature was f o u n d to be associated with significant growth o f the pre-existing crystals. Extensive nucleation o f the crystalline phase in the amorphous matrix followed by a limited growth culminatl~ag in early impingement marked the crystallization process at the higher temperature.
line and amorphous phases. Several characteristic features of the early stages of the solidification process, which otherwise would be difficult to observe, are expected to be preserved in this partly crystalline solid. Thermal t r e a t m e n t of this solid subsequent to solidification results in transformation of the amorphous phase to one or more crystalline phases. This transformation is brought about by the growth of the pre-existing crystalline particles as well as by fresh nucleation and growth of crystalline phases in the amorphous matrix. The examination of the as-solidified structure and of the structural changes accompanying the subsequent thermal treatment enables a comparison to be made between the processes of solidification and crystallization. In this paper the identification and the morphological characterization of the crystalline phase formed during the rapid solidification and the subsequent heat treatment operations in Zr-24at.%Fe alloy are dealt with. The crystalline phase was identified as a basecentred orthorhombic phase which has the stoichiometric composition Zr3 Fe. The formation of this phase in a fully amorphous alloy of identical composition during crystallization has been reported by Buschow [1]. Since the composition o f the crystalline phase and that of the melt from which it forms are almost identical, the solidification and crystallization processes do n o t involve solute partitioning between the parent and product phases. This is the reason why the Zr-24at.%Fe alloy provided a good o p p o r t u n i t y for a comparison to be made between the solidification and the crystallization processes.
1. INTRODUCTION 2. EXPERIMENTAL DETAILS Rapid solidification at cooling rates insufficient to suppress crystalline phase formation completely may yield a composite of crystal-
Crystal bar zirconium (oxygen content, 200 ppm) and high purity iron were melted
188 u n d e r a p u r e a r g o n a t m o s p h e r e in an arc furn a c e t o p r o d u c e t h e alloy b u t t o n s . T h e b u t t o n s were r e m e l t e d several t i m e s t o e n s u r e h o m o genization. R a p i d solidification was carried o u t in a m e l t - s p i n n i n g a p p a r a t u s consisting o f a c o p p e r w h e e l 20 c m in d i a m e t e r . A wheel surface s p e e d o f 20 m s-1 a n d an argon shield w e r e used d u r i n g m e l t spinning. T h e r i b b o n s p r o d u c e d w e r e a b o u t 3 m m wide a n d 50 p m thick. T h i n foils f o r e l e c t r o n m i c r o s c o p y w e r e p r e p a r e d b y t h e w i n d o w t e c h n i q u e using a s o l u t i o n c o n t a i n i n g 300 c m 8 o f m e t h a n o l , 170 c m 3 o f b u t a n o l a n d 30 c m a o f p e r c h l o r i c acid. T h e t e m p e r a t u r e o f t h e e l e c t r o l y t e was m a i n t a i n e d b e l o w 220 K using a m e t h a n o l d r y ice b a t h . T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y ( T E M ) was carried o u t in a Siemens E l m i s k o p 102 i n s t r u m e n t . D i f f e r e n t i a l scanning c a l o r i m e t r y was carried o u t in a P e r k i n - E l m e r DSC II i n s t r u m e n t .
TABLE 1 X-ray diffraction data for Zr3Fe
hkl d spacing (,~) l/I o
hkl d spacing (.~) I/I 0
001 020 021 002 022 110 111 040 041 023 112 130 131 042 004 113 132 024 043 133 150 060 114
151 061 044 152 062 025 200 134 201 220 221 202 153 063 115 222 045 006 203 135 240 170
8.800 5.490 4.658 4.400 3.433 3.170 2.985 2.745 2.620 2.582 2.576 2.455 2.366 2.327 2.200 2.156 2.142 2.042 2.004 1.885 1.832 1.830 1.809
NO 40 NO NO VW VW VW 20 10 100 100 20 10 5 5 5 VW 5 VVW 20 NO 20 NO
1.793 1.792 1.717 1.691 1.685 1.676 1.660 1.640 1.631 1.589 1.564 1.555 1.554 1.553 1.539 1.494 1.481 1.466 1.443 1.434 1.420 1.419
NO 10 NO VW VW NO VW VW NO NO 20 VW VW NO VW VW NO NO W W W VW
3. RESULTS
3.1. Solidified structure 3.1.1. X-ray diffraction X - r a y d i f f r a c t i o n carried o u t o n t h e meltspun ribbons showed a superimposition of b r o a d m a x i m a t y p i c a l o f an a m o r p h o u s structure and a n u m b e r of sharp peaks normally o b t a i n e d f r o m a crystalline phase, suggesting t h e p r e s e n c e o f an a m o r p h o u s - c r y s t a l l i n e phase m i x t u r e in t h e s p e c i m e n s . T a b l e 1 shows t h e X - r a y d i f f r a c t i o n results. T h e o b s e r v e d p e a k s could be i n d e x e d in t e r m s o f a basec e n t r e d o r t h o r h o m b i c p h a s e having t h e foll o w i n g lattice p a r a m e t e r s : a = 3 . 3 2 0 A; b = 1 0 . 9 8 0 ti,; c = 8 . 8 0 0 A. B u s c h o w [1] has previously r e p o r t e d t h e f o r m a t i o n o f a Z r 3 F e p h a s e o f similar s t r u c t u r e , w i t h lattice param e t e r s close to t h o s e o b t a i n e d here, in crystallized metallic glass o f Z r - 2 4 a t . % F e . T h e c r y s t a l l i z a t i o n s t u d y carried o u t b y B u s c h o w was o n a fully a m o r p h o u s m e l t - s p u n alloy. It s h o u l d be m e n t i o n e d t h a t t h e cooling r a t e a c h i e v e d during m e l t spinning d e p e n d s , a m o n g o t h e r factors, o n t h e surface s p e e d o f t h e w h e e l [2]. It is possible t o o b t a i n t h e s a m e alloy in a fully a m o r p h o u s state or a p a r t l y crystalline s t a t e b y altering t h e surface s p e e d o f t h e wheel. A relatively l o w surface s p e e d o f t h e wheel used in t h e p r e s e n t s t u d y is
NO, not observed; VVW, very very weak; VW, very weak ; W, weak.
possibly responsible for the formation of a p a r t l y crystalline s t r u c t u r e o f t h e r i b b o n c o m p a r e d w i t h t h e fully a m o r p h o u s s t r u c t u r e obtained by Buschow. A c o m p a r i s o n o f p e a k intensities o f certain s t r o n g r e f l e c t i o n s o b t a i n e d f r o m t h e wheelside surface (the surface in c o n t a c t w i t h t h e w h e e l d u r i n g m e l t spinning) a n d t h e air-side surface (the s u r f a c e a w a y f r o m t h e wheel) ind i c a t e d t h a t a smaller a m o u n t o f t h e crystalline p h a s e was p r e s e n t close to t h e wheel-side surface t h a n at t h e air-side surface.
3.1.2. Transmission electron microscopy T h i n foils o f t h e m e l t - s p u n r i b b o n s w e r e f o u n d t o c o n t a i n isolated single crystals a n d aggregates o f crystals d i s t r i b u t e d in an a m o r p h o u s m a t r i x . T h e d i f f r a c t i o n p a t t e r n (Fig. 1) f r o m a r e p r e s e n t a t i v e region o f t h e a m o r p h o u s m a t r i x s h o w s characteristic halos. Bright field e l e c t r o n m i c r o g r a p h s (Fig. 2) f r o m t h e a m o r p h o u s region s h o w a fine-scale s t r u c t u r e consisting o f a lighter a n d a d a r k e r region interweaved. T h e scale o f t h e s t r u c t u r e a n d t h e c o n n e c t i v i t y o f t h e t w o regions h a v e r e m a r k -
189
Fig. 1. Selected area electron diffraction pattern from the amorphous region, showing a single broad intense halo.
Fig. 2. Bright field electron micrograph from the amorphous region, showing contrast typical of amorphous phase separation. Darker and brighter regions suggestive of the presence of two amorphous phases can be seen.
able similarity to those encountered in the spinodally decomposed structure in elastically isotropic systems [3]. The average wavelength of the structure was found to be 300 .~. A similar mottled contrast observed in the amorphous structure of Zr36Ti24B40 alloy was attributed to the presence of two amorphous phases, as confirmed by differential scanning calorimetry [4]. In the present study, microdensitometer traces were taken on diffraction
patterns from the amorphous region to obtain corroborative evidence of two intensity peaks within the first diffused halo but no such intensity distribution could be seen. Moreover, chemically thinned specimens did n o t exhibit any mottled contrast. Electron diffraction from the crystals confirmed that the crystals were of the Zr 3 Fe intermetallic phase (Fig. 3). The isolated single crystals were generally f o u n d to have very regular geometric morphologies. Two of these morphologies are illustrated in Fig. 4. Extinction contours can be seen to extend across the entire length of these crystals, establishing their single-crystal nature. A wide range of crystal sizes was encountered, indicating different extents of crystal growth. The crystal aggregates could be grouped into two types. The first type can be described as aggregates with a nearly spherical central core surrounded by a number of crystals emanating from the core. In several such aggregates, the core is seen to be surrounded by six crystals (Fig. 5), a sixfold s y m m e t r y being present for the aggregate as a whole. A more detailed account of the crystallography and the origin of such crystal aggregates will be presented elsewhere. In the remaining aggregates of the first type, a large number of crystals are seen to be present around the central core. The second type of aggregate does n o t exhibit a core but several geometric crystals are found to be in a group with c o m m o n interfaces between them. Selected area diffraction patterns taken from the second type of aggregate could be indexed in terms of the Zr3Fe structure as observed in isolated crystals. Interfaces separating crystals from the amorphous matrix were seen to be sometimes planar and sometimes undulating. Singlesurface trace analysis showed that some of the planar interfaces lie along (100} planes. A careful examination of the modulated interfaces showed that the interfaces often assumed a zigzag morphology made up of facets of different rational planes such as (111) and (110) (Fig. 6). It was observed that the undulations were very small in size. Their existence was not due to the presence of a third phase. The crystals in aggregates were also seen to have these two types of interface. In the text that follows, the term "preexisting crystals" means crystals formed during solidification.
190
0
020
021
0
000
001
o}i
o2o
o~
ZONE
Ax,s :- [,oo3 z , 3 Fe
(a)
,~r
,To
,~,
o~r
o~o
oo,
Ill
IlO
ZONE AXIS:-
III
[J~o] Z~3Fe
(b)
Fig. 3. Selected area diffraction patterns from two isolated Zr3Fe crystals ((a) and (b)). The keys to the patterns are shown alongside.
3.2. Crystallization In o r d e r t o d e t e r m i n e t h e n u m b e r and the n a t u r e o f t h e r m a l events a c c o m p a n y i n g crystallization, t h e r i b b o n s were h e a t e d in a differential scanning calorimeter. A solitary sharp e x o t h e r m i c p e a k was observed at a heating rate o f 20 K min -1. A s c h e m a t i c p l o t o f the t h e r m o g r a m is s h o w n in Fig. 7. T h e t e m p e r a t u r e Tx at which crystallization starts and the peak t e m p e r a t u r e Tp at this heating rate were f o u n d to be 653 K and 6 5 6 . 5 K respectively. No e n d o t h e r m i c p e a k suggestive o f glass transition was observed. This aspect has been e l a b o r a t e d elsewhere in this paper. In o r d e r t o s t u d y t h e progress o f the crystallization process b y TEM, specimens o f the
as-solidified ribbons were h e a t t r e a t e d at 598 K for 1 h or at 723 K f o r 0.5 h. These specimens are r e f e r r e d t o as specimen A and specim e n B respectively.
3. 2.1. X-ray diffraction X-ray d i f f r a c t i o n carried o u t o n specimen A indicated t h a t n o n e w phase had f o r m e d . The p e a k intensities o f t h e crystalline phase f o r m e d during solidification s h o w e d an increase, suggesting an increase in the v o l u m e f r a c t i o n o f t h e Zr 3 Fe phase. The presence o f a b r o a d m a x i m u m characteristic o f t h e amorp h o u s phase was indicative o f an i n c o m p l e t e a m o r p h o u s - t o - c r y s t a l l i n e t r a n s f o r m a t i o n in t h e specimen heat t r e a t e d at 598 K f o r 1 h. This
191
Fig. 5. Bright field electron micrograph of a crystal aggregate in an amorphous matrix, depicting a small central core surrounded by six crystals.
Fig. 4. Crystals of the Zr3Fe phase in an amorphous matrix, showing some of the crystal morphologies and crystal-amorphous matrix interfaces encountered. The crystal in (a) has undulating interfaces on the sides and planar interfaces on the top and bottom.
Fig. 6. Bright field electron micrograph of a crystalamorphous matrix interface showing zigzag morphology.
3.2. 2. Transmission electron microscopy o b s e r v a t i o n was f u r t h e r c o n f i r m e d b y TEM imaging. The X-ray d i f f r a c t i o n o f s p e c i m e n B, h o w e v e r , i n d i c a t e d t h a t the a m o r p h o u s to-crystalline t r a n s f o r m a t i o n was c o m p l e t e .
S p e c i m e n A e x h i b i t e d g r o w t h o f t h e preexisting crystals and the n u c l e a t i o n o f a limited n u m b e r o f fresh crystals in t h e a m o r p h o u s m a t r i x (Fig. 8), the f o r m e r c o n t r i b u t i n g m o r e t o w a r d s the a m o r p h o u s - t o - c r y s t a l l i n e trans-
192
t lg W "r X W
u =E n~ UJ
"1I-0 Q Z i,i
tt
T Tp
1
I
610
620
I
I
630
I
640
I
650
.
I
J
660
670
TEMPERATURE
I
680
I
690
I
•700
I
J
710
720
(K)
Fig. 7. Thermogram obtained at a heating rate of 20 K rain -1 showing the crystallization peak temperature Tp./:/ indicates the rate of enthalpy change.
Fig. 8. Bright field electron micrograph obtained from specimen A, showing the growth of the preexisting crystals and the nucleation of crystals in the amorphous matrix.
f o r m a t i o n . The interfaces which advanced during t h e g r o w t h o f t h e crystalline particles were almost all f o u n d to be planar. Undulating interfaces were v e r y rarely e n c o u n t e r e d in the crystallized samples. Planar faces were again f o u n d t o be along ( 1 0 0 ) planes, suggesting t h a t the t r a n s f o r m a t i o n f r o n t t e n d s t o assume these orientations. Fresh n u c l e a t i o n within t h e m a t r i x did n o t o c c u r t o such an
Fig. 9. Bright field electron micrograph showing the high density of crystals formed in specimen B.
e x t e n t as t o restrict t h e g r o w t h o f pre-existing crystals. A fairly wide d i s t r i b u t i o n o f t h e size o f freshly f o r m e d crystals was r e c o r d e d . F r o m the m e a s u r e d average size o f the crystals a f t e r growth, t h e e s t i m a t e d rate o f a d v a n c e m e n t o f the t r a n s f o r m a t i o n f r o n t was f o u n d to be 0 . 0 3 p m min -1. In specimen B, crystallization o c c u r r e d mainly t h r o u g h n u c l e a t i o n and g r o w t h o f new crystals in the a m o r p h o u s m a t r i x {Fig. 9). As the n u c l e a t i o n density was very high (11 × 1015 nuclei cm-3), the n e w l y f o r m e d crystals did
193 not have much chance to grow. The growth of pre-existing crystals was also inhibited by homogeneously nucleated crystals.
4. DISCUSSION
4.1. Solidification The Z r - F e phase diagram has not y e t been fully established. Some controversy exists about the existence of different zirconiumrich intermetallic phases and on the nature of different phase reactions. A diagram in ref. 5 shows three intermetallic phases Zr 2 Fe, Zr 3 Fe and Zr 4Fe on the zirconium-rich side. However, recent investigations have indicated that the Zr4Fe phase does n o t exist [6]. The Zr3Fe phase (which is base centred orthorhombic with lattice parameters a = 3.320 A, b = 10.980 .~ and c = 8.800 ,~) has been found to be the first intermetallic c o m p o u n d on the zirconium-rich side of the phase diagram. It is interesting to note that the Zr 3 Fe phase is very close to the eutectic composition and therefore partitionless solidification is quite likely in a eutectic alloy under rapid solidification. Recent experiments with an alloy close to the eutectic composition have indeed demonstrated this. It should be mentioned here that, although the exact stoichiometry of the Zr3Fe phase required the alloy to contain 25 at.% Fe, it is possible for the phase to exist over a small composition range. The formation of Zr3Fe crystals from the liquid of nearly the same composition under rapid solidification processing involves atomic transport only at the crystal-liquid interface. As there is almost no solute redistribution between the solid and the liquid phase, morphological instability cannot set in because of constitutional supercooling. The stability of the interface under thermal undercooling also needs to be considered. It has been observed by some investigators that the crystal-liquid interface remains planar if it moves very rapidly. The morphological instabilities can set in only after the velocity of the interface becomes sufficiently low [7, 8]. In the melt-spinning process the cooling rate varies across the thickness of the ribbon. The different cooling rates in different parts of the ribbon have been f o u n d to cause different solidification morphologies across the thickness in the same ribbon [9, 10]. The o
variation in cooling rate across the thickness of the ribbon could be a possible source of the different types of crystal aggregate and crystal-amorphous matrix interface encountered here. In the melt-spinning process of the type used in this study, the ribbon leaves the heat-extracting surface of the wheel while it is still hot. In the melt-spinning process the relaxation of the glassy structure during the cooling of the amorphous phase from the temperature at which the liquid had become glassy to ambient temperature has been reported by some investigators [ 11 ]. By the same analogy, growth of the crystals formed during solidification as a result of slow postsolidification cooling is possible. However, the extent of growth is likely to be small and the crystals are likely to retain most of the solidification features. Although the electropolished samples showed a contrast typical of spinodal phase separation, it was not possible to obtain any supporting evidence in favour of phase separation. In the absence of this, it is concluded that the mottled contrast observed in the amorphous structure is due to a non-uniform attack of the electropolishing solution. Dong et al. [12] have also arrived at a similar conclusion for Zr-Ni amorphous alloys. The absence of mottled contrast in the chemically thinned specimens supports this conclusion.
4.2. Crystallization Crystallization involved transformation of the amorphous matrix to the Zr s Fe phase by growth of pre-existing Zr3Fe crystals as well as by fresh nucleation and growth. The Zr3Fe phase had the same composition as the amorphous matrix from which it formed. Such a crystallization process has been referred to as polymorphic crystallization [ 13 ]. Polymorphic crystallization occurs at concentrations close to those of the pure elements or compounds. It has been observed in, among other glasses, Fe75B25 glass. In Fe75B25 glass the crystallization of the amorphous matrix involves formation of the Fe3B intermetallic phase [13]. Generally, crystals forming by polymorphic crystallization have been found to have planar interfaces of the type encountered here. In categorizing the crystallization process as of a polymorphic type it was assumed that the matrix does n o t have any phase separation and consists of a single-amorphous phase. The
194 crystallization process in this case can be represented by the following reaction: amorphous (Zr-24at.%Fe) -~ Zr3Fe The appearance of a single sharp exothermic peak in the thermogram indicated that the amorphous matrix was transforming to a single crystalline phase. The appearance of a single peak could also mean the simultaneous formation o f more than one crystalline phase. However, X-ray diffraction as well as TEM studies showed that the former reaction had taken place. As mentioned earlier, two distinct modes of crystallization predominate at the two temperatures studied here. At 598 K, crystallization occurred, mainly by the growth of preexisting crystals. The formation and growth of a limited number of fresh nuclei did n o t contribute significantly. At 723 K, crystallization t o o k place by large-scale nucleation. The distance between the crystal nuclei depends on the temperature of crystallization. At temperatures where nucleation is very rapid and homogeneous, a large number of closely spaced nuclei are likely to form. These closely spaced nuclei do not undergo a considerable a m o u n t of growth because of early impingement. This type of crystallization behaviour was observed in specimen B. Specimens treated at the lower temperature (specimen A) showed slower nucleation. This observation suggests that the crystallization temperatures chosen in the present study are in the range in which nucleation rate decreases with temperature. According to classical nucleation theory, the nucleation rate which is essentially controlled by the extent of undercooling and the atomic mobility goes through a maximum at Tc as the temperature is lowered [14]. As it is difficult to approach the temperature range above Tc during the crystallization of metallic glasses, the majority of investigations have shown a temperature dependence of nucleation rate similar to that observed in the present work [15,16]. Rapid solidification invariably fails to suppress the formation of atomic clusters having a compositional and structural similarity to the crystalline phase. These quenched-in embryos have been found to play an important role in crystallization [15]. At sufficiently low temperatures of crystallization, the quenchedin embryos become critical and start growing
[ 17 ]. At higher temperatures of crystallization, these become subcritical and dissolve; therefore, thermally activated homogeneous nucleation becomes necessary for crystallization to proceed. Crystallization brought about by quenched-in embryos alone shows a very narrow size distribution. This is because all the embryos become critical at almost the same instant and grow to the same extent after the elapse of a certain period. Crystals formed by thermally activated nucleation show a wide size distribution. The wide size distribution of freshly formed crystals even in specimen A suggested a limited role of quenched-in embryos in the nucleation process in this system. The interfaces separating the pre-existing crystals and the amorphous matrix did not act as preferential sites for the nucleation of fresh crystals. Instead the pre-existing crystals grow presumably by atoms jumping across the advancing transformation front to occupy positions falling on the extension of the lattice of a given pre-existing crystal. The fact that sympathetic nucleation does not occur can be rationalized from the consideration of surface energy required in creating high energy interfaces between pre-existing and fresh crystals. The formation of intermetallic phases by polymorphic crystallization from the amorphous matrix has been studied in a large number of alloy systems. However, very few studies have been reported on their formation from the melt. Partitionless solidification of intermetallic phases from alloy melts of the same composition has been considered by Massalski [18] to be one of the factors responsible for the poor glass-forming ability of some alloy compositions. This is because the growth rate of crystals during partitionless solidification is much faster than that in a solidification process involving long-range atom transport. However, the growth rate decreases rapidly as the temperature approaches the glass transition temperature Tg at which the remaining liquid phase can vitrify into glass. The partly crystalline structure observed in the present case suggests that the cooling rate employed was such that the nucleation of some crystals prior to attaining Tg could not be avoided but the growth was not allowed to proceed beyond a certain extent. As mentioned earlier, polymorphic crystallization of ZraFe occurs during the annealing treatment, the mechanism of the process being remarkably
195 similar t o t h a t o f p a r t i t i o n l e s s solidification. B o t h t h e s e processes are c o m p o s i t i o n i n v a r i a n t a n d involve s h o r t - r a n g e d i f f u s i o n across an incoherent transformation front. The difference b e t w e e n t h e t w o arises b e c a u s e o f a higher d i f f u s i v i t y in t h e liquid p h a s e t h a n in t h e amorphous phase and the difference between the liquid-crystal and amorphous-crystal interfacial energies.
5. CONCLUSIONS (1) Melt s p i n n i n g o f Z r - 2 4 a t . % F e alloy at a relatively l o w w h e e l s p e e d results in a p a r t l y crystalline s t r u c t u r e consisting o f Z r 3 F e crystals in an a m o r p h o u s m a t r i x . (2) T h e f o r m a t i o n o f Z r 3 F e crystals f r o m t h e liquid p h a s e o c c u r s in a partitionless solidification process. (3) During annealing, t h e t r a n s f o r m a t i o n o f t h e a m o r p h o u s m a t r i x t o t h e crystalline p h a s e (Zr 3 Fe) o c c u r s b y p o l y m o r p h i c crystallization. T h e t r a n s f o r m a t i o n is b r o u g h t a b o u t b y t h e g r o w t h o f t h e p r e - e x i s t i n g crystals as well as b y t h e fresh n u c l e a t i o n a n d g r o w t h o f crystals in t h e a m o r p h o u s m a t r i x .
ACKNOWLEDGMENTS T h e a u t h o r s w o u l d like to t h a n k Dr. M. K. A s u n d i f o r his i n t e r e s t in this w o r k . Assistance in t h e e x p e r i m e n t a l w o r k b y Mr. E. G. Baburaj is g r a t e f u l l y a c k n o w l e d g e d . O n e o f t h e a u t h o r s (S.B.) w o u l d like to t h a n k his f o r m e r colleagues, p a r t i c u l a r l y Dr. M. G. S c o t t a n d Dr. G. Gregan, o f t h e R a p i d Solidification G r o u p ,
U n i v e r s i t y o f Sussex, f o r m a n y useful discussions.
REFERENCES 1 K. H. J. Buschow, J. Less-Common Met., 79 (1981)243. 2 A. Inoue, H. Tomioka and T. Masumoto, J. Mater. Sci., 18 (1983) 153. 3 J. W. Cahn and R. J. Charles, Phys. Chem. Glasses, 6 (1965) 181. 4 L. Tanner and R. Ray, Scr. Metall., 14 (1980) 657. 5 F. A. Shunk, Constitution o f Binary Alloys, McGraw-Hill, New York, 1969, p. 354. 6 L. Kumar, G. B. Kale and P. Mukhopadhyay, personal communication, 1984. 7 C. J. Lin and F. Spaepen, Scr. Metall., 17 (1983) 1259. 8 W. M. Mullins and R. F. Serkerka, J. Appl. Phys., 35 (1964) 444. 9 J. A. Van Der Hoexen, P. Van Mourik and E. J. Mittemeijer, J. Mater. Sci. Lett., 2 (1983) 158. 10 R. K. Garrett and T. H. Sanders, Mater. Sci. Eng., 60 (1983) 269. 11 D. Akhtar, B. Cantor and R. W. Cahn, Acta Metall., 30 (1982) 1571. 12 Y. D. Dong, G. Gregan and M. G. Scott, J. NonCryst. Solids, 43 (1981) 403. 13 U. Harold and U. Koster, in B. Cantor (ed.), Proc. 3rd Int. Conf. on Rapidly Quenched Metals, Brighton, July 3-7, 1978, Vol. 1, Metals Society,
London, 1978, p. 281. 14 J. Burke, The Kinetics o f Phase Transformation in Metals, Pergamon, Oxford, 1965, p. 36. 15 U. Koster and U. Herold, in T. Masumoto and K. Suzuki (eds.), Proc. 4th Int. Conf. on Rapidly Quenched Metals, Sendai, August 1981, Japan Institute of Metals, Sendal, 1982, p. 717. 16 D. G. Morris, Acta Metall., 29 (1981) 1213. 17 U. Koster and U. Herold, Glassy metals, Top. Appl. Phys., 46 (1980) 225. 18 T. B. Massalski, in T. Masumoto and K. Suzuki (eds.), Proc. 4th Int. Conf. on Rapidly Quenched Metals, Sendai, 1981, Japan Institute of Metals, Sendai, 1982, p. 203.
187
Rapid Solidification and Crystallization of a Zr-24at.%Fe Alloy G. K. DEY and S. BANERJEE
Physical Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085 (India) (Received May 30, 1984; in revised form November 1, 1984)
ABSTRACT
Rapidly solidified ribbons of Zr-24at.%Fe alloy produced in a melt-spinning apparatus have been found to contain a crystalline phase (which is base-centred orthorhombic with lattice parameters a = 3.320.7i, b = 1 0 . 9 8 0 . 4 and c = 8.800 A ) in an amorphous matrix. This phase is believed to be the Zr3Fe intermetallic c o m p o u n d which is produced through diffusionless solidification from a melt o f the same composition. The morphologies o f crystalline particles which are present as isolated crystals or as aggregates o f contiguous particles in the amorphous matrix are described and their possible mode o f formation discussed. The origin o f different types o f interface between these crystals and the amorphous ma trix is discussed in terms o f solute rejection. The phase formed after crystallization in the amorphous matrix was identified as the orthorhombic Zr3Fe phase. The morphology and distribution o f the crystals originating from the amorphous phase were examined and compared with those formed during solidification. The effects o f pre-existing crystals in the amorphous matrix during the crystallization process are discussed. The crystallization process was studied at two temperatures, -- 598 and 723 K. Crystallization at the lower temperature was f o u n d to be associated with significant growth o f the pre-existing crystals. Extensive nucleation o f the crystalline phase in the amorphous matrix followed by a limited growth culminatl~ag in early impingement marked the crystallization process at the higher temperature.
line and amorphous phases. Several characteristic features of the early stages of the solidification process, which otherwise would be difficult to observe, are expected to be preserved in this partly crystalline solid. Thermal t r e a t m e n t of this solid subsequent to solidification results in transformation of the amorphous phase to one or more crystalline phases. This transformation is brought about by the growth of the pre-existing crystalline particles as well as by fresh nucleation and growth of crystalline phases in the amorphous matrix. The examination of the as-solidified structure and of the structural changes accompanying the subsequent thermal treatment enables a comparison to be made between the processes of solidification and crystallization. In this paper the identification and the morphological characterization of the crystalline phase formed during the rapid solidification and the subsequent heat treatment operations in Zr-24at.%Fe alloy are dealt with. The crystalline phase was identified as a basecentred orthorhombic phase which has the stoichiometric composition Zr3 Fe. The formation of this phase in a fully amorphous alloy of identical composition during crystallization has been reported by Buschow [1]. Since the composition o f the crystalline phase and that of the melt from which it forms are almost identical, the solidification and crystallization processes do n o t involve solute partitioning between the parent and product phases. This is the reason why the Zr-24at.%Fe alloy provided a good o p p o r t u n i t y for a comparison to be made between the solidification and the crystallization processes.
1. INTRODUCTION 2. EXPERIMENTAL DETAILS Rapid solidification at cooling rates insufficient to suppress crystalline phase formation completely may yield a composite of crystal-
Crystal bar zirconium (oxygen content, 200 ppm) and high purity iron were melted
188 u n d e r a p u r e a r g o n a t m o s p h e r e in an arc furn a c e t o p r o d u c e t h e alloy b u t t o n s . T h e b u t t o n s were r e m e l t e d several t i m e s t o e n s u r e h o m o genization. R a p i d solidification was carried o u t in a m e l t - s p i n n i n g a p p a r a t u s consisting o f a c o p p e r w h e e l 20 c m in d i a m e t e r . A wheel surface s p e e d o f 20 m s-1 a n d an argon shield w e r e used d u r i n g m e l t spinning. T h e r i b b o n s p r o d u c e d w e r e a b o u t 3 m m wide a n d 50 p m thick. T h i n foils f o r e l e c t r o n m i c r o s c o p y w e r e p r e p a r e d b y t h e w i n d o w t e c h n i q u e using a s o l u t i o n c o n t a i n i n g 300 c m 8 o f m e t h a n o l , 170 c m 3 o f b u t a n o l a n d 30 c m a o f p e r c h l o r i c acid. T h e t e m p e r a t u r e o f t h e e l e c t r o l y t e was m a i n t a i n e d b e l o w 220 K using a m e t h a n o l d r y ice b a t h . T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y ( T E M ) was carried o u t in a Siemens E l m i s k o p 102 i n s t r u m e n t . D i f f e r e n t i a l scanning c a l o r i m e t r y was carried o u t in a P e r k i n - E l m e r DSC II i n s t r u m e n t .
TABLE 1 X-ray diffraction data for Zr3Fe
hkl d spacing (,~) l/I o
hkl d spacing (.~) I/I 0
001 020 021 002 022 110 111 040 041 023 112 130 131 042 004 113 132 024 043 133 150 060 114
151 061 044 152 062 025 200 134 201 220 221 202 153 063 115 222 045 006 203 135 240 170
8.800 5.490 4.658 4.400 3.433 3.170 2.985 2.745 2.620 2.582 2.576 2.455 2.366 2.327 2.200 2.156 2.142 2.042 2.004 1.885 1.832 1.830 1.809
NO 40 NO NO VW VW VW 20 10 100 100 20 10 5 5 5 VW 5 VVW 20 NO 20 NO
1.793 1.792 1.717 1.691 1.685 1.676 1.660 1.640 1.631 1.589 1.564 1.555 1.554 1.553 1.539 1.494 1.481 1.466 1.443 1.434 1.420 1.419
NO 10 NO VW VW NO VW VW NO NO 20 VW VW NO VW VW NO NO W W W VW
3. RESULTS
3.1. Solidified structure 3.1.1. X-ray diffraction X - r a y d i f f r a c t i o n carried o u t o n t h e meltspun ribbons showed a superimposition of b r o a d m a x i m a t y p i c a l o f an a m o r p h o u s structure and a n u m b e r of sharp peaks normally o b t a i n e d f r o m a crystalline phase, suggesting t h e p r e s e n c e o f an a m o r p h o u s - c r y s t a l l i n e phase m i x t u r e in t h e s p e c i m e n s . T a b l e 1 shows t h e X - r a y d i f f r a c t i o n results. T h e o b s e r v e d p e a k s could be i n d e x e d in t e r m s o f a basec e n t r e d o r t h o r h o m b i c p h a s e having t h e foll o w i n g lattice p a r a m e t e r s : a = 3 . 3 2 0 A; b = 1 0 . 9 8 0 ti,; c = 8 . 8 0 0 A. B u s c h o w [1] has previously r e p o r t e d t h e f o r m a t i o n o f a Z r 3 F e p h a s e o f similar s t r u c t u r e , w i t h lattice param e t e r s close to t h o s e o b t a i n e d here, in crystallized metallic glass o f Z r - 2 4 a t . % F e . T h e c r y s t a l l i z a t i o n s t u d y carried o u t b y B u s c h o w was o n a fully a m o r p h o u s m e l t - s p u n alloy. It s h o u l d be m e n t i o n e d t h a t t h e cooling r a t e a c h i e v e d during m e l t spinning d e p e n d s , a m o n g o t h e r factors, o n t h e surface s p e e d o f t h e w h e e l [2]. It is possible t o o b t a i n t h e s a m e alloy in a fully a m o r p h o u s state or a p a r t l y crystalline s t a t e b y altering t h e surface s p e e d o f t h e wheel. A relatively l o w surface s p e e d o f t h e wheel used in t h e p r e s e n t s t u d y is
NO, not observed; VVW, very very weak; VW, very weak ; W, weak.
possibly responsible for the formation of a p a r t l y crystalline s t r u c t u r e o f t h e r i b b o n c o m p a r e d w i t h t h e fully a m o r p h o u s s t r u c t u r e obtained by Buschow. A c o m p a r i s o n o f p e a k intensities o f certain s t r o n g r e f l e c t i o n s o b t a i n e d f r o m t h e wheelside surface (the surface in c o n t a c t w i t h t h e w h e e l d u r i n g m e l t spinning) a n d t h e air-side surface (the s u r f a c e a w a y f r o m t h e wheel) ind i c a t e d t h a t a smaller a m o u n t o f t h e crystalline p h a s e was p r e s e n t close to t h e wheel-side surface t h a n at t h e air-side surface.
3.1.2. Transmission electron microscopy T h i n foils o f t h e m e l t - s p u n r i b b o n s w e r e f o u n d t o c o n t a i n isolated single crystals a n d aggregates o f crystals d i s t r i b u t e d in an a m o r p h o u s m a t r i x . T h e d i f f r a c t i o n p a t t e r n (Fig. 1) f r o m a r e p r e s e n t a t i v e region o f t h e a m o r p h o u s m a t r i x s h o w s characteristic halos. Bright field e l e c t r o n m i c r o g r a p h s (Fig. 2) f r o m t h e a m o r p h o u s region s h o w a fine-scale s t r u c t u r e consisting o f a lighter a n d a d a r k e r region interweaved. T h e scale o f t h e s t r u c t u r e a n d t h e c o n n e c t i v i t y o f t h e t w o regions h a v e r e m a r k -
189
Fig. 1. Selected area electron diffraction pattern from the amorphous region, showing a single broad intense halo.
Fig. 2. Bright field electron micrograph from the amorphous region, showing contrast typical of amorphous phase separation. Darker and brighter regions suggestive of the presence of two amorphous phases can be seen.
able similarity to those encountered in the spinodally decomposed structure in elastically isotropic systems [3]. The average wavelength of the structure was found to be 300 .~. A similar mottled contrast observed in the amorphous structure of Zr36Ti24B40 alloy was attributed to the presence of two amorphous phases, as confirmed by differential scanning calorimetry [4]. In the present study, microdensitometer traces were taken on diffraction
patterns from the amorphous region to obtain corroborative evidence of two intensity peaks within the first diffused halo but no such intensity distribution could be seen. Moreover, chemically thinned specimens did n o t exhibit any mottled contrast. Electron diffraction from the crystals confirmed that the crystals were of the Zr 3 Fe intermetallic phase (Fig. 3). The isolated single crystals were generally f o u n d to have very regular geometric morphologies. Two of these morphologies are illustrated in Fig. 4. Extinction contours can be seen to extend across the entire length of these crystals, establishing their single-crystal nature. A wide range of crystal sizes was encountered, indicating different extents of crystal growth. The crystal aggregates could be grouped into two types. The first type can be described as aggregates with a nearly spherical central core surrounded by a number of crystals emanating from the core. In several such aggregates, the core is seen to be surrounded by six crystals (Fig. 5), a sixfold s y m m e t r y being present for the aggregate as a whole. A more detailed account of the crystallography and the origin of such crystal aggregates will be presented elsewhere. In the remaining aggregates of the first type, a large number of crystals are seen to be present around the central core. The second type of aggregate does n o t exhibit a core but several geometric crystals are found to be in a group with c o m m o n interfaces between them. Selected area diffraction patterns taken from the second type of aggregate could be indexed in terms of the Zr3Fe structure as observed in isolated crystals. Interfaces separating crystals from the amorphous matrix were seen to be sometimes planar and sometimes undulating. Singlesurface trace analysis showed that some of the planar interfaces lie along (100} planes. A careful examination of the modulated interfaces showed that the interfaces often assumed a zigzag morphology made up of facets of different rational planes such as (111) and (110) (Fig. 6). It was observed that the undulations were very small in size. Their existence was not due to the presence of a third phase. The crystals in aggregates were also seen to have these two types of interface. In the text that follows, the term "preexisting crystals" means crystals formed during solidification.
190
0
020
021
0
000
001
o}i
o2o
o~
ZONE
Ax,s :- [,oo3 z , 3 Fe
(a)
,~r
,To
,~,
o~r
o~o
oo,
Ill
IlO
ZONE AXIS:-
III
[J~o] Z~3Fe
(b)
Fig. 3. Selected area diffraction patterns from two isolated Zr3Fe crystals ((a) and (b)). The keys to the patterns are shown alongside.
3.2. Crystallization In o r d e r t o d e t e r m i n e t h e n u m b e r and the n a t u r e o f t h e r m a l events a c c o m p a n y i n g crystallization, t h e r i b b o n s were h e a t e d in a differential scanning calorimeter. A solitary sharp e x o t h e r m i c p e a k was observed at a heating rate o f 20 K min -1. A s c h e m a t i c p l o t o f the t h e r m o g r a m is s h o w n in Fig. 7. T h e t e m p e r a t u r e Tx at which crystallization starts and the peak t e m p e r a t u r e Tp at this heating rate were f o u n d to be 653 K and 6 5 6 . 5 K respectively. No e n d o t h e r m i c p e a k suggestive o f glass transition was observed. This aspect has been e l a b o r a t e d elsewhere in this paper. In o r d e r t o s t u d y t h e progress o f the crystallization process b y TEM, specimens o f the
as-solidified ribbons were h e a t t r e a t e d at 598 K for 1 h or at 723 K f o r 0.5 h. These specimens are r e f e r r e d t o as specimen A and specim e n B respectively.
3. 2.1. X-ray diffraction X-ray d i f f r a c t i o n carried o u t o n specimen A indicated t h a t n o n e w phase had f o r m e d . The p e a k intensities o f t h e crystalline phase f o r m e d during solidification s h o w e d an increase, suggesting an increase in the v o l u m e f r a c t i o n o f t h e Zr 3 Fe phase. The presence o f a b r o a d m a x i m u m characteristic o f t h e amorp h o u s phase was indicative o f an i n c o m p l e t e a m o r p h o u s - t o - c r y s t a l l i n e t r a n s f o r m a t i o n in t h e specimen heat t r e a t e d at 598 K f o r 1 h. This
191
Fig. 5. Bright field electron micrograph of a crystal aggregate in an amorphous matrix, depicting a small central core surrounded by six crystals.
Fig. 4. Crystals of the Zr3Fe phase in an amorphous matrix, showing some of the crystal morphologies and crystal-amorphous matrix interfaces encountered. The crystal in (a) has undulating interfaces on the sides and planar interfaces on the top and bottom.
Fig. 6. Bright field electron micrograph of a crystalamorphous matrix interface showing zigzag morphology.
3.2. 2. Transmission electron microscopy o b s e r v a t i o n was f u r t h e r c o n f i r m e d b y TEM imaging. The X-ray d i f f r a c t i o n o f s p e c i m e n B, h o w e v e r , i n d i c a t e d t h a t the a m o r p h o u s to-crystalline t r a n s f o r m a t i o n was c o m p l e t e .
S p e c i m e n A e x h i b i t e d g r o w t h o f t h e preexisting crystals and the n u c l e a t i o n o f a limited n u m b e r o f fresh crystals in t h e a m o r p h o u s m a t r i x (Fig. 8), the f o r m e r c o n t r i b u t i n g m o r e t o w a r d s the a m o r p h o u s - t o - c r y s t a l l i n e trans-
192
t lg W "r X W
u =E n~ UJ
"1I-0 Q Z i,i
tt
T Tp
1
I
610
620
I
I
630
I
640
I
650
.
I
J
660
670
TEMPERATURE
I
680
I
690
I
•700
I
J
710
720
(K)
Fig. 7. Thermogram obtained at a heating rate of 20 K rain -1 showing the crystallization peak temperature Tp./:/ indicates the rate of enthalpy change.
Fig. 8. Bright field electron micrograph obtained from specimen A, showing the growth of the preexisting crystals and the nucleation of crystals in the amorphous matrix.
f o r m a t i o n . The interfaces which advanced during t h e g r o w t h o f t h e crystalline particles were almost all f o u n d to be planar. Undulating interfaces were v e r y rarely e n c o u n t e r e d in the crystallized samples. Planar faces were again f o u n d t o be along ( 1 0 0 ) planes, suggesting t h a t the t r a n s f o r m a t i o n f r o n t t e n d s t o assume these orientations. Fresh n u c l e a t i o n within t h e m a t r i x did n o t o c c u r t o such an
Fig. 9. Bright field electron micrograph showing the high density of crystals formed in specimen B.
e x t e n t as t o restrict t h e g r o w t h o f pre-existing crystals. A fairly wide d i s t r i b u t i o n o f t h e size o f freshly f o r m e d crystals was r e c o r d e d . F r o m the m e a s u r e d average size o f the crystals a f t e r growth, t h e e s t i m a t e d rate o f a d v a n c e m e n t o f the t r a n s f o r m a t i o n f r o n t was f o u n d to be 0 . 0 3 p m min -1. In specimen B, crystallization o c c u r r e d mainly t h r o u g h n u c l e a t i o n and g r o w t h o f new crystals in the a m o r p h o u s m a t r i x {Fig. 9). As the n u c l e a t i o n density was very high (11 × 1015 nuclei cm-3), the n e w l y f o r m e d crystals did
193 not have much chance to grow. The growth of pre-existing crystals was also inhibited by homogeneously nucleated crystals.
4. DISCUSSION
4.1. Solidification The Z r - F e phase diagram has not y e t been fully established. Some controversy exists about the existence of different zirconiumrich intermetallic phases and on the nature of different phase reactions. A diagram in ref. 5 shows three intermetallic phases Zr 2 Fe, Zr 3 Fe and Zr 4Fe on the zirconium-rich side. However, recent investigations have indicated that the Zr4Fe phase does n o t exist [6]. The Zr3Fe phase (which is base centred orthorhombic with lattice parameters a = 3.320 A, b = 10.980 .~ and c = 8.800 ,~) has been found to be the first intermetallic c o m p o u n d on the zirconium-rich side of the phase diagram. It is interesting to note that the Zr 3 Fe phase is very close to the eutectic composition and therefore partitionless solidification is quite likely in a eutectic alloy under rapid solidification. Recent experiments with an alloy close to the eutectic composition have indeed demonstrated this. It should be mentioned here that, although the exact stoichiometry of the Zr3Fe phase required the alloy to contain 25 at.% Fe, it is possible for the phase to exist over a small composition range. The formation of Zr3Fe crystals from the liquid of nearly the same composition under rapid solidification processing involves atomic transport only at the crystal-liquid interface. As there is almost no solute redistribution between the solid and the liquid phase, morphological instability cannot set in because of constitutional supercooling. The stability of the interface under thermal undercooling also needs to be considered. It has been observed by some investigators that the crystal-liquid interface remains planar if it moves very rapidly. The morphological instabilities can set in only after the velocity of the interface becomes sufficiently low [7, 8]. In the melt-spinning process the cooling rate varies across the thickness of the ribbon. The different cooling rates in different parts of the ribbon have been f o u n d to cause different solidification morphologies across the thickness in the same ribbon [9, 10]. The o
variation in cooling rate across the thickness of the ribbon could be a possible source of the different types of crystal aggregate and crystal-amorphous matrix interface encountered here. In the melt-spinning process of the type used in this study, the ribbon leaves the heat-extracting surface of the wheel while it is still hot. In the melt-spinning process the relaxation of the glassy structure during the cooling of the amorphous phase from the temperature at which the liquid had become glassy to ambient temperature has been reported by some investigators [ 11 ]. By the same analogy, growth of the crystals formed during solidification as a result of slow postsolidification cooling is possible. However, the extent of growth is likely to be small and the crystals are likely to retain most of the solidification features. Although the electropolished samples showed a contrast typical of spinodal phase separation, it was not possible to obtain any supporting evidence in favour of phase separation. In the absence of this, it is concluded that the mottled contrast observed in the amorphous structure is due to a non-uniform attack of the electropolishing solution. Dong et al. [12] have also arrived at a similar conclusion for Zr-Ni amorphous alloys. The absence of mottled contrast in the chemically thinned specimens supports this conclusion.
4.2. Crystallization Crystallization involved transformation of the amorphous matrix to the Zr s Fe phase by growth of pre-existing Zr3Fe crystals as well as by fresh nucleation and growth. The Zr3Fe phase had the same composition as the amorphous matrix from which it formed. Such a crystallization process has been referred to as polymorphic crystallization [ 13 ]. Polymorphic crystallization occurs at concentrations close to those of the pure elements or compounds. It has been observed in, among other glasses, Fe75B25 glass. In Fe75B25 glass the crystallization of the amorphous matrix involves formation of the Fe3B intermetallic phase [13]. Generally, crystals forming by polymorphic crystallization have been found to have planar interfaces of the type encountered here. In categorizing the crystallization process as of a polymorphic type it was assumed that the matrix does n o t have any phase separation and consists of a single-amorphous phase. The
194 crystallization process in this case can be represented by the following reaction: amorphous (Zr-24at.%Fe) -~ Zr3Fe The appearance of a single sharp exothermic peak in the thermogram indicated that the amorphous matrix was transforming to a single crystalline phase. The appearance of a single peak could also mean the simultaneous formation o f more than one crystalline phase. However, X-ray diffraction as well as TEM studies showed that the former reaction had taken place. As mentioned earlier, two distinct modes of crystallization predominate at the two temperatures studied here. At 598 K, crystallization occurred, mainly by the growth of preexisting crystals. The formation and growth of a limited number of fresh nuclei did n o t contribute significantly. At 723 K, crystallization t o o k place by large-scale nucleation. The distance between the crystal nuclei depends on the temperature of crystallization. At temperatures where nucleation is very rapid and homogeneous, a large number of closely spaced nuclei are likely to form. These closely spaced nuclei do not undergo a considerable a m o u n t of growth because of early impingement. This type of crystallization behaviour was observed in specimen B. Specimens treated at the lower temperature (specimen A) showed slower nucleation. This observation suggests that the crystallization temperatures chosen in the present study are in the range in which nucleation rate decreases with temperature. According to classical nucleation theory, the nucleation rate which is essentially controlled by the extent of undercooling and the atomic mobility goes through a maximum at Tc as the temperature is lowered [14]. As it is difficult to approach the temperature range above Tc during the crystallization of metallic glasses, the majority of investigations have shown a temperature dependence of nucleation rate similar to that observed in the present work [15,16]. Rapid solidification invariably fails to suppress the formation of atomic clusters having a compositional and structural similarity to the crystalline phase. These quenched-in embryos have been found to play an important role in crystallization [15]. At sufficiently low temperatures of crystallization, the quenchedin embryos become critical and start growing
[ 17 ]. At higher temperatures of crystallization, these become subcritical and dissolve; therefore, thermally activated homogeneous nucleation becomes necessary for crystallization to proceed. Crystallization brought about by quenched-in embryos alone shows a very narrow size distribution. This is because all the embryos become critical at almost the same instant and grow to the same extent after the elapse of a certain period. Crystals formed by thermally activated nucleation show a wide size distribution. The wide size distribution of freshly formed crystals even in specimen A suggested a limited role of quenched-in embryos in the nucleation process in this system. The interfaces separating the pre-existing crystals and the amorphous matrix did not act as preferential sites for the nucleation of fresh crystals. Instead the pre-existing crystals grow presumably by atoms jumping across the advancing transformation front to occupy positions falling on the extension of the lattice of a given pre-existing crystal. The fact that sympathetic nucleation does not occur can be rationalized from the consideration of surface energy required in creating high energy interfaces between pre-existing and fresh crystals. The formation of intermetallic phases by polymorphic crystallization from the amorphous matrix has been studied in a large number of alloy systems. However, very few studies have been reported on their formation from the melt. Partitionless solidification of intermetallic phases from alloy melts of the same composition has been considered by Massalski [18] to be one of the factors responsible for the poor glass-forming ability of some alloy compositions. This is because the growth rate of crystals during partitionless solidification is much faster than that in a solidification process involving long-range atom transport. However, the growth rate decreases rapidly as the temperature approaches the glass transition temperature Tg at which the remaining liquid phase can vitrify into glass. The partly crystalline structure observed in the present case suggests that the cooling rate employed was such that the nucleation of some crystals prior to attaining Tg could not be avoided but the growth was not allowed to proceed beyond a certain extent. As mentioned earlier, polymorphic crystallization of ZraFe occurs during the annealing treatment, the mechanism of the process being remarkably
195 similar t o t h a t o f p a r t i t i o n l e s s solidification. B o t h t h e s e processes are c o m p o s i t i o n i n v a r i a n t a n d involve s h o r t - r a n g e d i f f u s i o n across an incoherent transformation front. The difference b e t w e e n t h e t w o arises b e c a u s e o f a higher d i f f u s i v i t y in t h e liquid p h a s e t h a n in t h e amorphous phase and the difference between the liquid-crystal and amorphous-crystal interfacial energies.
5. CONCLUSIONS (1) Melt s p i n n i n g o f Z r - 2 4 a t . % F e alloy at a relatively l o w w h e e l s p e e d results in a p a r t l y crystalline s t r u c t u r e consisting o f Z r 3 F e crystals in an a m o r p h o u s m a t r i x . (2) T h e f o r m a t i o n o f Z r 3 F e crystals f r o m t h e liquid p h a s e o c c u r s in a partitionless solidification process. (3) During annealing, t h e t r a n s f o r m a t i o n o f t h e a m o r p h o u s m a t r i x t o t h e crystalline p h a s e (Zr 3 Fe) o c c u r s b y p o l y m o r p h i c crystallization. T h e t r a n s f o r m a t i o n is b r o u g h t a b o u t b y t h e g r o w t h o f t h e p r e - e x i s t i n g crystals as well as b y t h e fresh n u c l e a t i o n a n d g r o w t h o f crystals in t h e a m o r p h o u s m a t r i x .
ACKNOWLEDGMENTS T h e a u t h o r s w o u l d like to t h a n k Dr. M. K. A s u n d i f o r his i n t e r e s t in this w o r k . Assistance in t h e e x p e r i m e n t a l w o r k b y Mr. E. G. Baburaj is g r a t e f u l l y a c k n o w l e d g e d . O n e o f t h e a u t h o r s (S.B.) w o u l d like to t h a n k his f o r m e r colleagues, p a r t i c u l a r l y Dr. M. G. S c o t t a n d Dr. G. Gregan, o f t h e R a p i d Solidification G r o u p ,
U n i v e r s i t y o f Sussex, f o r m a n y useful discussions.
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