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Stacking fault tetrahedra in fatigued copper
95
Materials Science and Engineering, 20 (1975) 95--96 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Short Communication Stacking Fault Tetrahedra in Fatigued
Copper
R. GONZALEZ, J. PIQUERAS and L. BRU
Departamento de Fisica Fundamental, Laboratorio de Microscopia Electr6nica, Facultad de Ciencias, Universidad Complutense, Madrid-3 (Spain) (Received February 20, 1975)
Since Silcox and Hirsch's [1] first observations with the electron microscope, the appearance of stacking fault tetrahedra has often been noted as a consequence of quenching various f.c.c. metals. Many properties of these defects were studied by Cotterill e t al. [2]. A special characteristic of copper is that, even though the appearance of stacking fault tetrahedra after quenching is theoretically possible, it has n o t been possible to reproduce observations of these tetrahedra in the electron microscope. However, the copper tetrahedron is a stable cluster up to a size of 235 A [3]. Studies of annealing after quench hardening in copper, as described b y Kimura and Maddin [4], indicate the possible presence of tetrahedra in quenched copper. Stacking fault tetrahedra have been observed in plastically deformed copper [5,6]. The tetrahedra could be formed by means of the clustering of vacancies produced b y moving jogs [3], b u t the results found by L o r e t t o e t al. [6] seem to exclude this possibility. They propose instead a dislocation reaction as the mechanism of tetrahedra formation during deformation. During copper fatigue, large quantities of small point defect clusters [7] are formed, primarily in the dislocation-rich zones. The density of these clusters is greater than that observed in tensile deformed copper. Consequently, the tetrahedra density should be relatively high in a fatigued sample even though the n u m b e r of tetrahedra is a small part of the total n u m b e r of clusters. In this study, copper specimens (made from 99,999% pure copper rods from Koch Light Labs.) measuring 3 mm in diameter and 15 mm
in gauge length were used. The samples were annealed in vacuum for ten hours at 720°C which produced a grain size of approximately 40 pm. A Hounsfield tensometer, modified according to a m e t h o d similar to the one described b y King [8] in order to make fatigue tests, was used to fatigue the samples during 5 and 80 cycles, with total strain amplitudes of 0.036 and 0.025, respectively. Disks were cut from the fatigued samples perpendicular to the axis of the specimen using a Servomet spark cutting machine; they were then electrolytically thinned in order to observe them in the electron microscope at 200 kV. Stacking fault tetrahedra large enough to be recognized were observed in these samples. The largest size of the tetrahedra observed was about 180 A. This upper limit is in agreement with the value of 235 A mentioned previously and also with that of 190 A observed by Loretto e t al. [6]. The tetrahedra were most clearly observed in zones where the total defect density was lowest. Several specimens were annealed in the electron microscope hot stage at various temperatures up to 500°C. At this temperature most of the other defects present in the crystal have been annealed out, leaving the tetrahedra which can be observed more easily than before annealing (Fig. 1). In copper the tetrahedra are annealed out at temperatures higher than 600°C and in gold, at temperatures above 650°C [9,10]. In accordance with this, the tetrahedra in this experiment remained after annealing. Clarebrough e t al. [10] found that, in gold, the temperature at which the tetrahedra were annealed o u t depends on their size and that the smaller tetrahedra disappear at lower tem-
96
From the above observations the presence of stacking fault tetrahedra in fatigued copper can be deduced and it is concluded that copper deformation, and in particular fatiguing followed by annealing, produce a configuration in which an appreciable proportion of the remaining small defects are stacking fault tetrahedra.
REFERENCES
Fig. 1. Fatigued copper after being annealed at temperatures not exceeding 500°C. Stacking fault tetrahedra are visible.
peratures. In our work, no annealing o u t of tetrahedra was observed in the range of temperatures considered. After annealing at 500°C, resolved tetrahedra of a b o u t 70 A were still present. Some of the tetrahedra present in the dense dislocation zones could only be recognized as such after most of the dislocations had disappeared, owing to the fact that t h e y had previously been masked b y them.
1 J. Silcox and P.B. Hirsch, Phil. Mag., 4 (1959) 72. 2 R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals. Academic Press, 1965. 3 R.M.J. Cotterill, in R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals, Academic Press, 1965. 4 H. Kimura and R. Maddin, in R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals, Academic Press, 1965. 5 J.E. Bailey, personal communication to R.M. Cotterill, mentioned in Lattice Defects in Quenched Metals, Academic Press, 1965. 6 M.H. Loretto, L.M. Clarebrough and R.L. Segall, Phil. Mag., 10 (1945) 459. 7 J. Piqueras, J.C. Grosskreutz and W. Frank, Phys. Status Solidi. (a), 11 (1972) 567. 8 A.E. King, J. Sci. Instr., 1 (1968) 1038. 9 H. Kimura and R.R. Hasiguti, J. Phys. Soc. Japan, 17 (1962) 1724. 10 L.M, Clarebrough, R.L. Segall and M.H. Loretto, Phil. Mag., 9 (1964) 377.
Materials Science and Engineering, 20 (1975) 95--96 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Short Communication Stacking Fault Tetrahedra in Fatigued
Copper
R. GONZALEZ, J. PIQUERAS and L. BRU
Departamento de Fisica Fundamental, Laboratorio de Microscopia Electr6nica, Facultad de Ciencias, Universidad Complutense, Madrid-3 (Spain) (Received February 20, 1975)
Since Silcox and Hirsch's [1] first observations with the electron microscope, the appearance of stacking fault tetrahedra has often been noted as a consequence of quenching various f.c.c. metals. Many properties of these defects were studied by Cotterill e t al. [2]. A special characteristic of copper is that, even though the appearance of stacking fault tetrahedra after quenching is theoretically possible, it has n o t been possible to reproduce observations of these tetrahedra in the electron microscope. However, the copper tetrahedron is a stable cluster up to a size of 235 A [3]. Studies of annealing after quench hardening in copper, as described b y Kimura and Maddin [4], indicate the possible presence of tetrahedra in quenched copper. Stacking fault tetrahedra have been observed in plastically deformed copper [5,6]. The tetrahedra could be formed by means of the clustering of vacancies produced b y moving jogs [3], b u t the results found by L o r e t t o e t al. [6] seem to exclude this possibility. They propose instead a dislocation reaction as the mechanism of tetrahedra formation during deformation. During copper fatigue, large quantities of small point defect clusters [7] are formed, primarily in the dislocation-rich zones. The density of these clusters is greater than that observed in tensile deformed copper. Consequently, the tetrahedra density should be relatively high in a fatigued sample even though the n u m b e r of tetrahedra is a small part of the total n u m b e r of clusters. In this study, copper specimens (made from 99,999% pure copper rods from Koch Light Labs.) measuring 3 mm in diameter and 15 mm
in gauge length were used. The samples were annealed in vacuum for ten hours at 720°C which produced a grain size of approximately 40 pm. A Hounsfield tensometer, modified according to a m e t h o d similar to the one described b y King [8] in order to make fatigue tests, was used to fatigue the samples during 5 and 80 cycles, with total strain amplitudes of 0.036 and 0.025, respectively. Disks were cut from the fatigued samples perpendicular to the axis of the specimen using a Servomet spark cutting machine; they were then electrolytically thinned in order to observe them in the electron microscope at 200 kV. Stacking fault tetrahedra large enough to be recognized were observed in these samples. The largest size of the tetrahedra observed was about 180 A. This upper limit is in agreement with the value of 235 A mentioned previously and also with that of 190 A observed by Loretto e t al. [6]. The tetrahedra were most clearly observed in zones where the total defect density was lowest. Several specimens were annealed in the electron microscope hot stage at various temperatures up to 500°C. At this temperature most of the other defects present in the crystal have been annealed out, leaving the tetrahedra which can be observed more easily than before annealing (Fig. 1). In copper the tetrahedra are annealed out at temperatures higher than 600°C and in gold, at temperatures above 650°C [9,10]. In accordance with this, the tetrahedra in this experiment remained after annealing. Clarebrough e t al. [10] found that, in gold, the temperature at which the tetrahedra were annealed o u t depends on their size and that the smaller tetrahedra disappear at lower tem-
96
From the above observations the presence of stacking fault tetrahedra in fatigued copper can be deduced and it is concluded that copper deformation, and in particular fatiguing followed by annealing, produce a configuration in which an appreciable proportion of the remaining small defects are stacking fault tetrahedra.
REFERENCES
Fig. 1. Fatigued copper after being annealed at temperatures not exceeding 500°C. Stacking fault tetrahedra are visible.
peratures. In our work, no annealing o u t of tetrahedra was observed in the range of temperatures considered. After annealing at 500°C, resolved tetrahedra of a b o u t 70 A were still present. Some of the tetrahedra present in the dense dislocation zones could only be recognized as such after most of the dislocations had disappeared, owing to the fact that t h e y had previously been masked b y them.
1 J. Silcox and P.B. Hirsch, Phil. Mag., 4 (1959) 72. 2 R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals. Academic Press, 1965. 3 R.M.J. Cotterill, in R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals, Academic Press, 1965. 4 H. Kimura and R. Maddin, in R.M.J. Cotterill, M. Doyama, J.J. Jackson and M. Meshii (eds.), Lattice Defects in Quenched Metals, Academic Press, 1965. 5 J.E. Bailey, personal communication to R.M. Cotterill, mentioned in Lattice Defects in Quenched Metals, Academic Press, 1965. 6 M.H. Loretto, L.M. Clarebrough and R.L. Segall, Phil. Mag., 10 (1945) 459. 7 J. Piqueras, J.C. Grosskreutz and W. Frank, Phys. Status Solidi. (a), 11 (1972) 567. 8 A.E. King, J. Sci. Instr., 1 (1968) 1038. 9 H. Kimura and R.R. Hasiguti, J. Phys. Soc. Japan, 17 (1962) 1724. 10 L.M, Clarebrough, R.L. Segall and M.H. Loretto, Phil. Mag., 9 (1964) 377.