54 R.A. Andrievski u VhSCUW V HfgVk W CfWd a AfV Rev. Adv. Mater. Sci. 29 (2011) 54-67 Corresponding author: R.A. Andrievski, e-mail: [email protected]BEHAVIOR OF RADIATION DEFECTS IN NANOMATERIALS R.A. Andrievski Institute of Problems of Chemical Physics, Russian Academy of Sciences, Semenov prospect, 1, Chernogolovka, Moscow Region 142432, Russia Received: March 23, 2011 Abstract. The much attention is given to some possible results of the nanomaterials irradiation such as the interphase role as the radiation defect sinks, the nanostructure amorphization, the nanocrystallization of amorphous state, the alloying component segregation, and the enhance- ment of recrystallization. The radiation effects are analyzed in connection with the structure and physical/mechanical properties of nanomaterials-based metals, alloys and compounds (inter- metallics, nitrides, oxides, carbides). The results of microscopic approaches and the molecular dynamics modeling are also described. Some low investigated questions are pointed and dis- cussed. 1. INTRODUCTION The nanomaterials are the non-equilibrium objects in general due to the great interface presence (in- cluding concentration segregations, residual stresses, etc.) and it causes to relate with a spe- cial attention to the real exploitation conditions in- fluence on these materials structure and properties. It is obvious that such objects under the thermal action or the stress, corrosion and radiation fields will suffer some processes connected with recrys- tallization, segregation, homogenization and relax- ation, phase transitions, phase decay and arising, amorphization, sintering and filling of micro- and nanopores (nanocapillars). All these processes lead to a nanostructure evolution or even to its annihila- tion that is accompanied with the changes of its physical, chemical, mechanical and other proper- ties. The questions of the nanostructures stability are of special interest because this problem has not only a theoretical value but have the big applied importance too, for both the nanomaterial stability and their economic effectiveness predictions [1-4]. The information about the nanomaterials behav- ior under an irradiation is very important especially for the design and creation the perspective materi- als for the nuclear units new generation (fast-neu- tron reactors, high-temperature gaseous reactors, thermonuclear reactors, etc), because their opera- tional conditions presuppose a long-term exploita- tion in the strong radiation fields under the high tem- peratures, severe stresses and corrosive surrounds. The new materials for the nuclear and thermonuclear power engineering must be very stable ones. The simplest picture of the radiation defects arise into usual metals assumed that under the material bombardment by high energy particles or neutrons the energy is partially transmitted to the displaced atoms of the crystal lattice and the primary knock- on atoms (PKA) are arising. The so called displace- ment cascades involve the Frenkel pairs in a form of the interstitial atoms and vacancies (IAV), their complexes (clusters) in a form of loops and the va- cancies nanopores. The point radiation defects can mutually recombinate (annihilate), are absorbed by pores and dislocations or remove along the inter- faces acting in this instance as the sinks. The irra- diation leads to the next main changes in the mate- rial structure: an amorphization, a ductility decrease
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Semenov prospect, 1,Chernogolovka, Moscow Region 142432, Russia
Received: March 23, 2011
Abstract. The much attention is given to some possible results of the nanomaterials irradiationsuch as the interphase role as the radiation defect sinks, the nanostructure amorphization, thenanocrystallization of amorphous state, the alloying component segregation, and the enhance-ment of recrystallization. The radiation effects are analyzed in connection with the structure andphysical/mechanical properties of nanomaterials-based metals, alloys and compounds (inter-metallics, nitrides, oxides, carbides). The results of microscopic approaches and the moleculardynamics modeling are also described. Some low investigated questions are pointed and dis-cussed.
1. INTRODUCTION
The nanomaterials are the non-equilibrium objectsin general due to the great interface presence (in-cluding concentration segregations, residualstresses, etc.) and it causes to relate with a spe-cial attention to the real exploitation conditions in-fluence on these materials structure and properties.It is obvious that such objects under the thermalaction or the stress, corrosion and radiation fieldswill suffer some processes connected with recrys-tallization, segregation, homogenization and relax-ation, phase transitions, phase decay and arising,amorphization, sintering and filling of micro- andnanopores (nanocapillars). All these processes leadto a nanostructure evolution or even to its annihila-tion that is accompanied with the changes of itsphysical, chemical, mechanical and other proper-ties. The questions of the nanostructures stabilityare of special interest because this problem hasnot only a theoretical value but have the big appliedimportance too, for both the nanomaterial stabilityand their economic effectiveness predictions [1-4].
The information about the nanomaterials behav-ior under an irradiation is very important especially
for the design and creation the perspective materi-als for the nuclear units new generation (fast-neu-tron reactors, high-temperature gaseous reactors,thermonuclear reactors, etc), because their opera-tional conditions presuppose a long-term exploita-tion in the strong radiation fields under the high tem-peratures, severe stresses and corrosive surrounds.The new materials for the nuclear and thermonuclearpower engineering must be very stable ones.
The simplest picture of the radiation defects ariseinto usual metals assumed that under the materialbombardment by high energy particles or neutronsthe energy is partially transmitted to the displacedatoms of the crystal lattice and the primary knock-on atoms (PKA) are arising. The so called displace-ment cascades involve the Frenkel pairs in a formof the interstitial atoms and vacancies (IAV), theircomplexes (clusters) in a form of loops and the va-cancies nanopores. The point radiation defects canmutually recombinate (annihilate), are absorbed bypores and dislocations or remove along the inter-faces acting in this instance as the sinks. The irra-diation leads to the next main changes in the mate-rial structure: an amorphization, a ductility decrease
(the radiation hardening) and an embrittlement (witha possible rise of the creep rate at high tempera-tures) due to the defect content increase. In addi-tion, there can be observed a swelling (the volumeincrease due to the non-compensated vacancysinks) and a transmutation due to the nuclear reac-tions or radioactive decay. The radiation processesin the nanoobjects under study until now had notbeen adequately investigated (as compared with theusual coarse crystallized materials, see for example[5,6]), and the study of the radiation defects innanomaterials is still in its infancy [4,7].
From the general considerations and existingexperimental data the following next scenario canbe proposed for the nanomaterials behavior underan irradiation:•the great quantity of the interfaces (in the forms
of the grains and triple joints boundary) can func-tion as the sinks for the radiation defects removaland it leads to the nanomaterial stability increaseas compared with the usual coarse-grained ma-terials;•the radiation defects will assist the nanostructure
annihilation and its transformation to an amor-phous state;•the irradiation will give rise of the recrystallization
processes;•the alloying component segregation can be sig-
nificant in irradiated nanomaterials;•the nanocrystallization induced by irradiation can
be observed in metallic glasses.As it will shown below all these scripts (some of
them is contradictory one to another) take placesand must be considered by different approaches.
2. EXPERIMENTAL RESULTS
As far as is known the higher radiation stability (ascompared with the coarse-grained materials) wasfirstly marked for the nanocrystalline samples of ZrO2
and Pd at irradiation by Kr ions [8,9], when the in-terface role (as the sinks for the radiation defects)manifests itself clearly. In Fig.1 the radiation defectdensity into the irradiated samples of ZrO2 and Pdis shown depending on the grain size L. The de-fects were indicated by a deformation contrast intransmission electron microscope (TEM). The re-sults show that the defects are not present for thegrain sizes below 15 nm (ZrO2 and 30 nì (Pd , i.e.they either are passing out the sample sizes alongthe interface boundaries and/or are regenerating onthe grain boundaries. The difference between thedefect-free grain sizes of ZrO2 and Pd reflects thehigher diffusion mobility of the point defects in pal-
ladium comparing with zirconium oxide. At presenttime, the methods of the irradiated materials inves-tigation are broadened and now they include notonly using the accelerators but the irradiation byneutrons in the reactor conditions. In Table 1 someresults of the nanomaterials irradiation are pre-sented. For comparison the data are given concern-ing the coarse-grained materials irradiation.
56 R.A. Andrievski
Subject Grain size, Particle Irradiation conditions Dose (dpa); Result nm Energy (MeV); fluence
Temperature (K) (ions/cm2;n/cm2)
MgCa2O4 4-12 Kr 0.3;100 12-96 dpa Nanocrystals are stable to [10] amorphization
~104 Amorphization starts at thedose of 12 dpa
TiNi 31±6 Ar 1.5;293 0.2-5.6 dpa Amorphization is not[11] observed
8·104 Amorphization starts at thedose of 0.4-2.5 dpa
3C-SiC 36 Au 4;293 1013-1015 Amorphization starts at the [12] ions/cm2 fluence of 1014 ion/cm2
~103 Amorphization starts at thefluence of 1013 ion/cm2
W-0.3%TiC 50-200 He 3;823 Up to 2 x·1019 Blistering is observed at the [13] ions/cm2 fluence of 1019 ion/cm2
Conven- Blistering is observed at thetional W fluence of 1018 ion/cm2
W-0.5%TiC 50-200 Neutrons > 1;873 2·x1020 ions/cm2 Radiation hardening is not [13] observed
Conven- Hardness is increased uptional W to 20%
Nanostruc- ~500 Neutrons > 0.1; 1.5 dpa Small radiation hardeningtured ferritic (grains); 573-900 and little loss of ductility
steel 14 2-5YWT (inclusions)
[14]
Table 1. Effect of the irradiation by ions at accelerators and neutrons in reactor conditions on nanomaterials(dpa – displacements per atom .
The results show that a nanostructure positiveinfluence at the ion and neutron irradiation is fixedfor the consolidated nanoobjects with various bond-ing nature (complex oxide MgCa2O4, intermetalliccompound TiNi, cubic modification of silicon car-bide, disperse hardened tungsten, nanostructuredchromium steel). In all cases the amorphization andradiation hardening are absent and the blistering (asurface spalling) manifests itself to a lesser degree.The analogous results were obtained for thenanocrystalline samples of nickel, copper and chro-mium-nickel stainless steel (see, for example[15,16]).
The mechanical properties change under irradia-tion was most extensively studied for the ODS-EUROFER and 14YWT ferritic/martensitic steels[17]. Former is an European variant of the oxidedispersion strengthened (ODS) steels that is pro-duced by mechanical alloying small amounts (0.2-
0.3 wt.%) of Y2O3 powder with gas atomizedEUROFER 97 powder followed by consolidationusing hot isostatic pressing. The 14YWT steel is ananostructured ferritic/martensitic steel (Fe-14wt.%Cr-3W-0.4Ti) obtained in the Oak Ridge Na-tional Laboratory. Its powder with 0.3 wt.% of Y2O3
nanoparticles was mechanically alloyed using high-energy attritor type ball mill and consolidated byhot extrusion at 850 °C with the following repeatedhot rolling. Such consolidation regime allows to cre-ate a matrix structure with the submicron size grainsthat is near to nanocrystalline structure. As is shownin Fig.2, the grain sizes are between 100 – 1000nm, the relation length to width for the grains is about1 – 5 and the oxide inclusions size is about 2 – 5nm.
The test results for the initial and irradiatedsamples are presented in Table 2, they demonstratethe mechanical characteristics advantage of the
57Behavior of radiation defects in nanomaterials
Steel Condition Y (MPa) B (MPa) (%) kIC (MPa·m0.5) TDBT(° C)
Table 2. Yield strength ( Y), ultimate strength ( B), total elongation (), fracture toughness (KIC) at roomtemperature, and temperature of ductile-brittle transition (TDBT) of ODS-EUROFER and 14YWT steels inunirradiated and irradiated conditions (E > 0.1 MeV, dose 1.5 dpa, T = 573K) [17].
14YWT nanostructured samples over the morecoarse crystalline ODS-EUROFER objects both inthe initial and irradiated states. Of particular inter-est is that the irradiation weakly affects on the tem-perature of ductile-brittle transition and a high valueof fracture toughness that can be connected withthe good cohesion of the oxide nanosize inclusions(of Y2Ti2O7 and Y2TiO5 type) with the matrix. Thevolume content of such inclusions is about 1.4х10-24 m-3 and they are characterized by a high ther-mal stability up to 1000 °C [18]. The irradiation ofthe 14YWT steel samples at temperature up to 900Êand the characteristics study in a wide temperaturerange (from nitrogen ones up to 900Ê also had dem-onstrated a high radiation stability of this material[17].
The detailed survey of technology and charac-teristic of the Japanese nanostructured ferritic steelsMA957 and J12YWT was presented in [19]. Apartfrom the mentioned high thermal stability of theMA957 steel (Fe-14Cr-0.9Ti-0.3Mo-0.25Y2O3; extru-sion at 1150 °C; the grain sizes from 200 to 1000nm, the relation length to width for the grains about1 – 5 and the mean diameter of oxide inclusions2.1±0.4 nm [18], the interesting results were ob-
tained from the radiation resistance tests. The neu-tron irradiation at T = 370-750 °C (doses up to 40dpa) and T = 500-750 °C (doses up to 100 dpa), aswell as the irradiation by heavy ions at 670 °C (dosesup to 150 dpa) have revealed the slight changes ofthe dislocation and grain structures for MA957 steel.The study of creep under the irradiation at T =400-600 °C revealed its non-thermal nature. It wasmarked also a radiation hardening decreasing withthe temperature rise. More detailed study of thehelium pores had shown that in the irradiatednanostructured steels the He concentration (andtheir brittle decay correspondingly) can be regulatedusing the interface surfaces. At the same irradia-tion doses the He concentration in MA957 steel isless than in EUROFER97 steel by a factor of three.In general these investigations leads to the conclu-sion that the radiation stability of MA957 steel issufficiently high but the irradiation is attended withsome accumulation of the radiation defects [19].
The tomographic atom probe methods are usedin studies of the irradiated material properties ingeneral (and the steels especially). These methodsallow trace the behavior and segregation of the dop-ing elements under the radiation processes (e.g.,see [20,21]). It was found that an irradiation by fer-rous ions (E = 0.16 MeV, dose 5-10 dpa) thenanostructured 316 austenitic stainless steel leadsto the grain boundary depletion by chromium andtheir enrichment by nickel and silicon [20]. Tomo-graphic atom probe characterization of ODSEUROFER steel revealed that fast neutron irradia-tion (T = 330 °C, the dose up to 32 dpa) leads to theconcentrations change of nanoclasters (with diam-eter 1-3 nm; their number density is 2-4 x 1024 m-3)as compared with unirradiated state [21]. Irradia-tion resulted in escaping vanadium from clustersinto surrounding matrix. The concentrations of yt-trium and oxygen in the matrix increase after irra-diation by six and three times (respectively) as apossible result of dissolution of the Y2O3
The radiation induced chromium and silicon seg-regations were discovered by the X-ray emissionspectroscopy method on the surface of the ferritic/martensitic steel samples after an irradiation by Ni(E = 7 MeV) and He (E = 30-70 KeV) ions [22]. Theinvestigations of the radiation-induced segregationsin nanomaterials are only beginning and an under-standing of such materials strength or their corro-sion characteristics nature under irradiation will beobtained in future.
It must be noted that ODS type steels have longbeen developed in former USSR (Russia, Ukraine(e.g., see [23-25]). The low temperature electron (E= 5 MeV) and neutron (E > 0.1 MeV) irradiation ofODS steel K5 (13Cr1TiMo-Y2O3) and the model fer-ritic steels (Fe-2, 13, and 20 Cr) had shown thatthere is not any qualitative difference in the radia-tion defects behavior in these objects [25]. The irra-diation at T = 77K leads to a sufficient radiationembrittlement and a strong decrease of the plastic-ity characteristics. The study of the irradiated sampleelectric and mechanical properties allow to registeran overlapping of the displacement cascades undera neutron fluence more than 1.5x1014 m-2. A com-parison between the annealed samples being irra-diated by electrons and neutrons shows an agree-ment of the defect behavior regularities under thecascade and cascade-less (electronic) irradiations.
In general, it can be said that the creation andstudies of the matrix nanostructures had receivedthe little researchers attention though more then 20years ago it was shown that the radiation swellingcan be limited by a non-traditional method using a
batten (rod) structure with a high density of the in-terface boundaries in the austenitic/martensiticCr16Ni19Mo3 and Cr13Mg7V2 steels [26,27].
The interesting results were obtained for the filmsbeing irradiated by the accelerators. In Fig. 3 thehigh resolution TEM-image of the nanostructuredTiN film, irradiated by He ions flow (E = 12 keV,total fluence of·1016 ions/cm2; T = 293K) [28] Asindicated in the figure, at the boundaries of the L~8size crystallites boundaries the amorphization signsare absent and therefore the authors [28] correctlyreasoned that it is a manifestation that the grainboundaries are functioning as the sinks for radia-tion defects. The films electrical resistance mea-surements have revealed only a little rise after theirradiation and it once more attested that the radia-tion defects are removed in a large extent. The X-ray and electron microscopy studies (including thesurface investigation by an atomic force microscopy)of the TiN films (thickness 240 nm; L = 13-16 nm;deposited on Si at T = 293K and 423K) have shownthat the argon ions irradiation with energy E = 120keV with the fluence rise from 1011 ions/m2 to 1012
ions/m2 is accompanied by a decrease of the lat-tice period, grain sizes and surface roughness aswell as an increase of the microstresses and dislo-cations density [29]. An amorphization was not ob-served in the studied fluence interval but a partialdisappearing of the films columnar structure wasmarked that can be connected with the peculiari-ties of the radiation defects removal.
Very interesting results were obtained in work[30], the authors of which irradiated the multiplayerCu/V films (with different thickness of the individuallayers, from 1 to 200 nm, and a total thickness1.5-2 m) at room temperatures by helium ions (E= 50 keV, total fluence of 6x1020 ions/m2). In thissystem copper and vanadium are forming an im-miscible system that is a very convenient object forstudying the interfaces influence by changing of thelayers number in the films. In Figs. 4a and 4b it isshown that the decrease of the individual Cu/V lay-ers thickness h leads to a decrease of the heliumpores content and swelling under the irradiation. Anestimation of these values (from the rule-of-mixtureconsiderations for the individual Cu and V layers)gives the far exceeding values by comparison withthe experimental data for the multilayer films.
The hardness changes of the initial and irradi-ated multiplayer films with the thickness h decreaseis shown in Figs. 4c and 4d, demonstrating that thedifference of the hardness between the initial andirradiated films is lowered with the h decrease andbecomes vanishing small at h = 2.5 nm, i.e. a ra-
diation hardening disappears. For a comparison atthese figures the hardness values are given calcu-lated from the additive considerations (Fig. 4c) andfor the irradiated Cu and V films with 1 m thick-ness (Fig. 4d). In summary, the results presentedat Fig. 4 unambiguously manifest that in the multi-layer films the swelling and radiation hardening aredecreasing with the individual layers width, i.e. withthe growth of the interface areas acting as the sinksfor the radiation defects.
Some peculiarities of the hardness and the he-lium pores content in the Fe/W, Al/Nb and Cu/Nbmultilayer films are investigated and discussed indetails in works [31-33]. The nanostructure stabilityof the irradiated copper-based film alloys is studiedin [34,35]. The temperature changes of the Mo andW nanosize inclusions into the as-deposited andirradiated Cu90Mo10 and Cu90W10 nanocompositefilms are shown in Fig. 5 as well as the grain growthin the copper matrix [34]. The irradiation was car-ried out by Kr ions with energy 1.8 MeV having runrange ~ 300 nm (the films thickness is ~ 200 nm).
The irradiation dose was ~ 75 dpa (the fluence is3x1012 ions/m2). The data reflect the role of both anadditional annealing and the irradiation temperature.As is illustrated in Fig. 5a, the Mo and W nanosizeinclusions are nucleated and growing at tempera-tures ~ 400 °C and ~ 500 °C, correspondingly buttheir marked increase is detected at ~ 0.65 TM and~ 0.85 TM (TM – the copper melting point tempera-ture). When the irradiation temperature is elevatedfrom room temperature to 550 °C the inclusions sizeis increasing from ~3 to ~5 nm with the fast follow-ing growth for Mo, but only at T > ~800 °C for W.The authors of [34] have theoretically estimated theprocess of the Mo nanosize inclusions nucleationand growth under the thermal peaks action in theradiation displacement cascades and obtained agood agreement with the experimental data withinthe temperature range up to ~600 °C (solid curve atFig. 5a).
The nanosize inclusions presence is favoring toa stability of the copper matrix nanostructure wherethe grain size at level below 40 nm remains practi-
60 R.A. Andrievski
Fig. 5. The temperature influence on the nanoinclusions growth: (a) Mo and W ( – Cu-Mo (annealing ; – Cu-Mo (grinding + annealing ; – Cu-Mo (irradiation ; – Cu-W (annealing ; – Cu-W (irradiation
cally unchangeable up to the annealing tempera-ture 900 °C (Fig. 5b). At the same time, the anneal-ing of the pure nanocrystalline copper at 600 °Cleads to the grain sizes growth up to ~100 nm.
The peculiarities of the Kr ions irradiation influ-ence on the decomposition of films from Cu1-XMX
type alloys (where 0.1 < x < 0.15; M – Ag, Co, Fe,Mo, and Nb) and the changes of the mutual solubil-
ity and sizes of nanoinclusions into the matrixnanostructure were analyzed in [35].
In work [36], the grains growth in thenanostructured metallic films (Au, Pt, Cu, Zr andZr-Fe) was studied in detail under the irradiation byAr (E = 0.5 MeV) and Kr (E = 0.5-1 MeV) ions. Thenanostructure evolution presented in Fig. 6 for theirradiation fluence rise at room temperatures showsthat the crystallite sizes are increasing in a widetemperature range including the low ones (20-773K).From these data the difference of the Au, Pt, andCu films can be clearly connected with their melt-ing point temperature differences. It is supposed todivide the temperature range tî three intervals: a purethermal regime (where the temperature influence onthe recrystallization is dominating), the thermo-in-duced (corresponding to a sum of the thermal andirradiation actions) and low-temperature (where thetemperature influence is insignificant). The transi-tion point from the low-temperature interval to themixed one depends on the object, but in the meanover the homological scale is changing from 0.15TM to 0.20 TM (TM – melting point temperature . Inthe theory of the grains radiation-induced growth itis suggested that under an irradiation in the cas-cades and subcascades the thermal peaks are form-ing. On this base the ion irradiation-induced graingrowth can be described by the relation of Ln – Lo
n ~K (Ft) type, where n~3, F – the beam intensity (ions%m2·sec , t – time, K – a constant, depending on thegrain mobility and driving force, Lo – the grain initialsize.
It must be noted that in some studies under anirradiation not only the grain growth was observed
but their size decrease too. Thus such effect wasmarked for the nickel samples with the initialnanostructure (with L
î= 115 nm) obtained by the
intensive plastic deformations. After the irradiationby protons and nickel ions at room temperature(EH = 590 MeV; dose 0.56 dpa; ENi = 590 keV) thegrain mean size equals 38 nm [15, 37]. The sameauthors have observed the grain size growth fromLî = 178 nm to L = 493 nm under the irradiation of
Cu-0.5Al2O3 samples. It can be added that the irra-diation at 350 °C of the 316 stainless steel by Feions (E = 0.16 MeV, dose 5-10 dpa) was accompa-nied by the grain growth (Lo~40 nm and L~60 nm)[20]. At the same time the irradiation of Zr films bythe high energy electrons (Å~1 MeV) had shownthat even at 180 dpa the grain growth did not pro-ceed [36]. It can be explained by the fact that thedefect generation under the electron irradiation pro-ceeds generally without cascade formation and theirnumber is deficient (and mainly in the MAV forms)as distinct from the irradiations under the accelera-tor or reactor conditions.
The further study of the irradiation influence onthe nanomaterial behavior (with account of the cas-cades overlapping and other factors) is importantfrom the other point of view because the nanomaterialrecrystallization can be accompanied with an ab-normal grain growth and it can lead to the materialdegradation and the drastic decrease of its irradia-tion resistance and another physical and chemicalproperties. In Fig. 7 the grain growth kinetics andthe microstresses fall at room temperatures areshown for a nanocrystalline Pd samples with theinitial grain size ~7 nm and porosity ~4% [38]. As
62 R.A. Andrievski
Subject Size of Irradiation conditions Result nanocrystal Ion Å (MeV) Dose (dpa);
(nm) Fluence (ions/m2)
ZrO2/SiO2 [39] ~3 Xe 1 ~0.8 dpa Amorphization ZrO2 [46] Single crystal Xe 0.4 680 dpa Crystal state Cu/SiO2 [40,41] ~2.5 Sn 5 0.16 dpa Amorphization
~8 Sn 5 1019 Crystal stateAu/SiO2 [39,42] 3 Xe 1 ~0.8 dpa Crystal state
3-5 Sn 2.3 1019-20 Crystal stateGe/SiO2 [43,44] 4-8 Si 5 1015-19 Amorphization of nano-
crystals is observedfirstly
Co/SiO2 [45] 3.7±1.0 Au 9 1017 Amorphization
Table 3. Irradiation effect on nanocrystals in amorphous SiO2 matrix.
the data show, after about 8-hour conditioning thereis observed a growth rate rise with the result thatthe nanostructure after 60 days turns into an usualmicrostructure with the grain sizes about 20-40 m.It is obvious that any nanomaterial usage as theirradiation-resistant materials must take into accounta possibility of such situation of the abnormal graingrowth. In any case the methods of such effect pre-diction and guarding against must be developed.
Let us consider the data concerning thenanostructure amorphization under an irradiationaction. Using the crystalline ZrO2, Si, Cu, Co, andGe nanoparticles, it was found that their irradiationinto the inert matrixes (amorphous silicon oxide type)leads to an amorphization (e.g., see [39-45]). Someof the results presented in Table 3 were obtainedusing the nanocrystals implanted into the amor-phous SiO2 layers with thickness ~2 m. The initialand irradiated nanocrystals state and compound wascarried out using several methods (high resolutionTEM, Rutherford backscattering, low-angle scatter-ing, absorption X-ray spectroscopy, etc.) and themolecular dynamics modeling.
Especially characteristic results of Table 3 con-cern to the amorphization of zirconium oxide (whichin single crystalline state is not amorphized even atthe very high irradiation level doses [46]) and to thenon-amorphized nanocrystals of gold and copperwith dimensions about 8 nm. The experimental data[39,41,42] have shown that the dimensional factor(in the event of copper) and the irradiated nanocrystalnature (as in case of gold) have a pronounced influ-ence on the process of the nanocrystalline statetransformation into amorphous one under irradiation.The detailed study of the irradiated copper
nanocrystals [41] has revealed also a role of Cu2Oformation in the amorphization process. The com-prehensive description of the irradiated nanoparticlesbehavior (both in matrix and free) is given in review[47] where also some results are analyzed differentfrom Table 3 results (for example, the results con-cerning cobalt nanocrystals) and is marked the ne-cessity of the following investigations.
3. MODELLING AND MICROSCOPICAPPROACHES
The molecular dynamics methods (e.g., see [44,45, 48-59], including the many scale methods [50,53]) are widely used now for the nanomaterial be-havior study under irradiation. Under thenanostructures irradiation by the high- energy ionsand neutrons, the next situations were modeling:an amorphization of the irradiated nanocrystals inamorphous matrixes [44,45]; the displacement cas-cades into Ni nanograins (L = 5 and 12 nm) at bom-barding particles energy of 5, 20, and 30 keV [48];the radiation-stimulated grain growth of nanosize Niparticles (L = 5 and 10 nm) for cascade induced bya particle impact with energy 5 keV [49]; the inter-stitial atoms and tetrahedral stacking faults behav-ior into the damage cascades [50]; the evolution ofthe nanocrystalline film morphology under an irra-diation [51,52]; the behavior under irradiation of thenanomaterials with the bcc- and fcc-structures [53];the nucleation and growth of the vacancy clustersinto a cubic modification of silicon carbide [55]; thegrain boundaries behavior in the nanostructuresacting as the defects sinks and sources [56], etc.In these works an important role of the grain inter-
faces and triple joints (as the radiation defect sinks)was demonstrated and the radiation-stimulated graingrowth processes are studied in details as well asthe corresponding changes of the surface rough-ness and stressed states. It is found that under theirradiation of the nanocrystalline materials with bcc-and fcc-structures the vacancy clusters are formeddominantly in the more loosen bcc-lattice. Fig. 8shows the structures of nano-Ge/amorph SiO2 sys-tem before and after ion irradiation [44,47]. the per-fect crystal structure degradation of nano-Ge afterirradiaton is obvious.
Let us consider in details the results of the cas-cade process modeling in vanadium [57] and cop-per [58,59]. The bcc-structured vanadium crystal-lites had the inclined 13<320>[001] and
17<410>[001] symmetric tilt grain boundaries withan atom numbers into the calculated cell from 65000to 450000 in accordance with the PKA energy from<0.5 keV to >0.5 keV at temperature T = 10K [57].The cascade evolution was studied for the threestages, namely the ballistic one (an energy propa-gation by the PKA atoms over the all volume of themodel object with maximal number of the defectsand the thermal peaks achieve), recombination one(the defect number decreases to a some stablevalue), and diffusion one (a following decrease ofthe defect number limited by their interaction andtransport processes). The stimulation results haveshown that the great interface surfaces sufficientlyaffect on the displacement cascade formation be-cause the boundaries are accumulating the big partof the radiation defects and are preventing the cas-cade developing, being sometimes (depending on
PKA values) even a non-permeable barrier for thecascades.
A modeling of the Frenkel pair IAV interactionwith the grain boundaries for the fcc copper allowsto suggest the next mechanism of this defect re-combination: the boundaries are firstly saturated bythe mobile interstitial atoms and after the satura-tion the inverse emission proceeds with a vacancyabsorption in the boundary regions [58]. Represent-ing the interface as a 11 <110> {131} symmetrictilt grain boundary and modeling a situation for 15cascades (with the PKA kinetic energy 4 keV; thetotal atom number in system about 160.000, andthe moving atoms number ~130.000), a tempera-ture influence on the vacancy and interstitial atomremoval along the grain interfaces was estimated.The calculation results have shown that in the 10-15K temperature range the process duration is verygreat (t > 1010 sec) and no one of the mechanismsis relevant. In the range 70-100K (t < 1 sec) theinterstitial atoms play the main role and at T = 300Kall mechanisms for radiation defects are possible.These results allow to explain the experimental datashown in Fig. 9 concerning the electrical resistanceof the irradiated Au foils with dimensions (width,length, height) of 0.5х10х5.3·10-3 mm at tempera-tures 15K and 300K [60]. On the basis of resis-tance increments the results manifest that nanosizesamples (L~23 nm), as compared with their coarsecrystalline analogs, are more prone to an irradiationat low temperatures, when by an estimation of [58]all radiation defect removal mechanisms are notrelevant. On the contrary, at the room temperatures(when all three mechanisms of the radiation defect
removal are working) the nanostructure demon-strated the higher radiation stability.
The modeling of the competition processes ofthe radiation defect absorption by the grain bound-
aries and their volume recombination have shownthat under the copper cascade-less irradiation byelectrons at little times the accumulation of theFrenkel pairs IAV is progressing on the boundarieswith the grain size decrease (in range from 40 nmto 15 nm), but subsequently the absorption by thebigger grains begins to prevail [59].
The microscopic approaches to the irradiationinfluence on the nanostructures were developed inworks [61-63] where the Frenkel pairs IAV forma-tion was studied for both highand low-energetic in-teractions. In former case the IAV were forming onthe nanograin boundaries as well as into the grainitself, but in latter case – the vacancies arise onlyon the boundaries and the interstitial atoms ariseinto the grains [61]. The defects behavior evolutionwas analyzed for the next stages: 1) a radiation-induced defect generation; 2) the defects absorp-tion by the boundaries between the grains; 3) theannihilation of IAV; 4) the stable clusters formationfrom the point defects. For the high-energetic inter-actions, stages 1) and 2) are assumed to be domi-nant. On the basis of the energy considerations aregion of the advantaged amorphization was deriveddepending the nanograins size. From one side, theinterfaces (grain boundaries) developed network isfavorable for the excess free energy rise and is low-ering the energy barrier of the amorphization, butfrom other side it is promoting the radiation defectsremoval, i.e. is preventing the amorphization.
65Behavior of radiation defects in nanomaterials
The energetic approach was developed also inwork [62], where a qualitative picture was presentedfor the free energy changes depending thenanograins size and a conclusion was drawn thatfor every material there is a special optimal grainsize, providing the most effective amorphization re-sistance and the radiation defects removal. The re-sults are shown schematically in Fig. 10, where thefive zones are defined, taking into account the sumsof the boundary grains and point defects free ener-gies, as well as the barrier of amorphization (a tran-sition from crystalline state to the amorphous one).These zones correspond to the next situations:1. The transition to the amorphous state is possible
without any irradiation (L < L1), that is observedfor example for the silicon nanoparticles with sizesbelow ~3 nm [64];
2. The transition to the amorphous state is initiatedby a weak irradiation (L1 < L < L2);
3. In this size interval an irradiation doesn’t lead toan amorphization (L2 < L < L3);
4. In this size interval an irradiation leads to anamorphization (L3 < L < L4);
5. In this size interval the defects free energy valueis dominating and the boundaries play only a littlerole in the defects removal (L4<L<Lmax). The de-fect annihilation by a volume recombination isprevailing at L > Lmax.The events in the 4 and 5 zones are qualitatively
supported by above described results of ZrO2 andPd irradiation (Fig. 1) [8].
The peculiarities of the radiation defect forma-tion in nanocrystals being inserted into an inert solid-body matrix have been analyzed in work [63]. It wasshown that the crystalline nanocluster amorphizationinto an inert matrix may be either accelerated (aradiation damage) or slowed-down (a radiation re-sistance). These variants realization depends on thesituation at the interface nanocluster–matrix, whichcan be (as compared with the coarse crystallineobjects) either compressed (the acceleratedamorphization) or extended (the retarded defect for-mation). These results qualitatively correspond tothe data of Table 3 but it is hard to derive the morecorrect information for the many factors influenceand various assumptions.
4. CONCLUSIONS
The presented data clearly demonstrate that theinterfaces play an important role in the consolidatednanomaterial characteristics and a rise of their ra-diation stability. This fact gives a serous orientationin the practical design of the new materials espe-
cially for the nuclear and thermonuclear power en-gineering but, in spite of the currently rising interestto the nanomaterial radiation stability, many aspectsof this problem invite further investigations. The firstquestion is connected with the radiation defectsnature and their evolution in dependence of the irra-diation dose, annealing temperature, stressed stateand environment conditions. In this connection itshould be noted that the radiation processes of thenanostructures stabilization, amorphization and re-crystallization have a very important role that mustbe carefully investigated for the materials behaviorprediction and control.
It is obvious that the technologies of the futureradiation resistant materials will have a crucial im-portance. The further improvements of the technol-ogy regimes must provide a production of the stablenanostructured materials with high physical-me-chanical and physical-chemical properties alongwith their economical efficiency. It must be also notedthat above analyzed questions are far from exhaust-ing all the problems connected with thenanomaterials radiation resistance. There are manyrelated problems: a hydrogen behavior into the irra-diated nanostructures and their erosion (e.g., see[65,66]), the radiation influence on thenanocrystallization in metallic glasses [67,68] anda nanodomain formation under the metastable al-loys irradiation [69,70]. More over the radiation sta-bility problems exist not only for the main construc-tional materials used in the nuclear and thermo-nuclear power engineering but also for many auxil-iary objects (scintillation counters, sensors, cos-mic units, etc. . The frames of this survey don’t al-low consider all the problems but a necessity of thegeneral approach to the radiation defects behaviorin irradiated nanomaterials designed for the variouspurposes or in the relative objects. Some questionson the radiation stability of nanomaterials are alsodiscussed in author’s review [71].
ACKNOWLEDGEMENTS
The author is grateful to Profs. A. Didyk, S. Golubov,I. Ovid’]o, V. Sagaradze, and X. Zang for discus-sions on radiation damage and paper sending. Thiswork was supported by the Program for Fundamen-tal Researches of the RAS Presidium (P21) and bythe Russian Foundation for Basic Research(Projects No 09-08-11005 and No 08-03-00105).
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