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Microstructural stability in a Cu-Cr-Si alloy under 300 keV Cu+ ion irradiation
1030
Journal of Nuclear Materials 179-181 (1991) 1030-1033 North-Holland
~icrostru~tural ion irradiation C. Ramachandra
stability in a Cu-Cr-Si
alloy under 300 keV Cu+
I, N. Wanderka *, R.P. Wahi ’ and H. Wollenberger
*
’ Department of Me~allutgicul Engineering, Banaras Hindu University, Varanasi, India ’ ~~n-~eitner-r~~~~u~,
Postjach 390128, D-f000 Berlin 39, Fed. Rep. Germany
The stability of precipitate phases in a Cu-base alloy under irradiation with 300 KeV CIA+ ions has been investigated. The irradiations were carried out in the temperature range of 200 o C to 530 ’ C at displacement rates of 2.3 X lo-’ and 2.3 x low5 dpa/s, respectively. Of the two types of precipitates which form in this alloy under thermal ageing, the metastable, coherent precipitates of fee chromium either do not form or dissolve, if already present, under most irradiation conditions. The second population of precipitates, i.e. incoherent particles of the stable phase Cr,Si, grow to a temperature dependent stable size under all irradiation conditions. The results are interpreted in terms of mechanisms of cascade mixing and radiation enhanced diffusional growth.
1. Introduction A copper base alloy containing small amounts of Cr, Si and Zr is one of the candidate materials being considered for use as a cooling tube material to cool the divertor plates in fusion reactors. The alloy exhibits a good combination of strength and thermal conductivity [l-11]. Precipitation hardening and work hardening together contribute to the final strength. Previous investigations on similar alloys have been carried out to study the influence of irradiation on the stability of work hardening in aged and cold-worked conditions [12-131. These studies have shown a considerable loss of yield strength due to irradiation [12,13]. This loss in yield strength has been associated with radiation-enhanced r~~stal~tion observed in Cu-Cr and Cu-Cr-Zr al-
loys [14]. In the present investigation, the effect of copper ion irradiation on the stability of the other factor contributing to the final strength of a Cu-Cr-Si alloy (CL-l.146 at% Cr-0.144 at% Si-0.030 at% Zr), namely the precipitate phases, has been investigated. 2. Experiment The specimens were irradiated with 300 keV Cu+ ions in two different heat treated states: (i) solution treated (900 “C/OS h) and water quenched, and (ii) aged at 480 o C for 25 h and 600 o C for 1 h respectively after treatment (i). The second treatment produces precipitates of (a) metastable, coherent fee chromium and (b) stable, incoherent Cr,Si (fig. 1). The displacement cross section u within the specimen was computed using
Fig. 1. TEM micrographs of the Cu-Cr-Si alloy aged at 480 o C for 25 h, showing (a) coherent fee Cr precipitates with strong strain field contrast in the form of pair of lobes about a line of no contrast, DF with [220]m, E = [ITO], and (b) incoherent precipitates identified to be of the Cr,Si phase and imaged with (210) precipitate reflections, B = [ill]. ~22-3115/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holl~d)
1031
C. Ramachandra et al. / Microstructural stability in Cu-Cr-Si
the TRIM code [15]. The first 100 nm below the irradiated surface, having a relatively uniform damage cross section (less than 30% variation), were examined by transmission electron microscopy (TEM). From the average value of (I = 2 x lo-l9 m2 within the first 100 nm the displacement rate K,, was calculated using the modified Kin&in-Pease expression with a displacement energy of 25 eV. The irradiated specimens (200 pm thick, 3 mm diameter) were electrolytically thinned from the unirradiated side until1 perforation occurred.
Table 1 Irradiation conditions causing dissolution (open symbols)/no dissolution (closed symbols) of coherent precipitates of 3.7 run in diameter (circles) and 7.9 nm in diameter (triangles) after a minimum fluence of 2.3 dpa (Kc = 2.3 X lo-’ dpa/s) and 0.23 dpa (Kc = 2.3 X 10m5 dpa/s) Displacement
rate
Irradiation
temperature
( o C)
(dpa/s)
530
480
430
380
300
200
2.3~10-~ 2.3~10-~
A
0
0
0
A
0
0
0
3. Results and discussion Irradiation of solution-treated and quenched specimens with 300 keV Cu+ ions, at temperatures and for times which lead to the incoherent and the coherent precipitates under thermal ageing, produced only the incoherent precipitates (fig. 2). The coherent precipitates could not be detected in the solution-treated specimens after any condition of irradiation employed. Irradiation of pre-aged specimens containing both coherent and incoherent precipitates caused growth of the incoherent precipitates under all the irradiation conditions and dissolution of the coherent precipitates under most conditions. 3.1. Coherent precipitates Table 1 shows the displacement rates and irradiation temperatures at which the stability of the coherent precipitates in the aged specimens was investigated. The open and closed symbols indicate the disappearance (dissolution) and presence, respectively, of the precipitates after irradiation to minimum fluences of 0.23 dpa (K,, = 2.3 X 10e5 dpa/s) and 2.3 dpa (K,, = 2.3 x low2 dpa/s). The circles and triangles correspond to initial
Fig. 2. DF images
precipitate diameters of 3.7 nm and 7.9 nm, respectively. Most of the experiments so far have been performed on specimens containing the smaller sized precipitates. These disappeared on irradiation under all combinations of temperature and displacement rate employed. However, two experiments on the larger precipitates of 7.9 nm diameter show that at a displacement rate of 2.3 X lop2 dpa/s they dissolve at 300°C but not at 530°C. These observations are qualitatively similar to those made earlier on Cu-Ni-Fe alloys [1618] and can be rationalized by invoking the concept of two opposing mechanisms which occur simultaneously during heavy ion irradiation [16-201: cascade mixing causing homogenization of the structure and irradiation-enhanced diffusion seeking to rebuild the two-phase structure. The rate of cascade damage for coherent precipitates under all, except one, of the irradiation conditions employed (table 1) is obviously much larger than that of the diffusional rebuilding of the precipitates. Thus, under irradiation the coherent precipitates do not form in the solution-treated and quenched specimens and those present in the pre-aged specimens are
of incoherent precipitates unaged with (210) precipitate reflections after irradiating (Ka = 2.3 solution-treated and water quenched specimens at 480 o C to (a) 1.15 dpa and (b) 23 dpa.
x
10e2 dpa/s)
the
1032
C. Ramachandra et al. / Microstructural stabiliry in Cu-Cr-Si
C, is the concentration of point defect sinks. Thus depending on the mode of point defect annihilation, d, can be written as:
GROWTH OF INCOHERENT PRECIPITATES UNDER 300KeV Cu+ ION IRRADIATION (K&O-*dpa/s) I
d, - ( Q/K, d, - ( l/C,)“2
>“4
recombination
case,
sink case.
The observed temperature dependence of d, implies that the recombination case is applicable to the present data obtained at the displacement rate of 2.3 x 1O-2 dpa/s. The ratio of the two sizes therefore can be expressed as: d,(530”C) 00°
5
10
15
20
FLUENCE
(dpa)
25
30
Fig. 3. Size of incoherent precipitates as a function of fluence in dpa.
forced to dissolve. More experimental data are necessary for a quantitative analysis of this behavior in terms of the above model. 3.2. Incoherent precipitates Fig. 3 shows the growth behavior of incoherent precipitates as a function of fluence in the solution-treated material. After an initial rapid growth, the precipitate diameter tends to approach a stable value and the stable diameter is larger for the higher irradiation temperature. These two observations together suggest that also in the case of incoherent precipitates the two opposing processes i.e., cascade damage and irradiation-enhanced diffusional growth, determine the growth behavior and lead to a stable precipitate size. As has been shown earlier [19], the stable size d, is given by: d, - ( Dim/K,)*‘2
Here, Di, is the diffusion coefficient under irradiation, is the rate constant for precipitate dissolution by cascades (K, - K, [19]), D, and C, are the diffusion coefficient and concentration of vacancies and K0 is the displacement rate. If we assume that the point defects under the present irradiation conditions have reached a stationary concentration, C, can be written as [21]: K,
defect annihilation
essentially
by mutual recombination,
_
point defect annihilation
The experimental value of the ratio of the two stable sizes ( = 1.3) agrees well with the derived value ( = 1.2) from the two diffusion coefficients D, assuming the migrating energy of vacancies in copper to be 0.76, which is in reasonable agreement with the experimentally found values [22]. Specimens pre-aged to produce the incoherent precipitates with an average size of 5 nm in diameter were irradiated with a displacement rate of 2.3 x 10e5 dpa/s at 300 o C and 480 o C. At both irradiation temperatures they grew to an average size of around 9 nm in diameter. This behavior at the small displacement rate of 2.3 X 10e5 dpa/s implies that the diffusion coefficient under these irradiation conditions is independent of temperature (sink case). The present results show that the response of the two types of precipitates to irradiation is different. The coherent precipitates completely dissolve under most experimental conditions, whereas the incoherent ones grow to a stable size under the same irradiation wnditions. 4. ConeIlI&g
- ( D$JKo)1’2.
point
d,(480=‘C)
essentially at
sinks. Here, K,’ is the production rate of freely migrating point defects (K,’ -f&, with f being 0.01 to 0.1) and
remarks
A commercial Cu-Cr-Si alloy was irradiated with 300 keV Cu+ ions in two different initial microstructural states. The following are the main findings: (1) Irradiation of the pm-aged alloy containing wherent precipitates of nearly pure chromium and incoherent precipitates of CrsSi results in the dissolution of the former and growth of the latter under most irradiation wnditions. (2) Irradiation of the solution-treated alloy causes the formation and growth of the incoherent precipitates of the CrsSi phase only. (3) The dissolution of the wherent precipitates and the growth behavior of the incoherent precipitates can be rationalized in terms of two opposing processes - cascade mixing and radiation-enhanced growth of precipitates - operating under the conditions of irradiation employed.
C. Ramachandra et al. / Microstructural stability in Cu-Cr-Si
References [l] M. Karma, 2. Metallk. 79 (1988) 684, [2] N.Y. Tang, D.M.R. Taphn and G.L. Dunlop, Mater. Sci. Technol. 1 (1985) 270. [3] W. Korten and W. Knorr, Z. Metallk. 45 (1954) 350. [4] W. Grnhl and RW. Fisher, Z. Metallk. 46 (1955) 742. [S] B. Lynch, Ph.D. Thesis, Aston University, UK, 1968. [6] J. Rezek, Can. Met. Quart. 8 (1969) 179. [7] R.W. Knights and P. Wilkes, Metall. Trans. 4 (1973) 2389. [8] D.W. Borland and R.L. SegaB, in: Proc. 3rd Int. Conf. on Strength of Metals and Alloys, Cambridge, UK, 1973, Vol. 1, p. 26. [9] Y. Komen and J. Rezek, Metall. Trans. 6A (1975) 549. [lo] G.C. Weatherby, P. Humble and D. Borland, Acta MetaB. 27 (1979) 1815. [ll] Z. Rdzawski and J. Stobrawa, Scripta Metall. 20 (1986) 341. (121 H.R. Brager, H.L. Heir&h and F.A. Garner, J. Nucl. Mater. 133 & 134 (1985) 676.
1033
1131 M. Appello and P. Fenici, J. Nucl. Mater. 152 (1988) 348. [14] S.J. Zinkle, G.L. Ku&in&i and K.L. Mansnr, J. Nucl. Mater. 141-143 (1986) 188. [15] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257 [16] U. Scheuer, Doctoral Thesis D83, TU Berlin, 1986. [17] U. Scheuer, R.P. Wahi and H. Wollenberger, J. Nucl. Mater. 140-143 (1986) 767. 1181 U. Scheuer, Radiat. Eff. 105 (1987) 85. 1191 R.P. Wahi, R Koch, C. Abromeit and H. Woll~berg~, J. Nucl. Mater. 127 (1985) 175. 1201Wollenberger, C. Abromeit and V. Nanndorf, J. Nucl. Mater. 155-157 (1988) 1174. [21] R. Sirmann, J. Nucl. Mater. 69 Br 70 (1968) 386. [22] H. Wollenberger, Physical Metalhtrgy Eds. RW. Cahn and P. Haasen (Elsevier, Amsterdam, 1983) Chap. 17, p. 1140.
Journal of Nuclear Materials 179-181 (1991) 1030-1033 North-Holland
~icrostru~tural ion irradiation C. Ramachandra
stability in a Cu-Cr-Si
alloy under 300 keV Cu+
I, N. Wanderka *, R.P. Wahi ’ and H. Wollenberger
*
’ Department of Me~allutgicul Engineering, Banaras Hindu University, Varanasi, India ’ ~~n-~eitner-r~~~~u~,
Postjach 390128, D-f000 Berlin 39, Fed. Rep. Germany
The stability of precipitate phases in a Cu-base alloy under irradiation with 300 KeV CIA+ ions has been investigated. The irradiations were carried out in the temperature range of 200 o C to 530 ’ C at displacement rates of 2.3 X lo-’ and 2.3 x low5 dpa/s, respectively. Of the two types of precipitates which form in this alloy under thermal ageing, the metastable, coherent precipitates of fee chromium either do not form or dissolve, if already present, under most irradiation conditions. The second population of precipitates, i.e. incoherent particles of the stable phase Cr,Si, grow to a temperature dependent stable size under all irradiation conditions. The results are interpreted in terms of mechanisms of cascade mixing and radiation enhanced diffusional growth.
1. Introduction A copper base alloy containing small amounts of Cr, Si and Zr is one of the candidate materials being considered for use as a cooling tube material to cool the divertor plates in fusion reactors. The alloy exhibits a good combination of strength and thermal conductivity [l-11]. Precipitation hardening and work hardening together contribute to the final strength. Previous investigations on similar alloys have been carried out to study the influence of irradiation on the stability of work hardening in aged and cold-worked conditions [12-131. These studies have shown a considerable loss of yield strength due to irradiation [12,13]. This loss in yield strength has been associated with radiation-enhanced r~~stal~tion observed in Cu-Cr and Cu-Cr-Zr al-
loys [14]. In the present investigation, the effect of copper ion irradiation on the stability of the other factor contributing to the final strength of a Cu-Cr-Si alloy (CL-l.146 at% Cr-0.144 at% Si-0.030 at% Zr), namely the precipitate phases, has been investigated. 2. Experiment The specimens were irradiated with 300 keV Cu+ ions in two different heat treated states: (i) solution treated (900 “C/OS h) and water quenched, and (ii) aged at 480 o C for 25 h and 600 o C for 1 h respectively after treatment (i). The second treatment produces precipitates of (a) metastable, coherent fee chromium and (b) stable, incoherent Cr,Si (fig. 1). The displacement cross section u within the specimen was computed using
Fig. 1. TEM micrographs of the Cu-Cr-Si alloy aged at 480 o C for 25 h, showing (a) coherent fee Cr precipitates with strong strain field contrast in the form of pair of lobes about a line of no contrast, DF with [220]m, E = [ITO], and (b) incoherent precipitates identified to be of the Cr,Si phase and imaged with (210) precipitate reflections, B = [ill]. ~22-3115/91/$03.50
0 1991 - Elsevier Science Publishers B.V. (North-Holl~d)
1031
C. Ramachandra et al. / Microstructural stability in Cu-Cr-Si
the TRIM code [15]. The first 100 nm below the irradiated surface, having a relatively uniform damage cross section (less than 30% variation), were examined by transmission electron microscopy (TEM). From the average value of (I = 2 x lo-l9 m2 within the first 100 nm the displacement rate K,, was calculated using the modified Kin&in-Pease expression with a displacement energy of 25 eV. The irradiated specimens (200 pm thick, 3 mm diameter) were electrolytically thinned from the unirradiated side until1 perforation occurred.
Table 1 Irradiation conditions causing dissolution (open symbols)/no dissolution (closed symbols) of coherent precipitates of 3.7 run in diameter (circles) and 7.9 nm in diameter (triangles) after a minimum fluence of 2.3 dpa (Kc = 2.3 X lo-’ dpa/s) and 0.23 dpa (Kc = 2.3 X 10m5 dpa/s) Displacement
rate
Irradiation
temperature
( o C)
(dpa/s)
530
480
430
380
300
200
2.3~10-~ 2.3~10-~
A
0
0
0
A
0
0
0
3. Results and discussion Irradiation of solution-treated and quenched specimens with 300 keV Cu+ ions, at temperatures and for times which lead to the incoherent and the coherent precipitates under thermal ageing, produced only the incoherent precipitates (fig. 2). The coherent precipitates could not be detected in the solution-treated specimens after any condition of irradiation employed. Irradiation of pre-aged specimens containing both coherent and incoherent precipitates caused growth of the incoherent precipitates under all the irradiation conditions and dissolution of the coherent precipitates under most conditions. 3.1. Coherent precipitates Table 1 shows the displacement rates and irradiation temperatures at which the stability of the coherent precipitates in the aged specimens was investigated. The open and closed symbols indicate the disappearance (dissolution) and presence, respectively, of the precipitates after irradiation to minimum fluences of 0.23 dpa (K,, = 2.3 X 10e5 dpa/s) and 2.3 dpa (K,, = 2.3 x low2 dpa/s). The circles and triangles correspond to initial
Fig. 2. DF images
precipitate diameters of 3.7 nm and 7.9 nm, respectively. Most of the experiments so far have been performed on specimens containing the smaller sized precipitates. These disappeared on irradiation under all combinations of temperature and displacement rate employed. However, two experiments on the larger precipitates of 7.9 nm diameter show that at a displacement rate of 2.3 X lop2 dpa/s they dissolve at 300°C but not at 530°C. These observations are qualitatively similar to those made earlier on Cu-Ni-Fe alloys [1618] and can be rationalized by invoking the concept of two opposing mechanisms which occur simultaneously during heavy ion irradiation [16-201: cascade mixing causing homogenization of the structure and irradiation-enhanced diffusion seeking to rebuild the two-phase structure. The rate of cascade damage for coherent precipitates under all, except one, of the irradiation conditions employed (table 1) is obviously much larger than that of the diffusional rebuilding of the precipitates. Thus, under irradiation the coherent precipitates do not form in the solution-treated and quenched specimens and those present in the pre-aged specimens are
of incoherent precipitates unaged with (210) precipitate reflections after irradiating (Ka = 2.3 solution-treated and water quenched specimens at 480 o C to (a) 1.15 dpa and (b) 23 dpa.
x
10e2 dpa/s)
the
1032
C. Ramachandra et al. / Microstructural stabiliry in Cu-Cr-Si
C, is the concentration of point defect sinks. Thus depending on the mode of point defect annihilation, d, can be written as:
GROWTH OF INCOHERENT PRECIPITATES UNDER 300KeV Cu+ ION IRRADIATION (K&O-*dpa/s) I
d, - ( Q/K, d, - ( l/C,)“2
>“4
recombination
case,
sink case.
The observed temperature dependence of d, implies that the recombination case is applicable to the present data obtained at the displacement rate of 2.3 x 1O-2 dpa/s. The ratio of the two sizes therefore can be expressed as: d,(530”C) 00°
5
10
15
20
FLUENCE
(dpa)
25
30
Fig. 3. Size of incoherent precipitates as a function of fluence in dpa.
forced to dissolve. More experimental data are necessary for a quantitative analysis of this behavior in terms of the above model. 3.2. Incoherent precipitates Fig. 3 shows the growth behavior of incoherent precipitates as a function of fluence in the solution-treated material. After an initial rapid growth, the precipitate diameter tends to approach a stable value and the stable diameter is larger for the higher irradiation temperature. These two observations together suggest that also in the case of incoherent precipitates the two opposing processes i.e., cascade damage and irradiation-enhanced diffusional growth, determine the growth behavior and lead to a stable precipitate size. As has been shown earlier [19], the stable size d, is given by: d, - ( Dim/K,)*‘2
Here, Di, is the diffusion coefficient under irradiation, is the rate constant for precipitate dissolution by cascades (K, - K, [19]), D, and C, are the diffusion coefficient and concentration of vacancies and K0 is the displacement rate. If we assume that the point defects under the present irradiation conditions have reached a stationary concentration, C, can be written as [21]: K,
defect annihilation
essentially
by mutual recombination,
_
point defect annihilation
The experimental value of the ratio of the two stable sizes ( = 1.3) agrees well with the derived value ( = 1.2) from the two diffusion coefficients D, assuming the migrating energy of vacancies in copper to be 0.76, which is in reasonable agreement with the experimentally found values [22]. Specimens pre-aged to produce the incoherent precipitates with an average size of 5 nm in diameter were irradiated with a displacement rate of 2.3 x 10e5 dpa/s at 300 o C and 480 o C. At both irradiation temperatures they grew to an average size of around 9 nm in diameter. This behavior at the small displacement rate of 2.3 X 10e5 dpa/s implies that the diffusion coefficient under these irradiation conditions is independent of temperature (sink case). The present results show that the response of the two types of precipitates to irradiation is different. The coherent precipitates completely dissolve under most experimental conditions, whereas the incoherent ones grow to a stable size under the same irradiation wnditions. 4. ConeIlI&g
- ( D$JKo)1’2.
point
d,(480=‘C)
essentially at
sinks. Here, K,’ is the production rate of freely migrating point defects (K,’ -f&, with f being 0.01 to 0.1) and
remarks
A commercial Cu-Cr-Si alloy was irradiated with 300 keV Cu+ ions in two different initial microstructural states. The following are the main findings: (1) Irradiation of the pm-aged alloy containing wherent precipitates of nearly pure chromium and incoherent precipitates of CrsSi results in the dissolution of the former and growth of the latter under most irradiation wnditions. (2) Irradiation of the solution-treated alloy causes the formation and growth of the incoherent precipitates of the CrsSi phase only. (3) The dissolution of the wherent precipitates and the growth behavior of the incoherent precipitates can be rationalized in terms of two opposing processes - cascade mixing and radiation-enhanced growth of precipitates - operating under the conditions of irradiation employed.
C. Ramachandra et al. / Microstructural stability in Cu-Cr-Si
References [l] M. Karma, 2. Metallk. 79 (1988) 684, [2] N.Y. Tang, D.M.R. Taphn and G.L. Dunlop, Mater. Sci. Technol. 1 (1985) 270. [3] W. Korten and W. Knorr, Z. Metallk. 45 (1954) 350. [4] W. Grnhl and RW. Fisher, Z. Metallk. 46 (1955) 742. [S] B. Lynch, Ph.D. Thesis, Aston University, UK, 1968. [6] J. Rezek, Can. Met. Quart. 8 (1969) 179. [7] R.W. Knights and P. Wilkes, Metall. Trans. 4 (1973) 2389. [8] D.W. Borland and R.L. SegaB, in: Proc. 3rd Int. Conf. on Strength of Metals and Alloys, Cambridge, UK, 1973, Vol. 1, p. 26. [9] Y. Komen and J. Rezek, Metall. Trans. 6A (1975) 549. [lo] G.C. Weatherby, P. Humble and D. Borland, Acta MetaB. 27 (1979) 1815. [ll] Z. Rdzawski and J. Stobrawa, Scripta Metall. 20 (1986) 341. (121 H.R. Brager, H.L. Heir&h and F.A. Garner, J. Nucl. Mater. 133 & 134 (1985) 676.
1033
1131 M. Appello and P. Fenici, J. Nucl. Mater. 152 (1988) 348. [14] S.J. Zinkle, G.L. Ku&in&i and K.L. Mansnr, J. Nucl. Mater. 141-143 (1986) 188. [15] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257 [16] U. Scheuer, Doctoral Thesis D83, TU Berlin, 1986. [17] U. Scheuer, R.P. Wahi and H. Wollenberger, J. Nucl. Mater. 140-143 (1986) 767. 1181 U. Scheuer, Radiat. Eff. 105 (1987) 85. 1191 R.P. Wahi, R Koch, C. Abromeit and H. Woll~berg~, J. Nucl. Mater. 127 (1985) 175. 1201Wollenberger, C. Abromeit and V. Nanndorf, J. Nucl. Mater. 155-157 (1988) 1174. [21] R. Sirmann, J. Nucl. Mater. 69 Br 70 (1968) 386. [22] H. Wollenberger, Physical Metalhtrgy Eds. RW. Cahn and P. Haasen (Elsevier, Amsterdam, 1983) Chap. 17, p. 1140.