oxide precipitates in an internally oxidized Cu-Al alloy

Materials

Chemistry

and Physics,

32 (1992)

207

207-211

Investigation of the interface structure an internally oxidized &-Al alloy F. R. Chen, HRTEM

Laboratory,

of Cu/oxide precipitates

in

T. C. Hsu Materials

Science

Center, National

Tsing-Hua

University

Hsinchu,

(Taiwan,

ROC)

I. F. Tsu and L. Chang Materials

(Accepted

Research

Laboratories,

Industrial

Technology

Research

Institute

Chutung,

(Taiwan,

ROC)

March 13, 1992)

Abstract &Al,O,, (Y-AI~O~and CuAIOz precipitates were found in an internally oxidized Cu-2.6wt.% Al alloy. The orientation relationships between these oxides and the matrix Cu were determined from the selected area diffraction technique. The &Alu203 has a monoclinic structure and fie orientation relationship between the matrix Cu and &AlaOa was determined to be [211]Cu//[132]19 and (113)Cu//(201)~. The CuAIOZ is a hexagonal crystal structure. The orientation relationship between Cu and CuA102 was determined to be [llO]Cu//[liOO]CuAlO, Ed (lB)Cu// (OOO1)CuAIOa. The interphase boundary plane between matrix Cu and both oxides a_re near (113)Cu. An aA&O3 oxide observed within a CuAlOa precipitate had the orientation relationships [llOO]CuAlO~/[2llO]a and (0001)CuAl0~/(0001)c~. The high resolution imaging technique was applied to study the atomic structure of a Cu/CuAIOZ interface. The image simulation suggests that the terminated plane of CuAlOa in the interphase boundary is an oxygen plane. The Cu atoms in (113)Cu are all in ‘lock-in’ position in the interface.

Introduction As mentioned elsewhere [l-3], metal/ceramic interfaces play an important role in the application of many advanced technologies such as microelectronic packaging, structural composites and ceramic coatings on metals for abrasion or corrosion protection and sometimes are the controlling functions in the macroscopic properties of the materials. It is very important to gain insight on the atomistic structure of the interface in order to understand the correlation between the interface structure and the macroscopic properties of the metal/ceramics. High-resolution transmission electron microscopy (HRTEM) has been extensively used to study the atomic structure of many metal/oxide interfaces which were prepared by different techniques such as Nb/a-A&O3 by diffusion bonding [4, 51, Nb/cuA1203 by molecular beam epitaxy (MBE) [5], Ag/ CdO, Cu/MnO, Cu/q’-A&O3 by internal oxidation technique [3,6,7] and Cu/NiO by internal reduction technique [8]. In our study, (Y-A~*O~,8-A1203 and CuAlO, precipitates in the Cu matrix were found by internally oxidizing the Cu-2.6 wt.% Al alloy. The morphology as well as orientation relationship

0254-0584/92/$5.00

between these oxides and Cu were studied by conventional transmission electron microscopy. High-resolution transmission electron microscopy was used to study the atomic structure of a Cu/ CuA102 interface.

Experimental

Cu-2.6wt.% Al alloy was prepared by the arc melting technique and followed by a high temperature homogenization. Cu-2.6wt.% Al specimens of diameter 3 mm and thickness 100 pm packed with CuZO, Cu and AlZ03 powders sealed by a copper foil (Rhines pack) [9] were internally oxidized in an evacuated quartz tube for 40 hours at 900 “C. TEM specimens were produced by mechanical pre-thinning and ion-milling. A JEOL 4000EX microscope with Cs = 1.0 mm, focal spread 6f = 8 nm, beam divergence (Y= 0.55 mrad and tilting capability + 25” was used for the TEM work. Conventional transmission electron microscopy was performed using a JEOL 2000FX microscope.

0 1992 - Elsevier Sequoia. All rights reserved

208

3. Results

relationship of Cu and CuAlO, precipitates. The diffraction pattern is sketched in Fig. l(b) and some of the diffraction spots are indexed for clarity. The interphase boundary plane is determined to be parallel to the basal plane of CuAlO,. The zone axis of the inset diffraction pattern is the [liO]Cu pole in the Cu crystal which iLclose to [llOO]CuAIO, and the angle between (113)Cu and the basal plane of CuA102 is about 2”. The 8-A1203 is a monoclinic crystal structure and is a metastable phase which is modified from a distorted spine1 structure with the lattice constants a = 1.18 nm, b =0.29 nm, c =0.563 nm and /3= 104.1” [13]. Isolated f3-A1203 precipitates were observed in the matrix, however, Fig. l(c) shows

and discussion

3.1. Orientation relationship of Cu and oxides In our research, two different polymorphic phases of Al,O,: 8-A1203 and a-Alz03 and a copper aluminate CuAIO, were found. CuAlO* has a hexagonal crystal structure with the lattice constants a =0.286 nm and c = 1.696 nm [lo]. The binary oxide CuA102 was found to be a product of the bonding between pre-oxidized copper and alumina [ll, 121. The CuAlO, precipitates in the Cu matrix were found to be of plate shape. Figure l(a) is a low magnification bright field image of CuA102 precipitates in the Cu matrix. The inset diffraction pattern in Fig. l(a) shows the orientation

b

Fig. 1. (a) A low magnification bright field image of CuAIOz precipitates in the Cu matrix; (b) a sketched diffraction pattern of the inset in (a); (c) a &A1203 phase coexisting with the CuAIOz phase; (d) a sketched diffraction pattern of inset in (c); (e) (IA1203 is present within a CuAIOz precipitate. A sketched diffraction pattern of the inset in (e) was given in Fig. l(f). The index of diffraction spots of the CuA102 precipitate were not subscripted.

209

a 8-A1203 coexisting with the CuAlO, phase. The orientation relationship of 8-A1203 and Cu was determined from the inset diffraction pattern in Fig. l(c). The inset diffraction pattern is resketched in Fig. l(d) and some of the important spots are indexed. The zone axis of the inset diffraction pattern is [2il]Cu and is close to [1?12]19.This orientation relationship was also checked from the other zone axis in which case [i%JCu is parallel to (n1]6. The interface plane (201)9 is about 2” away from (1g)Cu. This implies that the basal plane of CuAIO, and (201)6 is parallel to each other in Fig. l(c). The bright field image in Fig. l(e) shows that ~Al,0, is present within a CuA102 precipitate. A diffraction pattern from these two oxides is shown in Fig. l(e) as an inset. A sketched diffraction pattern was, again, given in Fig. l(f). The orientation relationship between these two oxides was determined to be [liOO]~uAlO~//[2~0]~ and (0001)CuA10,//(0001)a. Atomic ~~~~~ra~~~n of the CuiCuAiO,

interfhce

High resolution imaging technique was applied to study the atomic structure of a Cu/CuAlO, interface that was found in the thin region near the edge of the specimen. A unit cell of the CuAIOz structure viewed from [liOO] is depicted in Fig. 2. CuAIO, has four different types of basal planes which are denoted as {O}l, {Cu}, {0}2 and {Al}, which yield four possible interface structure models which are shown in Figs. 3(a) to (d). {Cu> and {Al} represent the basal planes of CuAlO, that contain only Cu and Al atoms respectively,

@ClJ

0 0

[1TO~Ol out of page

Fig. 2. CuAIOz has a hexagonal crystal structure with lattice constants a =0.286 nm and c = 1.696 nm. An unit cell of the CuA102 structure is viewed from [liOOJ.

id1

Fig. 3. Four possibfe models for the Cu(l~)/(O~l)Cu~O~ interface. See text regarding the detailed discussion of the modelling of the interface structure.

while {O)l and (0]2 are the oxygen lattice planes above and below gu}. These four models correspond to the (113)Cu lattice plane in contact with the basal planes {Cu}, {O}l, {Al} and {0}2 of CuAlO, respectively. The experimental high resolution images shown in Figs. 4(a) and (b) were taken along [llO]Cu at an underfocus value near -50 nm, which is close to ‘optimum underfocus’, - 1.2(Csh)“” of the JEOL 4OOOEX.However, the simulated images of Cu and CuAlO, structures alone for the underfocus -64 nm match better with the experimental image than those for underfocus -50 nm. The error in the underfocus value may be due to the fact that the specimen was in high angle tilt position when the underfocus value was estimated from the thin amorphous region at the edge of specimen. From the image simulation, the Cu atom columns show the most intense contrast, while the Al as well as the 0 atom columns appear to be of darker contrast at the underfocus -64 nm. The distance between two crystals in these four models was assumed so that the separations of Cu-Cu, 0-Cu, and Al-Cu across the interface were close to the shortest interatomic distances in Cu, Cu,O and CuAl, respectively. For the model 1, the spacing between the terminated {Cu} lattice plane in CuAlO, and (1fi)Cu lattice plane in the

210

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owe-owe-0 080808080 Fig. 4. Projection of a close-packed oxygen layer of CuA102 and a (1s)Cu onto the plane of the Cu/CuAlO, interface. A twodimensional coincidence-site-lattice (CSL) unit cell in the boundary plane can be constructed if the strains -0.031 and 0.012 were applied to the Cu lattice along the [llO]Cu and [332]Cu directions, respectively. The unit cell of CSL was outlined by solid lines. The coincidence sites were marked as ‘x’.

interface was set to be 0.22 nm so that the distance between the Cu positions in the terminating atomic layer of CuA102 and the closest Cu atoms in the Cu crystals varies from 0.22 nm to 0.28 nm. This interval is reasonably close to that of the closest Cu-Cu distance, 0.255 nm in the Cu crystal. The model 2 was derived from the model 1 by inserting a layer of {O}l between {Cu} and (1e)Cu lattice planes. The ionic radius of 02- is 0.138 nm so that there is sufficient space for the {O}l to be inserted at the interface. Projection of a closepacked oxygen layer of CuAlO, and a (1B)Cu onto the plane of the Cu/CuAlO, interface is depicted in Fig. 5. Following Balluffi et al. [14], a two-dimensional coincidence-site-lattice (CSL) unit cell in the boundary plane can be identified if the strains -0.031 and 0.012 were applied to the Cu lattice along [llO]Cu and [332]Cu directions, respectively. The unit cell of CSL was outlined by solid lines. The coincidence sites were marked as ‘x’ in Fig. 5. Considering the spacing of {O}l and (lD)Cu to be 0.09 nm, the distance between 0 in the terminated layer to the Cu atoms in (1n)Cu ranged from 0.162 nm to 0.191 nm which is close to the minimum distance of 0-Cu in cuprite (Cu,O), 0.18 nm. The Cu atoms in the

positions of coincidence sites are in the ball-onball position relative to 0 ions such that the O-Cu distance across the interface is too small. 0 ions in the ball-on-ball positions are therefore assumed to be vacant, so that all the Cu atoms are above the interstices in the close packed oxygen layer, i.e. all the Cu atoms are in the ‘lock-in’ positions. At the Cu/MnO interface Cu atoms were found in the ‘lock-in’ positions above an oxygen terminating layer of the MnO oxide [7]. This model may not be the same as the ‘lock-in’ model that proposed by Fecht and Gleiter [15] in which case close packed rows of metal atoms fit into the valleys between close packed rows of 0 atoms at the interface. In the case of model 3, the distance between the Al positions in the terminating atomic layer and the closest Cu atoms varies from 0.22 nm to 0.28 nm which coincides approximately with the Al-Cu spacings of 0.24 nm to 0.29 nm in CuAl. In the model 4, as {0}2 atomic layer was inserted and the distance between the {0}2 atomic layer and (1B)Cu was set according to the same reason given for the model 2. The construction of models for interface and image simulation were carried out by using the ‘CrystalKit’ and ‘MacTempas’ programs, respectively, which were developed by Kilass [16]. The corresponding simulated images of these four models for an underfocus value -64 nm and thickness 4 nm are shown as insets with the experimental images in Figs. 4(a) and (b). It can be readily seen that the simulated images in Fig. 4(b) based on models 3 and 4 do not match the contrast details at the interface with the experimentally observed images. The blurred elongated bright contrast at the interface for the simulated image of model 1 does not appear in the experimental image. The image character as simulated in model 2 in Fig. 4(a) gives a superior match to the others, in which case the {O}l in CuA102 is a terminating lattice plane and all the Cu atoms in the (1B)Cu are in the ‘lock-in’ position. An oxygen termination in the metal/oxide interface was also observed in other internally oxidized alloy systems such as Cu/q’-A1,03 [3], Ag/CdO [6] and Cu/MnO [7]. These authors concluded that an oxygen termination layer in an oxide formed by internal oxidation in Rhines pack is favorable in which case oxide precipitates were formed in an environment of oxygen excess. Misfit dislocations were usually observed in the metal side with a stand-off distance from the Nb/ A1203 interface [4, 17, 181. However, misfit dislocations were not observed in the Cu/CuAIOz interface from the high resolution image. Ernst

211

a

b

Fig. 5. The corresponding simulated images of four possible models in Fig. 3 for an underfocus are shown as insets with the experimental images in Figs. 5(a) and (b).

et al. [3] explained that the absence of localized misfit dislocation cores may be due to the weak and undirectional bonds in the CU/~‘-A1203 oxygen terminated interface. The absence of localized misfit dislocations in the Cu/CuAlO, interface may be attributed to the same reason. However, the misfit dislocation may possibly be inclined to the electron beam such that only a small component of the Burgers vector is perpendicular to the electron beam. The detail dislocation structure in this interface is under investigation using the conventional two-beam technique.

value -64

nm and thickness 4 nm

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Acknowledgments

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The authors would like to thank the support from the Materials Science Center, National Tsing Hua University and the support from the Republic of China National Science Council by grant No NSC 81-0208-M-007-519.

30 (1982)

1453.

15 H. J. Fecht and H. Gleiter, Acta Met., 33 (1985) 557. 16 R. Kilaas, Proceedings of49th EMSA Meeting, San Francisco, CA, USA, (1991) 528. 17 W. Mader, Mater. Res. Sot. Symp. Proc., 82 (1987) 403. 37 (1991) 247. 18 D. Knauss and W. Mader, Ultramicroscopy,