Crystallization phases of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy after slow solidification

Transcription

1 Crystallization phases of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy after slow solidification Q. Wei a) Universität Potsdam, Institut für Berufspädagogik, Karl-Liebknecht-str , D-14476, Golm, Germany N. Wanderka, b) P. Schubert-Bischoff, and M-P. Macht Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, Berlin, Germany S. Friedrich Universität Potsdam, Institut für Berufspädagogik, Karl-Liebknecht-str , D-14476, Golm, Germany (Received 29 November 1999; accepted 12 May 2000) A systematic study was carried out on the equilibrium phases after slow solidification of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy. The crystalline microstructure of the slowly cooled melt of the alloy shows polygons and plates embedded in a fine-grained two-component matrix. To analyze the crystal structure of the different components, microdiffraction technique combining convergent beam electron diffraction and conventional selected-area electron diffraction were used. The stoichiometry of these phases was confirmed by field ion microscopy with atom probe and energy-dispersive x-ray analysis in a transmission electron microscope. The polygons were determined to be cubic (a nm) with space group Fm3m (cf116). The plates were found to be tetragonal (a 0.37 nm, c nm) with space group I4/mmm (ti6). Its composition is (Cu + Ni)(Zr + Ti) 2. One phase of the fine-grained two-component matrix was rich in Ti and poor in Be; the other one was rich in Be and poor in Ti. The Ti-rich phase was determined to be hexagonal (a nm, c nm) with space group P6 3 /mmc. I. INTRODUCTION One way to design multicomponent metallic alloy systems with good glass-forming ability, such as ZrTiCu- NiBe alloys 1 is to combine binary and ternary systems of early (e.g., Zr, Ti, Hf) and late transition metals (e.g., Cu, Ni, Co) and simple metals (e.g., Be, Al) with deep eutectics and large differences of the atomic sizes for different alloy components. Such bulk metallic glasses show strong resistance against crystallization in the supercooled liquid state within a wide temperature range above glass transition. The Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 bulk glass has been investigated intensively with respect to amorphous phase separation and crystallization during heat treatment 2 7 ; however, only few investigations of the equilibrium phases exist. 7,8 The aim of this work is to study the morphology, chemical composition, and structure of the crystalline phases after slow solidification of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy. a) Present address: Max-Planck-Institut für Mikrostrukturphysik Halle, Weinberg 2, Halle, Germany. b) wanderka@hmi.de For composition analysis of the different phases of a heterogeneous microstructure, energy dispersive x-ray analysis in a transmission electron microscope (TEM/EDX) was applied successfully after careful sample preparation. As Be cannot be detected by EDX, an additional analysis by field ion microscopy with atom probe (FIM/AP) was carried out, which allowed a full analysis of all the components, even for very small crystals. X-ray diffraction (XRD) techniques for structure determination have been used frequently because of their high precision compared to the conventional electron diffraction methods. 9 However, it is difficult to measure the structure of small volume fractions of crystalline phases and of small crystals by these techniques. Moreover, if the microstructure contains several unknown complicated phases, the structure of the respective phases must be analyzed locally. In recent years convergent beam electron diffraction (CBED) has been adopted for identifying unknown phases in small regions (<1 m). 10 This technique reveals three-dimensional symmetry information which enables rapid determination of the crystallographic point group. When this technique is combined with selected-area diffraction (SAED) patterns, one can J. Mater. Res., Vol. 15, No. 8, Aug Materials Research Society 1729

2 discern possible space groups of the crystal structure. 11 However, the CBED method usually requires suitable specimens. Many specimens, especially those composed of fine crystals or crystals with large lattice parameters, give relatively poor CBED patterns showing little or no intensity variations within the disks. As a result, point group identification becomes impossible under these circumstances. An alternative technique is microdiffraction. 12,13 With this method the disk diameter in CBED pattern can be reduced by using a nearly parallel electron beam, thereby improving the angular resolution and reducing the diffuse scattering in the diffraction pattern. Considering both, geometrical symmetry (reflection position) and intensity symmetry of the zero-order Laue zone (ZOLZ) and first-order Laue zone (FOLZ) patterns, one can identify both the point and space groups of a crystal phase. A systematic procedure to determine the crystal point and space group using microdiffraction patterns was proposed by Morniroli et al. 12,13 In the present paper notations and terms are used according to their definitions. 12,13 The microdiffraction technique combining both CBED and SAED was applied to determine the crystal structures of the unknown phases. Combining this structural information with the stoichiometry of the phases supplied by compositional analysis of TEM/EDX and FIM/AP, the equilibrium phases in Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy were identified. Additional XRD spectra were used. II. EXPERIMENTAL The alloy was prepared by alloying a master alloy from Zr (99.5%) and Be (99%) and by melting it together with pure Ti, Ni, and Cu (99.99% each) in a special induction levitation melting device under purified argon (oxigen partial pressure 10 9 Pa). The resulting amorphous ingot was encapsulated in an argon-filled sealed silica tube, then melted and held at 1300 K for 6 h in a tube furnace, and finally cooled with a rate of 0.4 K/s. Slices approximately 1 mm thick were cut by a diamond saw from the resulting alloy rod (10-mm diameter). They were mechanically ground and polished to a thickness of about 0.3 mm. An ultrasonic disc cutter was used to cut the samples of 3 mm in diameter, which were ion beam milled to transparency. The specimens were very brittle and had to be prepared with great care. To trace the different phases, it was necessary to combine optical and electron microscopic (TEM) observations. Details of the preparation procedure are published elsewhere. 14 The electron diffraction pattern was obtained at 100 kv on a Philips EM400. The small convergence angle was obtained by using a 50- m condenser aperture, and the spot size was in the range of nm. The TEM/EDX was carried out on a Philips CM30. FIM/AP was used to measure the concentration of Be, because this element cannot be detected with EDX. The FIM tips with a tip radius of about 100 nm were prepared from crystal fragments by mechanical grinding and ion milling. 15 The XRD spectra were taken from different cross sections of the alloy rod with Cu K radiation in the 2 configuration by use of a Bruker AXS D8 diffractometer equipped with a graphite monochromator and a scintillation counter for 0.05 to 0.3 nm. III. RESULTS AND DISCUSSION A. Morphology and composition Figure 1 shows an optical micrograph of the multiphase microstructure. The microstructure shows polygons and plates embedded in a fine-grained two component matrix. According to their morphology the different phases are named polygons (A), plates (B) and matrix I and II (C). The crystals of the different phases are of different sizes ranging from the subnanometer scale to several hundred micrometers. It has been shown that the size depends on the cooling rate 16 and on the undercooling of the melt. 7,8 Even under the slow cooling conditions in the present case the distribution of the different crystalline phases and the size and the alignment of the crystals depict the history of the solidification of the melt. For example, there are many more polygons in the upper part of the alloy rod, whereas at the bottom of the rod the fine-grained matrix is coarser. Thus the volume fractions of the different phases cannot be determined. Moreover, the complicated crystallization sequence and impeded convection of the highly viscous melt may lead to local deviations from the equilibrium composition which modify the matrix microstructure. 17 FIG. 1. Optical micrograph of the microstructure obtained by slow cooling of the liquid Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 melt. According to their morphology the different components are named (A) polygons, (B) plates, and (C) fine-grained two-phase matrix J. Mater. Res., Vol. 15, No. 8, Aug 2000

3 The relative composition (without Be %) of each phase measured by TEM/EDX and the composition of three phases measured by FIM/AP are presented in Table I. The result of FIM/AP shows that the finegrained two-component matrix is composed of two phases, one is rich in Be and poor in Ti, while the other one is rich in Ti and poor in Be. The plates (B) are practically free of Be and the ratio of (Ni + Cu):(Zr + Ni) is about 1:2. A Zr 2 Cu-type phase was also found in the same alloy after isothermal crystallization in the undercooled melt. 8 It has been further stated by the same authors that a MgZn 2 -type Laves phase crystallizes from the liquid melt; howeveer, no such phase could be observed during the present investigation. The polygons could not be analyzed with FIM/AP because of difficulties with the sample preparation. From the dark appearance of the polygons in the SEM image, 17 it is supposed that the polygons are rich in low atomic order elements, e.g., Be. Concentration fluctuations of the constituents Zr and Ti in the crystals without significant change of the lattice parameter were revealed by combined TEM/EDX and TEM/SAD microanalysis, 14 indicating that the polyhedric/cubic phase can exist in a wide concentration range. B. Structure 1. Polygons Figures 2(a) 2(c) show [111], [001], and [011] microdiffraction patterns taken from polygons. The highest net symmetries obtained from these patterns are 4mm in [001] [Fig. 2(a)] and 6mm in [111] [Fig. 2(b)], which according to Table 7 of Ref. 12 corresponds to a cubic system. To characterize the Bravais lattice and glide plane, identification of the 100 and 110 zone axis patterns (ZAP) is required. 12 On the [001] ZAP [Fig. 2(a)], a shift between the nets of reflections situated in the ZOLZ and in the FOLZ implies that the Bravais lattice cannot be primitive. Furthermore, the smallest squares drawn in the ZOLZ and in the FOLZ are identical. This means that there is no periodicity difference between ZOLZ and FOLZ reflection nets. According to Fig. 9(d) in Ref. 12, the corresponding individual partial extinction symbol is either I-..or F-...The distinction between the I and F Bravais lattices should be combined with the additional observation of the 011 ZAP. The [011] ZAP [Fig. 2(c)] exhibits a centered rectangle net of the reflections with a net 2mm symmetry in ZOLZ. The lack of the FOLZ in this pattern satisfies the structure factor extinction conditions for a face-centered lattice (h + k, h + l, k + l odd). 12 Comparison with Fig. 9(d) of Ref. 12 leads to the unique determination of the face centered Bravais lattice with partial extinction symbol F..-. Furthermore, no forbidden reflection was observed FIG. 2. Microdiffraction pattern of polygons: (a) [001] zone-axis showing 4mm net symmetry in ZOLZ, (b) [111] zone axis showing 6mm net symmetry in ZOLZ and 3mm intensity symmetry in FOLZ, (c) [110] zone axis showing 2mm symmetry in ZOLZ. TABLE I. Composition of the equilibrium phases of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy as measured by TEM/EDX (without consideration of Be) and FIM/AP. The error bars for EDX analysis are 1 at.%, and for FIM/AP they are of magnitude 2. Chemical composition (at.%) State Analyses Zr Ti Cu Ni Be Amorphous nominal 41 (53) 14 (18) 12.5 (16) 10 (13) 22.5 (0) FIM/AP 46.4 ± ± ± ± ± 1.1 Polygons EDX (41) (11) (31) (17) Plates EDX (59) (9) (28) (4) FIM/AP 49.0 ± ± ± ± ± 0.9 Matrix I EDX (23) (39) (17) (21) FIM/AP 34.5 ± ± ± ± ± 2.5 Matrix II EDX (45) (16) (17) (22) FIM/AP 38.3 ± ± ± ± ± 1.5 J. Mater. Res., Vol. 15, No. 8, Aug

4 in the [110] zone axis along [001] which excludes the existence of a screw axis being parallel to 001. From measurements of diffraction spot distances using [111], [001] and [011] ZAPs the lattice parameter of this phase was calculated to be a nm. A careful examination of the intensity of a 111 ZAP shows that the beam intensity is compatible with a 6mm symmetry in ZOLZ but only with a 3m symmetry in FOLZ. Therefore, the whole pattern (WP) can only have a 3m symmetry. According to the Table 4 in Ref. 12, the point group corresponding to such a symmetry is m3m, which belongs to a centrosymmetric crystal. The possible space group in this case could be either Fm3m, Fm3c, Fd3m, or Fd3c. Considering the partial extinction symbols, as mentioned above, one can exclude the existence of glide planes d // (001) and c // (110). It is concluded, therefore, that the space group of the polygons must be Fm3m. Examples for this space group with a similarly large lattice parameter as the polygons are the Be-rich Be 15 Ni 8 Zr 6,Be 15 Ni 8 Ti 6, and Be 15 Cu 8 Zr 6, which correspond to the Mn 23 Th 6 (cf116) structure type Plates The tetragonal structure of the plates was determined by tilting the specimen around several zone axes in the SAED mode and by indexing the corresponding SAED patterns. The lattice parameters are a 0.34 nm and c nm. Figure 3(a) shows the [100] zone axis microdiffraction pattern. Both ZOLZ and FOLZ reflections have a (2mm), 2mm net, and (2mm), 2mm ideal symmetries and exhibit a centered rectangle pattern. The absence of ZOLZ/FOLZ periodicity difference indicates that there is no glide plane perpendicular to the [100] direction. According to Fig. 9(c) in Ref. 12, the partial extinction symbol is I.-.. CBED was used to identify the point group of the plate crystal. A [110] CBED ZOLZ pattern in Fig. 3(b) exhibitsa2mm symmetry. The FOLZ reflections are weak and far away from the center of the screen. To observe them, the specimen was carefully tilted along the mirrors m1 and m2 until FOLZ areas appear on the pattern. Figure 3(c) shows the FOLZ pattern along the mirror m1. The intensity of the FOLZ pattern presents a 2mm symmetry. The 2mm WP symmetry observed on [110] ZAP corresponds to the point group 4/mmm as given in Table 4 in Ref. 12. The possible space groups satisfying such conditions are listed in Table II together with the reflection conditions. A careful examination of the [100] SAED pattern [Fig. 3(a)] shows that reflection planes of the form (00l) are present for all l 2n which, referring to Table II, eliminates the I4/amd case. Thus the space group for the plates must be I4/mmm. FIG. 3. (a) [100] zone axis microdiffraction pattern of plates with (2mm), 2mm net symmeries in ZOLZ and (2mm), 2mm ideal symmetries in FOLZ, obtained by tilting the specimen along the mirrors m1 and m2; (b) [110] CBED ZOLZ pattern with 2mm symmetry and FOLZ pattern with a 2mm symmetry, obtained by tilting the specimen along the mirror m J. Mater. Res., Vol. 15, No. 8, Aug 2000

5 The corresponding FIM/AP analysis of this phase shows that it contains practically no Be (Be <1 at.%) and that the ratio of (Zr + Ti) to (Cu + Ni) is 2:1. Therefore, this phase is of the Zr 2 Cu type, which is a dominating phase of the Zr(Ti) Cu(Ni) boundary system. The additions of Ti and Ni may cause slightly different lattice constants in comparison to Zr 2 Cu. 3. Matrix I and II As mentioned above the matrix I and II consists of two phases, one of them is rich in Be and poor in Ti. The other is rich in Ti and poor in Be. The structure of the Ti-rich phase was determined to be hexagonal by using a series of SAED patterns. The lattice parameters are a nm and c nm. TABLE II. Possible space groups and the reflection conditions for the plates. Space group 00l Reflecting planes I4/mmm l 2n k+ l 2n I4/amd l 4n k + l 2n 0kl Figure 4(a) shows the [1121] zone axis microdiffraction pattern for the Ti-rich phase. Both the geometrical and the intensity symmetry of the ZOLZ reflection nets are 2mm. But the whole pattern has only an m symmetry, if the intensity of FOLZ reflection nets is also taken into account. According to Table 4 in Ref. 12, the corresponding point group in a hexagonal system satisfying such a condition is 6/mmm. The possible space groups are therefore P6/mmm, P6 3 /mmc, P6 3 /mcm, or P6/mcc. To determine whether a screw axis exists, the specimen was tilted around the [0001] direction. Figures 4(b) 4(d) show [1120], [4150], and [1010] ZAPs. By tilting from [1120] to [1010], the intensity of the (000l, l 2n +1) reflections is strongly reduced and eventually vanishes. This suggests that a 6 3 screw axis parallel to [0001] exists, which produces extinction. Such forbidden reflections are located along the row parallel to the screw axis. The forbidden reflections (000l, l 2n + 1) may appear by double diffraction 12 and can only be distinguished from the allowed reflections by tilting around the [0001] axis. In this way P6/mmm and P6/mcc were excluded. Furthermore, the examination of the [1120] pattern shows that reflection planes of the form (hh0l) are present for all l, which from Table 3.2 in Ref. 12 eliminates the P6 3 /mcm case, in which the reflection condition l 2n in (hh0l) exists. Consequently the space group of this crystal must be P6 3 /mmc. However, the actual composition of this phase (cf. Table 1) does not allow its description in terms of any known structure type. 19 The structure of the Ti-poor phase could not be identified because of difficulties with the sample preparation. Two of the three phases identified by the TEM analysis, plates and matrix I, can be also found in the XRD spectrum in Fig. 5. The identification of the proper FIG. 4. Microdiffraction pattern of the hexagonal, Ti-rich phase of the fine-grained two-phase matrix: (a) [1121] zone axis microdiffraction pattern showing (2mm) ideal symmetry in ZOLZ and m symmetry in FOLZ, (b) [1120] zone axis, (c) [4150] zone axis, (d) [1010] zone axis. FIG. 5. X-ray diffraction spectrum of the slowly solidified Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 melt (Cu K, 1.541Å). The Bragg peaks of two of the different phases are indicated. J. Mater. Res., Vol. 15, No. 8, Aug

6 Bragg peaks was only possible because the structure of the respective phases is known from the local analysis of discrete crystals. Thus no Bragg peaks could be associated with the phase matrix II. Moreover, only qualitative conclusions can be drawn from the XRD spectrum, because of the inhomogeneity of the microstructure and a texture caused by the alignment of the crystals during solidification (e.g., the plates). In particular the diffraction pattern of the polygons could not be discovered in the XRD spectrum. Although these crystals can be large (see Fig. 1), the volume fraction is rather small and they are mainly concentrated at the top of the alloy rod. 20 IV. SUMMARY During slow cooling, the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 melt crystallizes into a four-phase microstructure (polygons and plates embedded into a fine-grained two-phase matrix). The crystal structure, point and space groups, and the compositions of three of these phases were determined respectively by using microdiffraction combining CBED and SAED methods as well as by use of TEM/EDX and FIM/AP analysis. The first phase (polygons) was identified to be facecentered-cubic with space group Fm3m (cf116) and a lattice parameter a nm. It is probably relative to the phases Be 15 Ni 8 Zr 6,Be 15 Ni 8 Ti 6 and Be 15 Cu 8 Zr 6. The second phase (plates) is tetragonal with space group I4/mmm and lattice parameters a 0.37 nm and c nm. The composition of this phase is determined to be (Cu + Ni)(Zr + Ti) 2, which is similar to CuZr 2 (ti6), with a slight difference of the lattice parameters. One phase of the fine-grained two-phase matrix is rich in Ti and poor in Be. The other one is rich in Be and poor in Ti. The crystal structure of the Ti-rich phase is hexagonal with space group P6 3 /mmc and lattice parameters a nm and c nm. ACKNOWLEDGMENTS The authors thank Dr. W. Mieckeley and Mr. W. Rönnfeld for alloy preparation and realization of the solidification experiments, Dr. M. Müller for performing the XRD analysis, Dr. Mukherji for valuable discussions and suggestions, and Prof. Dr. H. Wollenbereger and Dr. V. Naundorf for the critical reading of the manuscript. REFERENCES 1. W.L. Johnson, Mater. Sci. Forum , 35 (1996). 2. A. Wiedenmann, U. Keiderling, M-P. Macht, and H. Wollenberger, Mater. Sci. Forum , 71 (1996). 3. M-P. Macht, N. Wanderka, A. Wiedenmann, H. Wollenberger, Q. Wei, H.J. Fecht, and S.G. Klose, Mater. Sci. Forum , 65 (1996). 4. S. Schneider, P. Thiagarajan, U. Geyer, and W.L. Johnson, Physica B , 918 (1998). 5. N. Wanderka, Q. Wei, I. Sieber, U. Czubayko, and M-P. Macht, Mater. Sci. Forum , 369 (1999). 6. N. Wanderka, Q. Wei, R. Dole, M. Jenkins, S. Friedrich, M-P. Macht, and H. Wollenberger, Mater. Sci. Forum , 773 (1998). 7. H-J. Fecht, in Structure and Dynamics of Glasses and Glass Formers, edited by C.A. Angell, K.L. Ngair, J. Kieffer, T. Egami, and G.U. Nienhaus (Mater. Res. Soc. Symp. Proc. 455, Pittsburgh, PA, 1997), p J. Schroers, R. Busch, A. Masuhr, and W.L. Jonson, Appl. Phys. Lett. 74, 2806 (1999). 9. H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures (John Wiley and Sons, New York, 1974). 10. B.F. Buxton, J.A. Eades, J.W. Steeds, and G.M. Rackham, Philos. Trans. R. Soc. A 284, 171 (1976). 11. B.G. Demczyk and S.F. Cheng, J. Magn. Mater. 88, 376 (1990). 12. J.P. Morniroli and J.W. Steeds, Ultramicroscopy 45, 219 (1992). 13. A. Redjaimia and J.P. Morniroli, Ultramicroscopy 53, 305 (1994). 14. D. Mühlhausen, P. Schubert-Bischoff, Q. Wei, and M-P. Macht, in Progress in Metallography, edited by M. Kurz and M.M. Pohl (special edition of the Practical Metallography 27, Informationsgesellschaft Verlag Oberursel/FRG, 1995), p N. Wanderka, P. Schuber-Bischoff, V. Naundorf, M-P. Macht, and H. Wollenberger, in Proceedings of the 11th european congress on electron microscopy, EUREM 96, T10, edited by W. Baumeister and D. Cottell (EUREM 96 U.C.D. Belfield, Dublin 4, Ireland, 1996). 16. M-P. Macht, N. Wanderka, A. Wiedenmann, H. Wollenberger, Q. Wei, S. Klose, A. Sagel, and H-J. Fecht, in Thermodynamics and Kinetics of Phase Transformations, edited by J.S. Imn, B. Park, A.L. Greer, and G.B. Stephensen (Mater. Res. Soc. Symp. Proc. 398, Pittsburgh, PA, 1996), pp M-P. Macht, N. Wanderka, Q. Wei, I. Sieber, N. Deyneka, Proceedings of the tenth International Conference on Rapidly Quenched and Metastable Materials (RQ10), , edited by K. Chattopadhyay and S. Ranganathan (Bangalore, India, in press). 18. E. Ganglberger, H. Nowotny, and F. Benesowsky, Monatsh. Chem. 96, 1206 (1965). 19. W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys (Pergamon Press, Oxford, New York, 1967), Vol. 2, p M-P. Macht, Q. Wei, N. Wanderka, I. Sieber, and N. Deyneka, Mater. Sci. Forum , 173 (2000) J. Mater. Res., Vol. 15, No. 8, Aug 2000