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Frank loop development in neutron-irradiated cold-worked type 316 stainless steel
JOURNAL
OF NUCLEAR
MATERIALS
47 (1973)
FRANK IN NEUTRON-IRRADIATED
105-109.0
LOOP
NORTH-HOLLAND
PUBLISHING
CO., AMSTERDAM
DEVELOPMENT
COLD-WORKED
TYPE
3 16 STAINLESS
STEEL
*
H.R. BRAGER and J.L. STRAALSUND Westinghouse Hanford Company, Richland,
Received
30 November
Experimental results are presented here to show that neutron irradiation of 20% cold-worked steel at moderate temperatures (- 450°C) induces the replacement of nearly the entire dislocation network with interstitial Frank loops. The microstructure of a solution-treated sample of the same steel, irradiated under identical conditions, consisted of voids and interstitial Frank loops. This low-fluence data indicates that Frank loop formation is not strongly dependent upon defect sink (dislocation) density. The material used in the present study was Type 3 16 stainless steel having the following chemical composition: 0.056 wt% C, 16.52 wt% Cr, 13.66 wt% Ni, 2.41 wt%Mo, 1.64wt%Mn,0.46wt%Si,0.013 wt% P, 0.08 wt% Cu, 0.006 wt% S, 0.006 wt% N, and 0.0008 wt % B. The preirradiation microstructure of the 2% coldworked (drawn) tubing consists of two different features: (1) a ‘coarse’ structure of microtwins, stacking faults and deformation bands, and (2) a dislocation network distributed through the balance of the matrix. The amount of ‘coarse’ stucture in this polycrystalline steel varies significantly probably due to differences in crystal orientation relative to the applied stresses during cold working. The ‘coarse’ structure is generally stable for temperatures up to recrystallization while the dislocation density decreases at much lower temperatures due to the annihilation of mobile dis-
Washington, USA
1972
locations. The dislocation network varies from a nearly random dislocation array to that of cell formation and is related to the material’s stacking fault energy and to the direction and magnitude of strain in the crystal. A typical dislocation network in the unirradiated 2% cold-worked Type 3 16 stainless steel tubing which contains a fairly high dislocation density (- 3 X 101’ cm/cm3) is shown in fig. 1. The dislocation density was determined by using the line intercept method [l] on weak beam dark field micrographs [2]. Corrections were made to take into account the fraction of dislocations which are invisible for use with the particular reflection [3]. Foil thicknesses were determined from dynamical stereomicrographs. Irradiation of this material at about 450°C in the unstressed condition to neutron fluences of only 0.6 X 1O22 n/cm2 resulted in a decrease in the dislocation density by about an order of magnitude to 3 X lOlo cm/cm3, fig. 2. However, a high density (- 3 X 1015 loops/cm3) of Frank faulted loops having amean diameter of about 350 A was formed. These loops have a total loop line length per unit volume of about 3 X lOlo cm/cm3. The loops appear to be uniformly distributed throughout the gram and from gram to grain with no denuded zone. Using standard loop identification techniques [4, 51, all the Frank loops which were analyzed were identified to be of the interstitial type. No noticeable change in ‘coarse’ microstructure was evident. A typical micrograph of loops formed in the same steel except irradiated in the solution-treated condition is shown in fig. 3. The structure contained a low density of voids as well as
* This paper is based on work performed
by Hanford Engineering Development Laboratory, Richland, WA, operated by Westinghouse Hanford Company, a subsidiary of Westinghouse Electric Corporation, under United States Atomic Energy Commission Contract AT (45-l)-2170.
105
106
H.R. Brager, J.L. Straalsund, Frank loop development
Fig. 1. 20% cold-worked
Type 316 stainless
w 3 X 1015 Frank loops/cm3 with a mean size of w 305 A which is comparable to that of the coldworked steel. It has been previously reported that irradiation of solution-treated austenitic stainless steels at moderate temperatures produces voids and a high density of Frank loops [6-101. The classical mechanism for
steel unirradiated.
void formation is that interstitials cause nucleation and growth of Frank loops which result in an excess of vacancies. These vacancies, probably assisted by neutron-induced helium, nucleate and grow voids. However, the formation of voids and Frank loops is a coupled mechanism requiring some driving force or bias to disperse these vacancies and interstitials to their respective sinks.
HR. Brager, J.L. Straalsund, Frank loop development
Fig. 2. 20% cold-worked Type 316 stainless steel; $t - 0.6 X 102* n/cm2 (E > 0.1 MeV), irradiation temperature
Current theoretical models [ 1 I] of void formation are based upon a biased attraction [ 12, 131 of interstitials to dislocation loops or to free dislocations. The preferential attraction for interstitial.9 is only slight however and the primary effect of an increased dislocation density is thought to be a comparable reduction in the supersaturation of both vacancies and
107
- 45O’C.
interstitials. This reduction in supersaturation would thus be expected to result in lower nucleation rates for both voids and loops. Recently, Russell and Powell [lS] extended the analysis of Loh [ 141 on homogeneous loop nucleation in irradiated materials. Their calculations indicate that interstitial loops form much more readily than vacancy
H.R. Brager, J.L. Straalsund, Frank loop development
108
Fig. 3. Solution-treated
Type
316
stainless
steel;
@t - 0.6
loops and that a larger change in defect supersaturation would be required to substantially influence the interstitial loop nucleation rate than the void nucleation rate. Nevertheless, the calculations do indicate that some supersaturation dependence is expected for interstitial loop nucleation.
X
10”
n/cm2,
irradiation
temperature
- 450°C.
However, the present data for cold-worked and solution-treated steel samples show that increasing the preirradiation dislocation density by about three orders of magnitude suppresses void formation but does not inhibit nucleation and growth of Frank loops at these moderate temperatures.
H.R. Brager, J.L. Straalsund, Frank loop development
Clearly, Frank loop formation is not strongly dependent on defect sink density. This means that the interstitials, formed during fast reactor irradiation, preferentially nucleate Frank loops and diffuse to these loops rather than to dislocations for a wide range of loop sizes. The vacancies produced during irradiation are probably diffusing to dislocations thereby causing climb and aiding dislocation recovery. In the present study, voids are not formed in the cold-worked steel at the neutron fluences where they do appear in solution-treated material. One possible explanation is that the neutron produced helium is swept up and trapped by the dislocations which are moving during recovery. Previous results on cold-worked Type 3 16 steel, irradiated to higher fluences than that reported here, have indicated that voids are fomred at these moderate temperatures [16]. However, the void number density in cold-worked steel is much less than that of solution-treated steel irradiated to the same conditions. With increasing neutron fluence (and helium), the lower density of dislocations in the recovered steel could become saturated with helium and/or become immobile thereby allowing the concentration of helium and vacancies to build up in the matrix and assist void formation.
109
References [l] R.T. DeHoff and F.N. Rhines, Quantitative Microscopy (McGraw-Hill, New York, 1968) p. 87. [2] D.J.H. Cockayne, I.L.F. Ray and M.J. Whelan, Phil. Mag. 20 (1969) 1265. [3] P.B. Hirsch et al., Electron Microscopy of Thin Crystals (Butterworth, London, 1965) p. 423. [4] B. Edmondson and G.K. Williamson, Phil. Mag. 9 (1964) 277. [5] W.J. Tunstall, Phil. Mag. 20 (1969) 701. [6] J.J. Holmes, R.E. Robbins, J.E. Brimhall and B. Mastel, Acta Met. 16 (1968) 955. [7] J.O. Stiegler and E.E. Bloom, J. Nucl. Mater. 33 (1969) 173. [8] H.R. Brager, J.L. Straalsund, J.J. Holmes and J.F. Bates, Met. Trans. 2 (1971) 1893. [9] E.E. Bloom, J.O. Stiegler and C.T. McHargue, Rad. Effects 14 (1972) 231. [lo] H.R. Brager and J.L. Straalsund, J. NucL Mater. 46 (1973) 134. [ 111 See various articles in Radiation-Induced Voids in Metals. ed. J.W. Corbett and L.C. Ianiello, Albany, N.Y. June 1971, USAEC Symposium Series 26. [12] S.D. Harkness, J.A. Tesk and Che-Yu Li, Nucl. Appl. Tech. 9 (1970) 24. [13] R. Bullough, B.L. Eyre and R.C. Perrin, Nucl. Appl. Tech. 9 (1970) 346. [14] B.T:M. Loh, Scripta Met. 5 (1971) 1049. [15] K.C. Russell and R.W. PowelI, Acta Met., to be published. [16] J.L. Straalsund, H.R. Brager and J.J. Holmes, Met. Trans. 2 (1971) 142.
OF NUCLEAR
MATERIALS
47 (1973)
FRANK IN NEUTRON-IRRADIATED
105-109.0
LOOP
NORTH-HOLLAND
PUBLISHING
CO., AMSTERDAM
DEVELOPMENT
COLD-WORKED
TYPE
3 16 STAINLESS
STEEL
*
H.R. BRAGER and J.L. STRAALSUND Westinghouse Hanford Company, Richland,
Received
30 November
Experimental results are presented here to show that neutron irradiation of 20% cold-worked steel at moderate temperatures (- 450°C) induces the replacement of nearly the entire dislocation network with interstitial Frank loops. The microstructure of a solution-treated sample of the same steel, irradiated under identical conditions, consisted of voids and interstitial Frank loops. This low-fluence data indicates that Frank loop formation is not strongly dependent upon defect sink (dislocation) density. The material used in the present study was Type 3 16 stainless steel having the following chemical composition: 0.056 wt% C, 16.52 wt% Cr, 13.66 wt% Ni, 2.41 wt%Mo, 1.64wt%Mn,0.46wt%Si,0.013 wt% P, 0.08 wt% Cu, 0.006 wt% S, 0.006 wt% N, and 0.0008 wt % B. The preirradiation microstructure of the 2% coldworked (drawn) tubing consists of two different features: (1) a ‘coarse’ structure of microtwins, stacking faults and deformation bands, and (2) a dislocation network distributed through the balance of the matrix. The amount of ‘coarse’ stucture in this polycrystalline steel varies significantly probably due to differences in crystal orientation relative to the applied stresses during cold working. The ‘coarse’ structure is generally stable for temperatures up to recrystallization while the dislocation density decreases at much lower temperatures due to the annihilation of mobile dis-
Washington, USA
1972
locations. The dislocation network varies from a nearly random dislocation array to that of cell formation and is related to the material’s stacking fault energy and to the direction and magnitude of strain in the crystal. A typical dislocation network in the unirradiated 2% cold-worked Type 3 16 stainless steel tubing which contains a fairly high dislocation density (- 3 X 101’ cm/cm3) is shown in fig. 1. The dislocation density was determined by using the line intercept method [l] on weak beam dark field micrographs [2]. Corrections were made to take into account the fraction of dislocations which are invisible for use with the particular reflection [3]. Foil thicknesses were determined from dynamical stereomicrographs. Irradiation of this material at about 450°C in the unstressed condition to neutron fluences of only 0.6 X 1O22 n/cm2 resulted in a decrease in the dislocation density by about an order of magnitude to 3 X lOlo cm/cm3, fig. 2. However, a high density (- 3 X 1015 loops/cm3) of Frank faulted loops having amean diameter of about 350 A was formed. These loops have a total loop line length per unit volume of about 3 X lOlo cm/cm3. The loops appear to be uniformly distributed throughout the gram and from gram to grain with no denuded zone. Using standard loop identification techniques [4, 51, all the Frank loops which were analyzed were identified to be of the interstitial type. No noticeable change in ‘coarse’ microstructure was evident. A typical micrograph of loops formed in the same steel except irradiated in the solution-treated condition is shown in fig. 3. The structure contained a low density of voids as well as
* This paper is based on work performed
by Hanford Engineering Development Laboratory, Richland, WA, operated by Westinghouse Hanford Company, a subsidiary of Westinghouse Electric Corporation, under United States Atomic Energy Commission Contract AT (45-l)-2170.
105
106
H.R. Brager, J.L. Straalsund, Frank loop development
Fig. 1. 20% cold-worked
Type 316 stainless
w 3 X 1015 Frank loops/cm3 with a mean size of w 305 A which is comparable to that of the coldworked steel. It has been previously reported that irradiation of solution-treated austenitic stainless steels at moderate temperatures produces voids and a high density of Frank loops [6-101. The classical mechanism for
steel unirradiated.
void formation is that interstitials cause nucleation and growth of Frank loops which result in an excess of vacancies. These vacancies, probably assisted by neutron-induced helium, nucleate and grow voids. However, the formation of voids and Frank loops is a coupled mechanism requiring some driving force or bias to disperse these vacancies and interstitials to their respective sinks.
HR. Brager, J.L. Straalsund, Frank loop development
Fig. 2. 20% cold-worked Type 316 stainless steel; $t - 0.6 X 102* n/cm2 (E > 0.1 MeV), irradiation temperature
Current theoretical models [ 1 I] of void formation are based upon a biased attraction [ 12, 131 of interstitials to dislocation loops or to free dislocations. The preferential attraction for interstitial.9 is only slight however and the primary effect of an increased dislocation density is thought to be a comparable reduction in the supersaturation of both vacancies and
107
- 45O’C.
interstitials. This reduction in supersaturation would thus be expected to result in lower nucleation rates for both voids and loops. Recently, Russell and Powell [lS] extended the analysis of Loh [ 141 on homogeneous loop nucleation in irradiated materials. Their calculations indicate that interstitial loops form much more readily than vacancy
H.R. Brager, J.L. Straalsund, Frank loop development
108
Fig. 3. Solution-treated
Type
316
stainless
steel;
@t - 0.6
loops and that a larger change in defect supersaturation would be required to substantially influence the interstitial loop nucleation rate than the void nucleation rate. Nevertheless, the calculations do indicate that some supersaturation dependence is expected for interstitial loop nucleation.
X
10”
n/cm2,
irradiation
temperature
- 450°C.
However, the present data for cold-worked and solution-treated steel samples show that increasing the preirradiation dislocation density by about three orders of magnitude suppresses void formation but does not inhibit nucleation and growth of Frank loops at these moderate temperatures.
H.R. Brager, J.L. Straalsund, Frank loop development
Clearly, Frank loop formation is not strongly dependent on defect sink density. This means that the interstitials, formed during fast reactor irradiation, preferentially nucleate Frank loops and diffuse to these loops rather than to dislocations for a wide range of loop sizes. The vacancies produced during irradiation are probably diffusing to dislocations thereby causing climb and aiding dislocation recovery. In the present study, voids are not formed in the cold-worked steel at the neutron fluences where they do appear in solution-treated material. One possible explanation is that the neutron produced helium is swept up and trapped by the dislocations which are moving during recovery. Previous results on cold-worked Type 3 16 steel, irradiated to higher fluences than that reported here, have indicated that voids are fomred at these moderate temperatures [16]. However, the void number density in cold-worked steel is much less than that of solution-treated steel irradiated to the same conditions. With increasing neutron fluence (and helium), the lower density of dislocations in the recovered steel could become saturated with helium and/or become immobile thereby allowing the concentration of helium and vacancies to build up in the matrix and assist void formation.
109
References [l] R.T. DeHoff and F.N. Rhines, Quantitative Microscopy (McGraw-Hill, New York, 1968) p. 87. [2] D.J.H. Cockayne, I.L.F. Ray and M.J. Whelan, Phil. Mag. 20 (1969) 1265. [3] P.B. Hirsch et al., Electron Microscopy of Thin Crystals (Butterworth, London, 1965) p. 423. [4] B. Edmondson and G.K. Williamson, Phil. Mag. 9 (1964) 277. [5] W.J. Tunstall, Phil. Mag. 20 (1969) 701. [6] J.J. Holmes, R.E. Robbins, J.E. Brimhall and B. Mastel, Acta Met. 16 (1968) 955. [7] J.O. Stiegler and E.E. Bloom, J. Nucl. Mater. 33 (1969) 173. [8] H.R. Brager, J.L. Straalsund, J.J. Holmes and J.F. Bates, Met. Trans. 2 (1971) 1893. [9] E.E. Bloom, J.O. Stiegler and C.T. McHargue, Rad. Effects 14 (1972) 231. [lo] H.R. Brager and J.L. Straalsund, J. NucL Mater. 46 (1973) 134. [ 111 See various articles in Radiation-Induced Voids in Metals. ed. J.W. Corbett and L.C. Ianiello, Albany, N.Y. June 1971, USAEC Symposium Series 26. [12] S.D. Harkness, J.A. Tesk and Che-Yu Li, Nucl. Appl. Tech. 9 (1970) 24. [13] R. Bullough, B.L. Eyre and R.C. Perrin, Nucl. Appl. Tech. 9 (1970) 346. [14] B.T:M. Loh, Scripta Met. 5 (1971) 1049. [15] K.C. Russell and R.W. PowelI, Acta Met., to be published. [16] J.L. Straalsund, H.R. Brager and J.J. Holmes, Met. Trans. 2 (1971) 142.