Structure and mechanical properties of irradiated magnesium aluminate spinel

ELSEVIER

Journal of Nuclear Materials 232 (1996) 59-64

Structure and mechanical properties of irradiated magnesium aluminate spinel R. Devanathan *, N. Yu, K.E. Sickafus, M. Nastasi Materials Science and Technology Division, Los Alamos National Laboratoo', Los Alamos, NM 87545, USA

Received 20 February 1996; accepted 15 May 1996

Abstract The relationship between structure and mechanical properties of MgA1204 spinel single crystals following 400 keV Xe 2+ irradiation at 100 K to doses up to 1 X 1020 ions/m 2 was examined. The structural changes in the irradiated layer were studied using electron diffraction from cross-sectionai samples. The nano-indentationtechnique was used to determine the mechanical properties. At low doses, the material transformed into a metastable crystalline phase characterized by the rearrangement of cations. At the onset of this transformation, the Young's modulus and hardness rose to values about 10 and 15% higher, respectively, than those of the unirradiated crystal. Upon further irradiation, the metastable crystal became amorphous. The Young's modulus and hardness of the amorphous state were about 30 and 60% less, respectively, than the corresponding values of unirradiated spinel. These results, in conjunction with the findings of a recent computer simulation study, provide important insights into the exceptional radiation resistance of magnesium aluminate spinel.

1. Introduction Fusion reactors of the future and accelerator-based technologies require radiation-resistant ceramics for structural and insulating applications. In order to design new materials for these applications, it is important to examine the structure-property relationships in irradiated ceramics. Magnesium aluminate spinel is an ideal material for such a study, because it is known to resist radiation damage when irradiated with energetic particles at elevated temperatures. Irradiation with fast neutrons [1,2] or a variety of ions [3,4], at or above room temperature, has been shown to produce negligible void swelling and no loss of crystallinity. The elastic properties of spinel have also been found to be unchanged under neutron irradiation at temperatures in the range 660-1025 K [5]. However, a recent study [6] reported, for the first time, the amorphization of spinel by Xe ion irradiation at 100 K. This transformation was preceded by the extinction of first-order Bragg reflections in the electron diffraction pattern.

* Corresponding author. Tel.: + 1-505 665 3936; fax: + 1-505 665 3935; e-mail: [email protected].

In an effort to understand the remarkable radiation resistance of spinel and the loss of this resistance at cryogenic temperatures, we have studied the structural evolution and mechanical behavior of spinel irradiated with 400 keV Xe 2+ at 100 K. This report presents evidence of phase transformations in low temperature ionirradiated spinel obtained using electron diffraction in conjunction with nano-indentation measurements. In addition, it discusses the nature and significance of these changes in the light of a recent computer simulation study [7] of the energetics of lattice defects in spinel.

2. Experimental details Synthetic MgA1204 single crystal wafers with (001) orientation were cooled to 100 K using liquid nitrogen and irradiated with 400 keV Xe 2+ ions at the Los Alamos Ion Beam Materials Laboratory. The details of the irradiation are discussed elsewhere [6]. The damage distribution and implanted ion profile were calculated using the TRIM [8] computer code and are shown in Fig. 1 for the highest dose used - - 1 X 1020 Xe2+/m 2. At this dose, the peak con-

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R, Devanathan et al. / Journal of Nuclear Materials 232 (1996) 59-64

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Fig. 1, The distributions of Xe ions and displacement damage in MgAI204 irradiated with 400 keV Xe 2+ to a dose of 1 × 10 20 ions/m 2. centration of Xe accumulated in the sample is about 2 at%. The thickness of the radiation damaged layer predicted by TRIM is about 200 nm. An average displacement threshold energy of 40 eV was used in the calculation. The elastic modulus, E, and hardness, H, of the irradiated layer were determined using a Nano-indenter II (Nano-indenter is a registered trademark of Nano Instru-

ments, Knoxville, TN). One of the principal advantages of the nano-indenter is its ability to deform the sample on the scale of several nanometers. It is generally accepted [9], that the indentation depth should not exceed 10-25% of the layer thickness in order to minimize the influence of the substrate on the measured value. Since, in this case, the thickness of the layer modified by radiation is estimated to be 200 nm, the nano-indentation technique is ideal for probing the mechanical properties of this layer without significant influence from the unirradiated substrate. For each sample, l0 indentations were made at depths of 20, 40 and 80 nm using a three-sided pyramidal diamond indenter and loads of 0.1-3 raN. The location of the first indent was selected at random and the remaining indents were placed at a separation of 15 t~m from each other in the form of a 5 × 2 rectangular array. The values of E and H were averaged over the l0 indents and scatter bars representing one standard deviation on either side were determined. Thermal drift rate corrections made to the data from each indent had an average value of 0.02 n m / s , indicating good thermal stability during testing. A fused silica sample was tested in each run as a control sample. Even though the samples were single crystals, the modulus is expressed as E, because plastic flow around the indenter produces displacements in many directions [9].

Fig. 2. [001] convergent beam electron diffraction patterns of spinel irradiated with 400 keV Xe 2+ following doses of (a) 0; (b) 5 × 10o8; (c) 1.75 × 1019; (d) 3 × 1019; (e) 5 X 1019 and (f) 1 X 1020 Xe2+/m 2.

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R. Decanathan et al. / Journal of Nuclear Materials 232 (1996) 59-64

Under these conditions, the measured modulus may be an average of the elastic constants and thus is better represented by a Voigt-Reuss average. A detailed discussion of the determination of H and E from the load-displacement curves for loading and unloading can be found in the reports by Oliver and Pharr [9,10]. The microstructural changes in the irradiated region were determined by electron diffraction from XTEM samples using a Philips CM30 microscope operating at 300 kV. The cross-sectional samples were prepared by gluing two halves of the irradiated sample with the irradiated layers face to face, mechanically polishing both sides using a tripod polisher [11] to achieve a thickness of less than 10 Ixm, and milling with 5 keV Ar ions at 12° from the surface. During ion-milling, the specimen stage was cooled by liquid nitrogen.

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3. Results Fig. 2 shows a series of [001] convergent-beam electron diffraction patterns from spinel irradiated to various doses. For the unirradiated sample, the (220) first-order fundamental reflections are quite intense as shown in Fig. 2(a). Following irradiation to a dose of 5 × 1018 X e 2 + / m 2, no significant change is evident in the diffraction pattern in Fig. 2(b). However, the intensity of the (220) reflections is considerably weaker following irradiation to 1.75 × 1019 X e 2 + / m 2, as shown in Fig. 2(c). At higher doses, the first-order fundamental reflections become extinct. In addition, all fundamental reflections diminish in intensity upon irradiation to a dose of 3 × 1019 X e a + / m 2. At and above 5 x 1019 X e 2 + / m 2, the diffraction pattern shows an amorphous halo. The irradiation-induced changes in the mechanical properties of spinel are shown in Figs. 3 and 4. In Fig. 3, the Young's modulus E of spinel is plotted as a function of irradiation dose for three different indentation depths. The value of E for the unirradiated crystal was found to be around 280 GPa, which shows good agreement with previous reports [5,12]. The modulus increases with dose and reaches a maximum at 1 × 1019 X e 2 + / m 2. The magnitude of this increase drops with increasing indentation depth from about 10% for indentations made at 20 nm to 6% at 80 nm. At higher doses, E decreases sharply with increasing dose up to 3 × 1019 X e 2 + / m 2 and then decreases gradually. The value of E following a dose of 1 × 10 2o X e 2 + / m 2 is about 30% less than the unirradiated spinel value for indentations made at 20 and 40 rim. At a depth of 80 nm, the corresponding decrease is about 17%. In general, the size of the scatter bars was found to decrease with increasing indentation depth. Fig. 4 shows the variation of the hardness H of spinel with irradiation dose, which is qualitatively similar to the variation of E with dose. At low doses, H increases with dose and reaches a value about 15% higher than that of

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Dose (xl019 Xe÷+/m 2) Fig. 3. The Young's modulus, E, of spinel as a function of irradiation dose obtained by nano-indentation from depths of 20, 40 and 80 nm. unirradiated spinel at a dose of 1 × 1019 X e 2 + / m 2. Upon further irradiation, H decreases rapidly up to a dose of 3 × 1019 X e 2 + / m ~. Around 5 × 1019 X e 2 + / m 2, H attains its saturation value. For spinel irradiated to the highest dose of 1 × 10 20 X e 2 + / m 2, the hardness is about 60% less than that of unirradiated spinel. The measured value of H seems to be much less sensitive to the indentation depth than the value of E. However, the scatter bars show the same trend in both cases-decreasing with increasing indentation depth. 4. Discussion The disappearance of the first-order fundamental reflections, such as (220), following low dose Xe 2+ irradiation

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R. Devanathan et al. / Journal of Nuelear Materials 232 (1996) 59-64

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(up to 3 × 1 0 1 9 X e 2 + / m 2) indicates that spinel undergoes a phase transformation to a metastable crystalline state. Diffraction patterns, taken at several different orientations (not shown here), indicate that the new phase is single crystalline, cubic and has a lattice spacing that is half that of unirradiated spinel. In an effort to determine the arrangement of cations and anions in this phase, we recently calculated the structure factors of several possible model structures for irradiated spinel [l 3]. Spinel belongs to the cubic Fd3m space group and retains its crystal structure over a wide range of compositions represented as x M g O . A l 2 0 3 , where x can vary from 0 (-/-AI20 3) to 1 (MgA1204). In the 'normal' ordered state, the unit cell of MgA1204 has a periodicity of 0.808 nm and contains an anion sublattice with a spacing of 0.404 nm - - the same as the lattice spacing of the metastable phase. The anions are in a distorted cubic close-packed arrangement with each O anion bonded to one tetrahedral Mg cation and three octahedral Al cations as shown in Fig. 5. This arrangement leaves seven-eighths of the tetrahedral and one-half of the octahedral cation sites vacant thus creating conditions favorable for site exchange between Mg and A1 cations known as inversion. The fact that the lattice spacing of the metastable phase is the same as that of the anion sublattice in irradiated spinel suggests that the anion lattice remains intact while the cation occupancy becomes randomized. However, cation exchange on existing sites (inversion) cannot account for the observed changes in structure and mechanical properties. Recently, it has been demonstrated [5] that the Young's modulus of spinel remains unchanged after more than 35% of the Mg cations have exchanged sites with AI cations as a result of neutron irradiation. In addition, inversion cannot lead to changes in the electron diffraction pattern, because the electron scattering factors of Mg and AI cations are nearly the same. To account for the observed changes in structure and properties, the cations must occupy other interstices in the anion lattice in addition to the octahedral and tetrahedral.

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R. Devanathan et al. / Journal of Nuclear Materials 232 (1996) 59-64

Our electron diffraction analysis [13] has shown that the observed changes in the diffraction pattern can be best accounted for if the cations are randomly distributed among octahedral, tetrahedral and threefold coordinated interstices. This new threefold interstice is unstable relative to the octahedral and tetrahedral interstices at elevated temperatures where the point defects are mobile. Hence, at elevated temperatures irradiation of spinel with energetic particles leads, merely, to inversion. However, at cryogenic temperatures irradiation leads to the accumulation of displacement damage in the material. Due to the large number of vacant octahedral and tetrahedral sites in the spinel lattice, it is much easier to displace a cation than an anion. Low dose irradiation at cryogenic temperatures causes the cation lattice to collapse while causing merely local distortions in the anion lattice. In such a distorted lattice the threefold interstice can become as stable as the octahedral and tetrahedral sites and thus favor the formation of the metastable phase. The rearrangement of the cations enroute to the formation of the metastable phase alters the separation between cations and anions leading to changes in bonding. This may be responsible for the elastic stiffening seen at low doses. We believe that the onset of the metastable transformation is around 5 × 1018 X e 2 + / m 2, when the mechanical properties begin to increase. At higher doses, the spinel phase is completely transformed to the metastable phase and eventually to the amorphous state. A decrease of about 40% in the elastic constants of irradiated Nb3Ir has been previously observed, even though the material remained crystalline [14]. An elastic softening of about 50% has also been reported in Zr3AI following radiation-induced amorphization [15]. This suggests that both the metastable and amorphous transformations may contribute to the observed softening in low-temperature ion-irradiated MgAl204. Our observation of considerable elastic softening upon amorphization is also consistent with recent experiments, molecular dynamics simulations and the generalized melting criterion of amorphization [16,17]. According to this criterion, a material becomes amorphous when the sum of the static and thermal displacements exceeds a critical value. The present study demonstrates that static displacements can be accumulated in spinel only at cryogenic temperatures. A two-stage damage accumulation model has been proposed to explain the observed phase transformations in spinel [ 18]. In the initial stage of ion irradiation, the cation sublattice is more susceptible to radiation because of the large number of structural vacancies. Following the redistribution of the cations, displacements begin to accumulate on the anion sublattice leading to amorphization of spinel. At elevated temperatures, the existence of structural vacancies and the enhanced mobility of point defects preclude the accumulation of lattice displacements. These interpretations in terms of the generalized melting criterion are supported by the results of an in situ high-voltage microscopy (HVEM) study of radiation dam-

63

age in spinel [19]. It was observed that a 100 nm thick HVEM sample of spinel was not amorphized by 1.5 MeV Kr ÷ irradiation to a dose of 1 X 1020 i o n s / m 2 at 20 K. However, pre-irradiating the spinel with 400 and 50 keV Ne + at 100 K prior to Kr + irradiation led to partial amorphization under the same conditions. In this case, the pre-irradiation introduces displacements in the cation lattice and favors amorphization upon subsequent Kr + irradiation. Moreover, the observation of radiation-induced amorphization in a 100 nm thick sample, where the irradiating species is not retained, indicates that displacement effects dominate over chemical effects. The findings of the present study are also in good agreement with the results of a computer simulation [7] of the energetics of point defects in spinel. The calculation used short-range shell model potentials along with longrange Coulombic potentials to model the ionic interactions. The study found that cation disorder is the most favored defect in spinel, which supports the idea of initiation of damage on the cation sublattice. In addition, it was observed that the interstitial position with the lowest energy was neither octahedral nor tetrahedral but an intermediate interstice. This suggests that it is possible for the cations to occupy the threefold site in addition to octahedral and tetrahedral interstitial positions. The anion lattice defects were found to have a much higher energy than the cation lattice defects. Thus, the anion lattice is likely to sustain only local distortions during the first stage of cryogenic radiation damage when the cations undergo rearrangement. Finally, the present work confirms the potential of the nano-indentation technique to determine the elastic properties of irradiated layers with a thickness of the order of 200 nm. In this case, the data from the 20 nm indentations shows more scatter than those from 40 and 80 nm, probably due to surface roughness. For the 80 nm indentation, the depth is 40% of the layer thickness and hence substrate effects are unavoidable. The changes in the elastic properties obtained from the 80 nm indents is much smaller than those from shallower indents, which clearly shows the influence of the substrate. These results demonstrate that a proper choice of indentation depth is essential to obtain reliable data using the nano-indentation technique.

5. Conclusions

We have studied the response of magnesio-aluminate spinel (MgA1204) to 400 keV Xe 2÷ irradiation at 100 K. At low doses MgAI204 undergoes a transformation to a metastable crystalline phase with half the lattice spacing of unirradiated spinel. This change is brought about by the rearrangement and randomization of cations among the interstices of the anion sublattice. During this first stage of damage accumulation, the anion lattice undergoes local distortions but retains its periodicity. The bonding changes during the initial stages of formation of the metastable

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phase lead to an elastic stiffening of about 10%. At higher doses, the accumulation of displacement damage on the anion sublattice leads to its collapse and amorphization of spinel. Upon completion of these phase transformations, there is a decrease of about 30% in the Young's modulus and 60% in the hardness of the material. These results, which can be interpreted in terms of the generalized melting criterion, are consistent with experimental and simulation studies of radiation damage in spinel.

Acknowledgements The authors wish to thank S.P. Chen, Los Alamos National Laboratory (LANL), for fruitful discussions and are grateful to J.R. Tesmer (LANL) for technical assistance with the ion irradiation. The microscopy was performed at the Center for Materials Science, LANL. This research is sponsored by the US Department of Energy, Office of Basic Energy Sciences, Department of Materials Sciences.

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