- Email: [email protected]
Crystallization in oxygen doped amorphous Cu60Zr40: A metallic glass-ceramic?
CRYSTALLIZATlON IN OXYGEN DOPED AMORPHOUS Cu,,Zr,: A METALLIC GLASS-CERAMIC? J. P. CHEVALIER, C.E.C.M.-C.N.R.S.,
Y. CALVAYRAC, A. QUfVY, M. HARMELIN and J. BIGOT 15 rue Georges Urbain, 94400 Vitry-sur-Seine, France (Received 26 August 1982)
Abstract-The effect of doping a Cu&rW glass with 0.5 at.% oxygen on the crystallization behaviour and morphology has been studied by differential thermal analysis, X-ray diffraction and electron microscopy. A two stage crystallization is observed, contrasting with a single stage for undoped C%Zr,. and after the first stage a uniform dispersion of small (N 500 A) crystals with the equilibrium CuJr, structure, in a remanent amorphous matrix, is obtained. This suggests a similarity with glass-ceramics. R&m&-Nous avons hudit, par analyse thermique diffe~ntielle, diffraction des rayons X et microscopic &ctronique, l’effet de l’addition de 0,5 at.‘? d’oxygene sur le comportement et la morphologic du verre mktallique Cu&ra tars de la cristallisation. La cristallisation se produit en deux &tapes, alors que le verre C&&r,,, non do@ cristallise en une seule &ape. Apr& la premidre &tape on observe une dispersion unifonne de petits cristaux (N 500 A) ayant la structure d’bquilibre Cu,Jr,, dans une matrice amorphe r&iduelle. Ces rksultats suggkrent une analogie avec les vitrtiramiques. Z~arn~f~~-Der EinfluB einer Dotienmg des Me~ll~~ Cu&r& mit 0,s At.-% Sauerstoff auf das K~sta~lisation~erhaiten und die Morphologic wurde mit di~e~tieller Thermoanalyse, Riintgenbeugung und Elektronenmikroskopie untersucht. Die Kristallisation 1Wt in zwei Stufen ab, im Gegensatz zu dem einstufigen Ablauf beim reinen Metallglas. Nach der ersten Stufe beobachtet man eine gleichmiil3ige Verteilung kleiner Kristalle (Durchmesser -500 A) mit der Gleichgewichtsstruktur Cu,Jr, in einer amorphen Restmatrix. Diese Beobachtung legt eine Analogie zu Glaskermiken nahe.
1. INTRODUCIION
Crystallization of amorphous alloys has been extensively studied (e.g. (1,2]) but there have been only a few reports on the effects of additions on the crystallization temperature [3-;1 and no results conceming the effects of small additions on the ensuing morphoJogy of the crystallized phase. Our own study [8,9] of the effects of oxygen contamination (i.e. oxygen which is introduced through surface diffusion) on amorphous Cur4 has shown that this modifies considerably the thermal behaviour of the glass. C~stal~~tion as observed by ~ff~~tial thermal analysis (DTA) now occurs in two stages, but the crystallization products remain unchanged. Furthermore, electron microscopy has shown that in the initial stages of the ensuing crystallization the degree of crystallization was inhomogeneous in Ithe foil thickness. These results suggested that oxygen acted as a nucleating agent for this glass. In this paper we report results concerning the crystallization behaviour of the Cu&Q,, glass for which oxygen has been added uniformly in the initial melt. The aim of this is to demonstrate that it is possible to obtain a metaIlic equivalent
to the vitro-ceramics.
2. EXPERIMENTAL
The materials used are Kroll zirconium (major impurities: 0, 3OOpg.g-‘; Fe, 12Opg.g-‘; C, 465
Hf, 200 pg. g-l) and OFHC copper 150 pg+g-‘; (major impurities: 0, 3Opg*g-‘; S, 25pg*g-‘; Ag, lOpg*g-‘). For the ternary (doped) alloy, oxygen is introduced as copper oxyde. The constituents are melted together by levitation under pure helium (99.998%) and cast in a cooled copper mould. Fragments of the ingot thus prepared are used to prepare the amorphous samples by melt-spinning under locally injected helium. Alloys with composition Cu,&r, and Cu59.7Zrjg.l10,,lwere prepared. The melt spun ternary alloy was found to be completely amorphous by XRD showing that CuO is in solid solution. Occasionally, we have detected slight traces of cubic Zr% for both doped and undoped ribbons, but these can be eliminated by slightly abrading the surface. The DTA curves are obtained using a SETARAM micro-differential thermal anaiyser at a heating rate of 10 I( min-‘, and samples wanting about X5mg. Isothermal heat-treatments are all operated under ion-pumped ultra high vacuum, since heat treatments carried out in the DTA apparatus under purified argon still leads to oxygen contamination [9]. X-ray diffraction spectra are obtained in the reflection mode with a step-scan diffractometer, using C!ol(, radiation. It has also proved necessary to obtain transmissionx-ray diagrams using a Guinier camera with a rotating sample. Specimens for electron microscopy are punched from the ribbons prior to heat-treatment, thinned using an ion beam thinner
466
CHEVALIER ef tit.: CRYSTALLIZATION IN OXYGEN
I
700
I a00
I
T50
I
650
Temperature , K
1. Influence of the introduction of 0.5 at.% oxygen on the DTA curves of amorphous CU&~ alloys: (a) asquenched Cu&Zr& (b) as-melt-spun CU&~~.~O& (c) Cu,,ZrBSOO,Safter imneahg for 3Omin at 790 K. fig.
The crystallization process of amorphous Cu&!& alloys, as observed by DTA and DSC under continuous heating, is now well kRqwn [IO, 11,121 and occurs by a single exothermic step pig. la), For the as melt-spun Cu~~,Zr~~.oOO,[j glass, DTA curves (Fig. lb) show that this specific thermal behaviour has been altered by the addition of oxygen. Both the glass transition and crystallization temperatures are shifted towards higher temperatures (N 20 K for T,, - 25 K for Q and a second exothermic peak occurs over a large temperature range (from about 823 to 883 K). Note that this shift of Ta and 7” towards higher temperatures is significant since the reypective standard deviations are mtich smaller>(3 K for 7” 7 K for T, frable 11) and that the total value of the heat evolved by the two exotherrnic effects (4.9 kJ.mol-‘) is higher than the average value corresponding to the 17 ribbons of Ref. [12]. Such modifications of the crystallization process due to the introduction of oxygen in the molten Cu&&, alloy are different from those observed by oxygen diffusion through the surface of the amor-
AMORPHOUS
Cu,Zr,
phous undoped Cu&r, ribbons (for instance, when they are annealed under argon [8]). When such an oxygen contamination intervenes, a second crystallization peak is also observed but always below 800K. Moreover, the thermal stability of the glass and the total value of the two stage crystalli~tion enthalpy are decreased by about 15x, depending on the degree of oxidation. In the case of doped specimens which have been annealed for 30 min at 790 K, that is just after the first exothermic DTA peak, the DTA curve (Fig. lc) now only shows the second crystallization peak, whose position in temperatute and amplitude are only slightly modified (Table 1). On the contrary, after similar annealing treatment, Cu&Zr, amorphous ribbons contaminated by oxygen diffusion are entirely crystallized.
operating at a voltage of 5 kV, and are examined in a JEOL 200 01: electron microscope. 3, THERMAL BEI-UVIOUR
DOPED
4. X-RAY
DIFFRACI-ION
For the as-melt spun Cu,~,ZrH~sOO.sglass the XRD scan shows that the scattered intensity is identical to that of the Cu.&r@ to within lx, which is the limit of accuracy of the measurement. After annealing the doped glass for 30min at 790 K, XRD shows that extensive crystallization has occurred. The crystalline phase can be identified as being the stable Cu&r, phase which has the Ni,,,Zr, structure (space group Cmca [13,14]). This phase is the same as that observed after crystallization of the undoped alloy. At this stage it appears that there remai.ns some amorphous phase (Fig. 2a). After annealing the doped alloy at temperatures above that of the second exothermic c~stalli~tion peak (for 1 h at 930 K), the X-ray diagram is shown in Fig. 2b. This diagram appears very different from that shown in Fig. 2a. In particular (hO0) reflections are very strong. In order to check that this is due to a crystallization texture the sample was ground to a pow&r. The X-ray spectra was then found to be nearly comparable to that of Fig. 2a, the jhO0) lines having a normal intensity. Nevertheless this is not quite sufficient to enable a complete indexation of both diagrams. To overcome these difficulties, specimens were thinned for examination in trans~ssion
Table 1. Comparison of the thermai behaviour of amorphous C&&r, and CU,,,Z~~,,,O,~ribbons Crystallization Glass II I transition Conditions Composition Tgf K NJrw (seeII 21)
Average values
702 f 3
A& (kJ.mol-‘)
T,,t (K) 744+7
4.2% 0.2
-
-
Mel&m
722
770
3.4
848
1.5
Annealed 30 min
-
-
-
856
1.7
UI 790 K tOnset extrapolated tcmpcrdrums.
Fig. 2. X-ray difftactograms (COK, radiation) for amorphous Cu so.7Zr,,,C&,foils: (a) after annealing For 30 min at 790 K. There remains some amorphous phase; (b) after annealing for 69 ruin at 930 K. The Cu,&, crystals arc strongly textured with the (hO0) planes parallel to the surface of the ribbon.
using rt Guinb camerawith 8 rotating sample.The diagrams co~~pond~ng to the two ~~l~~tio~ stages are then similar and can be indexed as the stable Cu,&r, phase. 5. ELECTRON MCROSCOPY Figure 3 is a Iow magnifkation dark-field electron micrograph of a specimen of Cu,,Zr,qsOC., annealed for 30 min at 790 K. The crystals are homogeneously distributed in the specimen and are equiaxed in projection. The average grain size is of the order of
SO A and the size d~st~b~~o~ is rer_atvely narrow. There are also some distinctly smaller crystals, and these can be identified as being due to a monoclinic zirconia phase. We believe that this is caused by oxygen contamination during ion-beam thinning, since XRD does not reveal such a structure in the bulk. At higher magni~~~on~ dark-field images (J?ig. 4) show Moire patterns due to overlapping crystals. The regularity of the Moire fringes suggests that the crystals do not have a high density of faults. High resolution bright field imaging (Fig. 5) contirms this.
468
CHEVALIER
rf ul.:
CRYSTALLIZATION
IN OXYGEN DOPED AMORPHOUS
Cu,Zr,
Fig. 3. Low magnification dark field image of Cu,,,Zr,,,O,,, annealed for 30 min at 790 K. The objective aperture is placed on the Debye rine;s which are around the position of the amorphous halo.
Armour the specimen thickness and deiocus conditions do not fulfill the conditions for interpretation of the image in terms of a simple projected charge density, it is nevertheless possible to say that the crystals imaged in the centre are relatively perfect and that they are in .an amorphous matrix. The latter point is consistent with the observation of a speckle contrast, superposed on the contrast due to the crystals, in the dark field images (Figs 3 and 4). This contrast is typical of a homogeneously disordered system [IS, 161. Electron. diffraction patterns (Fig. 6a) show isotropic Debye rings which are to be expected from the observed fine grain size. The rings can be indexed on thebasis of the Cu,,Zr, structure, except for a few low
angle Debye rings which correspond to a mono&&c zirconia phase. From the diffraction pattern shown in Fig. 6b there is a suggestion of a diffuse halo from a remanent amorphous phase, but the high density of Debye rings makes this observation difficult. However if we obtain a selected area diffraGton pattern from a thinner part of the specimen (i.e. close to the edge) we can now clearly observe (Fig. 6~) the diffiise halo due to the remaining amorphous phase, The inner diffuse halo which is also visible here has been observed previously and is attributed to an amorphous zirconia phase produced by oxygen conemanation either in the microscope or during ionbeam thinning B,lq. In comparison, crystallization morphology for an undopcd &Jr, glass is drastically different, although the crystallization product is the same. For such a specimen, annealed under identical canditions (i.e. UHV; Xlmin at 790 K) etectron microscopy reveals that very jnhomogeneous crystallization has occurred. For example, in some regions crystallization is complete with large heavily faulted crystals of size up to 2pm. In other regions, some amorphous phase remains, with only relatively small crystals (- 100 A). Since the electron micrographs reveal a considerable range ofcrystal shapes and sizes depending on the particular specimen area observed, we cannot describe a characteristic crystallization morphology. One of the few notable features is that the large crystals (_ I pm) tend to be rather heavily faulted [see Fig. 7). Finally, for the doped sample heat-treated for 30 min at 790 K and then further annealed for 60 min
CHEVALIER
PI al.:
CRYSTALLIZATION
IN OXYGEN DOPED AMORPHO~jS
Cu,,Zr,,
459
Fig. 5. High resotution axial bright field image of Cu,,,ZrS&ar anncahd for 30 min at 790 K. The crystals imaged here are embedded in an amorphous matrix.
at 930 K, electron micrographs now show that the crystallization morphology consists of small crystals (from 50 to 2~~~ and much Iarger crystah whose size is of order of several thousand Angstroms. Figure 8 is a high resolution axial bright field image which shows a large faulted singIe crystal region, in which much smaller crystals are imaged. This suggests that the smaller crystals occur through crystallization of the remaining amorphous phase and that the large crystals are due to the “secondary recrystallization” of the uniform small crystals observed in Figs 3, 4 and 5. 6. DISCUSSION
We have shown that after doping Cu&Zr, glassy alloys with 0.5 at.% oxygen, crystahiition (as observed in a dynamic DTA experiments) occurs in two stages, and with a slightly greater heat of crystallization than for the undoped glass. On the other hand X-ray diffraction shows that the as prepared
doped glass has the same diffracted intensity as the undoped glass and that the crystallization product is the same in both cases: CuJr,. It is-important to note that in this alloy crystallization occurs without change in composition, and hence no long range diffusion is required. Electron microscopic observations show that the crystallization morphology for the first stage of crystallization consists of a uniform dispersion of regularly sized crystals with a small proportion of remaining amo~hous phase. From the measured values of crystallization enthalpy (see Table 1) we can estimate the amount of remaining amorphous phase after the first crystallization peak as between about 20-30x, depending on whether the enthalpy of the first peak is compared respectively to the total enthalpy of the doped sample or to that of the undoped sample. This value appears to be consistent with both the X-ray and electron microscopy results. As far as the slightly larger value of total enthalpy for the doped glass is concerned, we suggest that the first peak is associated with the
470
CHEVALIER
UI ui.:
CRYSTALLIZATION
N OXYGEN DOPED AMORPHOUS Cu,,Zr,
Fig. 7. Dark-field image of Cu&&, anneated for 30 min at 790K. the contrast suggests severely faulted crystals.
after the first exothermic peak suggests that oxygen reduces the rate of crystal growth.
7. CONCLUSION
Fig. 6. (a, b, c) Electron diffraction patterns of Cu,,,Zr,,O,, annealed for 30min at 790 K taken from successively thinner regions.
crystallization of very fine grained Cu&r, and the second peak with the ~~stalli~tio~ of the remaining amorphous phase together with secondary recrystallization of the initial small crystals. The difference in total enthatpy between doped and undoped glass would then he due to recrystallization. These results suggest that, in this case, oxygen plays the role of a nucleating agent. but does not otherwise strongly affect the glass or crystal structures. However. that crystallization is not complete
The addition of a small amount a ternary eIement, oxygen, to a Cu-Zr glass considerably modifies the thermal bchaviour of the glass-crystal transition-as we11 as the ~s~l~~tion mo~holo~, but apparently does not strongly modify either the glass structure or the crystallization product. We have shown that after partial crystallization of the doped alloy a homogeneous distribution of small crystals is obtained, with a slight proportion of remaining amorphous phase. Thus, oxygen acts as a nucleating agent and we believe that the crystallization behaviour observed in the case of the doped glass resembles that of a glass-ceramic. Fundamentally, this approach (i.e. the effect of doping on the c~stal~i~tion of binary glasses) may yield, important information on the nucleation mechanism and technologically this may be. a method of obtaining, in a controlled manner, very fine grain size material for application in powder metallurgy.
~c~~~~~e~~e~~~~-We would like to thank J. the electron micrograph used in Fig. 8.
Rzepski for
CHEVALIER
<‘,
IN OXYGEN
DOPED
AMORPHOUS
&,zr,
471
Fig. 8. High resolution axial bright field image of CU~,,Z~,,,O~,~ annealed for 30 min at 790 K and then for 60 min at 930 K.
REFERENCES
2,
3. 4. 5.
6. 7.
8.
Kiister and U. Herold, Topics in Applied Physics Fhi lted by H. J. Gtintherodt and H. Beck), Vol. 46, p. 225. Sorinsrer. Berlin (1981). Proc. &h &. konf. on ‘Rapidly Quenched Metals (edited by T. Masumoto and K. Suzuki, Vol. I, Chap. 3. The Japan Institute of Metals, Sendai (1982). A. Inoue, A. Kitamura and T. Masumoto, J. Mater. Sci. 16, 1895 (1981). J. L. Walter, Mater. Sci. Engng SO, 137 (1981). M. Naka, U. Gonser, C. Giirlitz and H. Ruppersberg, J. Non-Cryst. Solicfs 45, 99 (1981). I. W. Donald and H. A. Davies, in Rapidly Quenched Metals III (edited by B. Cantor), Vol. I, p. 273. Metals Society, London (I 978). D. E. Polk, C. E. Dube and B. C. Giessen, in Rapidfy Quenched Metals III (edited by B. Cantor), Vol. I, p. 220. Metals Society, London (1978). J. Bigot, Y. Calvayrac, M. Harmelin, J. P. Chevalier
and A. Quivy, in Proc. 4th in!. CO@ on Rap idly Quenched Metals (edited by T. Masumoto and K.
Suzuki), Vol. II, p. 1463. The Japan Institute of Met:als, SendaJ (1982). 9. Y. Calvavrac. M. Harmelin. A. Ouiw. J. P. Chevr tlier and J. Bcgot,‘J. Phys. 41, d8-I 14 (1980). IO. A. J. Kerns, Ph. D. thesis, Northeastern University, Boston (I 974). I I. A. J. Kerns, D. E. Polk, R. Ray and B. C. Giessen, Mater. Sci. Engng 38, 49 (1979). 12. M. Harmelin, Y. Calvayrac, A. Quivy, J. P. Chevalier and J. Bigot, Scripta metall. 16, 703 (1982). 13. T_..Bsenko, J. less-common Metais 40, 365 (1975). 14. M. E. Kirkpatrick, J. F. Smith and W. L. Larsen, Acta crystafiogr.-15, 894 (t962). IS. A. Howie. J. Non-Crvst. So&& 31. 41 119781.
I6 J. P. Chevalier, J. M~eroscopy 119; I l3‘( 1984). 17. J. P. Chevalier, in Electron Microscopy and Analysis 1981, p. 391 Inst. Phys. Con$ Ser. 61. Inst. Phys., London (1982).
Y. CALVAYRAC, A. QUfVY, M. HARMELIN and J. BIGOT 15 rue Georges Urbain, 94400 Vitry-sur-Seine, France (Received 26 August 1982)
Abstract-The effect of doping a Cu&rW glass with 0.5 at.% oxygen on the crystallization behaviour and morphology has been studied by differential thermal analysis, X-ray diffraction and electron microscopy. A two stage crystallization is observed, contrasting with a single stage for undoped C%Zr,. and after the first stage a uniform dispersion of small (N 500 A) crystals with the equilibrium CuJr, structure, in a remanent amorphous matrix, is obtained. This suggests a similarity with glass-ceramics. R&m&-Nous avons hudit, par analyse thermique diffe~ntielle, diffraction des rayons X et microscopic &ctronique, l’effet de l’addition de 0,5 at.‘? d’oxygene sur le comportement et la morphologic du verre mktallique Cu&ra tars de la cristallisation. La cristallisation se produit en deux &tapes, alors que le verre C&&r,,, non do@ cristallise en une seule &ape. Apr& la premidre &tape on observe une dispersion unifonne de petits cristaux (N 500 A) ayant la structure d’bquilibre Cu,Jr,, dans une matrice amorphe r&iduelle. Ces rksultats suggkrent une analogie avec les vitrtiramiques. Z~arn~f~~-Der EinfluB einer Dotienmg des Me~ll~~ Cu&r& mit 0,s At.-% Sauerstoff auf das K~sta~lisation~erhaiten und die Morphologic wurde mit di~e~tieller Thermoanalyse, Riintgenbeugung und Elektronenmikroskopie untersucht. Die Kristallisation 1Wt in zwei Stufen ab, im Gegensatz zu dem einstufigen Ablauf beim reinen Metallglas. Nach der ersten Stufe beobachtet man eine gleichmiil3ige Verteilung kleiner Kristalle (Durchmesser -500 A) mit der Gleichgewichtsstruktur Cu,Jr, in einer amorphen Restmatrix. Diese Beobachtung legt eine Analogie zu Glaskermiken nahe.
1. INTRODUCIION
Crystallization of amorphous alloys has been extensively studied (e.g. (1,2]) but there have been only a few reports on the effects of additions on the crystallization temperature [3-;1 and no results conceming the effects of small additions on the ensuing morphoJogy of the crystallized phase. Our own study [8,9] of the effects of oxygen contamination (i.e. oxygen which is introduced through surface diffusion) on amorphous Cur4 has shown that this modifies considerably the thermal behaviour of the glass. C~stal~~tion as observed by ~ff~~tial thermal analysis (DTA) now occurs in two stages, but the crystallization products remain unchanged. Furthermore, electron microscopy has shown that in the initial stages of the ensuing crystallization the degree of crystallization was inhomogeneous in Ithe foil thickness. These results suggested that oxygen acted as a nucleating agent for this glass. In this paper we report results concerning the crystallization behaviour of the Cu&Q,, glass for which oxygen has been added uniformly in the initial melt. The aim of this is to demonstrate that it is possible to obtain a metaIlic equivalent
to the vitro-ceramics.
2. EXPERIMENTAL
The materials used are Kroll zirconium (major impurities: 0, 3OOpg.g-‘; Fe, 12Opg.g-‘; C, 465
Hf, 200 pg. g-l) and OFHC copper 150 pg+g-‘; (major impurities: 0, 3Opg*g-‘; S, 25pg*g-‘; Ag, lOpg*g-‘). For the ternary (doped) alloy, oxygen is introduced as copper oxyde. The constituents are melted together by levitation under pure helium (99.998%) and cast in a cooled copper mould. Fragments of the ingot thus prepared are used to prepare the amorphous samples by melt-spinning under locally injected helium. Alloys with composition Cu,&r, and Cu59.7Zrjg.l10,,lwere prepared. The melt spun ternary alloy was found to be completely amorphous by XRD showing that CuO is in solid solution. Occasionally, we have detected slight traces of cubic Zr% for both doped and undoped ribbons, but these can be eliminated by slightly abrading the surface. The DTA curves are obtained using a SETARAM micro-differential thermal anaiyser at a heating rate of 10 I( min-‘, and samples wanting about X5mg. Isothermal heat-treatments are all operated under ion-pumped ultra high vacuum, since heat treatments carried out in the DTA apparatus under purified argon still leads to oxygen contamination [9]. X-ray diffraction spectra are obtained in the reflection mode with a step-scan diffractometer, using C!ol(, radiation. It has also proved necessary to obtain transmissionx-ray diagrams using a Guinier camera with a rotating sample. Specimens for electron microscopy are punched from the ribbons prior to heat-treatment, thinned using an ion beam thinner
466
CHEVALIER ef tit.: CRYSTALLIZATION IN OXYGEN
I
700
I a00
I
T50
I
650
Temperature , K
1. Influence of the introduction of 0.5 at.% oxygen on the DTA curves of amorphous CU&~ alloys: (a) asquenched Cu&Zr& (b) as-melt-spun CU&~~.~O& (c) Cu,,ZrBSOO,Safter imneahg for 3Omin at 790 K. fig.
The crystallization process of amorphous Cu&!& alloys, as observed by DTA and DSC under continuous heating, is now well kRqwn [IO, 11,121 and occurs by a single exothermic step pig. la), For the as melt-spun Cu~~,Zr~~.oOO,[j glass, DTA curves (Fig. lb) show that this specific thermal behaviour has been altered by the addition of oxygen. Both the glass transition and crystallization temperatures are shifted towards higher temperatures (N 20 K for T,, - 25 K for Q and a second exothermic peak occurs over a large temperature range (from about 823 to 883 K). Note that this shift of Ta and 7” towards higher temperatures is significant since the reypective standard deviations are mtich smaller>(3 K for 7” 7 K for T, frable 11) and that the total value of the heat evolved by the two exotherrnic effects (4.9 kJ.mol-‘) is higher than the average value corresponding to the 17 ribbons of Ref. [12]. Such modifications of the crystallization process due to the introduction of oxygen in the molten Cu&&, alloy are different from those observed by oxygen diffusion through the surface of the amor-
AMORPHOUS
Cu,Zr,
phous undoped Cu&r, ribbons (for instance, when they are annealed under argon [8]). When such an oxygen contamination intervenes, a second crystallization peak is also observed but always below 800K. Moreover, the thermal stability of the glass and the total value of the two stage crystalli~tion enthalpy are decreased by about 15x, depending on the degree of oxidation. In the case of doped specimens which have been annealed for 30 min at 790 K, that is just after the first exothermic DTA peak, the DTA curve (Fig. lc) now only shows the second crystallization peak, whose position in temperatute and amplitude are only slightly modified (Table 1). On the contrary, after similar annealing treatment, Cu&Zr, amorphous ribbons contaminated by oxygen diffusion are entirely crystallized.
operating at a voltage of 5 kV, and are examined in a JEOL 200 01: electron microscope. 3, THERMAL BEI-UVIOUR
DOPED
4. X-RAY
DIFFRACI-ION
For the as-melt spun Cu,~,ZrH~sOO.sglass the XRD scan shows that the scattered intensity is identical to that of the Cu.&r@ to within lx, which is the limit of accuracy of the measurement. After annealing the doped glass for 30min at 790 K, XRD shows that extensive crystallization has occurred. The crystalline phase can be identified as being the stable Cu&r, phase which has the Ni,,,Zr, structure (space group Cmca [13,14]). This phase is the same as that observed after crystallization of the undoped alloy. At this stage it appears that there remai.ns some amorphous phase (Fig. 2a). After annealing the doped alloy at temperatures above that of the second exothermic c~stalli~tion peak (for 1 h at 930 K), the X-ray diagram is shown in Fig. 2b. This diagram appears very different from that shown in Fig. 2a. In particular (hO0) reflections are very strong. In order to check that this is due to a crystallization texture the sample was ground to a pow&r. The X-ray spectra was then found to be nearly comparable to that of Fig. 2a, the jhO0) lines having a normal intensity. Nevertheless this is not quite sufficient to enable a complete indexation of both diagrams. To overcome these difficulties, specimens were thinned for examination in trans~ssion
Table 1. Comparison of the thermai behaviour of amorphous C&&r, and CU,,,Z~~,,,O,~ribbons Crystallization Glass II I transition Conditions Composition Tgf K NJrw (seeII 21)
Average values
702 f 3
A& (kJ.mol-‘)
T,,t (K) 744+7
4.2% 0.2
-
-
Mel&m
722
770
3.4
848
1.5
Annealed 30 min
-
-
-
856
1.7
UI 790 K tOnset extrapolated tcmpcrdrums.
Fig. 2. X-ray difftactograms (COK, radiation) for amorphous Cu so.7Zr,,,C&,foils: (a) after annealing For 30 min at 790 K. There remains some amorphous phase; (b) after annealing for 69 ruin at 930 K. The Cu,&, crystals arc strongly textured with the (hO0) planes parallel to the surface of the ribbon.
using rt Guinb camerawith 8 rotating sample.The diagrams co~~pond~ng to the two ~~l~~tio~ stages are then similar and can be indexed as the stable Cu,&r, phase. 5. ELECTRON MCROSCOPY Figure 3 is a Iow magnifkation dark-field electron micrograph of a specimen of Cu,,Zr,qsOC., annealed for 30 min at 790 K. The crystals are homogeneously distributed in the specimen and are equiaxed in projection. The average grain size is of the order of
SO A and the size d~st~b~~o~ is rer_atvely narrow. There are also some distinctly smaller crystals, and these can be identified as being due to a monoclinic zirconia phase. We believe that this is caused by oxygen contamination during ion-beam thinning, since XRD does not reveal such a structure in the bulk. At higher magni~~~on~ dark-field images (J?ig. 4) show Moire patterns due to overlapping crystals. The regularity of the Moire fringes suggests that the crystals do not have a high density of faults. High resolution bright field imaging (Fig. 5) contirms this.
468
CHEVALIER
rf ul.:
CRYSTALLIZATION
IN OXYGEN DOPED AMORPHOUS
Cu,Zr,
Fig. 3. Low magnification dark field image of Cu,,,Zr,,,O,,, annealed for 30 min at 790 K. The objective aperture is placed on the Debye rine;s which are around the position of the amorphous halo.
Armour the specimen thickness and deiocus conditions do not fulfill the conditions for interpretation of the image in terms of a simple projected charge density, it is nevertheless possible to say that the crystals imaged in the centre are relatively perfect and that they are in .an amorphous matrix. The latter point is consistent with the observation of a speckle contrast, superposed on the contrast due to the crystals, in the dark field images (Figs 3 and 4). This contrast is typical of a homogeneously disordered system [IS, 161. Electron. diffraction patterns (Fig. 6a) show isotropic Debye rings which are to be expected from the observed fine grain size. The rings can be indexed on thebasis of the Cu,,Zr, structure, except for a few low
angle Debye rings which correspond to a mono&&c zirconia phase. From the diffraction pattern shown in Fig. 6b there is a suggestion of a diffuse halo from a remanent amorphous phase, but the high density of Debye rings makes this observation difficult. However if we obtain a selected area diffraGton pattern from a thinner part of the specimen (i.e. close to the edge) we can now clearly observe (Fig. 6~) the diffiise halo due to the remaining amorphous phase, The inner diffuse halo which is also visible here has been observed previously and is attributed to an amorphous zirconia phase produced by oxygen conemanation either in the microscope or during ionbeam thinning B,lq. In comparison, crystallization morphology for an undopcd &Jr, glass is drastically different, although the crystallization product is the same. For such a specimen, annealed under identical canditions (i.e. UHV; Xlmin at 790 K) etectron microscopy reveals that very jnhomogeneous crystallization has occurred. For example, in some regions crystallization is complete with large heavily faulted crystals of size up to 2pm. In other regions, some amorphous phase remains, with only relatively small crystals (- 100 A). Since the electron micrographs reveal a considerable range ofcrystal shapes and sizes depending on the particular specimen area observed, we cannot describe a characteristic crystallization morphology. One of the few notable features is that the large crystals (_ I pm) tend to be rather heavily faulted [see Fig. 7). Finally, for the doped sample heat-treated for 30 min at 790 K and then further annealed for 60 min
CHEVALIER
PI al.:
CRYSTALLIZATION
IN OXYGEN DOPED AMORPHO~jS
Cu,,Zr,,
459
Fig. 5. High resotution axial bright field image of Cu,,,ZrS&ar anncahd for 30 min at 790 K. The crystals imaged here are embedded in an amorphous matrix.
at 930 K, electron micrographs now show that the crystallization morphology consists of small crystals (from 50 to 2~~~ and much Iarger crystah whose size is of order of several thousand Angstroms. Figure 8 is a high resolution axial bright field image which shows a large faulted singIe crystal region, in which much smaller crystals are imaged. This suggests that the smaller crystals occur through crystallization of the remaining amorphous phase and that the large crystals are due to the “secondary recrystallization” of the uniform small crystals observed in Figs 3, 4 and 5. 6. DISCUSSION
We have shown that after doping Cu&Zr, glassy alloys with 0.5 at.% oxygen, crystahiition (as observed in a dynamic DTA experiments) occurs in two stages, and with a slightly greater heat of crystallization than for the undoped glass. On the other hand X-ray diffraction shows that the as prepared
doped glass has the same diffracted intensity as the undoped glass and that the crystallization product is the same in both cases: CuJr,. It is-important to note that in this alloy crystallization occurs without change in composition, and hence no long range diffusion is required. Electron microscopic observations show that the crystallization morphology for the first stage of crystallization consists of a uniform dispersion of regularly sized crystals with a small proportion of remaining amo~hous phase. From the measured values of crystallization enthalpy (see Table 1) we can estimate the amount of remaining amorphous phase after the first crystallization peak as between about 20-30x, depending on whether the enthalpy of the first peak is compared respectively to the total enthalpy of the doped sample or to that of the undoped sample. This value appears to be consistent with both the X-ray and electron microscopy results. As far as the slightly larger value of total enthalpy for the doped glass is concerned, we suggest that the first peak is associated with the
470
CHEVALIER
UI ui.:
CRYSTALLIZATION
N OXYGEN DOPED AMORPHOUS Cu,,Zr,
Fig. 7. Dark-field image of Cu&&, anneated for 30 min at 790K. the contrast suggests severely faulted crystals.
after the first exothermic peak suggests that oxygen reduces the rate of crystal growth.
7. CONCLUSION
Fig. 6. (a, b, c) Electron diffraction patterns of Cu,,,Zr,,O,, annealed for 30min at 790 K taken from successively thinner regions.
crystallization of very fine grained Cu&r, and the second peak with the ~~stalli~tio~ of the remaining amorphous phase together with secondary recrystallization of the initial small crystals. The difference in total enthatpy between doped and undoped glass would then he due to recrystallization. These results suggest that, in this case, oxygen plays the role of a nucleating agent. but does not otherwise strongly affect the glass or crystal structures. However. that crystallization is not complete
The addition of a small amount a ternary eIement, oxygen, to a Cu-Zr glass considerably modifies the thermal bchaviour of the glass-crystal transition-as we11 as the ~s~l~~tion mo~holo~, but apparently does not strongly modify either the glass structure or the crystallization product. We have shown that after partial crystallization of the doped alloy a homogeneous distribution of small crystals is obtained, with a slight proportion of remaining amorphous phase. Thus, oxygen acts as a nucleating agent and we believe that the crystallization behaviour observed in the case of the doped glass resembles that of a glass-ceramic. Fundamentally, this approach (i.e. the effect of doping on the c~stal~i~tion of binary glasses) may yield, important information on the nucleation mechanism and technologically this may be. a method of obtaining, in a controlled manner, very fine grain size material for application in powder metallurgy.
~c~~~~~e~~e~~~~-We would like to thank J. the electron micrograph used in Fig. 8.
Rzepski for
CHEVALIER
<‘,
IN OXYGEN
DOPED
AMORPHOUS
&,zr,
471
Fig. 8. High resolution axial bright field image of CU~,,Z~,,,O~,~ annealed for 30 min at 790 K and then for 60 min at 930 K.
REFERENCES
2,
3. 4. 5.
6. 7.
8.
Kiister and U. Herold, Topics in Applied Physics Fhi lted by H. J. Gtintherodt and H. Beck), Vol. 46, p. 225. Sorinsrer. Berlin (1981). Proc. &h &. konf. on ‘Rapidly Quenched Metals (edited by T. Masumoto and K. Suzuki, Vol. I, Chap. 3. The Japan Institute of Metals, Sendai (1982). A. Inoue, A. Kitamura and T. Masumoto, J. Mater. Sci. 16, 1895 (1981). J. L. Walter, Mater. Sci. Engng SO, 137 (1981). M. Naka, U. Gonser, C. Giirlitz and H. Ruppersberg, J. Non-Cryst. Solicfs 45, 99 (1981). I. W. Donald and H. A. Davies, in Rapidly Quenched Metals III (edited by B. Cantor), Vol. I, p. 273. Metals Society, London (I 978). D. E. Polk, C. E. Dube and B. C. Giessen, in Rapidfy Quenched Metals III (edited by B. Cantor), Vol. I, p. 220. Metals Society, London (1978). J. Bigot, Y. Calvayrac, M. Harmelin, J. P. Chevalier
and A. Quivy, in Proc. 4th in!. CO@ on Rap idly Quenched Metals (edited by T. Masumoto and K.
Suzuki), Vol. II, p. 1463. The Japan Institute of Met:als, SendaJ (1982). 9. Y. Calvavrac. M. Harmelin. A. Ouiw. J. P. Chevr tlier and J. Bcgot,‘J. Phys. 41, d8-I 14 (1980). IO. A. J. Kerns, Ph. D. thesis, Northeastern University, Boston (I 974). I I. A. J. Kerns, D. E. Polk, R. Ray and B. C. Giessen, Mater. Sci. Engng 38, 49 (1979). 12. M. Harmelin, Y. Calvayrac, A. Quivy, J. P. Chevalier and J. Bigot, Scripta metall. 16, 703 (1982). 13. T_..Bsenko, J. less-common Metais 40, 365 (1975). 14. M. E. Kirkpatrick, J. F. Smith and W. L. Larsen, Acta crystafiogr.-15, 894 (t962). IS. A. Howie. J. Non-Crvst. So&& 31. 41 119781.
I6 J. P. Chevalier, J. M~eroscopy 119; I l3‘( 1984). 17. J. P. Chevalier, in Electron Microscopy and Analysis 1981, p. 391 Inst. Phys. Con$ Ser. 61. Inst. Phys., London (1982).