Comprehensive nuclear materials 5 07 performance of aluminum in research reactors

Comprehensive nuclear materials 5 07 performance of aluminum in research reactors Comprehensive nuclear materials 5 07 performance of aluminum in research reactors Comprehensive nuclear materials 5 07 performance of aluminum in research reactors Comprehensive nuclear materials 5 07 performance of aluminum in research reactors Comprehensive nuclear materials 5 07 performance of aluminum in research reactors v

5.07 Performance of Aluminum in Research Reactors K Farrell Formerly of Oak Ridge National Laboratory, Oak Ridge, TN, USA ß 2012 Elsevier Ltd All rights reserved 5.07.1 5.07.2 5.07.2.1 5.07.3 5.07.3.1 5.07.3.2 5.07.4 5.07.5 5.07.6 5.07.6.1 5.07.6.2 5.07.6.2.1 5.07.6.2.2 5.07.6.2.3 5.07.7 5.07.7.1 5.07.7.2 5.07.7.3 5.07.7.4 5.07.8 References Introduction Typical Applications History of Aluminum Applications in Research Reactors Properties of Aluminum Practical Characteristics Alloy Types, Temper Designations, and Tensile Properties Fuel Elements Corrosion Radiation Effects Basics Microstructures Fluence Temperature Transmutation products Property Changes Swelling Mechanical Properties Effects of Neutron Spectrum Radiation Softening, Creep, and Stress Relaxation Conclusion Abbreviations AIME ANL ANSI ASM ASTM ATR CRC CTE EBR-II Emod ETR GR HEU HFIR HPRR IAEA INL IRV-M2 American Institute of Mining, Metallurgical, and Petroleum Engineers Argonne National Laboratory American National Standards Institute American Society for Metals American Society for Testing Materials Advanced Test Reactor Chemical Rubber Company Coefficient of thermal expansion Experimental Breeder Reactor-II Modulus of elasticity Experimental test reactor Graphite Reactor Highly enriched uranium High Flux Isotope Reactor High performance research reactor International Atomic Energy Authority Idaho National Laboratory Acronym for a recent Russian research reactor LANL LEU MTR OPAL ORNL ORR PIE PIREX RERTR RR SNF STP TRIGA TEM UTS VPH YS 144 144 144 145 146 147 149 153 158 158 159 160 161 161 166 166 166 169 170 173 173 Los Alamos National Laboratory Low enriched uranium Specifically, MTR is the Materials Testing Reactor at Idaho National Laboratory Also used generically for materials test reactors Open Pool Australian Light water reactor Oak Ridge National Laboratory Oak Ridge Research Reactor Post irradiation examination Proton Irradiation Experiment facility Reduced enrichment for research and test reactors Research reactor Spent nuclear fuel Special Technical Publication Test, research, isotopes, general atomic Transmission electron microscopy Ultimate tensile stress Vickers pyramid hardness Yield stress 143 144 Performance of Aluminum in Research Reactors 5.07.1 Introduction Aluminum alloys are generally too weak or have temperature limitations that preclude their use in reactors built to produce electricity, high-temperature process heat, or marine propulsion But in the milder conditions in most research reactors (RRs) where bulk water coolant temperatures are usually 0.11 MeV, and the thermal fluence will be 4.5  1026 n mÀ2 In Al, this fast fluence will generate gas concentrations of 11 appm He and 63 appm H, and in Fe there will be appm He and 95 appm H In Zr, which is noted for its low cross-sections, only about 0.25 appm He and appm H would be produced The atomic displacement levels for the three metals can also be calculated They are 36 dpa for Al, 18 dpa for Fe, and 17 dpa for Zr The larger dpa level in Al is due primarily to its low effective displacement energy, 25 eV versus 40 eV for Fe and Zr There are three interesting reactions with thermal neutrons that produce gases from the foreign elements Li, B, and Ni, which may be present in some Al alloys The spatial distributions of gases from these sources are each different Lithium has high solubility in Al and generates a uniform distribution of gases Boron and Ni are insoluble and they produce localized concentrations of gas Lithium is not a common impurity in Al, but there is a commercial series of Al–Li alloys developed for their lightweight highstrength properties Laboratory-made Al–Li alloys have been used to study radiation hardening, helium embrittlement, and swelling.63,64 Natural Li contains 7.5% of the 6Li isotope, which has a capture crosssection of about 950b and decomposes to He and tritium via the reactions 6Li ỵ nth ! 7Li ! 4He ỵ 3H The present writer65 made an Al–0.052 wt% Li alloy using 6–9 high-purity Al and Li enriched to 98% with 6Li, and irradiated it to a dose of 5.5  1025 n mÀ2, E < 0.0253 eV and 2.2  1025 n mÀ2, E > 0.1 MeV at $55  C About 95–99% of the 6Li was burnt up to produce about 2200 appm each of He and tritium The atomic displacement level was about dpa, not including any displacements from the recoiled gases The effects of these high levels of gases were striking, see Figure The insert is an enlarged view of the matrix cavities Compared with irradiated pure Al control specimens, the concentrations of matrix cavities were increased 1000-fold, and their sizes decreased tenfold; dislocation densities were increased tenfold Most grain boundaries were crammed with large bubbles, many so interconnected that it was difficult to obtain thinned foils for TEM examination because the grain boundaries were eaten away before much thinning of the grain interiors 0.1 mm mm Figure Modification of void structure by very high helium and tritium levels from burnup of 6Li Reproduced from Farrell K.; Houston J T Combined Effects of Displacement Damage and High Gas Content in Aluminum, ORNL-TM-5395; Oak Ridge National Laboratory: Oak Ridge, TN, May 1976 Also available in Proceedings of International Conference on Radiation Effects and Tritium Technology for Fusion Reactors, Gatlinburg, TN, Oct 1–3, 1975, U.S Department of Commerce CONF-750989, Mar 1976; pp II-209–II-233 occurred The grain boundary in Figure is one with a low concentration of bubbles Hardness measurements gave a Vickers pyramid hardness (VPH) of 137 MPa for the annealed, unirradiated Al and the alloy, and 382 and 902 MPa for the irradiated specimens In bend tests made in air and liquid nitrogen (LN), the unirradiated materials and the irradiated pure Al were bent through full circles without rupture The irradiated alloy broke with an audible crack and with no detectable plastic strain Fracture was accompanied by release of tritium The fracture surfaces displayed 100% intergranular failure These are incredible hardening and embrittling effects of the gases Electron microcopy examination of carbon replicas taken from the fracture surfaces showed huge irregular interconnected bubble cavities Failure occurred by plastic tearing of the small areas of intact grain boundaries between the cavities Postirradiation annealing treatments caused the appearance of a coarse distribution of large facetted matrix cavities superimposed on the small matrix cavities, and with no denuding of the surrounding small cavities These enlarged cavities were frequently associated with large silicon particles that grew concurrently during the anneals An anneal at 500  C showed incipient disintegration of the specimens and TEM foils could not Performance of Aluminum in Research Reactors be obtained It was postulated that the large cavities grown during annealing were tritium bubbles Al often contains trace quantities of B in the form of small B4C inclusions Natural B contains 19.8% 10B, which has a large neutron capture cross-section of 3835b, producing Li and He via 10 B ỵ nth ! 11B ! 7Li ỵ 4He The range of the recoiled He atoms is about mm, and the He is segregated in a well-defined band in a halo around the parent inclusion The larger Li atom has a smaller range and is soluble; it forms a diffuse halo At low irradiation temperatures and low doses, the halos are very prominent because of heavy decoration with dislocation loops As the dose increases, the loops grow and move off leaving dense halos of voids, especially for the He halo The writer has seen hundreds of these halos Most of them were circular or near circular, with an occasional cigar shape, depending on the shape of the mother particle Most were isolated randomly, but some were in groups or were strung in chains on a grain boundary One is illustrated in Figure This particular halo is slightly squashed, following the elliptical contour of the central particle The dark region of the outer halo is actually filled with small cavities, resolvable at higher magnification Where a halo intercepts a grain boundary the voids seem to disappear, but during annealing they become visible as bubbles on the boundary that grow mm Figure Damage halos around a suspected B4C particle in 1100-OAl irradiated to 2.9  1026 n mÀ2 at about 55  C 163 faster than those elsewhere in the halo Such highly heterogeneous distributions of transmutant gas have been perceived more as a novelty than as a possible threat to the integrity of the host material This attitude may be unwarranted A highly localized concentration of helium in a patch on a grain boundary could be a prime site for premature helium embrittlement at stresses and temperatures below the ranges for normal helium embrittlement elsewhere in the specimen A spongy helium halo that intercepts the surface of Al cladding may provide a potential site for initiation of local corrosion For these reasons, it might not be a good idea to consider placing particles of B4C burnable poison in single-layer cladding on monolithic LEU fuel plates; a better location would be in the inner layer of a two-layer cladding Some of those considerations apply to He produced from Ni in Al It comes from the 59Ni isotope, which is not found in natural Ni The 59Ni must first be created from 58Ni that comprises 68.1% of natural Ni The two-step process66 to yield the He is 58 Ni ỵ nth ! 59Ni; 59Ni ỵ nth ! 56Fe þ 4He Helium generation via this route does not scale linearly with time It is slow to start while the 59Ni accrues, then it increases as the square of the fluence It is favored by long-term exposures or strongly thermalized neutron spectra Only trace quantities of Ni are found in most Al alloys except the X8001 and AlFeNi-type alloys, which contain a nominal wt% Ni These alloys were developed for cladding because early laboratory corrosion tests indicated they might have better corrosion resistance than existing cladding alloys Trials of the X8001 have not shown superior performance The alloying elements in X8001 are insoluble in the solid alloy and form intermetallic inclusions that are malleable and become deformed and extended into stringers during unidirectional rolling and extrusion processing The helium atoms formed from the Ni in the stringers are recoilimplanted into the near-matrix regions surrounding the stringers where they accelerate local formation of voids and dislocations The recoiled 56Fe atoms cause extra dpa locally.67 These He-enriched regions are not as obvious as those around B4C inclusions They are indicated by higher concentrations of voids, and the emergence of more numerous He bubbles during postirradiation annealing This localized damage offers an explanation of a hitherto inexplicable puzzle found in the corrosion response of X8001 alloy A characteristic of extruded X8001 tubes undergoing aqueous corrosion in reactors is that smooth shallow troughs or discontinuous ruts lying in the direction of 164 Performance of Aluminum in Research Reactors the tube axis are created in the corroded surfaces.68 Such troughs have not been reported in laboratory corrosion tests of X8001 It is suggested here that the troughs are stringer beds left when the Ni-rich stringers are eased out of the surface by selective corrosion/erosion of the more highly damaged He-rich matrix at the stringers Localized enhancement of He at Ni-rich stringers is also believed to play a major role in the occurrence of axial cracking of the X8001 cladding on HFIR long-term isotope target rods.69,70 The target material is a cylindrical compact of actinide oxides in an Al powder matrix, 6.3 mm diameter  14.5 mm long, each jacketed in 1100Al The meat contains about 10% porosity to accommodate fission gases A target rod consists of 35 jacketed capsules stacked in a tube of X8001 alloy that is hydrostatically compressed around them to form the outer cladding The tubes are made by extrusion and have six equally spaced longitudinal fins Before the target slugs are loaded into the tubes, most of the fins are machined off, leaving short lengths of fins at several locations along the active length of the rod The loaded target rod is slid into an X8001 tubular sheath with hexagonal ends, known as a hex can The groups of remnant fins along the length of the rod act as spacers that centralize the rod in the hex can and maintain an annular water-cooling channel around the rod A bundle of 31 sheathed target rods just fills the vertical HFIR trap Cooling water flows inside and outside the hex cans Cracks were found in the mid length, high flux regions of the target rods during a search for the source of a-contamination detected in the exiting coolant Investigation showed that the cracks were intergranular and were oriented in the length of the rods at locations where the fins had been removed Lengths were up to 66 mm The cracks originated in the target rod cladding, but some of the larger ones had penetrated the 1100Al jackets of the target slugs There was no evidence that corrosion was involved The hex cans were not cracked The exposure history of the rods is that they were first irradiated for about year in the D2O environment of the C reactor at Savannah River Nuclear Laboratory at a temperature of 20  C They were returned to ORNL and inserted in the HFIR at 46  C where, during their fifth fuel cycle, the a-leak was detected The summed fluences were $6.9  1026 n mÀ2, thermal, and 1.2  1026 n mÀ2, E > 0.82 MeV The conclusion from the investigation was that gas swelling of the target meat had imposed a hoop stress on the radiation-damaged cladding that had become too stiff to undergo plastic flow and had cracked instead Rupture tests on unirradiated lengths of the cladding tubes by internal pressure caused failure along the machined-off fin lines, indicating the lines were weak regions in the tubes A possible solution to the cracking problem was to decrease the swelling-related hoop stresses on the cladding by raising the pore volume in the target meat from the then current 10% level to 20% and 25% Trials were successful, and the cracking no longer occurs The questions of why the cracks were located only on the fin lines and why it was intergranular were not answered, but were pursued.71 The intergranular nature of the fractures made helium embrittlement a suspect However, calculations of helium levels from the 27Al(n, a) reaction with fast neutrons had given $7 appm, which was considered inadequate for helium embrittlement at the low irradiation temperatures experienced by the rods But supposing it was occurring, why did it favor the fin lines? Metallographic and TEM examinations of pieces of unirradiated cladding tubing showed that the extrusion process had stretched the inclusion particles into stringers and forced sheets of them into the fins from which they extended back into the tube wall Removal of the fins left behind an aggregation of stringers protruding into the tube wall Ergo the weakened regions along the fin lines seen in the rupture tests of the unirradiated tubing When the enhanced production of helium from Ni by thermal neutron capture was announced,66 a connection between aggregates of Ni-rich stringers and helium embrittlement was discerned Irradiated pieces of high-purity Al, 1100Al, and a X8001 hex can were sent to a specialist laboratory for helium analyses For a common thermal fluence of $1  1026 n mÀ2, the results were 1.8, 4.8, and 9.5 appm, respectively After a fluence of $3  1026 n mÀ2, the corresponding values were 7.2, 18.1, and 145 A piece of hex can irradiated to 5.8  1026 n mÀ2 yielded 220 appm A piece of 6061Al at 13.8  1026 n mÀ2 gave 47 appm, much less than the hex can at 5.8  1026 n mÀ2 These results leave no doubt that the presence of Ni in Al irradiated to high thermal neutron fluences greatly boosts the helium levels And since the He will remain close to the Ni particles, there must be very large concentrations around the stringers Any grain boundaries overlapped by those local helium clouds will be prime candidates for helium embrittlement cracking under the influence of hoop stresses Hence, the intergranular cracking at the fin lines In most metals, the gaseous transmutation products play a larger role in the development and effects of Performance of Aluminum in Research Reactors radiation damage structure than the nongaseous transmutants, one reason being that most construction metals not produce much nongaseous transmutants Al is different Depending on the degree of thermalization of the neutron spectrum, Al can produce large quantities of silicon from the two-step reaction 27Al ỵ nth ! 28Al ỵ g; 28Al ! 28Si ỵ b A rough guide to the quantity expected annually in the HFIR PTP spectrum can be obtained by multiplying the thermal neutron fluence by 230mb, the standard thermal neutron (0.0235 eV, 2200 m sÀ1) absorption crosssection for Al The result is 1.035 at.% Si (1.073 wt%) The Si is insoluble in Al at temperatures below about 350  C and is usually manifest as a precipitate of elemental Si.72 This precipitate makes a substantial contribution to radiation damage in Al, and is the dominant hardening agent at high thermal neutron fluences There is one outstanding qualifier to that generalization In the 5xxx-type Al–Mg solid solution series, the free Mg atoms dissolved in the Al will react with the atoms of transmutant Si to form a precipitate of Mg2Si.73 Thus, a 5xxx series alloy will be converted to a 6xxx-like alloy.73–76 Figure shows the Mg2Si microstructure formed in irradiated 5052-O alloy Because this precipitate occurred at a temperature below the usual 160  C aging temperature used to obtain the T6 tempered condition in 6061Al, the Mg2Si precipitate developed in the 5000 alloy is finer than in the 6061-T6 alloy The microstructure of heavily irradiated 6061-T6 alloy is illustrated in Figure Since there are usually larger quantities of Mg in the 5xxx alloys than in the 6xxx alloys, a greater volume of Mg2Si can be created in the former 5xxx alloys Hence, irradiated 5xxx alloys will undergo radiation hardening and precipitation hardening simultaneously, and their overall hardening rate will be larger than in other Al alloys exposed to the same neutron fluence Note that there are no voids in Figure Sparsely distributed voids are found75 at a higher fluence of 1.8  1027 n mÀ2 At half of that dose, the 6061-T6 alloy contains many more voids, Figure The association of the transmutant Si with voids is interesting We saw earlier that voids and particles of Si become visible in the microstructure at about the same dose The voids are larger and fewer than the Si particles A Si particle is usually attached to one facet of a void, and that particle is larger than its unattached brethren in the matrix It is also facetted As irradiation continues, a change occurs in the void-Si relationship The voids lose their facetted shape and become rounded.77 They are completely covered with a thin coating consisting of mostly Si 165 100 nm 200 000 220 020 Figure Precipitates of Mg2Si and excess Si in formerly 5052-OAl irradiated to 5.7  1026 n mÀ2 at $55  C Reproduced from Farrell, K J Nucl Mater 1981, 97, 33–43, with permission from Elsevier 0.1 mm Figure Voids with background precipitate of radiation-produced silicon and silicon-decorated original Mg2Si precipitates in 6061-T6Al irradiated to $1027 n mÀ2 at $55  C 166 Performance of Aluminum in Research Reactors 0.1 mm Figure 10 Si particles and Si-coated voids on a carbon extraction replica from 1100-OAl irradiated to 1.4  1027 n mÀ2 (E > 0.1 MeV) and 2.3  1027 n mÀ2 (E < 0.025 eV) at 55  C Reproduced from Farrell, K.; Bentley, J.; Braski, D N Scripta Metall 1977, 11, 243–248 with some Al The coating is noncrystalline and flexible The unattached Si particles in the matrix are also rounded but are crystalline The coated voids jut out of the thinned edge of the hole in TEM foils and can be lifted from the matrix on carbon extraction replicas Figure 10 is an example The larger features with the dark rims are the coated voids Four of them have partially collapsed without breaking, indicating a highly ductile coating Many of the Si particles seem to have a layered structure The Al–Si system is a simple eutectic; there are no compounds It is suspected that the small amount of Al found in the void coatings may be from the Al matrix that was not completely dissolved from the voids during the electrolytic extraction process Silicon is obviously involved in void formation and growth but its specific role is unclear 5.07.7 Property Changes 5.07.7.1 Swelling Radiation swelling is the increase in volume arising by accumulation of voids from excess vacancies and by formation of gas bubbles For Al, there are also small swelling contributions from build-up of particles of transmuted Si and, in 5xxx alloys, Mg2Si, which have densities of 2329 and 1990 kg mÀ3, respectively.78 Gas bubble swelling is not an issue for Al in RRs because the temperature is too low, except perhaps in fuel cladding where some pores found in the cladding may have been formed by the accumulation of hydrogen Swelling can be measured from dimensional changes More often it is determined from changes in immersion density values Swelling in various Al alloys is shown in Figure 11 These alloys were all irradiated in the core of the HFIR and they make the most comprehensive and consistent set of swelling data.79 For reference, the dotted line is estimated for the swelling from Si alone It is evident that the unirradiated chemical compositions and microstructures have major effects on the degree of radiation swelling The purest grades, sixnines and four-nines, show swelling earliest in dose and swell at the highest rates with dose The rates decrease above a dose of about  1025 n mÀ2 Swelling in the two-nines grade (1100-O) requires significantly higher doses, but the swelling rate is unchanged The 6061-T6 alloy, with its inherent Mg2Si phase, starts swelling appreciatively later in dose than the 1100-O This is traceable to reduced nucleation of voids, but its swelling rate is about the same as the other alloys The greatest resistance is in the 5052-O alloy There, the swelling is less than for the Si alone In this alloy, much of the early swelling is not due to voids; it is caused by the silicon and the new Mg2Si phase and by the increase in the original density of the matrix, r0, as Mg is drawn from solution to create the Mg2Si The effects of prior cold work on swelling in Al agree in general that the presence of cold work dislocation structure decreases the overall void swelling but the reduction is not massive; concurrence of dislocation recovery confuses the details.80–83 5.07.7.2 Mechanical Properties The major consequences of radiation damage structures on the mechanical properties of Al alloys are radiation hardening and associated loss in ductility There are too many data from too many sources to be described in detail here A good source of compiled data, including the sparse information on fracture toughness and weldments, is Marchbanks.84 For 6061Al in particular, see Farrell.85 Strengthening and loss of ductility are demonstrated best in tensile properties In Figure 12 we can directly compare the changes in strength and ductility of different alloys irradiated and tested under the same conditions.79 The most striking feature is the relatively rapid hardening displayed by the 5052-O alloy As explained earlier, this is caused by the combined effects of radiation damage and in-reactor Performance of Aluminum in Research Reactors wt % Si 0.1 dpa 0.1 10 10 10 100 6-9 4-9 2-9 (1100-O) 6061-T6 5052-O ( ri ) % Ti = 328 K (0.35 Tm) 6061-T6 Swelling ro−ri 167 1100-O 5052-O 0.1 Pure aluminum 28 Si 0.01 1024 1025 1026 Fluence (n m-2 > 0.1 MeV) 1027 Figure 11 Radiation-induced swelling in various Al alloys as a function of fast fluence Reproduced from Farrell, K In Proceedings of the Conference on Dimensional Stability and Mechanical Behaviour of Irradiated Metals and Alloys, Brighton, Apr 11–13, 1983; British Nuclear Energy Society: London, 1983; Vol 1, pp 73–76, with permission from British Nuclear Energy Society (now The Nuclear Institute) formation of a fine precipitate of Mg2Si In contrast, the 6061-T6 alloy, which contains Mg2Si before irradiation, begins radiation hardening at about the same fluence as the 1100-OAl, and hardens thereafter at the same rate The 1100-O alloy contains no Mg2Si before or after irradiation From which we deduce that preexisting Mg2Si precipitates play no role in radiation hardening This is an interesting conclusion It contradicts the expectation that the precipitates would diminish the degree of radiation hardening in 6061-T6Al by promoting the recombination of freely migrating vacancies and interstitials Perhaps that expectation is wrong But it seems satisfactory for explaining the delayed swelling in the 6061-T6 alloy, where nucleation of voids is retarded, perhaps until the transmutant gases enable achievement of critical size cavity nuclei Alternatively, maybe the radiation-produced Si dominates the hardening process It is a mystery The fluence for the onset of radiation hardening in the weak 4–9Al is about one order of magnitude less than in the other alloys, and the subsequent rate of hardening is less than for the others Here, again, we invoke the recombination argument This higher purity material contains less solutes and inclusions Thus there is less trapping and annihilation of freely migrating point defects, hence more point defect clusters are formed in the early stages of irradiation It is suspected that the reduced rate of hardening is connected with dynamic recovery of deformation during the tensile test It was pointed out in Section 5.07.3.2 that recovery from cold work occurs readily in high-purity Al at room temperature Loss in uniform elongation in all of the alloys is concomitant with increase in strength to a point At a fluence of about 1026 n mÀ2 the ductility reaches a plateau of 3–5% even though the strength continues to rise The 1100-OAl has the least ductility in the plateau region and displays an intergranular-like fracture mode that may be caused by tearing and void interconnection in the void-rich regions lying alongside the grain boundaries Performance of Aluminum in Research Reactors 600 Stress (MPa) 500 Tirr = 328 K Ttest = 323 K ~ 10-4 S-1 ' 168 UTS 0.2% FS 400 300 6061-T6 200 5052-O 100 1100-O 4-9AI Uniform elongation (%) 40 30 20 4-9AI 5052-O 1100-O 10 6061-T6 1023 1024 1025 1026 1027 -2 Fluence (n m > 0.1 MeV) Figure 12 Radiation-induced changes in room temperature tensile properties of various Al alloys Reproduced from Farrell, K In Proceedings of the Conference on Dimensional Stability and Mechanical Behaviour of Irradiated Metals and Alloys, Brighton, Apr 11–13, 1983; British Nuclear Energy Society: London, 1983; Vol 1, pp 73–76, with permission from British Nuclear Energy Society (now The Nuclear Institute) Irradiated metals undergoing plastic deformation are prone to strain localization, seen as dislocation channels swept free of point defect clusters Such channeling coincides with changes in the tensile test curve, notably introduction of a sharp yield point and a reduction in the slope of the strain-hardening portion of the curve In severe cases, a yield point drop occurs that leads directly into prompt necking Dislocation channeling has not been reported for neutron-irradiated Al Nor the tensile curves for neutron-irradiated Al display sharp yield points and pronounced decreases in work hardening rate The reason for this apparent resistance to channeling in Al is not known Al is not immune to channeling It occurs in unirradiated quenched-and-aged Al When high-purity Al crystals are quenched from near the melting point into water at room temperature or into iced brine, then aged at room temperature for 1–4 days or at 60  C for h, there is a considerable increase in yield stress (YS) and large decrease in strain hardening rate.86,87 Such quenchaging treatment produces many small vacancy clusters Yielding occurs abruptly, and large widely spaced slip bands appear on the specimen surfaces, forming mirror images from one surface to the opposite surface TEM examination87 reveals that Performance of Aluminum in Research Reactors A5 AI, ~35% CW, 77 K (0.08 Tm) 400 ~1 ϫ 1023 n m-2, E > MeV Stress (MPa) 300 ~5 ϫ 1021 n m-2 7.5 ϫ 1021 n m-2, then held 60 h at room temperature before test at 77 K 200 Unirradiated 100 Unirradiated, tested 293 K 0 10 20 30 40 Elongation (%) Figure 13 Tensile curves of pure Al irradiated and tested at cryogenic temperatures, showing recovery at room temperature Reproduced from Bochirol, L.; Brauns, P.; Claudet, G Prog Refrigeration Sci Technol 1973, 1, 643–650, with permission from International Institute of Refrigeration, AVI Publishing Company the slip bands are dislocation channels prominently displayed against the background of undisturbed vacancy clusters remaining in the matrix between the channels Another way to induce a sharp yield point and reduced strain hardening rate in Al is to irradiate and tensile test at cryogenic temperature Figure 13 gives tensile curves for an A5 alloy (>99.95% Al) in the half-hard, H24 (35% cold worked), condition after neutron irradiation and testing at 77 K in LN.88 A curve for an unirradiated specimen tested at room temperature is included as a reference condition Note that this unirradiated specimen has only 14% elongation, in keeping with its cold-worked condition When tested in LN the unirradiated material is not only stronger it is much more ductile, with an elongation of 44% This is an example of the enhancement of ductility at low temperature mentioned in Section 5.07.3.1 Irradiation to $5  1021 n mÀ2 in LN further increases the YS, and although the elongation falls a little, it is still larger than at room temperature; moreover, the initial part of the strain hardening curve to about 8% elongation is steeper than that for the unirradiated specimen, indicating no irradiation-induced dislocation channeling However, the reduced slope after 8% elongation and prior to onset of necking in the irradiated specimen signals possible late entry of channeling Note that for this dose at room temperature there would be no discernible radiation damage structure and no change in tensile properties In the LN specimen irradiated to the higher fluence 169 of $1  1023 n mÀ2 intervention of channeling is more likely There, very steep initial strain hardening passes though a peak at $1% elongation and plunges into prompt necking Even greater strengthening and more exaggerated prompt necking is found for both cold-worked and annealed specimens when the irradiation and testing are done at 27 K in liquid neon.88 The deformation modes were not determined for these specimens Identification of the deformation mode would have required TEM examinations of specimens cut from the gauge sections of the specimens and would probably not have revealed the channeling because the specimens would undergo rapid recovery from radiation hardening when they are brought up to room temperature Such recovery is illustrated by the curve for the specimen irradiated to $1  1022 n mÀ2 then held at room temperature for 60 h before testing in LN Almost complete recovery has occurred, indicating elimination of the point defect clusters needed to provide background contrast for detection of dislocation channels The good result of this recovery is that it relieves the stored energy built up by accumulation of the radiation damage microstructure at the cryogenic temperature Advantage is taken of this recovery process in Al cold neutron sources by performing periodic in situ room temperature anneals on them These anneals not erase the transmutation products Residual hardening from the gaseous products will be negligible but the accumulating Si, whose levels will be high due to the highly thermalized neutron spectrum at the cold source, will give increasing retained hardening as it forms precipitates at room temperature For cold source vessels constructed from 5xxx alloys, the transmutation-produced Si will react with dissolved Mg at room temperature to create naturally aged fine precipitates of Mg2Si, with greater hardening effects than from Si alone 5.07.7.3 Effects of Neutron Spectrum When tensile data for irradiated Al from different reactors or from different regions of a single reactor are compiled in a single traditional plot of property versus fast neutron fluence, there tends to be a large scatter in the data, particularly at the higher fluences It was noted89 that the scatter was reduced somewhat if the plot was made in terms of thermal fluence This improvement was attributed to a spectral effect involving the production of Si precipitates by the thermal neutrons and their modification by fast neutrons That explanation has since been taken a little 170 Performance of Aluminum in Research Reactors wt% silicon 0.1 0.01 10 Tensile strength (MPa) 800 6061-T6 aluminum grouping by fth/ft ratio 700 20 Unirradiated HFIR target HFBR surveillance HFBR V15 HFBR CRDF 600 500 21 21 21 5-81-3 22 400 1.3 57 53 0.5 300 200 1023 1024 1025 1026 Thermal fluence (n m- 2) 1027 1028 Tensile strength (MPa) 800 20 700 21 600 21 21 5-8 500 22 400 1-3 1.3 57 53 0.5 300 200 1023 1024 1025 1026 1027 1028 Fast fluence, E > 0.1 MeV (n m- 2) Figure 14 Radiation-induced strengthening of 6061-T6Al discriminated by thermal and fast fluence Reproduced from Farrell, K In Proceedings of Materials Research Society Symposium on Microstructure of Irradiated Materials, 1995; Vol 373, pp 165–170, with permission from Materials Research Society farther.90 Figure 14 presents tensile strength data for the 6061-T6 alloy irradiated in two reactors, the HFIR and the High Flux Beam Reactor (HFBR), at 50–65  C The same strength data are plotted against thermal fluence in the upper box and against fast fluence in the lower box Ignoring for the moment the lines in the plots, it is evident that at fluences above $1  1025 n mÀ2 the data points are more scattered when plotted against fast fluence The HFIR specimens were all irradiated at the same location where the ratio of thermal-to-fast flux, ’th/ ’f, was narrow, 1.7–2.3 The HFBR was cooled and moderated by heavy water, and the HFBR specimens were taken from regions with ’th/’f values between 0.5 and 22 When the data are discriminated by ’th/ ’f as indicated by the lines labeled with the large numerals, a pattern of dependence on ’th/’f emerges The trend is the same in both boxes but the separation is clearer in the fast fluence plot It is evident that when irradiation is conducted in a hard spectrum, ’th/’f ¼ 0.5, there is little increase in strength, even at high fluence As the spectrum softens, ’th/’f ! 21, the specimens harden This response is obviously due to greater generation of silicon precipitates in the more thermalized spectra TEM examinations of the more heavily irradiated specimens showed that the dominant microstructural feature introduced by the irradiation in both reactors was particles of Si They were more numerous and smaller in the softer spectrum, in agreement with the strengthening trend After eliminating possible contributions from differences in damage rates, irradiation temperatures, and exposure times, it was concluded that the spectrum controls the ripening behavior of the Si precipitates, which determines the degree of strengthening A hard spectrum causes more sputtering of the precipitates and more transport of the sputtered atoms, resulting in greater ripening of the precipitates and less strengthening per unit Si level or unit fluence 5.07.7.4 Radiation Softening, Creep, and Stress Relaxation There are some highly contentious reports of radiation softening of Al alloys that demand some discussion These are not claims of minor softening such as a decline in the strain-hardening region of a tensile curve They are full-blown changes from an agehardened or cold-worked condition to a dead soft, Performance of Aluminum in Research Reactors ‘annealed’ state And they occur in specimens that are not carrying a load The softening is usually interpreted as being due to destruction of dislocation tangles and precipitate particles through dynamic atomic displacements created by fast neutrons It is the present writer’s opinion that they are actually unintended instances of thermal softening by radiation heating, which can happen very easily and quickly in Al if adequate precautions are not taken to ensure adequate cooling during irradiation In each of these reports, the specimens were irradiated in sealed cans with external water cooling and no means of measuring the internal temperature Using sealed cans is an invitation to radiation heating of the contents unless very strict actions are taken to ensure satisfactory heat removal In the first91 of these reports, disks of 2024-O, 2024-T3, 1100, and sintered Al powder of undeclared Al2O3 content were placed in aluminum cans evacuated and back filled with helium The cans were irradiated in the MTR for 60 days to fast neutron doses of  1022,  1023, and 8.5  1023 n mÀ2 at estimated temperatures of 100, 119, and 138  C, respectively Hardness tests on the 2024-T3 disks showed: unirradiated control, Hv ¼ 170;  1022 ¼ 178;  1023 ¼ 68; 8.5  1023 ¼ 60, indicating a pronounced change from the age-hardened-T3 condition to a fully softened condition The hardnesses of the 1100 and the sintered aluminum powder alloy were unchanged by the irradiations This softening of 2024-T3 alloy conflicts with tensile and hardness data92 for a 2024-T3 or -T4 alloy irradiated to doses between  1023and 1.3  1026 n mÀ2 that show no change in properties to a dose of 1.2  1024 n mÀ2, above which there is hardening These latter data were for specimens that were irradiated in direct contact with flowing water to minimize nuclear heating The writer has also tested tensile specimens of 2024-T351Al that were irradiated in contact with flowing water; there was no change in properties for fluences between  1021 and  1024 n mÀ2, beyond which hardening was displayed.93 The second softening claim94 was made for tensile specimens of Al–3Mg in a cold-worked condition (degree of work not given), and 6061-T6 bombarded with 600–800 MeV protons in the Los Alamos Meson Physics Facility (LAMPF) Annealed specimens of the two alloys were included for reference Assemblies of seven specimens laid side-by-side were sealed in envelopes made from 0.13 mm thick foil of 1100Al Sixteen packages were stacked with gaps between them to allow cooling water to flow though at a rate of l sÀ1 and pressure of 17 bar (1700 kPa) to 171 ensure good contact of the foil with the specimens The beam spot size was 50 mm, the average current was 574 mA, and the exposure period was 750 h Displacement damage up to about 0.2 dpa was created by the direct impact of the protons and by highenergy neutrons spalled from surrounding materials and the nearby beam stop No Si was generated Helium and hydrogen contents were 67 and 275 appm From the beam parameters, it can be seen that the power input was about 200 MW mÀ2, which is about 20 times larger than the heat fluxes reached on fuel cladding The irradiation temperature for the LAMPF specimens was estimated to be 40–50  C, certainly not exceeding 100  C Postirradiation tests revealed that the yield strengths of the cold-worked Al–3Mg and 6061-T6 specimens were reduced by 70–80% and the ultimate strengths by 40–65%, which placed them at the same strength levels as those for unirradiated annealed specimens of the alloys The irradiated annealed alloys did not undergo irradiation hardening; they actually softened by 10–20% Metallographic studies and TEM examinations of the formerly cold-worked Al–3Mg alloy94,95 showed that recrystallization had occurred At a dose of only 0.1 dpa, the tightly packed dislocation cell structure was gone, replaced by very loosely tangled dislocations, and the grains were now equiaxed In the former 6061-T6 alloy, there were no longer any Mg2Si precipitates In the irradiated annealed Al–3Mg specimens, there were gas bubbles on the grain boundaries These microstructural changes are very much like those found for thermal anneals or high temperature irradiations; they require sustained, long range, high-intensity diffusion of vacancies and solutes Usually, irradiation with energetic particles does not create or support such condition unless radiation heating occurs Under adequate temperature control, radiation enhanced diffusion is highly local and sporadic and is rapidly quenched Normally, radiation attrition of precipitates results in precipitate atoms being recoiled into the adjacent matrix and there forming new satellite particles of the mother phase or a new phase A situation of total elimination, with no reappearance of the precipitate by aging, is more typical of a high-temperature solution anneal and a slow cool Nevertheless, the authors attributed these extreme changes in microstructure to the absence of transmutation-produced Si which stabilizes radiation damage microstructure in regular neutron irradiations, and to the different nature of the damage produced by high-energy protons, this despite efforts they and others were making 172 Performance of Aluminum in Research Reactors at the time to promote such irradiations as a substitute for slower neutron irradiations They were unable to account for the disagreement with an earlier 600 MeV proton irradiation of a solution-treated Al–Mg–Si alloy similar to 6061 at 150  C in the Swiss PIREX facility, which induced precipitation of a high density of Mg2Si precipitates.96 Strangely, they gave no consideration to the possibility that they might have overestimated the cooling efficiency of their irradiation experiment in LAMPF It should have been the prime consideration because at that time LAMPF was undergoing changes to make it suitable for materials irradiations None had been performed there previously The changes were incomplete but the experiment was made anyway It was a pseudo atest Since then, improvements have been made to the facility and many materials irradiations have been conducted that simulate reactor irradiations Later irradiations of 6061-T6 in LAMPF97,98 have not encountered the softening found in the inaugural proton irradiation The third softening claim99,100 was again for a cold-worked, nominal Al–3Mg alloy and for a precipitation hardened AlMgSi (6061-type) alloy Tensile specimens and annealed companion specimens were irradiated to fast fluences of 8.7  1021 and 2.5  1022 n mÀ2 in the wet channels of the Egyptian Nuclear Research Reactor The responses of the materials to both of these fluences were almost identical and are confusing The AlMgSi alloy in two different age-hardened conditions and its as-received cold rolled condition displayed large decreases in hardness, YS, and ultimate tensile stress (UTS) The Al–3Mg alloy in its as-received cold rolled condition showed large reduction in hardness and relatively small reductions in YS and UTS In a partially annealed condition (1 h at 240  C) its hardness was reduced 30% but its YS and UTS were increased by 55% and 27% In the fully annealed state (1 h at 400  C) the hardness was decreased by 5%, the YS was increased by a whopping 380%, and the UTS rose by 97% Although no microstructural studies were made, the authors concluded that the observed softening was due to removal of cold work dislocations and dissolution of Mg2Si precipitates as seen in Lohmann et al.94 and Singh et al.95 The conclusion that Mg2Si precipitates are unstable in a neutron flux is at odds with the well-established fact that the precipitates persist to very high fluences in 6061-T6 specimens that are properly cooled during irradiation, and they are actually generated by the irradiation in Al–Mg solid solution alloys The unusually large hardening caused in the partially and fully annealed Al–3Mg specimens was not discussed It was ascertained from the lead author that the specimens had been irradiated in sealed cans It was conveyed to the author that ongoing experiments at ORNL with 6061-T6 were not finding any signs of radiation softening at fluences similar to those in his experiments Some years later, he published a third paper101 on the AlMgSi alloy in four conditions: Cold rolled; annealed; naturally age hardened; and artificially age hardened The specimens were wrapped in Al foil and were packed in aluminum powder in sealed Al cans The fast neutron fluence was 3.7  1022 n mÀ2, somewhat higher than his previous irradiations Hardness and tensile tests revealed no softening of the cold-worked and age-hardened specimens The annealed specimens displayed a 23% increase in Vickers hardness and a 111% increase in yield strength This radical departure from the previous softening results was not credited to better cooling of the specimens during irradiation Rather, the newfound nonsoftening of the prehardened materials was attributed to the longer irradiation exposure It was explained that the prehardened alloys actually did go through a full softening process in the region of  1022 n mÀ2 as before, by the mechanism of radiation-induced dissolution of the cold work dislocations and Mg2Si precipitates This removal of these strong point defect sinks allowed irradiation damage microstructure to build up as irradiation continued At the exposure of 3.7  1022 n mÀ2 sufficient damage microstructure had formed to restore the strengths of the softened materials to their former values These claims of radiation softening, controversial as they are, serve a useful purpose They highlight the previously discussed temperature sensitivity of cold-worked and age-hardened aluminum Moreover, they send a message that in the event of an unplanned temperature excursion on such materials they should be examined carefully to ensure that they still meet the strength requirements for the application Radiation creep and stress relaxation are timedependent plastic deformation processes driven by applied external stresses or by stored internal stress They can be accelerated by point defects from atomic displacement events The few data that exist for radiation creep in aluminum are confusing and inconclusive Details are available in Farrell.85 Creep can lead to failure if the load is continuously applied Strain from stress relaxation ceases once the load is relaxed There are at least two cases81,102 of stress Performance of Aluminum in Research Reactors relaxation in Al reactor components Both were found in retired reactor components that had entered service in cold-worked conditions, one 1100Al and the other precipitation-hardened 6063Al During service they had been continuously in direct contact with the coolant water, so thermal interference was not involved Some softening occurred due to partial recovery of the cold work structure at low doses, followed by radiation hardening at higher doses Complete softening to an annealed level did not occur during irradiation 10 11 12 13 5.07.8 Conclusion 14 Aluminum and its alloys have contributed immensely to development of water-cooled RRs and to our understanding of radiation effects in metals They continue to so 15 16 Acknowledgments During preparation of this chapter, interactions with present and former Oak Ridge National Laboratory personnel, S A David, R D Godfrey, S J Pawel, J D Sease, and R E Stoller were sincerely appreciated Opinions and conclusions in this chapter are solely the responsibility of the writer 17 18 19 References West, C D Nuclear News, Oct 1997; 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