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FIM atom probe study of an Al2O3 dispersion strengthened copper alloy
Surface Science 266 ( !992) 337-341 North-Holland
;:. . . . . . . . . . . surface science
FIM atom probe study of an A1203 dispersion strengthened copper alloy F. Zhu, L. Jiao, N. Wanderka, R.P. W a h i a n d H. W o l l e n b e r g e r ltahn-Meitner-h~st~tut Berlin GmbH, Glienicker Str. 100, W-IO00 Berlin 39. Germany
Received 5 August 1991; accepted for publication 2 September 1991
The morphology, size distribution and mass spectrum of AI:O3 and the Cu/AI203 interface in a Cu-AI 25 alloyhave been studied by FIM-AP and TEM. All of the AI203 particles examined by FIM are single crystals. The field evaporation of alumina takes place predominantly in the form of aluminum oxygen clusters due to the large f' ld pcnct~aiiun in aluminum oxide particles. The binding between AI20 x particles and the Cu matrb¢ is weaker than within the Cu matrix or within the AI203 particles.
I. Introduction
2. Experimental
The Ai203 dispersion strengthened copper alloys have been developed in recent years. Their high strength is caused by dispersion hardening resulting from the hard A1203 particles and the high electrical conductivity resuRs from the copper matrix. Investigations showed [1,2] that as compared with o t h e r copper alloys, the a~umina copper alloys exhibited the best overall resistance to fast neutron and heavy ion d a m a g e as they show minimal swelling and retain their original values of yield strength and electrical resistivity after irradiation. They are being considered for several applications in magnetic fusion reactors. Microscopic details of the m e t a l / c e r a m i c interface play an important role in advanced structural composites. Ernst et al. [3] have studied a C u / A I 2 0 3 interface in an internally oxiJized C u - A I alloy by high-resolution electron microscopy. In the present work, field ion microscopy-atom probe ( F I M - A P ) and transmission electron microscopy ( T E M ) have been used to study the morphology, size distribution and mass spectrum of A120 3 and the C u / A I 2 0 3 interface in a C u - A i 25 alloy.
The Cu-A1 25 alloy was supplied by SCM in the form of a rod of about 20 mm diameter. The manufacturing process /or the dispersion hardened C u - A ! 25 alloy involves mixing an appropriate amount of C u . O powder with Cu-A1 (0.25 wt.C,; A] as A I 2 0 3) alloy powder and heating the mixture to a tcmpet'aturc of aNml q00°C for 40 rain. This causes internal oxidation of the aluminum in the mixture. Any excess oxygen remaining in the powder after complete oxidation of the alumip.um is removed by heati,,g the p o w d e r in a dissociated ammonia atmosphere at 820°C for 2 h. R o d stock is p r o d u c e d by canning the powder in copper tubing and hot extrusion at 900°C. T h e as-received rod material was cold-drawn to wires of 0.25 mm diameter. The wires for the FIM tip were annealed at 900°C for 1 h to eliminate the effect of the hot and cold treatment of the alloy. The FIM tips were then prepared by electropolishing in 1 g sodium chromate dissolved in 10 ml acetic acid (cone.) at room Lemperaturc and a voltage of 16 V. FIM and AP studies were performed by using the instrument described elsewhere [4]. In FIM, the tips were cooled to about 70 K and imaged in neon at a pressure of
{1039-~'~1}28/92/$05.00 ,e:, 1992 - Elsevier Science Publisher.~ B3,' All rights reserved
338
I'~ Zhu et al. / Study of an AI,_O.~ dispersion strengthened copper alloy
about 10 -6 Torr. The AP analyses were performed with a pulse fraction (VpuI~/VDc)of 0.16 under 1 X 10 -7 Torr Ne. TEM specimens were cut from the as-delivered material and prepared by standard techniques and examined in a Philips EM 400.
3. Results and discussion
The TEM studies show that the morphology of the particles is polyhedral. It can be seen in fig. 1 that some of these particles are of triangular and rectangular shapes, which is due to the anisotropic growth of AI20 3. Those crystalline planes, growing faster than others, disappear. Therefore, the grain shape is determined by those crystalline planes for which the growth velocities are mini-
mum. Triangular A 1 2 0 3 particles of about 100 nm also have been observed by Ashby and Smith [5] and Ernst et al. [3]. TEM studies indicated a large size range for the AIzO 3 particles. At the higher end of this range there are very large A1203 particles of about 160 rim. These are hardly observed at the magnification of 30000 because of their low concentration. Excluding these large particles ( > 100 rim), the measurements yield the size distribution shown in fig. 2. The density of the Al20 3 particles is about 1016/cm 3. Diffraction images of Cu-AI 25 alloys from TEM show that the Al203 particles are incoherent to the Cu matrix. A Ne field ion micrograph of Cu-Al 25 alloy is shown in fig. 3. Owing to its high binding energy, A1203 particles exhibit a very high image contrast with respect to the surrounding Cu matrix of
lOOnrn 4 Fig. 1. Bright field TEM micrograph of a Cu-AI 25 alloy showing different sizes of alumina.
F. Zhu et aL / Study of an AI20 ~ dispersion strengthened copper alloy SO
30 g 2o n
0
I
0
. . . .
I
10
. . . .
I
20
. . . .
!
. . . .
30
I
40
. . . .
50
Diameter [nm] Fig. 2. Size distribution of small alumina particles in a C u - A I 25 alloy.
which the concentric rings cannot be observed. Therefore, it is not possible to study the orientation relationship between AlzO 3 and the Cu ma-
330
trix. About 30 AI20.~ particles were examined. Most of the alumina particles are polyhedrons. but also some cubic AI203 particles ~ere observed (fig. 4). Alumina particles consisting of two grains have occasionally been observed by TEM. It is clearly shown in fig. 3 that the concentric rings are nearly perfect at the center of the AI203 particles but are not so perfect in the vicinity of the interface. If geometrical effects are excluded to cause this effect, then the structure a n d / o r the composition in the region close to the interface may deviate from that inside the Al203 crystal. We assume that the formation of AI,_O 3 particles involves two steps. At first, oxy.gen atoms (or ions) diffuse into the vicinity of Al atoms in the alloy and combine with AI atoms to form O - A I bonds. Then, Al atoms diffuse to the vicinity of these O - A l bonds and form more O - A I
Fig. 3. Ne field ion micrograph of a C u - A I 25 alloy obtained at 70 K.
F. Zhu et al. / Study of an AI20,~ dispersion strengthened copper alloy
340
Fig, 4. Ne field ion micrograph of a cubic A1203 particle in "1 Cu-A1 25 alloy obtained at 70 K.
b o n d s . T h i s r e a c t i o n occu:s r e p e a t e d l y . T h e s e c o n d s t e p is t h e p r o p e r r e a r r a n g e m e n t o f A I - O
8o
+i!t4o-]
~c,,+
and Cu atoms such that the perfect A120 3 can 200
o
"5~, E
z
30-
Ne+! 63Cu++
20-
l,
10-
It ~.++
i5Cu~ l
100
20.0 500 ¢0.0 50.0 80.0 7¢3.0 80.0
A t o m i c Mass Units Fig. 5..\tom-prohc mass spectrum from AI:()~ pa,ticlcs and Cu malrix in ~, Cu-AI 25 alloy obtained at 70 K. Except fo." ">('u ', "3Cu ', r's('u +', <'3Cu + ' and 2t~Ne+, peak idcntificalions are diffikult duc to the complex aluminum oxygen cluster ion form;_ilion.
,
AI203 Cu-mat:H~ I
/
[
(S) x
I I
f)-
120-
`5
80-
..~
~.¢1_
0 0.o
I
I
I I
m Y
/
E Z
o
~o 26o ago 46o s& +Go 7~o 8~o ~oo Totcl NumbeF of Ions
Fig. (~. (Tumulalb,,¢ ph)t from the AP analysis showing two AIzO ~ partides in the Cu matrix.
F. Zhu et aL / Study of an AI20t dispersion strengthened copper alloy
form. The intermediate state involving an imperfect AI203 crystal has been observed. By this way, an irregular incoherent phase boundary is to be expected. Such boundary would act as a sink for point defects produced by irradiation. Fig. 5 shows the preliminary mass spectrum of the Cu-AI 25 alloy. Besides four peaks of 6SCu+, 63Cu +, 6SCu ++ and 6 3 C u + + , peaks of m / n = 18, 19 and 29 are present. Because of the large electric field penetration in the insulator AI/O3 particles, the field evaporation takes place in the form of aluminum oxygen clusters rather than single ions as in the case of semiconductors. It is now difficult to decide to which clusters these peaks should be ascribed because of the complicated structure of AI203. Further studies are under way on this question. Fig. 6 shows a cumulative plot obtained from the AP analysis of two m1203 particles in the Cu-AI 25 specimen. From this ladder diagram, it is seen that the interfaces are not abrupt on the atomic scale. This result is in agreement with fig. 3. It seems that Cu atoms exist in the transition region between the A[203 particles and the Cu matrix. A field-evaporation rate of about one ion per 50 pulses was used to get the mass spectrum of A1203 particles and Cu matrix. Across the C u / A I 2 0 3 interface the evaporation rate rises to more than 8 ions per pulse. The result reveals the
34 t
relative weak binding between A I 2 0 3 particles and the Cu matrix. 4. Conclusion o The shape of ml203 particles is polyhedral. The mean diameter of the A1203 particles is 9 rim. All of alumina particles examined by FIM are single crystals. • The interface between the AI20 3 particles and the Cu matrix is incoherent. o The field evaporation of AI203 takes place predominantly in the form of aluminum-ox3~gen clusters rather than single AI ions, due to the large field penetration in the AIzO 3 particles. • The binding between the A1203 particles and the Cu matrix is much weaker than the binding within Cu or AI20 3. References [1] J.A. Spitznagel, N.J. Doyle, W.J. Choyke, J.G. Greggi Jr., J.N. McGruer and J.W. Davis, Nucl. Instrum. Methods Phys. Res. B 16 (1986) 27q. [2] R.J. Livak. T.G. Zocco and L.W. Hobbs, 3. Nud. Mater. 144 (1987) 121. [3] F. Ernst, P. Pirouz and A.H. Heucr. PhiMs. Mug. A ~3 (1'091) 259. [4] P, Mertens, V. Vidic and H, Becket, A computer cot> trolled FtM with AP, HMi Berlin Report (1985). [5] M.F. Ashby and G.C. Smith. J. ]nsL Met. t~l (19~3) tS2.
;:. . . . . . . . . . . surface science
FIM atom probe study of an A1203 dispersion strengthened copper alloy F. Zhu, L. Jiao, N. Wanderka, R.P. W a h i a n d H. W o l l e n b e r g e r ltahn-Meitner-h~st~tut Berlin GmbH, Glienicker Str. 100, W-IO00 Berlin 39. Germany
Received 5 August 1991; accepted for publication 2 September 1991
The morphology, size distribution and mass spectrum of AI:O3 and the Cu/AI203 interface in a Cu-AI 25 alloyhave been studied by FIM-AP and TEM. All of the AI203 particles examined by FIM are single crystals. The field evaporation of alumina takes place predominantly in the form of aluminum oxygen clusters due to the large f' ld pcnct~aiiun in aluminum oxide particles. The binding between AI20 x particles and the Cu matrb¢ is weaker than within the Cu matrix or within the AI203 particles.
I. Introduction
2. Experimental
The Ai203 dispersion strengthened copper alloys have been developed in recent years. Their high strength is caused by dispersion hardening resulting from the hard A1203 particles and the high electrical conductivity resuRs from the copper matrix. Investigations showed [1,2] that as compared with o t h e r copper alloys, the a~umina copper alloys exhibited the best overall resistance to fast neutron and heavy ion d a m a g e as they show minimal swelling and retain their original values of yield strength and electrical resistivity after irradiation. They are being considered for several applications in magnetic fusion reactors. Microscopic details of the m e t a l / c e r a m i c interface play an important role in advanced structural composites. Ernst et al. [3] have studied a C u / A I 2 0 3 interface in an internally oxiJized C u - A I alloy by high-resolution electron microscopy. In the present work, field ion microscopy-atom probe ( F I M - A P ) and transmission electron microscopy ( T E M ) have been used to study the morphology, size distribution and mass spectrum of A120 3 and the C u / A I 2 0 3 interface in a C u - A i 25 alloy.
The Cu-A1 25 alloy was supplied by SCM in the form of a rod of about 20 mm diameter. The manufacturing process /or the dispersion hardened C u - A ! 25 alloy involves mixing an appropriate amount of C u . O powder with Cu-A1 (0.25 wt.C,; A] as A I 2 0 3) alloy powder and heating the mixture to a tcmpet'aturc of aNml q00°C for 40 rain. This causes internal oxidation of the aluminum in the mixture. Any excess oxygen remaining in the powder after complete oxidation of the alumip.um is removed by heati,,g the p o w d e r in a dissociated ammonia atmosphere at 820°C for 2 h. R o d stock is p r o d u c e d by canning the powder in copper tubing and hot extrusion at 900°C. T h e as-received rod material was cold-drawn to wires of 0.25 mm diameter. The wires for the FIM tip were annealed at 900°C for 1 h to eliminate the effect of the hot and cold treatment of the alloy. The FIM tips were then prepared by electropolishing in 1 g sodium chromate dissolved in 10 ml acetic acid (cone.) at room Lemperaturc and a voltage of 16 V. FIM and AP studies were performed by using the instrument described elsewhere [4]. In FIM, the tips were cooled to about 70 K and imaged in neon at a pressure of
{1039-~'~1}28/92/$05.00 ,e:, 1992 - Elsevier Science Publisher.~ B3,' All rights reserved
338
I'~ Zhu et al. / Study of an AI,_O.~ dispersion strengthened copper alloy
about 10 -6 Torr. The AP analyses were performed with a pulse fraction (VpuI~/VDc)of 0.16 under 1 X 10 -7 Torr Ne. TEM specimens were cut from the as-delivered material and prepared by standard techniques and examined in a Philips EM 400.
3. Results and discussion
The TEM studies show that the morphology of the particles is polyhedral. It can be seen in fig. 1 that some of these particles are of triangular and rectangular shapes, which is due to the anisotropic growth of AI20 3. Those crystalline planes, growing faster than others, disappear. Therefore, the grain shape is determined by those crystalline planes for which the growth velocities are mini-
mum. Triangular A 1 2 0 3 particles of about 100 nm also have been observed by Ashby and Smith [5] and Ernst et al. [3]. TEM studies indicated a large size range for the AIzO 3 particles. At the higher end of this range there are very large A1203 particles of about 160 rim. These are hardly observed at the magnification of 30000 because of their low concentration. Excluding these large particles ( > 100 rim), the measurements yield the size distribution shown in fig. 2. The density of the Al20 3 particles is about 1016/cm 3. Diffraction images of Cu-AI 25 alloys from TEM show that the Al203 particles are incoherent to the Cu matrix. A Ne field ion micrograph of Cu-Al 25 alloy is shown in fig. 3. Owing to its high binding energy, A1203 particles exhibit a very high image contrast with respect to the surrounding Cu matrix of
lOOnrn 4 Fig. 1. Bright field TEM micrograph of a Cu-AI 25 alloy showing different sizes of alumina.
F. Zhu et aL / Study of an AI20 ~ dispersion strengthened copper alloy SO
30 g 2o n
0
I
0
. . . .
I
10
. . . .
I
20
. . . .
!
. . . .
30
I
40
. . . .
50
Diameter [nm] Fig. 2. Size distribution of small alumina particles in a C u - A I 25 alloy.
which the concentric rings cannot be observed. Therefore, it is not possible to study the orientation relationship between AlzO 3 and the Cu ma-
330
trix. About 30 AI20.~ particles were examined. Most of the alumina particles are polyhedrons. but also some cubic AI203 particles ~ere observed (fig. 4). Alumina particles consisting of two grains have occasionally been observed by TEM. It is clearly shown in fig. 3 that the concentric rings are nearly perfect at the center of the AI203 particles but are not so perfect in the vicinity of the interface. If geometrical effects are excluded to cause this effect, then the structure a n d / o r the composition in the region close to the interface may deviate from that inside the Al203 crystal. We assume that the formation of AI,_O 3 particles involves two steps. At first, oxy.gen atoms (or ions) diffuse into the vicinity of Al atoms in the alloy and combine with AI atoms to form O - A I bonds. Then, Al atoms diffuse to the vicinity of these O - A l bonds and form more O - A I
Fig. 3. Ne field ion micrograph of a C u - A I 25 alloy obtained at 70 K.
F. Zhu et al. / Study of an AI20,~ dispersion strengthened copper alloy
340
Fig, 4. Ne field ion micrograph of a cubic A1203 particle in "1 Cu-A1 25 alloy obtained at 70 K.
b o n d s . T h i s r e a c t i o n occu:s r e p e a t e d l y . T h e s e c o n d s t e p is t h e p r o p e r r e a r r a n g e m e n t o f A I - O
8o
+i!t4o-]
~c,,+
and Cu atoms such that the perfect A120 3 can 200
o
"5~, E
z
30-
Ne+! 63Cu++
20-
l,
10-
It ~.++
i5Cu~ l
100
20.0 500 ¢0.0 50.0 80.0 7¢3.0 80.0
A t o m i c Mass Units Fig. 5..\tom-prohc mass spectrum from AI:()~ pa,ticlcs and Cu malrix in ~, Cu-AI 25 alloy obtained at 70 K. Except fo." ">('u ', "3Cu ', r's('u +', <'3Cu + ' and 2t~Ne+, peak idcntificalions are diffikult duc to the complex aluminum oxygen cluster ion form;_ilion.
,
AI203 Cu-mat:H~ I
/
[
(S) x
I I
f)-
120-
`5
80-
..~
~.¢1_
0 0.o
I
I
I I
m Y
/
E Z
o
~o 26o ago 46o s& +Go 7~o 8~o ~oo Totcl NumbeF of Ions
Fig. (~. (Tumulalb,,¢ ph)t from the AP analysis showing two AIzO ~ partides in the Cu matrix.
F. Zhu et aL / Study of an AI20t dispersion strengthened copper alloy
form. The intermediate state involving an imperfect AI203 crystal has been observed. By this way, an irregular incoherent phase boundary is to be expected. Such boundary would act as a sink for point defects produced by irradiation. Fig. 5 shows the preliminary mass spectrum of the Cu-AI 25 alloy. Besides four peaks of 6SCu+, 63Cu +, 6SCu ++ and 6 3 C u + + , peaks of m / n = 18, 19 and 29 are present. Because of the large electric field penetration in the insulator AI/O3 particles, the field evaporation takes place in the form of aluminum oxygen clusters rather than single ions as in the case of semiconductors. It is now difficult to decide to which clusters these peaks should be ascribed because of the complicated structure of AI203. Further studies are under way on this question. Fig. 6 shows a cumulative plot obtained from the AP analysis of two m1203 particles in the Cu-AI 25 specimen. From this ladder diagram, it is seen that the interfaces are not abrupt on the atomic scale. This result is in agreement with fig. 3. It seems that Cu atoms exist in the transition region between the A[203 particles and the Cu matrix. A field-evaporation rate of about one ion per 50 pulses was used to get the mass spectrum of A1203 particles and Cu matrix. Across the C u / A I 2 0 3 interface the evaporation rate rises to more than 8 ions per pulse. The result reveals the
34 t
relative weak binding between A I 2 0 3 particles and the Cu matrix. 4. Conclusion o The shape of ml203 particles is polyhedral. The mean diameter of the A1203 particles is 9 rim. All of alumina particles examined by FIM are single crystals. • The interface between the AI20 3 particles and the Cu matrix is incoherent. o The field evaporation of AI203 takes place predominantly in the form of aluminum-ox3~gen clusters rather than single AI ions, due to the large field penetration in the AIzO 3 particles. • The binding between the A1203 particles and the Cu matrix is much weaker than the binding within Cu or AI20 3. References [1] J.A. Spitznagel, N.J. Doyle, W.J. Choyke, J.G. Greggi Jr., J.N. McGruer and J.W. Davis, Nucl. Instrum. Methods Phys. Res. B 16 (1986) 27q. [2] R.J. Livak. T.G. Zocco and L.W. Hobbs, 3. Nud. Mater. 144 (1987) 121. [3] F. Ernst, P. Pirouz and A.H. Heucr. PhiMs. Mug. A ~3 (1'091) 259. [4] P, Mertens, V. Vidic and H, Becket, A computer cot> trolled FtM with AP, HMi Berlin Report (1985). [5] M.F. Ashby and G.C. Smith. J. ]nsL Met. t~l (19~3) tS2.