Crystallization of amorphous Zr-rich alloys

Journal of Non-Crystalline Solids 54 (1983) 101-106 North-Holland Publishing Company

101

CRYSTALLIZATION OF A M O R P H O U S Zr-RICH ALLOYS

K.H.J. BUSCHOW Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands

I. VINCZE * and F. VAN DER W O U D E Solid State Physics Laboratory, University of Groningen, Groningen, The Netherlands Received 5 August 1982

The crystallization of amorphous alloys of the approximate composition ZrsM 2, where M represents Cu, Ni, Co, Fe, Rh and Ir was studied by means of differential scanning calorimetry X-ray diffraction and also, in the case M = Fe by 57Fe M6ssbauer effect spectroscopy. It is shown that the first step in the crystallization of these alloys involves the precipitation of a metastable modification of Zr metal (w-Zr).

1. Introduction

The crystallization behaviour of amorphous Z r l _ x M x ( M = Cu, Ni) has been studied in previous investigations [1,2]. It has been found that crystallization of amorphous alloys relatively rich in Zr gives rise to two distinct exothermal effects when heating in a differential scanning calorimeter. The first of these exothermal effects has been attributed to a transformation of the amorphous alloys into a-Zr and a metastable intermetallic compound. The second exothermal effect has been shown to be associated with the transformation of these crystalline phases into the stable crystalline phases expected on the basis of the corresponding phase diagrams, i.e. a-Zr and Zr2Cu (MoSi 2-type) in the case of Zrl_xCu ~ and a-Zr and Zr2Ni (CuA12-type) in the case of Zr~_~Ni~. In both systems the crystal structure of the metastable compound was reported to be cubic, although no further details could be obtained owing to the bad quality of the X-ray diagrams of the partially crystallized amorphous alloys [1,2]. Quite similar thermal behaviour was observed recently in amorphous Zr~_xFex alloys [3], opening up the possibility that the initial transformation of amorphous Zr 1_~M x alloys is more or less uniform and not much dependent on the nature of the M component. In the present investigation the crystallization modes of several amorphous Zr~_~M x alloys will be compared. The nature of the metastable phase will be studied in more detail * Permanent address: Central Research Institute for Physics, Budapest, Hungary.

0022-3093/83/0000-0000/$03.00 © 1983 North-Holland

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for the amorphous alloy Zr80Fe20 by means of a combination of X-ray diffraction and 57Fe M6ssbauer effect spectroscopy.

2. Experimental Zr-rich amorphous alloys of a composition close to Zr8M 2 ( M = Cu, Ni, Co, Fe, Rh and Ir) were prepared by arc melting followed by melt spinning in a purified argon gas atmosphere. Small parts of each ribbon were investigated by means of X-ray diffraction. We applied CuK~ radiation in combination with an X-ray monochromator. The thermal stability of the alloys was studied by means of a Du Pont 910 differential scanning calorimeter (DSC), using an atmosphere of purified argon gas and a heating rate of 50°C/min. The nature of the heat effects observed was studied by heating the ribbons to the distinct stages in the crystallization process, followed by rapid cooling and X-ray diffraction. The 57Fe MOssbauer effect measurements were made at room temperature with a conventional constant-acceleration spectrometer.

3. Experimental results and discussion X-ray diffraction performed on the as-spun alloy ribbons showed that sharp diffraction lines that would have indicated the presence of crystalline material were absent. Results of the DSC measurements are shown in fig. 1. In all these cases the general features of the DSC traces are similar: there is a relatively narrow exothermic heat effect (T2) which is preceded in temperature by a smaller and broadened exothermic effect ( T 1). For alloys of Zr with Cu or 3d metals the lower heat effect occurs near 350°C, for alloys with Rh and Ir it occurs near 450°C. A small quantity of the as-spun ribbons of all these alloys was heated in a second run in the DSC equipment to a temperature slightly higher than T 1, but still sufficiently below the minimum occurring between T 1 and T2 in the DSC trace. The alloys were then cooled quickly to room temperature and subjected to X-ray diffraction. A comparison of the X-ray diagrams obtained in this way revealed a great similarity. In all these cases the X-ray diagrams were composed of two sets of lines. One set belonged to a-Zr and the other belonged to the still unidentified metastable phase. In the case of Zr80 Fe20 this metastable phase was earlier designated as a-Zr4Fe and the corresponding X-ray diagram was indexed hexagonally with the lattice constants a = 5.045 .~, c = 3.121 A. The same indexing could be applied to the other metastable Z r - M phases ( M = Cu, Ni, Co, Rh or It). In previous investigations indexing with a more symmetrical type of structure was proposed for the metastable alloys obtained with Zr75Cu25 and Zr78Ni22 [1,2]. Cubic indexing now proved to be in error. This was found to originate from the fact that several of the

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low-intensity reflections of the hexagonal phase were not observed in the corresponding diagrams (owing to the inferior quality of the X-ray diagrams) suggesting a unit cell of higher symmetry than was actually present.

250

350

450 i

i

550 J

amorphous Zr- base alloys

Zr~8Ni2;

uJ

ZrBoRh2o

300

_

400

500

T('C)

600

Fig. 1. Exothermic heat effects observed when heating various amorphous Zr-M alloys in a differential scanning calorimeter, using a heating rate of 50°C/min (M = Cu, Ni, Co and Fe: top scale; M = Rh, Ir: bottom scale).

The mere fact that the same type of metastable phase is observed in the initial crystallization of amorphous alloys differing widely in composition and nature of the M component indicates that this phase might be a metastable modification of Zr metal itself. A more direct proof of this can be obtained by means of 57Fe M6ssbauer spectroscopy. The M6ssbauer spectrum of the amorphous alloy ZrsoFe20 when quenched from slightly above T I is shown in fig. 2, together with the M6ssbauer spectrum of the as-spun amorphous alloy Zr76Fe24 reported elsewhere [4]. Visual inspection, but even more so a more careful analysis of the spectra, makes it clear that these spectra are virtually identical. Since the Fe atoms will occupy symmetric positions in the crystalline metastable phase of hexagonal symmetry, whereas there is a considerable lack in symmetry for the Fe atom positions in the amorphous phase, one has to conclude that the amorphous and metastable crystalline phase cannot give rise to virtually the same M6ssbauer

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I t

;

o

b

-h

-'2

V

0m/s

Fig. 2. Room temperature 57Fe M6ssbauer spectra of (a) amorphous ZrsoFe2o after heating to 355°C and of (b) amorphous melt spun Zr76Fe24. spectrum. It follows therefore from these results that the initial crystallization of amorphous Zr80Fe2o near T t leads to the formation of a metastable phase not containing Fe (i.e. Zr), leaving an amorphous matrix richer in Fe than the original melt-spun alloy. The Mrssbauer spectrum shown in fig. 2a is, in fact, the representative for this more Fe rich amorphous phase, which explains its similarity to the amorphous Zr76Fe24. It is interesting to note that a search through the literature for metastable modifications of Zr metal revealed the occurrence of a metastable Zr-phase Qo-Zr) with similar lattice symmetry and lattice constants (a = 5.02 ,~, c = 3.00 A) to those observed here [5]. This metastable ~-Zr phase was reported to occur during quenching from fl-Zr and during Zr precipitation from crystalline alloys of Zr with small quantities of the metals V, Nb and U. In the following we shall therefore give the metastable phase occurring in the initial crystallization process of all the alloys investigated in this study the designation t0-Zr. The transient nature of o~-Zr in the crystallization process of the various Zr-rich amorphous alloys becomes apparent from investigations of the X-ray diffraction diagrams of melt-spun alloys heated in a third DSC run to

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temperatures exceeding the second exothermic peak ( T2). In the first two alloys shown in fig. 1 this leads to the attainment of the equilibrium phases i.e. a-Zr together with MoSi2-type Zr2Cu and a-Zr together with CuAl2-type Zr2Ni for Zr75Cu25 and Zr78Ni22, respectively. A mixture of a-Zr and the CuA12-type phase is also observed in Zr80Rh20, although here the X-ray diagram was found to contain a strong additional line of still unknown origin, having a spacing d = 2.92 A. Heating to still higher temperatures than T2 led to the disappearance of this line. With the alloy Zr84Ir~6 heating to beyond T2 resulted in the formation of a-Zr and Zr3Ir (fl-U type). The lattice constants found for Zr3Ir are equal to a = 10.77 ,~, c = 5.62 A, in agreement with those reported by Raman and Schubert [6]. A special situation occurs in the second stage of the crystallization of the Co and Fe alloys shown in fig. 1. The X-ray diagrams of melt-spun alloys heated to slightly above T2 showed the presence of a completely new pattern that could be indexed on the basis of an orthorhombic unit cell with the dimensions a = 3.283 ,~, b = 3.553 ,~, c = 6.867 ,~. This indexing is not unique. It also proved possible to index the pattern on the basis of the orthorhombic unit cell (a = 6.602 ,~, b = 6.375 ,~ and c = 5.560 ,~) which can be regarded as an orthorhombic distortion of the tetragonal CuA12 type of Zr2Fe (a = 6.385 A, c = 5.596 ,~, see ref. [7]). The Mrssbauer spectrum of this metastable phase (formerly designated [3] fl-Zr4Fe ) is shown in fig. 3a. Stable phases in thermal equilibrium with each other are reached only after heating to still higher temperatures (T>~ 500°C). Crystallization is then complete and has resulted in a mixture of a-Zr and the Re3B-type phase Zr3Fe [3,4]. The Mrssbauer spectrum of the latter phase is also shown in fig. 3b. A characteristic asymmetry in the line intensities of the Mrssbauer spectrum of the metastable crystalline ZrsoFe20 taken at T2 temperature (fig. 3a) can be observed. This observation clearly indicates the presence of more different Fe neighbourhoods in this sample than in the stable Re3B-type structure Zr3Fe which has a symmetric Mrssbauer spectrum (fig. 3b), The exact composition of the metastable crystalline Zr80Fe20 is difficult to assess. However, it follows from the results described above that this phase crystallized from an amorphous matrix richer in Fe than the original melt-spun alloy. A designation as fl-Zr3Fe rather than fl-ZraFe seems therefore more appropriate. It is very probable that the crystalline Zr80Fe20 sample contains a mixture of the stable orthorhombic Zr3Fe and the metastable phase with unknown structure found earlier [4] in the crystallization of amorphous Zr76Fe24. However, on the basis of the present Mrssbauer study this cannot be unambiguously verified due to the overlap of the Mrssbauer lines in fig. 3a. A similar metastable phase was also observed in the amorphous alloy ZrsoCo20 after heating to slightly above T2 and quenching to room temperature [8]. Concluding, we have shown that the crystallization of Zr-base amorphous alloys near 80 at% Zr proceeds in at least two steps. The first step involves the precipitation of a metastable Zr-phase (o~-Zr) and leads to the formation of a less Zr-rich amorphous matrix. The second step, occurring at higher tempera-

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.h

'

-'2

6

'

iv

tnats)

Fig. 3. Room temperature 57Fe M/Sssbauer spectra of (a) amorphous Zrs0Fe20 after heating to 390°C and (b) stable crystalline Zr3Fe.

tures, involves the crystallization of this less Zr-rich matrix. In alloys of Zr with Fe and Co the stable crystalline phases are not obtained directly, but via the formation of a metastable intermetallic compound. Two of the authors (I.V. and F.v.d.W.) acknowledge the financial support of the Foundation of Fundamental Research of Matter (FOM).

References [1] [2] [3] [4] [5] [61 [7] [81

K.H.J. Buschow, J. Appl. Phys. 52 (1981) 3319. K.H.J. Buschow and N.M. Beekmans, Phys. Rev. BI9 (1979) 3843. K.H.J. Buschow, J. Less-Common Met. 79 (1981) 243. I. Vincze, F. van der Woude and M.G. Scott, Sol. St. Commun. 37 (1981) 567. B.A. Hatt and J.A. Roberts, Acta Met. 8 (1960) 575. A. Raman and K. Schubert, Z. Metallk. 55 (1964) 704. E.E. Havinga, W. Damsma and P. Hokkeling, J. Less-Common Met. 27 (1972) 169. K.H.J. Buschow, J. Less-Common Met. 85 (1982) 221.