MOTA

775

Journal of Nuclear Materials 179-181 (1991) 775-778 North-Holland

Microstructural examination of simple vanadium alloys irradiated in the FFTF/MOTA S. Ohnuki I, D.S. Gelles *, B.A. Loomis 3, F.A. Gamer * and H. Takahashi ’ ’ Faculty of Engineering, Hokkaido Uniuersity, Sapporo 060, Japan ’ Battelie Pacific Northwest Laboratory, Richland, WA 99352, USA ‘Argonne National Laboratory, Argonne, IL 60439, USA

A series of V alloys including pure and impure V and alloys with MO, W, Cr, Ti and Y have been examined by transmission electron microscopy following irradiation in the FFTF/MOTA to 14 dpa at 870 K. All specimens were found to have developed features typical of radiation damage. Void, dislocation and precipitate structures were different even for specimens of similar composition. However, in general, specimens displayed precipitate development, often indicated by (001) stacking faults. About half of the alloys showed voids. Dislocation structures included large loops and dislocation tangles but much smaller loops similar to black spot damage could be frequently identified. 1. Introduction V alloys are candidate materials for first wall fusion reactor applications based on low induced activity and heat resistance response [1,2]. However, resent results have indicated that neutron irradiation damage in V alloys leads to degradation of mechanical properties [3-81. Some alloys which have a potential use, such as V-15Cr-5Ti and Vanstar, have been developed and examined. In order to understand the cases of property degradation and to identify most promising alloy compositions, it is necessary to understand the combined effect of neutron damage and composition on microstructure. In this study a number of simple V alloys are examined after irradiation in Materials Open Test Assembly of the Fast Flux Test Facility (FFTF/MOTA) at 870 K. 2. Experimental procedure Twelve alloys were examined by 200 kV transmission electron microscopy (TEM). Compositions of the speci-

mens are listed in table 1. Specimens had been irradiated in the form of 3 mm disks immersed in Li and contained in MO alloy capsules. Irradiation was performed in the FFTF/MOTA during cycle 7 and 8 in level 5. The operation temperature was 874 f 20 K. However, during irradiation cycle 7, specimens underwent a temperature of 1071-1079 K for 50 min. The accumulated neutron fluence was 2.4 x 1O26 n/m2 (E > 0.1 MeV) corresponding to a dose of 14 dpa. One specimen of each alloy was assigned for density change measurement and the second was provided for TEM. 3. Results Density measurement data are included in table 1. It can be noted that swelling was on the order of 1% or less for all irradiated specimens. The alloys contained with high Cr levels showed the largest swelling, but Ti addition appeared to reduce swelling. The purest V had a complex microstructure. Voids of about 50 nm in diameter were found at low density

Table 1 Compositions and results of density change measurement ANL ID

Material

Interstitial concentration (wt. ppm)

BL-20 BL-19 BL-1 BL-2 BL-4 BL-5 BL-25 BL-11 BL-12 BL-15 BL-8

Pure V Less-pure V V-4.0Mo v-8.6W V-lO.OCr-O.lAl V-14.1Cr-0.3Al V-14.4Cr-0.3Ti-0.3Al V-4.9Ti V-9.8Ti V-17.7Ti V-20Ti-5Y

570 1101 230 300 530 330 390 1820 1670 830

0

N 110 161 73 150 76 69 64 530 390 160

C 120 360 90 120 240 200 120 470 450 380

‘) ND: not detected. 0022-3115/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

Swelling (%) a) Si

Fe

325 300 110 59 -=z50 < 50 < 50 220 245 480

Cl00 < 100
ND ND ND 0.51 1.64 1.16 0.26 0.03 0.01 0.01 ND

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S. Ohnuki et al. / Examinution

non-uniformly distributed within grains. Much smaller voids or bubbles on the order of 5 nm in diameter were distributed more uniformly at higher density. At least

of simple oanndium alloys

two types of precipitates could be identified, one blocky in shape with sizes of about 30 nm, and the other in the form of targe platelets less than 10 nm in thickness and

TYI cal microstructures in vanadium and its alloys irradiated at 870 K to 2.4~ 102’ n/ml. (a, b) pure V in slightly differe nt g (h) V-4.9%, (i) V-17.7Ti and (i1 k‘-2OTi- -5Y. ion, c) impure V, (d) V-4M0, (e) V-8.6W, (f) V-IOCr), (g) V-14.4Cr-.0.3Ti, Line markers indicate 100 nm each.

S. Ohnuki et al. / Examinaiion

several hundred nm long. The dislocation contained a network of dislocation lines, and a high density of black could also be imaged in strain contrast. Examples of these features are shown in figs. la and lb, which show the smaller voids at high density and both the blocky and planar precipitates, respectively. However, a less pure V behaved quite differently. No voids were found, and the dislocation structure had not yet developed into a network and the density of platelet precipitates was higher. Fig. lc shows platelet precipitates in g = (110) contrast. Examples of loop formation near dislocation line segments, large dislocation loops, stacking fault features in three orientation, and black spot damage can be identified. The faulted features were found to be elongated in (001) directions. Therefore, heat to heat differences in impurity can delay irradiation-induced microstructural development and void formation. Addition of MO to V appeared to reduce, but not eliminate cavitation and to promote phase instability. An example V-4Mo is given in fig. Id showing the structure in void contrast. Blocky darker features associated with irregularly shaped cavities and smaller darker features can be identified throughout the structure. At higher magnification, larger irregularly-shaped cavities are coupled to a darker region believed to be a precipitate particle, and smaller spherical bubble-like features are scattered randomly through the matrix. It has not yet been possible to identify the precipitate that is forming; both intermetallic or interstitial compound are possibilities. The dislocation structure consists of climbing perfect dislocations, probably of Burgers vector a/2(111), black spot damage and large planar faulted features. Therefore, the dislocation evolution in V-MO appears to be similar to that found in the impure V. Fig. le shows the void structure in V-W at low magnification. A low density of voids can be identified. The largest voids which size was about 50 nm appear cuboidal whereas the smaller voids are more spherical, indicative of solute coatings on the cuboidal voids. The faint darker images show dislocations in weak contrast. A high density of faulted features appear in orthogonal arrays indicative of (100) faults similar to those found in impure V and V-MO, but at higher density. For the dislocation structure using g = (200) contrast, both black spot damage and larger loops can be seen restricted to regions adjacent to the climbing dislocations. Therefore, segregation of W to dislocation is expected to play an important role in microstructural evolution in this alloy. V-Cr alloys were found to be the highest swelling in this experiment. The V-1OCr alloy contained voids distributed uniformly with void diameter generally 100 nm or less, but in grain boundary regions, several voids were found as large as 200 nm. However, void sizes as small as 5 nm were found, indicating that nucleation was continuing and steady state microstructures may

of simple vanadium alloys

711

not have developed by 14 dpa. Voids ranging in size from 5 to 100 nm are shown in fig. If. The voids are clearly facetted, with both cuboidal (001) and dodecahedoral (110) surface showing. Such shape variations are generally ascribed to solute segregation. The dislocation structure using g = (002) contrast consists of a tangle of climbing dislocations, dislocation loops and black spot damage. Therefore, Cr additions promote void development. The increase in Cr content to 14.4% resulted in very little change in microstructure. Additions of 0.3Ti to V-14.4Cr alloy were found to have a large effect on void swelling response, that is, void sizes were greatly reduced. In typical area, as shown in fig. lg, no voids can be seen but there exists a coarse corduroy-like structure. In some regions containing large voids, the presence of hundreds of cavities as small as 2 nm in diameter are seen. Therefore, the observed reduction in swelling is apparently due to enhanced void or bubble nucleation. The dislocation structure consists of climbing dislocations, small loops and some black spot damage. In V-Ti alloys Ti additions resulted in irradiated microstructures with low swelling, and a tendency for phase instability. Examples for V-4.9Ti are given in fig. lh, which shows the major features of the microstructure in a (001) orientation using g = (110) contrast. The dislocation structure consists of a low density of a/2(111) dislocation line segments and a higher density of both long and short features elongated in (100) directions. The features elongated in (100) directions are expected to be associated with precipitation because (100) features do not otherwise form in irradiated V alloys. Larger precipitate structures are also present, decorated by small bubbles or cavities. These large precipitates are expected to be TiO,, present in the alloy prior to irradiation. Therefore, V-4.9Ti is found to be very swelling resistant, with cavities forming only in association with preexisting TiO,. A higher addition of 17.7% Ti produced striking microstructures. Precipitates grew both as large precipitates and as decorations on dislocations. An example of the microstructure near (111) orientation is given in fig. li at low magnification using g = (110). The dislocation structure consists of decorated climbing dislocation segments and loops. Large elongated platelets can be seen in three orientations. The climbing line segments are probably of Burgers vector type a/2(111) lying on (011) planes whereas the faulted loops lie on (001) planes and are probably of Burgers vector a/2(100) type. Several examples of multiple a/2(100) loop nucleation at the same site, can be identified as rings with unfaulted centers. Crystallographic analysis of thicker platelets in a thin foil verified a (001) habit plane, and demonstrated an hexagonal crystal structure with [OOOl] /][Oll] and [OIlO] ]I[211], and lattice parameters of c = 0.48 nm and a = 0.26 nm for hexagonal phase, in agreement with

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of srmple vanadium alloys

alpha Ti. Therefore, the microstructural development is controlled by the segregation of Ti to climbing dislocations and the precipitation on (001) planes. Large additions of both Ti and Y resulted in irradiated microstructures quite different from those in other alloys only containing Ti. Evidence for precipitation of alpha Ti was not found and dislocation structures were only of a/2(111) type. An example of the structure in V-20Ti-5Y alloy is given in fig. lj. A grain boundary is shown decorated with a blocky precipitate; in dislocation contrast on one side and in void contrast on the other. The dislocation structure consists of a tangle of climbing dislocations and small loops, and no void swelling is observed. Such features may arise due to segregation and precipitation on dislocations. Therefore, Y appears to prevent nucleation of alpha Ti on (001) planes. In same region, voids as large as 50 nm were decorated by precipitates. Such observations are in general associated with grain boundary migration and solute allowing early void nucleation and redistribution, growth. Therefore. it is likely that at higher dose. voids will nucleate and grow in this alloy.

sponse by causing formation of a very high density of small cavities. Also, different alloying additions are found to alter void shape. In general, voids are highly truncated, often appearing almost sphere-like. Example were pure V and Cr-containing alloys. However, alloys with W and Ti developed voids of cuboidal shape, and several alloys contained voids of unusual shape due to void-precipitate interactions, such as in VP20Ti-5Y. Dislocation evolution was also complex. In most cases examples of climbing perfect dislocation arrays with the standard a/2(1 11) Burgers vector were found. However, often smaller loops developed near some of the climbing dislocations, and black spot damage was also found. In many cases, faulted features associated with (001) planes were also found, for example, in impure V. V-MO, V-W, V-9.8Ti and V-17.7Ti. It appears likely that faults on {OOI } planes and black spot damage are indicative of precipitate formation, but further work is needed to verify that this is the case for each of the alloys studied.

4. Discussion

This work was performed laboration on FFTF/MOTA

Microstructural development from neutron damage in V and its alloys is found to be complex, and response can vary greatly as a result of minor changes in composition. For example, void swelling response following irradiation to 14 dpa at 870 K varies from void free structures in the less-pure heat of pure V to the high swelling in V-Cr. Intermediate cases included the purest of V, V-8.6W and V-4.9Ti, where uniform void swelling had not quite been reached, and V-4M0, V-9.8Ti and V-17.7Ti where swelling was very low, and voids were found associated with precipitate particles, either formed during irradiation or, as in the case of TiO,. probably present prior to irradiation. The most unusual case was that of V-14.4Cr-0.3Ti. where it appears that the addition of 0.3’%Ti has reduced the swelling re-

References

Acknowledgement

[l] [2] [3] [4] [5] [6] [7] [8] [9]

under the Japan/US irradiation program.

col-

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