Faulted defect aggregates in neutron-irradiated MgAl2O4 spinel

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Faulted defect aggregates in neutron-irradiated

MgAl2O4 spinel

L. Hobbs, F. Clinard

To cite this version:

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Faulted defect aggregates in neutron-irradiated MgA120, spinel

(*)

L. W. Hobbs (**) and F. W. Clinard Jr. (***)

(**) Case Western Reserve University, Cleveland, OH 44106, U.S.A.

(***) Los Alamos Scientific Laboratory, Los Alamos, NM 87545, U.S.A.

Resume. - On a etudit les agrkgats de defauts par microscopic electronique dans le spinelle MgA1204 s t ~ c h i o - mktrique, irradik aux neutrons avec des doses de 2,8 x loz5 n

k-'

(- 1,5 dpa) 8 1 015 K et

-

1,9 x loz6 n m1 (- 10 dpa) B 925 K et 1 100 K. A une dose plus ClevBe, on a trouvk des boucles de Frank de dislocation intersti- tielles dans les plans { 110

1

et { 111 ) ayant respectivement des vecteurs de Burgers b = 114 ( 110 ) (faute cationique) et b = 116

<

111

>

(faute cationique et anionique). Ce dernier conserve la stoechiomCtrie uniquement pour une distribution statistique des cations.

Abstract.

-

Single crystals of stoichiometric MgA1204 spine1 were neutron-irradiated to fluences (> 0.1 MeV) of 2.8 x loz5 n m-2 (- 1.5 dpa) at 1015 K and

-

1.9 x loz6 n m-2 (- 10 dpa) at 925 K and 1 100 K. The resulting point defect'aggregates were investigated using bright-field and weak-beam dark-field transmission electron microscopy. For the higher dose, faulted interstitial Frank dislocation loops were found on { 110 } and { 111 } planes with b = 114

<

110 ) (cation fault) and b = 116

<

111 ) (anion

+ cation fault) respectively.

The latter preserves stoichiometry only for a randomized cation distribution.

1 . Introduction. -The nuclear industry has cur- rent and projected need for refractory insulating solids characterized by low swelling and structural integrity in severe radiation environments [I-31. Density measurements [2] of fission-neutron irradiated material have indicated that certain complex ceramics, notably MgA1204, Y3Al5Ol2, Si2N20 and SiA10N7s exhibit low swelling to doses as high as 10 displacements per atom (dpa), in contrast to simpler ceramics such as Be0 and A1203 which swell catastrophically, and often anisotropically. Void swelling in ceramic solids typically occurs over the temperature range 0.2-0.5 T, (T,=melting point) [3]. MgA1204 spinel has the advantage of being cubic, and thus isotropic, and has a large and complex unit cell (a = 0.808 nm, 56 atomslunit cell). Single crystal MgA1204 additionally exhibits negligible change in thermal diffusivity even after 10 dpa [4], indicating that

a

large isolated point defect concentration does not remain after irradiation and that Frenkel defects either recombine efficiently or accumulate in aggregates which somehow do not lead to void swelling. We have sought evidence from TEM to distinguish these possibilities and thus explain the radiation resistance of MgA1204.

2. Irradiation and specimen preparation.

-

Single crystals of Czochralski-grown stoichiometric MgA1,04, obtained from Linde Division of Union

(*) Work performed under the auspices of the U.S. Department of Energy.

Carbide Corporation and containing

-

100 ppm Si, 20 ppm Fe, 5 ppm Ca and 8 ppm B impurity, were sawn to dimensions 10 mm x 10 mm x 0.5 mm and neutron irradiated in EBR-I1 to fast neutron

( > 0.1 MeV) fluences of 2.8 x loz5 n m-2 at 1015 1 4 0 K (0.42 T,) and

-

1.9 x n m-2 at 925

t-

20 K (0.38 T,) and 1 100

+

20 K (0.46 T,). These two fluences correspond to approximately 1.5 dpa and 10 dpa, based on an oxygen displacement energy of 130 eV 151. The resulting primary Frenkel defect spectrum is unlikely to have been stoichiometric, however [6]. TEN specimens were sectioned parallel to ( 111 }, thinned to electron transparency using argon-ion bombardment [7] and coated with

--

20 nm evaporated carbon to preclude charge acquisition in the electron beam [8].

3. Transmission electron microscopy.

-

Thinned discs (foil normal n = [I 111) were examined in conven- tional transmission [9] at 200 kV using bright-field and weak-beam dark-field [lo] imaging methods. Little or no aggregate damage was observable in the lower dose (1.5 dpa, 1 015 K) sample, but in those subjected to the higher dose (10 dpa ; 925 K, 1 100 K) two large defect aggregate species were observed

(A, B in figure 1) which are described separately below.

3.1 ( 110 ) LOOP ROSETTES.

-

Clusters of faulted dislocation loops on all six equivalent ( 110 } planes were observed to intersect centrally, forming rosettes ;

three of the loops are inclined by 350 to the (1 11)

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FAULTED DEFECT AGGREGATES IN NEUTRON-IRRADIATED MgAI,O, SPINEL C6-233

Fig. 1. - { 110 ) loops in (111) foil irradiated at 925 K and imaged in g = 2%. a ) Bright-field. b) Weak-beam dark-field showing fault fringes for { 110 } loops at A and inclined { 111 } loops at B.

foil plane at A in figure 1, the other three normal to the foil plane with their traces along ( 112 ). At 925 K, the mean loop size was

--

200 nm. At 1 100 K, the loops had grown to such large dimensions (> 1 pm) that they intersected the foil surfaces (Fig. 2). Contrast from the { 110 } loops using g = 2%-, 117- or 511- type reflections indicated a loop Burgers vector b along the ( 110 ) loop normals. The { 110 ) loops are therefore Frank [1 11 loops and represent addi- tion or removal of material on { 110 } planes.

The magnitude of the Burgers vector b (and thus of the fault vector R) was deduced from the presence of fault contrast for g = 220 and its absence for

g =

440

(Fig. 3) where g

.

b = g . R is an integer and

the phase shift across the fault 2 n. The fault is therefore a .n fault [12] with Burgers vector b = 114 ( 110

).

It is evident from figure 2 that this fault is not removed by an internal shear even when the loops become very large (> 1 pn). However, double layer loops at C, corresponding to nucleation of a second Frank loop on planes adjacent to an existing faulted loop, were observed which locally remove the faulted stacking sequence. The character of the { 110 } loops was deduced using the method of Groves and Kelly 1131 incorporating the precautions cited by Maher and Eyre [14]. The loops were determined to be of interstitial character and the faults extrinsic.

3.2 { 11 1 } LOOPS.

-

Figure 1 reveals as well loops (at B) inclined by 70° to the (111) specimen plane which exhibited fault fringes for a g = 220-

type reflection. Equivalent loops were found parallel to the (111) specimen plane for which g . b = 0 but

g.b x u # 0 (u = local tangent to the loop core) ;

these loops exhibit residual contrast except for a line normal to g where g . b x u = 0. This behavior indicates a loop Burgers vector b along [Ill]. These

{ 111 } loops are thus Frank loops also. They were, however, consistently smaller (< 100 nm diameter) and found in lower density than the { 110 } Frank loops formed at 925 K, and were absent in the 1 100 K samples.

The magnitude and character of the associated fault were deduced using image contrast for dark-field weak-beam diffraction conditions. For this determination, it was assumed that the fault vector R = 6 ( 111 ) was a simple 6 = 116, 114, 113 or 112

submultiple of the 12-layer { 11 1 ) stacking repeat. Fault contrast was observed for g = 220,311 and 131,

consistent with a b = 116 ( 111 ) loop fault and Burgers vector (see discussion,

5

4.3). A loop charac-

ter determination, carried out in parallel with the

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ions in a way which does not preserve the normal stacking sequence of { 110 } or { I l l ) planes in the spinel structure. The number of interstitials condensed (assuming stoichiometric condensation) was calculated from the measured total interstitial loop area and the magnitude of the Burgers vector and amount- ed to a stabilized interstitial concentration

c 2: 5 x for the 925 K sample (0.005

% of the

total displacements) and c = 9 x (0.009

%

of the total displacements) for the 1 100 K sample. These numbers do not differ greatly and confirm the suspected absence of a swelling peak in the 0.3-0.5 T ,

temperature range for spinel.

4.2 { ~ ~ O } L O O P S . - T h e 1 / 4 ( 1 1 0 ) { 1 1 0 } l o o p s can be explained as the condensation of two extra layers aa (or a@) in the idealized { 110 } planar stacking sequence of normal spinel

where the a layers contain 0 and A1 ions in the ratio AlO2, and the a and

fi

layers contain 0 , A1 and Mg ions in the ratio MgA102 (assuming no cation inversion). The 114 ( 110 ) { 110 } extrinsic fault is confi- gurationally equivalent to the pure shear 114 ( 172 ) { 110 ) and has been associated previously with both F I ~ . 2 - (1 11) foil of MgAI,O, neutron-irradiated to glide and climb dissociation of dislocations in stoichiometric and non-stoichiometric MgA1204 [15-191 and

-

1.9 x loz6 n m - 2 (- 10 dpa) _{with growth faults in MgA1204 [20], Fe304 [21] and} a t 1 100 K showing large faulted loops on { 110 ) planes mtersect- LiFesOs [22].

ing the foil surfaces and double-layer loops at C. Insertion of the two layers aa (or ap) preserves

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FAULTED DEFECT AGGREGATES IN NEUTRON-IRRADIATED MgA1204 SPINEL C6-235

stoichiometry, the degree of cation inversion and the anion stacking sequence, but introduces a cation fault which must be of sufficiently low energy (compared to the shear modulus) that the loops do not unfault by shear even when the loops become very large. Veyssikre et al. [23] calculated y = 3.4 J m F 2 as the

specific energy of this fault for strict cation ordering in normal MgA1204 spinel. However, the cation site occupation in synthetic MgA1204 particularly at the higher irradiation temperature (1 100 K) is almost certainly substantially inverted [24], and Veyssibre

et al. calculate a substantial lowering of fault energy in this case, to y = 1.4 J m-2 for a statistically ran-

dom distribution of cations. These are probably over- estimates 1191.

We do not yet understand why the { 110 ) loops nucleate in rosette form, but since no common junc- tion dislocation is required for an intersection (along

( 111 )) of two or more such 114 ( 110 ) faults [20], the loop area (and number of interstitials stabilized) are in this way maximized with respect to the dislocation core perimeter. Such fault junctions have been observed in Fe304 [21] and LiFe,O, [22] as well as in the present experiments. We suspect that the appea- rance of much larger ( 110 ) loops at l 100 K reflects the smaller number of stable nucleation sites a t the higher temperature. The lower of the two fault energies calculated above corresponds to

--

7 eV per MgA1204 unit condensed, or

-

1 eV per interstitial. Persistance of the fault amounts to failure of a 114 ( 112 ) partial dislocation to nucleate, sweep across the loop plane and so remove the fault.

4.3 { 1 1 1 ) LOOPS. - The idealized stacking of

{ 11 1 } planes in normal MgA1204 spinel occurs as the 12 layer sequence

1/3 ( 111 )

m

where a, b and c are oxygen layers, a,

P

and y are kagomt layers containing only A1 (in the ratio A13 : O4 to a neighboring oxygen layer) and a',

P',

y' are mixed layers containing A1 and Mg (in the ratio Mg2Al : O4 to a neighboring oxygen layer). Partial inversion substitutes Mg in the mixed layers for some of the A1 in the kagomC layers.

The stacking sequence in (2) has been annotated to indicate the extrinsic layer sequence associated with each of the four possible partial Burgers vectors b = 6

<

111 ), 6 = 116, 114, 113 or 112. These repre- sent the insertion of 2 , 3 , 4 or 6 ( 11 1

1

layers respectively. The Burgers vector b = 113 { 111 }, corresponding to insertion of four layers, preserves stoichiometry but introduces both a cation and an anion

fault. The b = 1/2 ( 111

>

fault produces only a cation fault but alters Mg/AI stoichiometry and charge balance; it also involves six layers, which makes it an unlikely candidate for nucleation as an extrinsic fault. The b = 114 ( 111 ) fault involves three layers and cannot therefore preserve anionlcation stoichiometry, leading to a large electrostatic fault energy term. The b = 116 ( 111 ) fault has the shortest Burgers vector, involving only two layers (oxygen

+

kagomC or oxygen

+

mixed) but introduces both anion and cation faults and alters Mg/Al stoichiometry and charge balance in normal spinel.

The difficulty with stoichiometry and charge balance is easily overcome for 116 ( 111 ) by partial inversion of the inserted cation layer. A choice exists between insertion of one oxygen layer

+

one kagome layer or one oxygen layer

+

one mixed layer. For the former, the fault composition is A1304 in normal spinel and A11.5Mgl,504 in inverse spinel; for the latter, it is Mg2A104 in normal spinel and A12.5Mg0.504 in inverse spinel. A random cation distribution preserves MgAI2O4 stoichiometry in both cases, which never- theless remain crystallographically distinguishable. For less-than-random cation distributions, the 116

( 11 1 ) loops offer a mechanism for local accommodation of a non-stoichiometric primary cation displacement spectrum, since the kagomblayer loop is Al- rich and the mixed layer loops Mg-rich for the case of normal spinel. Such loops could thus represent nucleation sites for the local decomposition of spinel into A1203 and MgO. These ( 111 } loops probably dissolve at higher temperatures in favor of the more stable { 110 ) loops.

4.4 RADIATION RESISTANCE. - By Way of COmpa- rison with MgA1204, A120, after 3 dpa at similar homologous temperatures exhibits an extensive void array [I] and a dense dislocation network derived from initially cation-faulted basal and prism plane loops which unfault at an early stage of irradiation [25]. These loops, stabilizing an interstitial concentration

--

4 %, accommodate

-

0.2

%

of the total displacements, compared to

--

0.01 % in spinel. The relative 'resistance of MgA1204 to defect aggregation may thus rest in the details of loop nucleation. The absence of void swelling can be attributed to the failure of the 'loops to unfault and develop into dislocation net- works; they therefore remain less than perfect interstitial sinks since the energy per added interstitial never drops below the fault energy. Vacancy-interstitial recombination thus remains the dominant mode of defect accommodation, and saturating defect kinetics inevitably ensue [9]. Support for this view comes from a parallel observation (Youngman, Hobbs and Clinard, in preparation) of void nucleation in polycrystalline MgA1204 spinel adjacent to grain boundaries, the latter apparently acting as far more efficient sinks for interstitials.

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faulted Frank dislocation loops on { 110 ] and to a lesser extent { 11 1 ) planes. The { 110 ) loops grow as rosettes involving mutually intersecting loops on all six equivalent { 110 ) planes with Burgers vectors b = 114 { 110 ) and incorporating a cation fault. The smaller ( 111 ) loops remain isolated, with b = 116

<

11 1 ) implying faulting on both anion and cation sublattices. Both loops represent condensation of Mg, Al and 0 interstitials, in stoichiometric proportions for the ( 110 ) loops but stoichiometri-

925 K and

-

9 x for 1 100 K irradiations. The

{ 110 ) loop size at 1 100 K is, however, roughly an order of magnitude larger than at 925 K. The faults are not easily removed by shearbbut only by double- layer loop nucleation, which may explain the resis- tance of MgA120, to void swelling.

Acknowledgment. - The authors acknowledge the contribution of Dana L. Rohr to a portion of the electron microscopy.

References [I] CLINARD, F. W., Jr., Critical Problems in Energy Production

(Academic Press, New York) 1976, pp. 141-63. [2] CLINARD, F. W., Jr., Ceramics for Application in Fusion

Systems, Paper R-I, Proc. First Topical Meeting on Fusion Reactor Materials, Miami Beach, Fla.. 29-31 January 1979; J. Nucl. Mat. 85-86 (1979) 393-404. [3] HOBBS, L. W., J. Amer. Ceram. Soc. 62 (1979) 267-78. [4] HURLEY, G. F., Structural and Thermal Changes in High Dose

Irradiated Ceramics, Paper T-8, op. cit. ref. [2]. [5] CRAWFORD, J. M., LEE, K. H. and WHITE, G. S., Bull. Am.

Phys. Soc. 22 (1978) 253.

[6] PARKIN, D. M. and COULTER, C. A,, Displacement Functions for Diatomic Materials, Paper H-6, op. cit. ref. [2], pp. 61 1-

615.

[7] BARBER, D. J., J. Mat. Sci. 5 (1970) 1-8.

181 HOBBS, L. W., An Introduction lo Analytical Microscopy, ed. Hren, J. J., Joy, D. C. and Goldstein, J. I. (Plenum Press, New York) 1979, pp. 437-80.

[9] HOBBS, L. W., Defects and their Structure in Non-Metallic Solids, ed. Henderson, B. and Hughes, A. E. (Plenum Press, New York) 1976, pp. 431-82.

[lo] COCKAYNE, D. J. H., 2. Naturforsch. 27a (1972) 452-60. 1111 FRANK, F. C., Proc. Phys. Soc. A 62 (1949) 202.

[12] VAN LANDUYT, J., GEVERS, J. and AMELINCKX, S., Phys. Status Solidi 7 (1964) 519-46.

[13] GROVES, G. W. and KELLY, A., Phil. Mag. 6 (1961) 1527-29; ibid. 7 (1962) 892

[14] MAHER, D. M. and EYRE, B. L., Phil. Mag. 23 (1971) 409-38. [15] HORNSTRA, J., Phys. Chem. Solids 15 (1960) 311-23. 1161 LEWIS, M. H., Phil. Mag. 17 (1968) 481-99.

(171 MITCHELL, T. E., HWANG, L. and HEUER, A. H., J. Mat. Sci. 11 (1976) 264-72.

WELSCH, G., HWANG, L., HEUER, A. H. and MITCHELL, T. E., Phil. Mag. 29 (1974) 1371-79.

[18] D u c ~ o s , R., DOUKAHN, N. and ESCAIG, B., J. Mat. Sci. 13 (1978) 1740-48.

DOUKHAN, N. and ESCAIG, B., J. Physique Lett. 35 (1974) L-181-84.

[19] DONLON, W. T., MITCHELL, T. E. and HEUER, A. H., Phil. Mag. 40 (1979) 351-66.

[20] Lewrs, M. K., Phil. Mag. 14 (1966) 1003-18.

[21] BAKER, G. S. and WHELAN, M. J., Microscopic Electronique 1970, ed. Favard, P. (Soc. Frangaise Micr. Electr., Paris) 1970, pp. 283-4.

[22] VAN DER BIEST, 0. and THOMAS, G., Phys. Status Solidi (a) 24 (1974) 65-77.

[23] VEYSSI~RE, P., RABIER, R., GAREM, H. and GRILH~, J., Phil. Mag. 38 (1978) 61-79.

[24] SCHMOCKER, U. and WILDNER, F., J. Phys. C . 9 (1976) L-235-37. 1251 MITCHELL, T. E., BARNARD, R. S., HOWITT, D. G. and HOBBS,