Comprehensive nuclear materials 2 13 properties and characteristics of zrc

Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc Comprehensive nuclear materials 2 13 properties and characteristics of zrc

2.13 Properties and Characteristics of ZrC H F Jackson and W E Lee Imperial College London, London, UK ß 2012 Elsevier Ltd All rights reserved 2.13.1 Introduction 339 2.13.2 2.13.3 2.13.3.1 2.13.3.2 2.13.3.3 2.13.3.4 2.13.4 2.13.4.1 2.13.4.2 2.13.4.3 2.13.4.4 2.13.5 2.13.5.1 2.13.5.2 2.13.5.3 2.13.5.4 2.13.5.5 2.13.5.6 2.13.5.7 2.13.6 2.13.6.1 2.13.6.1.1 2.13.6.1.2 2.13.6.1.3 2.13.6.1.4 2.13.6.2 2.13.6.2.1 2.13.6.2.2 2.13.6.2.3 2.13.6.2.4 2.13.7 References Crystallographic Properties and Chemical Bonding Thermodynamics of the Zr–C System Zr–C Phase Diagram Enthalpy of Formation Enthalpy and Heat Capacity Vaporization Thermal Properties Thermal Conductivity Electrical Resistivity Thermal Expansion Diffusion Mechanical Properties Elastic Properties Hardness Strength Fracture Toughness Plastic Deformation Creep and Stress Relaxation Thermal Shock Resistance Environmental Resistance Oxidation Oxidation products Oxidation kinetics Oxidation by water vapor Summary and outlook Performance Under Irradiation Thermal neutron capture cross-section Durability and dimensional stability under neutron irradiation Microstructural changes under heavy ion or proton irradiation Irradiation effects on electrical, thermal, and mechanical properties Summary and Outlook 340 341 341 343 343 346 347 347 350 351 353 355 355 355 356 357 359 361 363 363 363 363 364 364 364 365 365 365 367 368 368 369 Abbreviations fcc Face-centered cubic %FIMA Percent fissions per initial actinide metal atom CRSS Critical resolved shear stress CTE Linear coefficient of thermal expansion DBTT Ductile-to-brittle transition temperature dpa Displacements per atom DTA Differential thermal analysis EDX Energy-dispersive X-ray spectroscopy SEM TEM TRISO XRD Scanning electron microscopy Transmission electron microscopy Tri-structural isotropic (coated fuel particle) X-ray diffraction 2.13.1 Introduction Zirconium carbide, like other carbides of the transition metals of Groups IV, V, and VI, exhibits an 339 340 Properties and Characteristics of ZrC unusual combination of properties that are useful for refractory applications These carbides combine the cohesive properties of covalently bonded ceramics (high melting point, high strength, and hardness) with the electronic properties of metals (high thermal and electrical conductivity) Comparative properties of the refractory transition metal carbides have been reviewed previously by Schwarzkopf and Kieffer,1 Storms,2 Toth,3 Kosolapova,4 and Upadhyaya.5 A thorough understanding of the thermodynamic and heat transport properties of carbides is limited by a paucity of experimental data as a function of composition 2.13.2 Crystallographic Properties and Chemical Bonding In the Zr–C system, the monocarbide is the only intermetallic phase reported, crystallizing in the face-centered cubic NaCl structure (Fm"3m, space group 225) (Figure 1) Zr atoms form a close-packed lattice, and the smaller C atoms (rC ¼ 0.48rZr) fill the octahedral interstices.3 The ZrCx phase exists over a wide compositional range and, as further discussed in Section 2.13.3.1, is stable with up to 50% vacancies on the carbon sublattice Low-temperature ordered phases have been experimentally reported for the Ti–C, V–C, and Nb–C systems, but so far have been suggested only via thermodynamic calculations for the Zr–C system.6 Metallic vacancies comprise at most a few atomic percent.3 The effect of carbon vacancies on unit cell geometry has been investigated extensively (Figure 2), Zr C Figure Rocksalt crystal structure of ZrCx 4.705 Lattice parameter (Å) 4.700 4.695 Brown and Kempter86 Chang and Graham85 Storms2 Nickel et al.121 Ramqvist8 Baker et al.179 Morrison and Sturgess50 Shevchenko et al.194 Bukatov et al.69 Storms and Griffin13 Storms and Wagner35 Bulychev et al.180 Shevchenko et al.140 Kumashiro et al.123 Christensen182 4.690 Equation [1] y = Kempter and Fries185 Farr18 Henney and Jones184 Grossman46 Rudy et al.21 Sara15 Aronson et al.60 4.685 4.680 0.5 0.6 0.7 0.8 C/Zr ratio Figure ZrCx lattice parameter as a function of the carbon/zirconium ratio x 0.9 1.0 1.1 Properties and Characteristics of ZrC with the relationship between room temperature lattice parameter and C/Zr ratio difficult to establish conclusively Scatter in literature values is a common theme in the study of transition metal carbides because of the difficulty of preparing pure specimens and adequately characterizing them Oxygen and nitrogen readily substitute for carbon in the lattice, and their presence is correlated with reduced lattice parameter On the basis of literature values for a range of impurity contents, Mitrokhin et al.7 established a quantitative relationship between the lattice parameter of such oxycarbonitrides and carbon, as well as the oxygen–nitrogen impurity content: aZrCx ONịy ẳ 4:5621 0:2080x ỵ 0:3418x 0:80y1 xị ẵ1 where x is the C/Zr atomic ratio (0.62 < x < 1) and y is the (O ỵ N)/Zr atomic ratio ( y < 0.3) In general, lattice parameter increases with C/Zr ratio, with evidence for an increase and a decrease as C content increases above approximately ZrC0.8 toward ZrC1.0 Ramqvist8 qualitatively explained the peak in lattice parameter versus C/Zr ratio as being due to competing influences on lattice size: expansion with increasing carbon content due to the increased space required to accommodate interstitials, and contraction due to the increased bond strength The nature of chemical bonding in ZrCx is not fully understood, and electronic structure investigations have sought to establish the relative influences of covalent, metallic, and ionic contributions Carbon s- and p-orbitals and zirconium d-orbitals participate in bonding and contribute to strong metal–nonmetal bonding and octahedral coordination.9 Other authors10 emphasize the interstitial nature of carbon in the ZrC structure and the donation of electrons from carbon to metal, strengthening Zr–Zr bonds Lye and Logothetis11 proposed that some charge transfer from carbon to metal occurs and that carbon stabilizes the carbide structure by contributing bonding states Hollox12 and Storms and Griffin13 suggest that, depending on the carbide, lattice stability decreases with increasing carbon content if antibonding states become filled; this is consistent with observed hardness and melting temperature measurements for ZrCx The electronic structure of ZrC must be placed in context with the properties of Groups IV, V, and VI transition metal carbides, and the interested reader is referred to the comparative reviews seen earlier 341 2.13.3 Thermodynamics of the Zr–C System 2.13.3.1 Zr–C Phase Diagram The most recent critical assessment of the Zr–C system was carried out by Fernandez-Guillermet14 and is depicted in Figure The phase diagram shows the formation of a monocarbide phase which exists between 37.5 and 49.5 at.% C (ZrC0.6–0.98 with extent of phase field temperature-dependent), melts congruently at 3700 K and 46 at.% C (ZrC0.85), and forms a eutectic with carbon at 3200 K at 67.6 at.% C Solid solubility of C in Zr has not been established conclusively but is estimated to be between and at.% C by Sara15, Rudy,16 and Kubaschewski-von Goldbeck.17 The Zr ỵ ZrC eutectic is close to the melting temperature of bcc Zr, 2127 K, contributing to the assessment of low carbon solubility Solubility of Zr in C is taken as nil Figure shows the results of experimental phase diagram studies superimposed on the assessed diagram Phase boundaries of the ZrC phase were established via ceramography by Farr,18 Sara and Doloff,19 Sara et al.,20 Sara,15 and Rudy et al.,21 while Storms and Griffin13 used C and Zr activity values determined during a Knudsen effusion study Rudy et al.21 prepared mixtures of Zr, ZrH2, or graphite with ZrC and determined ZrCx solidus temperatures and ZrC–C eutectic temperature via differential thermal analysis (DTA), ceramography, or melting in a Pirani furnace As described by Rudy and Progulski,22 the Pirani technique subjects a bar specimen with a central blackbody hole to resistance heating; melting is determined by the temperature at which liquid forms in the blackbody hole The technique is noted to be most precise for isothermal transformations (i.e., congruent melting or eutectic), as the sample often collapses or the blackbody hole closes before the liquidus is reached Sara15 prepared zirconium carbides having various C/Zr ratios from mixtures of ZrH2 and graphite to determine melting temperatures and the congruent melting temperature and composition Adelsberg et al.23 performed ceramography on C–Zr diffusion couples to contribute data points to the low-carbon liquidus line; ZrC–C eutectic temperature was also determined by ceramography Zotov and Kotel’nikov24 placed ZrCx bars with a radial hole under axial loading and resistance heating; fracture of the sample at the temperature at which the hole melted determined ZrCx solidus For the ZrC0.88 sample, at least, their value is 342 Properties and Characteristics of ZrC 0.2 0.4 0.6 C/Zr ratio 0.8 1.4 4500 4500 4000 Liquid 4000 Graphite ~ZrC0.85 3700 K 3500 3500 3000 3200 K 3000 Liquid + ZrCx 2500 2500 2127 K 2000 ZrCx 2000 1500 ZrCx+ C β-Zr + ZrCx Temperature (ЊC) Temperature (K) Liquid + ZrCx 1500 1000 1000 α-Zr + ZrCx 500 Zr 0.1 0.2 500 0.3 0.4 0.5 0.6 Atomic fraction C 0.7 0.8 0.9 1.0 C Figure Zr–C phase diagram, as assessed by Fernandez-Guillermet.14 4000 3500 Temperature (K) 3000 2500 2000 Farr18 Congruent melting ZrC phase boundary, lattice parameter vs C/Zr Sara15 Quenched, liquid by ceramography Not melted ZrC–C eutectic, DTA/ceramography Zr–ZrC eutectic, same ZrC phase boundaries by ceramography 1500 1000 500 0.1 0.2 0.3 Rudy et al.,21 Rudy16 ZrC phase boundary, ceramography ZrC–C eutectic composition, ceramography lsothermally molten Incipient melting Quenched, liquid observed Specimen collapsed during melting By DTA Liquidus by chemical analysis C solubility in Zr at Zr–ZrC eutectic Adelsberg et al.26 C solubility in Zr ZrC–C eutectic temperature Storms and Griffin13 ZrC phase boundaries, activity vs C/Zr Zotov and Kotel’nikov 24 Specimen ruptured during melting ZrC–C eutectic, Ceramography/specimen rupture Bhatt et al.25 Zr–ZrC eutectic temp 0.4 0.5 0.6 Atomic fraction C 0.7 Figure Experimental phase diagram studies compared with the assessed diagram 0.8 0.9 1.0 Properties and Characteristics of ZrC 343 Standard enthalpy of formation ΔHf (J mol−1) −140 ϫ 103 −160ϫ103 −180ϫ103 Mah and Boyle,189 combustion calorimetry Pollock,37 Langmuir vaporization Same, Knudsen effusion Coffman et al.,38 Langmuir vaporization Same, Knudsen effusion Mah,188 combustion calorimetry Baker et al.,179 combustion calorimetry Equation [2] −200ϫ103 −220ϫ103 0.7 0.6 0.9 0.8 C/Zr ratio 1.0 Figure Standard molar enthalpy of formation of ZrCx as a function of C/Zr ratio anomalously high Heating the sample in an effusion cell, Bhatt et al.25 determined Zr–ZrC eutectic temperature by an optical pyrometric ‘spot technique.’ 2.13.3.2 Enthalpy of Formation Other properties on which the current phase diagram is based include enthalpy of formation, enthalpy increment or heat content, specific heat capacity (Cp), and activity of C and Zr in ZrC Standard  enthalpy of formation, ÁHf , of ZrCx as a function of the C/Zr ratio is plotted in Figure A quadratic fit to the reviewed data is provided by  ÁHf ¼ 2:03  105 x À 5:04  105 x À 9:92  104 ½2Š  where x is the C/Zr ratio and ÁHf is in units of joules per mole Within the compositional range,  ÁHf is most negative at the stoichiometric composition and the recommended value is À197 kJ molÀ1.26 Toth3 attributes this to decreasing ZrCx bond strength with removal of C from the lattice 2.13.3.3 Enthalpy and Heat Capacity Enthalpy increment of ZrCx with respect to 298 K (HT – H298) is plotted as a function of temperature in Figure and as a function of C/Zr ratio at 1600 K in Figure Storms and Griffin report the following equation to fit the experimental values of Mezaki et al.,27 Levinson,28 Kantor and Fomichev,29 and Turchanin and Fesenko30: HT H298 ẳ 2:14 104 ỵ56:86T 5:46103 T 1:456  106 ½3Š T where H is in units of joules per mole and T is absolute temperature, valid from 298 to 3200 K From their low-temperature heat capacity measurements on ZrC0.96, Westrum and Feick31 determined a value of H298 À H0 of 5.9 kJ molÀ1 and an entropy, S298 À S0 of 33.3 J molÀ1 Heat capacity of ZrCx is plotted as a function of temperature in Figure and as a function of C/Zr ratio at 298 K in Figure Heat capacity is equal to the first derivative of enthalpy with temperature, and the function recommended by Storms and Griffin13 is ỵ 1:86 106 T ỵ Cp ẳ 56:86 0:0109T ỵ 5:586 10À6 T 1:456  106 ½4Š T where Cp is in units of joules per mole per kelvin Low-temperature heat capacity of ZrC0.96 was measured by Westrum and Feick31 by adiabatic calorimetry between and 350 K No data are available for more carbon-deficient compositions, limiting efforts to quantify the entropy of mixing introduced by carbon vacancies High-temperature drop calorimetry À 344 Properties and Characteristics of ZrC 160ϫ103 Neel et al (1962), ZrC0.92 Mezaki et al (1963), ZrC0.986 Westrum and Feick (1963), ZrC0.96 140ϫ103 Levinson (1965), ZrC0.958 Storms and Griffin (1973), ZrC0.96 Enthalpy, HT –H298 (J mol−1) 120ϫ103 100ϫ103 80ϫ103 60ϫ103 40ϫ103 20ϫ103 0ϫ103 500 1000 1500 2000 Temperature (K) 2500 3000 Figure Enthalpy of ZrCx as a function of temperature 66 ϫ 103 1600 K Neel et al.32 Levinson28 Kantor and Fomichev29 Bolgar et al.33 Turchanin and Fesenko30 Storms and Griffin13 Enthalpy HT –H298 (J mol−1) 64 ϫ 103 62 ϫ 103 60 ϫ 103 58 ϫ 103 56 ϫ 103 54 ϫ 103 0.6 0.7 0.8 C/Zr ratio Figure Enthalpy of ZrCx at 1600 K as a function of C/Zr ratio 0.9 1.0 Properties and Characteristics of ZrC 80 70 Heat capacity (J mol−1 K−1) 60 50 40 30 Neel et al.,32 ZrC0.92 Mezaki et al.,32 ZrC0.986 Westrum and Feick,31 ZrC0.927 Levinson,28 ZrC0.958 Bolgar et al.,33 ZrC0.99 Kantor and Fomichev,29 ZrC1.0 Storms and Griffin,13 ZrC0.96 Petrova and Chekhovskoi,34 ZrC1.04 20 10 0 500 1000 1500 2000 2500 3000 Temperature (K) Figure Heat capacity of ZrCx as a function of temperature 40 Westrum and Feick31 Bolgar et al.33 Kantor and Fomichev29 Storms and Griffin13 Storms and Wagner35 Heat capacity at 298 K (J mol−1 K−1) 39 38 37 36 35 34 33 0.6 0.7 0.8 C/Zr ratio Figure Heat capacity at 298 K as a function of C/Zr ratio 0.9 1.0 345 346 Properties and Characteristics of ZrC measurements were made on ZrC0.92–1 by Neel et al.,32 Mezaki et al.,27 Levinson,28 Bolgar et al.,33 Kantor and Fomichev,29 and Turchanin and Fesenko.30 Petrova and Chekhovskoi34 determined heat capacity, using a pulsed electric current method to measure thermal diffusivity Storms and Wagner35 used the laser flash method to measure thermal diffusivity for ZrC0.64–1 at 300 K and estimated Cp for these compositions, using a known value for ZrC0.9631 and by assuming a curve parallel to that established for NbCx as a function of C/Nb ratio.36 Heat capacity increases sharply between and 500 K, saturates, then begins to increase more rapidly near the melting point Both roomtemperature heat capacity and high-temperature enthalpy increase with C/Zr ratio in the homogeneity range Room-temperature heat capacity of ZrC0.96 is 38 J molÀ1 KÀ1.31,35 2.13.3.4 Vaporization Vapor pressures have been established by Langmuir vaporization of C-saturated ZrC and by Knudsen effusion studies of ZrC in equilibrium with graphite These are plotted in Figure 10 Langmuir studies are internally consistent, but give higher pressures than for the Knudsen method Pollock37 and Coffman et al.38 assumed the congruent evaporation composition to be stoichiometric, that is, equal evaporation rates for Zr and C However, Langmuir evaporation of ZrC0.74–0.96 by Nikol’skaya et al.39 found the congruently evaporating composition to lie in the range ZrC0.8–0.87, decreasing with increasing temperature between 2300 and 3100 K Vidale40 computed Zr and C vapor pressures from tabulated H and S functions for  Zr and C, ÁHf for ZrC of À185.5 kJ molÀ1, and an  estimated ÁSf for ZrC of À11.3 kJ molÀ1 KÀ1, and the trend is consistent with Langmuir data Storms2 computed Zr vapor pressure over ZrC ỵ C from thermodynamic functions derived by the author for ZrC0.96, values in the 1963 JANAF thermochemical tables for  Zr(g) and C(s), ÁHf for ZrC of À196.6 kJ molÀ1, and  ÁHvap for ZrC of 608 kJ molÀ1, with the prediction consistent with Knudsen data Evaporation rate as a function of temperature is plotted in Figure 11 Standard enthalpy of vaporization of ZrC at 298 K has been reported as À1520 kJ molÀ1 for Langmuir studies and À805 kJ molÀ1 for Knudsen studies.37,38 Storms and Griffin13 coupled Knudsen effusion from TaC cells with mass spectrometry between 1800 and 2500 K to determine the Zr activity of ZrC0.55–‘‘1.97’’ by comparing ion currents from pure Zr with those of the carbide Carbon activity was obtained via a Gibbs–Duhem integration; activity of both as a function of C/Zr ratio at 2100 K is plotted Temperature (K) 100 3000 2800 2600 2400 2200 Pzr Pollock,37 Langmuir Same, Knudsen Coffman et al.,38 Langmuir Vidale,40 thermochemical calculations Storms,2 thermochemical calculations 10−1 Vapor pressure (Pa) Pc Pollock,37 Langmuir Coffman et al.,38 Langmuir Vidale,40 thermochemical calculations 10−2 10−3 10−4 10−5 3.4 ϫ 10−4 3.6ϫ10−4 3.8ϫ10−4 4.0ϫ10−4 1/T (K−1) Figure 10 Vapor pressures of C and Zr as a function of temperature 4.2ϫ10−4 4.4ϫ10−4 4.6 ϫ 10−4 Properties and Characteristics of ZrC 347 Temperature (K) 3000 2800 2600 2400 10−4 Pollock,37 ZrC1.0 Coffman et al.,38 ZrC0.92 Nikol’skaya et al.39 Evaporation rate (g cm−2 s−1) 10−5 10−6 10−7 10−8 10−9 3.2 ϫ 10−4 3.4ϫ10−4 3.6 ϫ 10−4 3.8 ϫ 10−4 4.0ϫ10−4 4.2ϫ10−4 4.4 ϫ 10−4 1/T (K−1) Figure 11 Langmuir rate of evaporation of ZrCx as a function of temperature in Figure 12 Activity of Zr exceeds that of C for carbon-deficient compositions up to the cross-over composition at 2100 K of ZrC0.89 The change in Zr activity with C/Zr ratio is most rapid at high-carbon compositions and becomes near-constant as the composition drops below approximately ZrC0.8 Partial standard molar enthalpies of vaporization for Zr and C as a function of C/Zr ratio are plotted in Figure 13 Total enthalpies obtained by Pollock37 and Coffman et al.38 are consistent with the values of Storms and Griffin.13 Partial enthalpy of Zr decreases monotonically as C is removed from the lattice Partial enthalpy of C exceeds that of Zr for most of the homogeneity range, approaching that of Zr at a composition of ZrC0.99 2.13.4 Thermal Properties 2.13.4.1 Thermal Conductivity It is appropriate to discuss thermal and electrical conductivity as coupled phenomena Thermal conductivity is considered a sum of phonon and electron contributions to conductivity The phonon contribution to thermal conductivity should decrease with temperature, as atomic vibrations inhibit phonon transport The contribution to thermal conductivity due to electrons is calculated by the Wiedemann– Franz law,41 according to ke ẳ LT r ẵ5 where ke is the electronic thermal conductivity, L is the Lorentz constant (2.44  10À8 W O KÀ2), T is absolute temperature, and r is electrical resistivity Generally, electrical resistivity of metals increases with temperature; in transition metal carbides, electron thermal conductivity increases with temperature At low temperatures heat is mainly conducted by phonons, which are scattered strongly by conduction electrons.42–44 At intermediate temperatures, both electrons and phonons contribute to thermal conductivity, but in the transition metal carbides the electronic component is dominant Phonon scattering by carbon vacancies becomes important above about 50 K, contributing to a decrease in thermal conductivity with increasing temperature At high temperatures, thermal conductivity increases approximately linearly with temperature The temperature dependence of electronic thermal conductivity is plotted in Figure 14; this was computed from the Wiedemann–Franz law and 348 Properties and Characteristics of ZrC 100 Zr 10−1 Activity at 2100 K 10−2 ZrC0.89 10−3 10−4 C 10−5 Storms and Griffin,13 Knudsen effusion 0.5 0.6 0.7 0.8 0.9 C/Zr ratio 1.0 1.1 1.2 Figure 12 Activity of Zr and C in ZrCx as a function of C/Zr ratio at 2100 K Partial standard molar enthalpy of vaporization ⌬HfЊ (J mol–1) 1100 ϫ 103 C 1000ϫ103 900ϫ103 ZrC0.99 800ϫ103 700ϫ103 Knudsen effusion studies 600ϫ103 Pollock37 Coffman et al.38 Storms and Griffin13 Zr 500ϫ103 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 C/Zr ratio Figure 13 Partial standard enthalpies of vaporization of Zr and C as a function of C/Zr ratio a linear fit to the electrical resistivity measurements of Taylor45 and Grossman46: r ẳ 0:79T ỵ 36:3 Experimental measurements of thermal conductivity of ZrCx as a function of temperature between 1.8 and 3400 K are also plotted in Figure 14 The overall trend is a steep increase of thermal conductivity with temperature up to 50 K, followed by a slight decrease in an intermediate temperature range 358 Properties and Characteristics of ZrC 34 Ramqvist,8 arc-melted Hot-pressed Ordan’yan et al.,190 fused 100 Funke et al., (100) single crystal Kohlstedt,101 single crystal ZrC0.94 Samsonov et al.,102 arc-melted Samsonov et al.,10 hot-pressed Ogawa et al.,111 pyrolytic Samsonov and Vinitskii,192 sintered Andrievskii et al.,104 hot-pressed Vahldiek and Mersol,105 (100) single crystal Kumashiro et al.,123 (100) single crystal Kosolapove,95 sintered He et al.,107 ion-beam deposited 32 30 Hardness (GPa) 28 26 Carburized Zr filament 24 22 20 18 16 0.4 0.5 0.6 0.7 C/Zr ratio 0.8 0.9 1.0 Figure 24 Room-temperature hardness of ZrCx as a function of C/Zr ratio Ultimate tensile strength (MPa) 200 Gangler,71 hot-pressed ZrC0.83–0.85 Same, specimen oxidized Leipold and Nielsen,72 hot-pressed ZrC0.936’ 6.5% porosity, 0.85% free C Same, hot-pressed, ZrC0.77–0.84’ 1–5% porosity, 1.6–2.5% free C Same, annealed in reducing atmosphere Turchin et al.,195 sintered ZrC0.95’ 7% porosity 150 Kosolapova,95 sintered, 5% porosity 100 50 0 500 1000 1500 2000 Temperature (K) Figure 25 Ultimate tensile strength of ZrCx as a function of temperature 2500 3000 Properties and Characteristics of ZrC 359 600 500 Bend strength (MPa) Gridneva et al.97 Sintered ZrC0.98’ 50 μm grains, 9% porosity Shaffer and Hasselman54 Hot-pressed, 4.4% porosity 8.8% porosity Lepie,96 pyrolytic ZrC1.03 ZrC0.95 10 μm grains, 2.5% porosity ’ Same, thicker specimen Fedotov and Yanchur 98 Sintered ZrC1.0’ 10 μm grains, 5% porosity ZrC0.96 from carburized Zr, 250 μm grains Same, thicker specimen Lanin et al.,99 ZrC0.96’ 8μm grains, 6% porosity 400 300 200 100 0 500 1000 1500 Temperature (K) 2000 2500 3000 Figure 26 Bend strength of ZrCx as a function of temperature 1.11 MPa m1/2 Lanin et al.108 reported a KIC of 2.8 MPa m1/2 based on cyclic compressive loading of notched ZrC0.96 in air at room temperature Toughness measurements based on cracking during Vickers indentation of magnetron-sputtered ZrC0.8–1 thin films are also reported in the range of 1.5– 2.5 MPa m1/2.112 Further study of the fracture toughness of ZrC must be undertaken, preferably according to standard tests of fracture toughness for ceramics, as the accuracy and consistency of indentation toughness results have been brought into question.113 2.13.5.5 Plastic Deformation Though strength and fracture properties are controlled by sample processing, the elastic and plastic deformation behavior inherent to ZrC may be understood in terms of its chemical bonding Transition metal carbides are known to be brittle at ambient temperatures but ductile at high temperatures The metal-like conductivity and the fcc structure of ZrC suggest that deformation along the fcc metal slip systems is possible In fcc, the {111}h110i system corresponds to the slip of close-packed planes along close-packed directions and requires the lowest stress to form and move a dislocation The same is assumed for the rocksalt structure However, many crystals with the NaCl structure are ionic, and slip along the above system is inhibited due to the energy required to overcome strong Coulombic repulsion when in the half-glided position.3 Instead, ionic rocksalt compounds prefer to slip along {110} planes, maintaining attractive Coulombic forces If ZrC is known to slip along {111} planes, the degree of ionic bonding must not be large Even if there is no ionic prohibition to slip in ZrC, the directed nature of its covalent bonding is still an impediment to slip Strong metal–carbon bonds in an octahedral coordination inhibit slip on close-packed {111} planes, leading to high shear stresses required for dislocation mobility at low temperatures The preferred mechanism of deformation is then brittle fracture, which occurs by cleavage on {100} planes.12,114 Brittle fracture persists at least to 1000 K.3 At elevated temperatures, however, a ductile–brittle transition has been observed Based on compressive loading of single-crystal ZrC0.9,115 ZrC0.945116, and arcmelted ZrC0.94,117 plastic yield was reported at 1172, 1353, and 1473 K Microplasticity was reported at 1273 K, with gross plastic yield above 1773 K, as observed by fractography of tensile specimens.118 Transmission electron microscopy (TEM) of dislocations in ZrC0.98 after elevated temperature compression revealed microplasticity above 1420 K.119 360 Properties and Characteristics of ZrC The crystallography of the slip system has been investigated Lee and Haggerty116 measured the critical resolved shear stress (CRSS) in the compression of single crystal ZrC0.945 samples grown along h100i and h111i directions, and one sample whose axis corresponded to the ‘0.5’ orientation for {111} h110i slip, indicating that one of these 12 equivalent slip systems was oriented at 45 to the crystal axis When loaded uniaxially along the crystal axis, a maximum resolved shear stress (t) of half of the applied stress (or 0.5s) would be achieved, according to Schmid’s law, t ẳ s cos f cos l ẵ11 where f and l are, respectively, the angles separating the slip plane normal and the slip direction from the load axis CRSS as a function of temperature for the various samples are plotted in Figure 27 The results of Williams115 for compression of h100i oriented single crystal ZrC0.875 are consistent with those of Lee and Haggerty, plotted in the same figure The slip planes were identified by slip traces on the samples, while the Burgers vector was confirmed to be 1=2h110i by diffraction contrast of dislocations, a result consistent with the TEM analysis of Britun 140 Williams,115 single crystal ZrC0.875 Lee and Haggerty,116 single crystal ZrC0.945 Loaded along {100} axis, assumed slip on {111} planes 120 Critical resolved shear stress (MPa) et al.119 Slip was induced on either {100}, {110}, or {111} planes, depending on the crystal axis and its slip system of maximum resolved shear stress For loading along the h100i direction, the maximum resolved shear stress was for the {110} plane; for h111i loading, slip was along {100} planes; and for the ‘0.5’ oriented sample, slip was along {111} planes CRSS for slip along {100} was highest, and along {110} and {111} was approximately equal over the temperature range where they overlapped Hannink et al.122 characterized the anisotropy of room-temperature Knoop hardness on the {100} surface of single crystal ZrC0.94, rotating the long axis of the Knoop indenter azimuthally; hardness as a function of rotation angle is plotted in Figure 28 Hardness varied sinusoidally with rotation, with a minimum occurring when the indenter was aligned along h100i directions, and maximum for indenter alignment with h110i directions This indicated that the slip system was {110}h110i, normally associated with ionic crystals The authors also measured Vickers hardness, which exhibited anisotropy in the same sense as the Knoop measurements, but with lower amplitudes, in agreement with the results of Kumashiro et al.123 100 Loaded along {111} axis, slip on {100} planes 80 111 60 Loaded at 45º to {111}{110} slip system, slip on {111} planes 40 100 110 Orientation of load axis 20 Loaded along {100} axis, slip on {110} planes 1000 1200 1400 1600 1800 2000 2200 2400 Temperature (K) Figure 27 Critical resolved shear stress of single crystal ZrCx loaded along different crystallographic axes as a function of temperature Properties and Characteristics of ZrC [100] [110] [010] [110] 361 [100] Hardness (GPa) 22 21 20 19 Hannink et al.,122 Knoop indentation on {100} single crystal ZrC0.94 Kumashiro et al.,123 Vickers indentation on {100} single crystal ZrC0.9 45 90 Indenter angle (º) 135 180 Figure 28 Room-temperature hardness of single crystal ZrCx as a function of indenter azimuthal angle Hannink et al.122 suggested that the active slip system is dependent on temperature, as they observed for TiC0.96 and VC0.83 At ‘low’ temperatures (room temperature for TiC and ZrC, 87 K for VC), the {110}h110i slip system was active, as seen by the hardness anisotropy described earlier At higher temperatures (883 K for TiC, 623 K for VC), maximum and minimum hardness occurred respectively for indenter alignment with h100i and h110i, which is the opposite to that observed for {110}h110i slip ‘High’ temperature slip in TiC and VC was on the {111}h110i or {100}h110i system Kohlstedt101 proposed that covalent, directional bonding dominates at low temperatures, prohibiting slip on {111} planes and resulting in high hardness, while at high temperatures, the degree of covalent bonding is decreased as the {111}h110i slip characteristic of fcc metals is favored, resulting in the observed hardness drop with temperature Capacity for plastic deformation has also been observed to vary with the C/Zr ratio A monotonic decrease in hardness with decreasing C/Zr ratio is seen in Figure 23 Although not determined for ZrC, CRSS of TiCx was observed to decrease with decreasing C/Ti ratio.115 The author explains this intuitively in terms of fewer C–Ti bonds that must be broken during dislocation motion Hollox12 attributed these results to a decrease in the contribution made by carbon atoms to cohesion in TiC as the C/Ti ratio is reduced, further citing the band structure calculations of Lye and Logothetis11 which indicated that carbon donates electrons to and strengthens metal–metal bonds Hollox also inferred that the DBTT might decrease as the carbon-tometal ratio is decreased, but this has not been demonstrated conclusively 2.13.5.6 Creep and Stress Relaxation Two relevant thermomechanical processes in hightemperature structural applications are creep and stress relaxation Steady-state creep deformation, or time-dependent strain under an elevatedtemperature stress, has been observed for ZrC In general, creep rate is dependent on applied stress (s) and temperature (T ) according to   Q ẵ12 "_ ẳ Asn exp RT where "_ is strain rate, A is a constant dependent on the material and creep mechanism, n is an exponent dependent on the creep mechanism, R is the gas constant, and Q is the activation energy of the creep mechanism Activation energies for creep under various conditions are summarized in Table Zubarev and Kuraev130 proposed a creep mechanism map of stress normalized to shear modulus versus homologous temperature, based on compressive creep in He atmosphere of ZrC1.0 with 14 mm grain size The authors distinguished between different temperature–stress regimes governed by creep processes having low or high activation energies Indeed, 362 Properties and Characteristics of ZrC Table Activation energy for creep of ZrC Temperature range (K) Activation energy (kJ molÀ1) C/Zr ratio Grain size (mm) Ref 1173–1373 307 308 331 501 Ỉ 19 460 314 837 485 Ỉ 75 510 Ỉ 31 582 Ỉ 33 657 Ỉ 40 728 Ỉ 44 678 Ỉ 42 703 Ỉ 42 761 Æ 46 531 523 515 711 Æ 42 0.9 sci a 0.94 0.945 0.76–0.84 250 sci 0.95 0.73 0.75 0.84 0.895 0.9 0.96 0.984 0.94 3–5 45 70 20 6–65 16 8.5–17 30 4.5 e 0.99 5–20 h 1473–2073 1673–2273 2073–2423 2473–2873 2450–2520 2400–3030 2423–2903 2473–3023 b c d f g a Kumashiro et al.,124 Vickers indentation in {100} surface, for (100)h001i, (110)h001i, and (111)h110i slip systems, respectively b Darolia and Archbold,117 compression in vacuum c Lee and Haggerty,116 compression in vacuum along h111i crystal axis d Leipold and Nielsen,72 1–5% porosity, 1.6–2.5 wt% free carbon e Miloserdin et al.,125 tension, 3.4–9.8 MPa, 7% porosity f Spivak et al.,126 creep in He atmosphere, 4–6% porosity g Zubarev and Dement’ev,127 in tension, bending, and compression, respectively, 0.96–19.6 MPa, inert atmosphere, 15–17% porosity h Zubarev and Shmelev,128,129 in tension, 0.96–73.5 MPa, Ar atmosphere, 3–5% porosity, 0.38–1.1 wt% free carbon i Single crystal the two activation energies provided by Leipold and Nielsen72 are attributed to a change in creep mechanism above 2423 K At low or intermediate temperatures (below about 1623–2473 K for ZrC, or 2073 K for ZrC) and intermediate or low stress, creep has a higher activation energy and is controlled by diffusion The activation energy for creep in this regime is close to that of bulk self-diffusion in ZrC, which is $500 kJ molÀ1 for C and $700 kJ molÀ1 for Zr, as detailed in Table The diffusion rate of the lower-mobility species should be rate-limiting, so creep in this regime is usually attributed to self-diffusion of Zr However, diffusion along grain boundaries may reduce the activation energy for creep relative to that of bulk diffusion Diffusional mass transfer (Nabarro–Herring creep) and grain boundary sliding are suggested mechanisms,130 with the latter confirmed by scanning electron microscopy (SEM) ceramography and not applicable to single crystals.131 Britun et al.119 also confirmed grain boundary shear and rotation by TEM ceramography of ZrC0.98 compressed at 2100–2500 K The dependence of creep mechanism on grain size has been studied by Zubarev et al.131 Analysis of creep mechanisms among polycrystalline (14–1000 mm grain size) and single-crystal ZrC1.0 revealed that with increasing grain size and with the single crystal, dislocation creep mechanisms occurred at lower threshold stresses, and the Nabarro–Herring and grain boundary sliding processes diminished in importance or disappeared Free carbon has been reported to facilitate grain boundary creep.131 Creep has been studied as a function of C/Zr ratio Creep in compression of ZrC0.89–0.96 at 2773– 2973 K132 showed a monotonically decreasing creep rate with decreasing C/Zr ratio In the same work, a v-shaped trend of creep rate with C/Zr ratio was found for creep in bending of NbC0.82–0.98 at 2273– 2473 K, decreasing to a minimum creep rate at a composition of approximately NbC0.85 They speculated that such a trend may exist for ZrCx, but that the associated minimum existed below the compositional range they investigated Based on creep of ZrC0.75–0.98 between 2400 and 3030 K, Spivak et al.126 found activation energy increased with increasing C/Zr ratio This would be consistent with expectations of enhanced diffusion with an increase in C vacancies However, their earlier work83 reported activation energy for self-diffusion of Zr in ZrC as being composition-independent between ZrC0.84–0.97, and that of C decreasing with increasing C/Zr ratio Some hypotheses have been put forth (see Section 2.13.4.4), but further study of Zr diffusion in ZrCx is required to explain this conclusively Stress relaxation, or an evolution in stress with time for a component at fixed strain, has been investigated to a lesser degree than creep Repeated fourpoint bend loading of ZrC0.95–1 (6–35 mm grains) at Properties and Characteristics of ZrC 1873–2273 K, with unloading at intervals, resulted in increased resistance to relaxation, via work hardening, upon subsequent loading cycles.91,133 The authors concluded that under these conditions slip occurs by diffusion along grain boundaries At higher temperatures, up to 2473 K, no beneficial effects were imparted by repeated loading, and the authors concluded that no work hardening occurred They judged stress relaxation and creep in ZrC to be controlled by different mechanisms 2.13.5.7 Thermal Shock Resistance Thermal shock has been evaluated qualitatively for ZrC by various means Susceptibility to failure by thermal shock is lowered in materials with high tensile strength, low elastic modulus, low thermal expansion coefficient, and high thermal conductivity Gangler’s71 test involved cyclic heating and quenching of hot-pressed ZrC0.83–0.85 between a 1255 K furnace and 300 K air stream ZrC withstood 22 cycles, though excessive oxidation was noted Shaffer and Hasselman54 subjected hot-pressed ZrC spheres to thermal shock on heating: roomtemperature specimens were drawn rapidly into the hot zone of a tube furnace at a temperature sufficiently high to cause fracture For ZrC this was determined to be 1725 K, and free carbon was found to improve thermal shock resistance Lepie96 subjected a pyrolytic ZrC–C alloy to firing in the nozzle–throat section of a solid-fuel rocket; no ill effects from the sudden exposure to the 3894 K exhaust flame were reported, and firing for 30 s at 5.5 MPa caused little erosion 2.13.6 Environmental Resistance 2.13.6.1 Oxidation Despite excellent refractory properties, ZrC suffers from poor oxidation resistance, with oxidation initiating in the range of 500–900 K (Table 5) The kinetics and mechanism of ZrC oxidation have been assessed in several studies, between room temperature and 2200 K, at oxygen partial pressures (PO2) between  10À4 and 101 kPa (0.79  10À6 and atm), with the oxidation products a function of both parameters 2.13.6.1.1 Oxidation products Oxidation resistance is imparted by the formation of a dense, adherent oxide scale which effectively restricts oxygen access to the carbide Since the oxides of carbon are gaseous, protection is only afforded Table Onset temperature of ZrC oxidation Oxidation temperature (K) 773 573 653–673, Zr 773–863, C 575 773 973 763, Zr 973, C 723–823 658 473–573 363 PO2 (kPa) Ref 0.007–101 0.66–39.5 1–40 a 5–50 21 21 21 d 21 21 21 h b c e f g i j a Bartlett et al.134 Shimada & Ishii,135 temperature at which sintered ZrC weight gain initiates c Shimada,136 DTA peaks indicating onset of oxidation of Zr and C in single crystal ZrC, respectively d Rama Rao and Venugopal.137 e Opeka et al.138 f Voitovich and Pugach.139 g Shevchenko et al.,140 DTA peaks indicating onset of oxidation of Zr and C, respectively h Tamura et al.141 i Zhilyaev et al.,142 ZrCxOy (x ¼ 0.7–0.85, y ¼ 0.15–0.25) j Zainulin et al.,143 ZrCxOy (x ¼ 0.43–0.97, y ¼ 0.09–0.36) b by the zirconium oxide However, low temperatures ( 0.5 kPa (i.e., sufficient for surface saturation with adsorbed oxygen) and PH2 O 21–42 kPa, water vapor did not appreciably oxidize ZrC but did accelerate the oxidation rate in the presence of O2(g) 2.13.6.1.2 Oxidation kinetics The kinetics of oxidation of ZrC is related to the nature of the oxidation products and microstructure of the oxide scale formed In general, oxidation accompanied by the formation of a protective scale is associated with low temperatures The oxide scale is reported to be protective and its growth to be parabolic with time, that is, controlled by diffusion of 2.13.6.1.4 Summary and outlook Clearly, oxidation resistance of monolithic ZrC is compromised at temperatures above about 700 K, and its use at higher temperatures is restricted to inert or reducing atmospheres To enable the prediction of structural component performance under accidental oxidizing conditions, it is important to Properties and Characteristics of ZrC understand the effects of oxidation on mechanical properties For instance, strength and toughness degradation must be characterized for ZrC partially oxidized to ZrO2 and ZrCxOy , and failure mechanisms must be identified, such as grain boundary attack or scale rupture Oxidation to ZrO2 will also lower thermal conductivity, which is relevant to applications where the high thermal conductivity of ZrC is exploited, but heat transport in ZrCxOy must also be established 2.13.6.2 Performance Under Irradiation ZrC is a material of interest for next-generation nuclear fuel applications – for example, as a replacement or supplement for the SiC diffusion barrier coating in Tri-structural isotropic coated fuel particle (TRISO) coated fuel particles in high-temperature gas-cooled reactors There are two main incentives to develop ZrC for this application First, higher fuel operating temperatures, and thus higher efficiency of next-generation reactors, would be enabled by the higher degradation temperatures ZrC offers over SiC There is evidence that fission product diffusion will be slower in ZrC and that ZrC is more resistant to fission product attack.158 Second, its sensitivity to oxidation recommends ZrC as an effective oxygen getter, inhibiting the so-called ‘amoeba effect’ insidious in SiC-based TRISO, which involves a thermal gradient-induced diffusion of carbon and effective migration of UO2 fuel out of its protective coating layers, in part, due to an excessive oxygen potential (see, for instance, Bullock and Kaae).159 2.13.6.2.1 Thermal neutron capture cross-section Implementation of zirconium-based components in a nuclear reactor core, in fuel cladding or advanced composite fuel coatings and matrix, is enabled by its low thermal neutron capture cross-section Lowactivation structural materials are desired, and particularly for ZrC in fuels, diversion of neutrons from fission reactions is to be avoided The isotopic average cross-section for Zr for 0.0253 eV thermal neutrons (at room temperature) is 0.19 barn (1.9  10À29 m2), and for C is 0.0035 barn (3.5  10À31 m2).160 Owing to similar chemistry, Zr naturally occurs together with up to wt% Hf unless specially purified for nuclear reactor applications (1 MeV) fluence of (0.4 –5.4)  1021 cmÀ2, including 63–142 thermal cycles from room temperature to the irradiation temperature during the experiment, in the Oak Ridge National Laboratory’s Engineering Test Reactor Damage due to thermal cycling alone was ruled out by out-of-pile tests Severe fracturing of the hot-pressed and slip-cast samples were noted for fluences above (2.5–3)  1021 cmÀ2, with the explosion-pressed (containing Co or Ni binder) samples showing minor to severe damage above 1.1  1021 cmÀ2 Of the 2–3% radiation-induced volumetric swelling measured for all sample forms, 1% was accounted for by lattice parameter expansion, with the authors proposing point defect clusters and gas bubbles as causing the remainder Helium gas is produced in carbides through fast-neutron reactions with carbon, and the authors considered the possibility of hydrogen produced by thermal neutron reactions with nitrogen impurities Swelling increased with fluence up to  1021 cmÀ2, but further fluence resulted in the same or lesser degree of expansion Keilholtz et al.163 expanded this study to irradiation at 1273–1373 K at a fluence of 2.4  1021 cmÀ2 Swelling was lower at the higher temperatures, with 0.18 MeV) fluence of  1021 cmÀ2, to a burnup of 70% fissions per initial actinide metal atom (%FIMA) for fissile and 8% FIMA for fertile particles, in the Commissariat a` l’Energie Atomique’s Siloe Test Reactor Postirradiation examination revealed that the only ZrC damage was radial cracking in almost all the particles with ZrC directly atop kernels; as this configuration lacked buffer volume to accommodate fission gases, such failures were expected ZrC in the standard TRISO configuration was crack-free No diffusion or reactions between actinides/fission products and ZrC was evident in any configuration Ogawa et al.168 fabricated TRISO particles with a 20–30 mm thick ZrC layer and UO2 kernel and compacted these in a graphite–resin matrix at 2073 K Irradiation by fast neutrons (>0.18 MeV) under various conditions (1173–1873 K, fluence (1–2.2)  1021 cmÀ2, burnup of 1.5–4% FIMA, during 81–156 effective full-power days) was carried out in the Japan Materials Test Reactor at the Japan Atomic Energy Research Institute Following deconsolidation of the fuel compacts to recover loose particles, ceramography showed no ZrC degradation in particles irradiated at the highest burnup, fluence, and temperature for over 135 effective full-power days A zero particle failure rate was assessed by in-reactor monitoring of fission gas 85Kr release rate during 80 effective days of irradiation at 1173 K, which was an order of magnitude smaller than that expected for a single particle rupture out of 7000 particles in this test Minato et al.169 irradiated similarly prepared compacts between 1673–1923 K with a fast neutron (>0.18 MeV) fluence of 1.2  1021 cmÀ2 to 4.5% FIMA burnup during 100 effective full-power days, finding a through-coating failure rate of 0.01%, that is, failure of less than one out of 2400 particles irradiated Some of the particles irradiated by Ogawa et al.168 were subsequently heat treated at 1173–2273 K Particles originally irradiated at 1173 K to a fluence of 1.2  1021 cmÀ2 and burnup of 1.5% FIMA during Properties and Characteristics of ZrC 80 effective days were annealed in flowing He by Minato et al.170 at 1873 K for a total of 4500 h and by Minato et al.120,171 at 2073 K for 3000 h and at 2273 K for 100 h In situ 85Kr release monitoring showed none of the 100 particles annealed during each test ruptured Ceramography and X-ray microradiography of particles from each test showed that at 1873 K, there was no thermal degradation or corrosion of ZrC coatings At 2073 K, the ZrC coating was intact, but there was some surface roughness attributed to thermal degradation and evidence of attack along the grain boundaries At 2273 K, all but of the 100 particles showed some failed or damaged coating layers, including the ZrC layer, and evidence of reaction or interdiffusion between ZrC and U Interpreting the results via a thermodynamic analysis of the Zr–C–U–O system, the authors attribute deterioration of ZrC at these temperatures, at least in part, to mechanical failure of the inner pyrocarbon layer combined with oxidation of carbon by the oxygen released during the transmutation of U in UO2 Subsequent exposure of ZrC to CO(g) oxidizes ZrC to ZrO2 and C and reduces the ZrC coating integrity Durability of ZrC is more likely to be limited by irradiation of the system contained within rather than by high-temperature degradation of ZrC alone Ogawa and Ikawa172 subjected unirradiated TRISO particles having either UO2 or (Th,U)O2 kernels to annealing in He atmosphere for h at 2073 K (as during fuel compact fabrication) followed by h at 2173–2823 K ZrC coatings on all (Th,U)O2 particles were intact after h at 2823 K, but durability of UO2 particles was guaranteed only to 2373 K, with swelling noted at 2723 K and U migration out of the ZrC coating at 2773 K ZrC grain growth and plastic deformation occurred, especially above 2723 K 2.13.6.2.3 Microstructural changes under heavy ion or proton irradiation Heavy ion irradiation by Kr has been used to simulate some aspects of fission neutron irradiation, such as high damage rate (up to 100 dpa) Gan et al.173 irradiated TEM foils of commercial hot-pressed ZrC0.99 (the authors report a C/Zr ratio of 1.01, but the composition was corrected to reflect the impurity content of 1.9 wt% Hf, 0.19 wt% Ti, 0.21 wt% O, and 0.61 wt% N, considering that the metals and nonmetals substitute for Zr and C on their respective sublattices) Irradiation was conducted at 298 or 1073 K to >1 MeV Kr ions to a fluence of 367 2.5  1015À1.75  1016 cmÀ2 (10–70 dpa), with in situ TEM of microstructural evolution during irradiation Lattice parameter swelled by 0.6–0.7% ($2% volume increase) at 10 dpa (298 and 1073 K), 0.9% ($3% volume increase) at 298 K and 30 dpa, and 7% (21% volume expansion) at 1073 K and 70 dpa Simultaneously, precipitation of a fcc phase with 8% larger lattice parameter (5.09 A˚) than the matrix (4.71 A˚) was detected by ring patterns superimposed on the single-crystal ZrC electron diffraction pattern Precipitate coarsening with temperature and fluence was observed Energy-dispersive X-ray spectroscopy (EDX) detected no change in stoichiometry during irradiation The authors linked the precipitate phase and the 7% lattice parameter increase at high temperature and fluence, but could not explain adequately its origin, hypothesizing that the expansion was related to Kr implantation Cubic ZrO2 formation is also plausible (a $ 5.1 A˚) They acknowledged that the large ratio of surface area to volume in a TEM foil may permit larger lattice expansion than is possible in the bulk Other microstructural features noted were grain boundary cracking at high fluence, defect clusters at low temperatures and fluence, and dislocation segments at high temperatures No irradiationinduced voids or amorphization were detected Because of very small irradiated volume (depth