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The effect of boron additions on irradiation-induced order changes in Ni3Al intermetallic compounds
Scrip~Metallurgica etMat&lis,Vol. 33, No. 9, pp. 13794385.1995
lz4sevie.rscience Ltd Copyright0 1995 Acta Metalhjca Inc. PcultedintheUSAAllrigbtsr-ed 0956-716x/95 $9.50 + .oo
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THE EFFECT OF BORON ADDITIONS ON IRRADIATION-INDUCED ORDER CHANGES IN N&Al INTERMETALLIC COMPOUNDS N. Njah’, D. Gilbon’ and 0. Dimitrog ‘Laboratoire de l’Etat Solide, Faculte des Sciences, Route de Soukra Km 4, Sfax (Tunisi.i). ‘CEA/DTA/DECM/SRMA, CE Saclay, F-9 119 1 Gif-sur-Yvette Cedex (France). 3Centre d’Etudes de Chimie MCtallurgique, 15 rue G. Urbain, F-94407 Vitry Cedex (France) (Received March 15,1995) (Revised May 29,1995) Introduction
N&Alis a long-range ordered alloy with an L 1z structure, which retains a highly ordered state up to its melting temperature. It is of potential interest for high-temperature applications, particularly since it exhibits a positive temperature dependence of the yield-strength [ 1,2]. Also, Ni,Al-based intermetallic compounds form the y ’ hardening phase of y I y’ nickel alloys, which are often used in a radiation environment. Disordering under irradiation has been investigated either in binary Ni,Al or internary Ni,(Al, Ti) alloys [3,4], but there are no studies concerning the effects of interstitial additions, especially boron which is often used as a ductilising element [5]. During irradiation it was observed that the long-range order parameter S decreased rapidly with increasing dose, then reached a steady-state value depending on the irradiation parameters. This steady-state degree of order was interpreted in terms of a competition between irradiationinduced disordering (arising from replacement sequences along close-packed directions or f_+omrandom recombination of point defects) and irradiation-enhanced reordering (due to the migration of point defects which promote atomic mobility). Several calculations have formulated the kinetics of order-disorder transformations under irradiation on the basis of a model of chemical reaction rates [6-91. We summarize below the main equations which will be used for a quantitative investigation of disordering kinetics. The Long Range Order (LRO) parameter will be defined by
s-
c; - CA 1-
c* -
c;- CB l-cB
where c is the fmction of w sites occupied by X atoms; C, is the atom traction of element X in the alloy. For a stoichiometric alloy, S = 1 when all a (resp. b) sites are occupied by A (resp. B) atoms.
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In N&Al alloys, we have shown [9] that the total disordering rate can be expressed by dS . e+S + K,(l dt
- S)2
(1)
E is the number of replacements per displacement, 4 is the atomic displacement rate (in dpa.s-‘), K,, is a reordering rate constaut which depends on the concentration and the mobility of defects. In a stationary regime of defect elimination, a solution of this equation can be written [9] as:
Where
f(s)-
1 - sms
(3)
s _ s cm
S- is the steady-state degree of order; it is related to
E
and Ko by setting equation (1) to zero:
Ko
!b
(1 - s+2
(4)
=z
The aim of the present work was to study the effect of small boron additions on the effective disordering kinetics under electron irradiation. The results were analysed on the basis of the above theoretical model. ExDerimental
The intermetallic alloys chosen for this investigation were a binary Ni,,& material and a boron-doped Ni &la (0.lwt% B) material; boron should be in solid solution in this latter material since Liu et al. [lo] have shown that the solubility of boron exceeds 0.2 wt % in Ni 3 Al with 24 at. % Al. The compositions of the alloys are given in table 1. They were prepared by melting together weighed quantities of high purity Ni, Al and N&B in an inductive plasma furnace. The N&A&., alloy was homogenized by annealing for 40 hours at 1323K under a vacuum of 6.10”’ Pa. Specimens, 3mm in diameter and 0.2 mm thick, were prepared by diamond sawing, spark-machining and mechanical polishing. They were subsequently annealed for 24 hours at 1273K under a vacuum of 10” Pa, and furnace cooled. The N&Al, (0.1 wt%B) alloy was annealed for 5 hours at 1273K under a vacuum of 6.10d Pa and rolled to sheet, 0.2 mm in thickness. Specimens were obtained by punching. These were annealed for 5 hours at 1273K then for 5 hours at 1023K under a vacuum of 10” Pa and furnace cooled. All specimens were thinned with a Tenupol electropolisher at 40 V in a mixture of ethanol (70 vol.%), 2butoxyetbanol(20%) and perchloric acid (10%). TABLE 1 Compositions
Alloy Alloy Nicks Ni&,(o.lwt%B) *referred to (Ni + Al)
Ni(at%)
Al(at%)
B(wi%)
76.04
23.96
-
76.04*
23.96*
0.1
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EFFECTOF BORON
1381
-b-
-a-
Figus 1. Bright-field micrographs of i&metallic mmpounds imdiakd with a displacement rate of 0.9 x lo” dpa.s”. a: Ni,&., (0.1 wt?h B) alloy irradiated at 293K. b: Ni,&, alloy irradiated at 2933 c: Ni,6Altialloyirradited at 4OOK
Irradiation experiments were performed in the 1MeV High Voltage Electron Microscope of the SRMA/CE Saclay equipped with a double-tilt heating stage, at temperatures of 293K and 41OK, with electron flux densities of 0.23 to 11.8 x 10’ge/cmz.s Mg to displacement rates of 0.09 x 10’ to 4.7 x 10” dpas’. Atomic displacement rates were obtained by measuring the transmitted current density and using a displacement cross-section of 40 barns (threshold displacement energy Td = 24 eV [ 111). Specimens were irradiated along orientations a few degrees away from [0 121, [0 1l] or [ 1111. These are sticiently close to minimize a possible effect on the disordering kinetics. Temperature was measured using a thermocouple in contact with the specimen. At regular intervals of time during irradiation, the electron beam was defocused and a picture of the diffraction pattern was taken. Bright field and dark field images were often recorded to follow the evolution of the microslructure. The intensities of diffraction spots were measured on the film using a two-beam microdensitometer. The ratio between the instantaneousLRO parameter S at time t and the LRO parameter So (at t = 0), S/So, was calculated from the intensity of superlattice and fundamental dif&acted beams using the well known relationship, in kinematical conditions of diffraction: -IS Se
(Is/If), [ W9,
l/2
_ cl1
(5)
This can be written as S2= k (Is/If),. The constant k depends on the chosen reflections [ 121. In the following, S is the average of-the values calculated from the intensities of di&rent superlattice spots, normalized to the average intensity of their two neighbouring fundamental spots. Results In the initial conditions, bright field images and aaction
patterns showed that the alloys were single-phased and fully recrystallized, although a few dislocations were observed in most of them. Typical micro-
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EFFECT OF BORON
3
- a
0
1
dose (dpa)
3
-b-
4
0
1
dose (dpa)
2
Figure 2. a: Dose depe&nce of the Long-Range Order parameter in Ni,&, (squares) and Ni,& (0.1 wt%B) alloys irradiated with adisplacemntrate sf 0.9 x 10” dpa.s”, at 293 K (Ml symbols) or at 410 K (open symbols). b: Representation of equation 2 (see text), leading to a detemunation of E, in Ni,& (0.1 wt% B) irradiated at 293 K with a displacement rata of 0.9 x 105 dpa.s”.
structures, observed during irradiation to displacements of = 1 $a at a displacement rate of 0.9 x low3 dpa.s-’ , are shown on the bright-field images of figure 1 for the alloys Ni,&lz4 and N&Al,, (0. lwt?X~B) irradiated at 293K, and for the alloy Ni,,&, irradiated at 4 1OK. For investigating the disordering kinetics, the LRO parameter was determined by using equation (5). In the early stages of irradiation of the Ni7,$& alloy at 293K and for displacement rates 0.09 x 10 -3and 0.9 x 10e3dpas’ the calculation of the S/So ratio from some of the superlattice spots lead to values greater than 1; this is not physically acceptable since such alloys have a LRO parameter already close to unity a& the appropriate thermal treatments. This effect might be related to an enhancement of the di@acted intensities, due to the creation of irradiation-induced defects, which could, at the beginning of the irradiation, overwhelm the weak decrease in intensity caused by the progressive disordering of the alloy. Therefore only reflections which presented a systematic decrease in intensity were used. Figure 2 shows typical evolutions of the LRO parameter with irradiation dose in the two alloys. In most of cases, the LRO parameter decreased rapidly to reach a steady-state value S 1 which depends on the irradiation conditions (table 2). However, in the case of the N&Al,, (O.lwt%B) alloy irradiated at 293K with a displacement rate of 0.09 x 10” dpa/s, the steady-state conditions were not reached even for the longest irradiation time, resulting in a larger uncertainty on S-.
Microstructural
Evolution
At both temperatures and in the two alloys, dislocation loops formed and grew for increasing doses. The comparison between figs la and lb shows that, at 293K, the loops are smaller in the boron-doped ahoy. Figures 1b and 1c show that at the same displacement rate and at comparable doses, the size of dislocation loops is larger and their density is smaller in Ni7&12,at 410K than at 293K. These loops, formed during
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TABLE 2 Steady-state Long Range Order Parameter S- for Diierent Irradiation Conditions, Obtained by Fitting Equation (2) to the Experimental Data. Ni,Jl~(O.lwt%B)
Ni&& 4 (lo” dpah)
0.9
0.09
2.9
4.7
0.09
0.9
2.9
0.5%=0.07
0.36+XM4
0.2wo.02
0.56*0.06
0.67HI.02
Sm at 293K
~ 0.72*0.04
0.74M.02
-
0.5UO.03
Smat410K
-
0.9OM.02
0.67M.02
-
-
Their size and density are related to the nucleation and and result from the defect di.fCsion kinetics. The effect of temperature is as expected since growthPro== diffusion should be accelerated at higher temperatures, leading to smaller stationary defect concentrations and, consequently, to smaller loop nucleation rates. The comparison between the undoped and boron-doped alloys at the same temperature shows that the defect dithtsionrate is greater in pure Ni,&lz4. This suggests that boron traps point defects and inhibits their difhrsion.
imdiatiou, are attributed to point defect condensation.
Kinetics of Order Changes The approach to steady-state: determination of the disordering efjciency. The disordering rate can be characterized by detenmning the number of replacements per displacement E from the decreasing part of S/So (4) curves. This was performed by fitting equation (2) to the experimental data, thus obtaining E and Sm. In order to check the validity of this relationship, figure 3 shows a plot of Ln cf(s)l , which should be 1 + sm proportional to 41.Within experimental uncertainty, a straight line is actually obtained; its slope yields an E value (1.3) cousistent with the one obtained by the fit of equation (2). In some cases, the number of experimental points in the decreasing part of the S(4) was not sufficient to calculate an accurate value of e, due to the fact that the steady state was too rapidly established. The values of E obtained in several irradiation conditions in the two alloys, are given in table 3. The values, 1 to 3 replacements per displacement, are comparable to those reported in Ni,Al alloys [3]. The data show that there is no systematic effect of boron additions on E and, therefore, on the disordering rate. Steadv-State
Conditions: Information on the Reordering Processes
Ni7,JlU exhibits high values of the steady-state order-parameter Sm even at the lower temperature and at high displacement rates so that, taking into account the experimental uncertainties, a significant temperature dependence of Sa is not clearly apparent in this material This observation suggests that the temperature range investigated in the present work (Tz293K) corresponds to free vacancy migration, leading to high reordering rates and high S- values. This agrees with the results of Liu and Mitchell [:3] in Nid; for a displacementrate of the same order of magnitude as ours, they have shown that the curve SW(T) reached a high level above 293 K, then did not depend any more on temperature.
Eficectrofirmdia&ion temperature.
The boron-doped alloy exhibited low values of S=Jat 293K and high values at 4 1OK. This suggests that two di&rent reordering mechanisms are operative at these temperatures, e.g. reordering by the migration of interstitial-typedetects at 293K, and by vacancies at 41 OK. By comparison with the results obtained in the Ni&l,, alloy, the effect of boron might be explained as follows:
E@cts of boron additions.
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TABLE3 Number of Replacementsper Displacemente Calculatedfrom the InitialPart of the S(+) Curves by FittingEquation(2) to the Data.N.D. Meansthat e was not Calculated, Due to the Fact that the SteadyStatewas too RapidlyEstablished. Ni,,+&(O.1wiXB)
NW, I$(10” dpa.s-’
0.09
0.9
e at 293K
N.D.
1.7*0.4
e at 41OK
-
3.Qo.4
2.9 0.8rtO.2
4.7
0.09
0.9
2.9
0.7rtO.2
N.D.
l.lrtO.2
3.4Hl.4
l.UO.3
0.7M.2
-
-
- At 293K boron would interact with vacancies by forming a vacancy-boron complex, so that vacancies are immobilized or, at least, become less mobile by comparison with the case of pure Ni+&. As a result, reordering is mainly due to split interstitials or/and low-mobility vacancy-boron complexes; this leads to low S=Jvalues. - At 41OK the vacancy-boron complexes would be dissociated and vacancies become sufliciently mobile to promote an important reordering. The onset of effective vacancy migration is thus shifted to a higher temperature range by small additions of boron. Similar trapping effects have been previously discussed by Takaki et al. [13] for electron-irradiation induced defects in iron in the presence of interstitial carbon atoms; It was concluded that successive reactions took place at increasmgtemperatures: formation of vacancy-carbon complexes by vacancy migration, further growth by carbon migration, and finally break-up of these clusters. Conclusions
The effects of boron additions (0.1 w-t%)on the kinetics of atomic order changes in a Ni7&l2Ainter-metallic compound, under 1 MeV electron irradiation, were investigated at temperatures of 293K and 410K and displacementrates of 0.09 x 10 -3to 4.7 x 10 -3dpa.s-‘. In these irradiation conditions, a state of residual order was obtained for long irradiation times, characterized by a steady state order parameter S-; it corresponds to a competition between two opposite features: irradiation disordering and thermal reordering enhanced by irradiation. Boron additions did not affect the efficiency of irradiation-induced disordering: the disordering crosssection (or, equivalently,the number of replacements per displacement a) were comparable with and without a boron addition. By contrast, the Smvalues at 293K were much lower in the alloy containing boron. Since boron does not change the disordering rate, the large difference between the values obtained in undoped and in boron-doped alloys shows that the reordering rate is strongly reduced by the presence of boron. Thus, boron modifies the mobility of the defects responsible for the irradiation-enhanced diffusion. The data on dislocation-loop size (section 4.1) and the reordering kinetics suggest that vacancies are trapped by boron at low temperatures and immobilized, probably by the formation of a boron-vacancy complex. The e&ct becomes weaker at higher displacementratea and higher temperatoms,probably due to the boron-vacancy complexes becoming unstable. It is proposed that two difFeremreordering mechanisms may be operative at 293 K, according to the presence of boron: reordering is promoted by vacancy migration in the N&J& alloy, whereas in the Ni,,&., (0. lwt%B) alloy, it is promoted by the migration of split-interstitials or/and of low-mobility vacancy-boron complexes.
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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
RG. Davies and N.S. Stoloff, Trans. Met. Sot. AIME 233,714 (1965). P.H. Thornton, RG. Davies and T.L. Johnson, Metall. Trans. 1,207 (1970). H.C. Liu and T.E. Mitchell, Acta Metall. 31,863 (1983). N. Njah, D. G&on and 0. Dimitrov, J. Nucl. Mat. 199,237 (1993). K. Aoki and 0. Izumi, J. Jap. Inst. Metals. 43,119O (1979). KY. Liou and P. Wilkes, J. Nucl. Mat. 87,317 (1979). R Zee and P. Wilkes, Phys. Status Solidi A42,463 (1980). S. Banerjee. and K. Urban, Phys. Status Solidi A81.14S (1984). N. Njah, J. Nucl. Mater. 170,232 (1990). C.T. Liu, CL White, J./L Horton, Actametall. 33,213 (1985). H.C. Liu and T.E. Mitchell, J. Nucl. Mat. 107,318 (1982). M.Z. Hameed, R.E. Smalhnan and M.H. Loretto, Phil. Msg. A, 46,707 (1982). S. Takaki, J. Fuss, H. Kugler, U. Dedek and H. Schultz, Radiation Ef%cts, 79,87 (1983).
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THE EFFECT OF BORON ADDITIONS ON IRRADIATION-INDUCED ORDER CHANGES IN N&Al INTERMETALLIC COMPOUNDS N. Njah’, D. Gilbon’ and 0. Dimitrog ‘Laboratoire de l’Etat Solide, Faculte des Sciences, Route de Soukra Km 4, Sfax (Tunisi.i). ‘CEA/DTA/DECM/SRMA, CE Saclay, F-9 119 1 Gif-sur-Yvette Cedex (France). 3Centre d’Etudes de Chimie MCtallurgique, 15 rue G. Urbain, F-94407 Vitry Cedex (France) (Received March 15,1995) (Revised May 29,1995) Introduction
N&Alis a long-range ordered alloy with an L 1z structure, which retains a highly ordered state up to its melting temperature. It is of potential interest for high-temperature applications, particularly since it exhibits a positive temperature dependence of the yield-strength [ 1,2]. Also, Ni,Al-based intermetallic compounds form the y ’ hardening phase of y I y’ nickel alloys, which are often used in a radiation environment. Disordering under irradiation has been investigated either in binary Ni,Al or internary Ni,(Al, Ti) alloys [3,4], but there are no studies concerning the effects of interstitial additions, especially boron which is often used as a ductilising element [5]. During irradiation it was observed that the long-range order parameter S decreased rapidly with increasing dose, then reached a steady-state value depending on the irradiation parameters. This steady-state degree of order was interpreted in terms of a competition between irradiationinduced disordering (arising from replacement sequences along close-packed directions or f_+omrandom recombination of point defects) and irradiation-enhanced reordering (due to the migration of point defects which promote atomic mobility). Several calculations have formulated the kinetics of order-disorder transformations under irradiation on the basis of a model of chemical reaction rates [6-91. We summarize below the main equations which will be used for a quantitative investigation of disordering kinetics. The Long Range Order (LRO) parameter will be defined by
s-
c; - CA 1-
c* -
c;- CB l-cB
where c is the fmction of w sites occupied by X atoms; C, is the atom traction of element X in the alloy. For a stoichiometric alloy, S = 1 when all a (resp. b) sites are occupied by A (resp. B) atoms.
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EFFECTOF BORON
In N&Al alloys, we have shown [9] that the total disordering rate can be expressed by dS . e+S + K,(l dt
- S)2
(1)
E is the number of replacements per displacement, 4 is the atomic displacement rate (in dpa.s-‘), K,, is a reordering rate constaut which depends on the concentration and the mobility of defects. In a stationary regime of defect elimination, a solution of this equation can be written [9] as:
Where
f(s)-
1 - sms
(3)
s _ s cm
S- is the steady-state degree of order; it is related to
E
and Ko by setting equation (1) to zero:
Ko
!b
(1 - s+2
(4)
=z
The aim of the present work was to study the effect of small boron additions on the effective disordering kinetics under electron irradiation. The results were analysed on the basis of the above theoretical model. ExDerimental
The intermetallic alloys chosen for this investigation were a binary Ni,,& material and a boron-doped Ni &la (0.lwt% B) material; boron should be in solid solution in this latter material since Liu et al. [lo] have shown that the solubility of boron exceeds 0.2 wt % in Ni 3 Al with 24 at. % Al. The compositions of the alloys are given in table 1. They were prepared by melting together weighed quantities of high purity Ni, Al and N&B in an inductive plasma furnace. The N&A&., alloy was homogenized by annealing for 40 hours at 1323K under a vacuum of 6.10”’ Pa. Specimens, 3mm in diameter and 0.2 mm thick, were prepared by diamond sawing, spark-machining and mechanical polishing. They were subsequently annealed for 24 hours at 1273K under a vacuum of 10” Pa, and furnace cooled. The N&Al, (0.1 wt%B) alloy was annealed for 5 hours at 1273K under a vacuum of 6.10d Pa and rolled to sheet, 0.2 mm in thickness. Specimens were obtained by punching. These were annealed for 5 hours at 1273K then for 5 hours at 1023K under a vacuum of 10” Pa and furnace cooled. All specimens were thinned with a Tenupol electropolisher at 40 V in a mixture of ethanol (70 vol.%), 2butoxyetbanol(20%) and perchloric acid (10%). TABLE 1 Compositions
Alloy Alloy Nicks Ni&,(o.lwt%B) *referred to (Ni + Al)
Ni(at%)
Al(at%)
B(wi%)
76.04
23.96
-
76.04*
23.96*
0.1
Vol. 33, No. 9
EFFECTOF BORON
1381
-b-
-a-
Figus 1. Bright-field micrographs of i&metallic mmpounds imdiakd with a displacement rate of 0.9 x lo” dpa.s”. a: Ni,&., (0.1 wt?h B) alloy irradiated at 293K. b: Ni,&, alloy irradiated at 2933 c: Ni,6Altialloyirradited at 4OOK
Irradiation experiments were performed in the 1MeV High Voltage Electron Microscope of the SRMA/CE Saclay equipped with a double-tilt heating stage, at temperatures of 293K and 41OK, with electron flux densities of 0.23 to 11.8 x 10’ge/cmz.s Mg to displacement rates of 0.09 x 10’ to 4.7 x 10” dpas’. Atomic displacement rates were obtained by measuring the transmitted current density and using a displacement cross-section of 40 barns (threshold displacement energy Td = 24 eV [ 111). Specimens were irradiated along orientations a few degrees away from [0 121, [0 1l] or [ 1111. These are sticiently close to minimize a possible effect on the disordering kinetics. Temperature was measured using a thermocouple in contact with the specimen. At regular intervals of time during irradiation, the electron beam was defocused and a picture of the diffraction pattern was taken. Bright field and dark field images were often recorded to follow the evolution of the microslructure. The intensities of diffraction spots were measured on the film using a two-beam microdensitometer. The ratio between the instantaneousLRO parameter S at time t and the LRO parameter So (at t = 0), S/So, was calculated from the intensity of superlattice and fundamental dif&acted beams using the well known relationship, in kinematical conditions of diffraction: -IS Se
(Is/If), [ W9,
l/2
_ cl1
(5)
This can be written as S2= k (Is/If),. The constant k depends on the chosen reflections [ 121. In the following, S is the average of-the values calculated from the intensities of di&rent superlattice spots, normalized to the average intensity of their two neighbouring fundamental spots. Results In the initial conditions, bright field images and aaction
patterns showed that the alloys were single-phased and fully recrystallized, although a few dislocations were observed in most of them. Typical micro-
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Vol. 33, No. 9
EFFECT OF BORON
3
- a
0
1
dose (dpa)
3
-b-
4
0
1
dose (dpa)
2
Figure 2. a: Dose depe&nce of the Long-Range Order parameter in Ni,&, (squares) and Ni,& (0.1 wt%B) alloys irradiated with adisplacemntrate sf 0.9 x 10” dpa.s”, at 293 K (Ml symbols) or at 410 K (open symbols). b: Representation of equation 2 (see text), leading to a detemunation of E, in Ni,& (0.1 wt% B) irradiated at 293 K with a displacement rata of 0.9 x 105 dpa.s”.
structures, observed during irradiation to displacements of = 1 $a at a displacement rate of 0.9 x low3 dpa.s-’ , are shown on the bright-field images of figure 1 for the alloys Ni,&lz4 and N&Al,, (0. lwt?X~B) irradiated at 293K, and for the alloy Ni,,&, irradiated at 4 1OK. For investigating the disordering kinetics, the LRO parameter was determined by using equation (5). In the early stages of irradiation of the Ni7,$& alloy at 293K and for displacement rates 0.09 x 10 -3and 0.9 x 10e3dpas’ the calculation of the S/So ratio from some of the superlattice spots lead to values greater than 1; this is not physically acceptable since such alloys have a LRO parameter already close to unity a& the appropriate thermal treatments. This effect might be related to an enhancement of the di@acted intensities, due to the creation of irradiation-induced defects, which could, at the beginning of the irradiation, overwhelm the weak decrease in intensity caused by the progressive disordering of the alloy. Therefore only reflections which presented a systematic decrease in intensity were used. Figure 2 shows typical evolutions of the LRO parameter with irradiation dose in the two alloys. In most of cases, the LRO parameter decreased rapidly to reach a steady-state value S 1 which depends on the irradiation conditions (table 2). However, in the case of the N&Al,, (O.lwt%B) alloy irradiated at 293K with a displacement rate of 0.09 x 10” dpa/s, the steady-state conditions were not reached even for the longest irradiation time, resulting in a larger uncertainty on S-.
Microstructural
Evolution
At both temperatures and in the two alloys, dislocation loops formed and grew for increasing doses. The comparison between figs la and lb shows that, at 293K, the loops are smaller in the boron-doped ahoy. Figures 1b and 1c show that at the same displacement rate and at comparable doses, the size of dislocation loops is larger and their density is smaller in Ni7&12,at 410K than at 293K. These loops, formed during
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TABLE 2 Steady-state Long Range Order Parameter S- for Diierent Irradiation Conditions, Obtained by Fitting Equation (2) to the Experimental Data. Ni,Jl~(O.lwt%B)
Ni&& 4 (lo” dpah)
0.9
0.09
2.9
4.7
0.09
0.9
2.9
0.5%=0.07
0.36+XM4
0.2wo.02
0.56*0.06
0.67HI.02
Sm at 293K
~ 0.72*0.04
0.74M.02
-
0.5UO.03
Smat410K
-
0.9OM.02
0.67M.02
-
-
Their size and density are related to the nucleation and and result from the defect di.fCsion kinetics. The effect of temperature is as expected since growthPro== diffusion should be accelerated at higher temperatures, leading to smaller stationary defect concentrations and, consequently, to smaller loop nucleation rates. The comparison between the undoped and boron-doped alloys at the same temperature shows that the defect dithtsionrate is greater in pure Ni,&lz4. This suggests that boron traps point defects and inhibits their difhrsion.
imdiatiou, are attributed to point defect condensation.
Kinetics of Order Changes The approach to steady-state: determination of the disordering efjciency. The disordering rate can be characterized by detenmning the number of replacements per displacement E from the decreasing part of S/So (4) curves. This was performed by fitting equation (2) to the experimental data, thus obtaining E and Sm. In order to check the validity of this relationship, figure 3 shows a plot of Ln cf(s)l , which should be 1 + sm proportional to 41.Within experimental uncertainty, a straight line is actually obtained; its slope yields an E value (1.3) cousistent with the one obtained by the fit of equation (2). In some cases, the number of experimental points in the decreasing part of the S(4) was not sufficient to calculate an accurate value of e, due to the fact that the steady state was too rapidly established. The values of E obtained in several irradiation conditions in the two alloys, are given in table 3. The values, 1 to 3 replacements per displacement, are comparable to those reported in Ni,Al alloys [3]. The data show that there is no systematic effect of boron additions on E and, therefore, on the disordering rate. Steadv-State
Conditions: Information on the Reordering Processes
Ni7,JlU exhibits high values of the steady-state order-parameter Sm even at the lower temperature and at high displacement rates so that, taking into account the experimental uncertainties, a significant temperature dependence of Sa is not clearly apparent in this material This observation suggests that the temperature range investigated in the present work (Tz293K) corresponds to free vacancy migration, leading to high reordering rates and high S- values. This agrees with the results of Liu and Mitchell [:3] in Nid; for a displacementrate of the same order of magnitude as ours, they have shown that the curve SW(T) reached a high level above 293 K, then did not depend any more on temperature.
Eficectrofirmdia&ion temperature.
The boron-doped alloy exhibited low values of S=Jat 293K and high values at 4 1OK. This suggests that two di&rent reordering mechanisms are operative at these temperatures, e.g. reordering by the migration of interstitial-typedetects at 293K, and by vacancies at 41 OK. By comparison with the results obtained in the Ni&l,, alloy, the effect of boron might be explained as follows:
E@cts of boron additions.
Vol. 33, No. 9
EFFECTOF BORON
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TABLE3 Number of Replacementsper Displacemente Calculatedfrom the InitialPart of the S(+) Curves by FittingEquation(2) to the Data.N.D. Meansthat e was not Calculated, Due to the Fact that the SteadyStatewas too RapidlyEstablished. Ni,,+&(O.1wiXB)
NW, I$(10” dpa.s-’
0.09
0.9
e at 293K
N.D.
1.7*0.4
e at 41OK
-
3.Qo.4
2.9 0.8rtO.2
4.7
0.09
0.9
2.9
0.7rtO.2
N.D.
l.lrtO.2
3.4Hl.4
l.UO.3
0.7M.2
-
-
- At 293K boron would interact with vacancies by forming a vacancy-boron complex, so that vacancies are immobilized or, at least, become less mobile by comparison with the case of pure Ni+&. As a result, reordering is mainly due to split interstitials or/and low-mobility vacancy-boron complexes; this leads to low S=Jvalues. - At 41OK the vacancy-boron complexes would be dissociated and vacancies become sufliciently mobile to promote an important reordering. The onset of effective vacancy migration is thus shifted to a higher temperature range by small additions of boron. Similar trapping effects have been previously discussed by Takaki et al. [13] for electron-irradiation induced defects in iron in the presence of interstitial carbon atoms; It was concluded that successive reactions took place at increasmgtemperatures: formation of vacancy-carbon complexes by vacancy migration, further growth by carbon migration, and finally break-up of these clusters. Conclusions
The effects of boron additions (0.1 w-t%)on the kinetics of atomic order changes in a Ni7&l2Ainter-metallic compound, under 1 MeV electron irradiation, were investigated at temperatures of 293K and 410K and displacementrates of 0.09 x 10 -3to 4.7 x 10 -3dpa.s-‘. In these irradiation conditions, a state of residual order was obtained for long irradiation times, characterized by a steady state order parameter S-; it corresponds to a competition between two opposite features: irradiation disordering and thermal reordering enhanced by irradiation. Boron additions did not affect the efficiency of irradiation-induced disordering: the disordering crosssection (or, equivalently,the number of replacements per displacement a) were comparable with and without a boron addition. By contrast, the Smvalues at 293K were much lower in the alloy containing boron. Since boron does not change the disordering rate, the large difference between the values obtained in undoped and in boron-doped alloys shows that the reordering rate is strongly reduced by the presence of boron. Thus, boron modifies the mobility of the defects responsible for the irradiation-enhanced diffusion. The data on dislocation-loop size (section 4.1) and the reordering kinetics suggest that vacancies are trapped by boron at low temperatures and immobilized, probably by the formation of a boron-vacancy complex. The e&ct becomes weaker at higher displacementratea and higher temperatoms,probably due to the boron-vacancy complexes becoming unstable. It is proposed that two difFeremreordering mechanisms may be operative at 293 K, according to the presence of boron: reordering is promoted by vacancy migration in the N&J& alloy, whereas in the Ni,,&., (0. lwt%B) alloy, it is promoted by the migration of split-interstitials or/and of low-mobility vacancy-boron complexes.
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EFFECTOF BORON
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
RG. Davies and N.S. Stoloff, Trans. Met. Sot. AIME 233,714 (1965). P.H. Thornton, RG. Davies and T.L. Johnson, Metall. Trans. 1,207 (1970). H.C. Liu and T.E. Mitchell, Acta Metall. 31,863 (1983). N. Njah, D. G&on and 0. Dimitrov, J. Nucl. Mat. 199,237 (1993). K. Aoki and 0. Izumi, J. Jap. Inst. Metals. 43,119O (1979). KY. Liou and P. Wilkes, J. Nucl. Mat. 87,317 (1979). R Zee and P. Wilkes, Phys. Status Solidi A42,463 (1980). S. Banerjee. and K. Urban, Phys. Status Solidi A81.14S (1984). N. Njah, J. Nucl. Mater. 170,232 (1990). C.T. Liu, CL White, J./L Horton, Actametall. 33,213 (1985). H.C. Liu and T.E. Mitchell, J. Nucl. Mat. 107,318 (1982). M.Z. Hameed, R.E. Smalhnan and M.H. Loretto, Phil. Msg. A, 46,707 (1982). S. Takaki, J. Fuss, H. Kugler, U. Dedek and H. Schultz, Radiation Ef%cts, 79,87 (1983).
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