Specimens of 1100 aluminum were exposed to several fluences of 23.5-GeV protons at the Brookhaven Alternating Gradient Synchrotron. Although this energy is above those currently being proposed for spallation-neutron applications, the results can be viewed as indicative of trends and other microstructural evolution with fluence that take place with high-energy proton exposures such as those associated with an increasing ratio of gas generation to dpa. TEM investigation showed significantly larger bubble size and lower density of bubbles compared with lower-energy proton results. Additional testing showed that the tensile strength increased with fluence as expected, but the microhardness decreased, a result for which an intepretation is still under investigation

Czajkowski, C. J.

Snead, C. L. Jr.

Todosow, M.

UNT Digital Library

Investigation of High-Energy-Proton Effects in Aluminum* Carl J. Czajkowski, C. Lewis Snead, Jr., and Michael Todosow Brookhaven National Laboratory Upton, NY 11973-5000 . USA Abstract Specimens of 1 100 aluminum were esposed to several fluences of 23.5-GeV protons at the Brookhaven Alternating Gradicnt Synchrotron. Although this energy is above those cu~~cndy being proposed for spallation-neutron applications, thc results can be viewed as indicative of trends and other microstructural evolution with fluence that take place with high- energy proton exposures such as those associated with an increasing ratio of gas generation to dpa. TEM investigation showed significantly larger bubble size and lower density of bubblcs compared with loweraerg proton results. Additional testing show& that the tensile strength increased with fluence as esqxcted, but the microhardness decreased, a result for which an intepretation is still under investigation. 1100 Aluminum Radiation will change the internal structure of many materials. In this research the effects of high-energy protons (23.5-GeV) on 1 IO0 aluminum are emphasized . The effects of radiation on a niatcrial, of course, differ from general heating, in that radiation may localix’large amounts of energy, whereas elevated temperatures will normally energize all of the atoms. The piimaiy processes of radiation damage in metals are transmutations and atomic displacements. Atomic displacements in a solid material can result in the alteration of many of its properties (e.g., increases in hardness and strength with concurrent decreases in toughness and ductility). Some of the earliest work [ 1 ] in this area considered this mechanism and subsequent gas production to be major contributors to mechanical-property changes. Proton Irradiation RECEIVED (for production of military-use tritium) was evaluated by the Energy Research Advisory Board (ERAB). ERAB concluded that an accelerator option, Accelerator Producer of Tritium (APT), could produce tritium. However, the amount of tritium then perceived as necessary would have required an APT accelerator needing 1000 MWe (mega-watts electric) for operation. It was concluded (by E M )  that such a power requirement would push rlie operating cost into a noncompetitive region. In November 199 1, the Secretary of Energy decided that APT would be reevaluated. This evaluation was accomplished in January 1992 by the Jason panel, a DOE assipcd senior consultant group. This group concludcd that the APT option was technically feasible and could meet the demands of the projected new (lower) goal quantity of tritium. In April 1992, the Secretary of Energ directed that a development program be initiated for the APT concept to support the record of decision (ROD) (a record of decision is the date at which one or two ofthe various tritium production methods \ti111 be selected for construction). The performance of these conceptual designs is to be determined through analysis and limited codirmatoly esperiments to characterize the production performance, the environmental impact, and safety of an accelerator facility. The materials effort directed by this program included evaluating possible alloys for use in a I -GeV proton beam to an anticipated 1.7 x lo’’ D/cm* for target/window life. This value (-1.7 x loB p/cm’) was used as a base line projection for one year’s operation of a window in the APT beam. Aluminuni alloys [2,3,4,5,6,  ] are the first choice as structural materials for the APT primarily due to their favorable low thermal neutron absorption cross section. Aluminum alloys have been in use on accelerators and have a much faster radioactivity decay (than stainless) leading to lower ALARA (As Low As Reasonably Achievable) human esposure considerations When the Department of Energy (DOE) initiated the New Production Reactors (NPR) Program to replace the aging defense production reactors, the use of a linear proton accelerator during maintenance operations. Note: aluminum alloys decay approsimately five times faster than stainbss steels. It was necessary to verify the integrity of tlie alloys already in use in the accelerator and reactor communities to assure the safety of the present design. The materials test program consisted of evaluation and characterization of previously-proton-irradi: ired windowvhrget materials. Evidence of materials degradation through either microstructural or mechanical-property deterioration were to be investigated and evaluated. Comparison ta archive specimen mechanical/mimavmicrostruchnal properties were pczfomed whaever possible. The material testdevaluated and reported here is: Aluminum alloy I 100, proton irradiated in the BNL Alternating Gradient Synchrotron (AGS) at 23.5 GeV tcl a fluence of -10” p/cm2. Unirradiated archival material WAS also evaluated. Tensile tests were Monned on the procured archival stock materials. Additionally, microhardncss measurements were ma& on the I 100 aluminum. Microhardness nieasuranenls were performed skce the irradiated materials were in the fix-ni of thin sheets and this method of testing would enwre consistency of measured data. Table I lists thc rcsults of the tensile tcsts performed OR the archival stock. Tahle 1 1 100 Aluminum Mechanical Prc’peilics Specimen Number (a), Displacemcnt at Pcak (nun) (b), % Strain at Pe3k (%) (c), Load at Pcak fig) [d)? Stress at Peak (Mpa) (e), Stress at 0.2% Yield (Mpa) (0, % Strain at Break (%) (g), Stress at Break (Mpa) (h) a b C d e f 2  11 1 3.05 12.00 209.3 104.0 44.5 19.30 46.5 2 2.73 10.76 209.7 -*104.1 43.2 1S.76 65.1 3 2.53 9.98 209. 103.8 40.5 21.SS 6.9 Mean 2.8 10.91 209.4 104.0 42.7 19.9s 39.5 Standard Deviation 0.26 1.02 0.32 0.02 2.1 1.67 29.7 The microhardness data for the aichi~al materials is tabulated in Table 2. Two sets of microhardness values were recorded for each material. This was done to dcteiniine if one of the hvo methods of transmission electron inicroscope (TEM) spechien cutting impened work hardening to die material. The questionable metliod of specinien cutting involved a punch, which could cut a 3-mm diameter specimcn. The second method of TEM specimen cutting utilized electric discharge machining (EDM). EDM is a process in which the work piece is machined or eroded by elecn-ical energy of high density. In EDM. machining spark erosion takes place in a nonconducting or dielectric fluid. The work piece and EDM electrode are submerged in ?he dielectric, which is usually oil. The dielectric fluid concentrates the arc enerm and flushes away the material eroded from the work picce. Although no work- hardening effccts on the specimens cut by either of these mcthods were obmved, it was determined that EDM would be used, whenever possible, for ALARA considerations. Table 2 1 100 Aluminum Microhardness Tcsi Results (KN) 47,43,43,43,43,44 Avg. 44 KN Punch 43,43,44,43,43 Avg. 43 KN Electron Micmscopv Scanniny Electron Microscopv (SEMI Tlie li-acture faces of each of thc materials tcnsile tested were esamined by SEM. This evaluation \vas used to cvaluate the mode of fracture from thc uniasial tcnsile pulls. Figure 1 is thc low magnification fracture suiface associated with the 1 100 aluminum. The fixture was of the dimpled rupture type (Figure 2 )  which is typical of a ductile material failure. These dimples Figure 1. Low- magnification SEM photograph of the resultant fracture fiom a tensile failure in 1 100 AI. Areas a and B are locations of high-magnification fiactopaphs. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employes, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, mom- mendirtion, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Table 3 Microhardness and Tensile Test Results on 1 100 Aluminum AGS Run (23.5 GeV, 1 O’$/cm?) Specimen Irradiated/ Tensile Str Hardness Unirradiated (Mpa) KN (Avg. of 5 )  1 Un 100.2 2 Un 100.7 3 Un 100.7 4 IIT 103.4 5 IIT 104.5 6 TIT 105.0 7 In- 101.3 44 44 44 35.66 31.9 3 2.04 3 1.42 The smaller (circular) specimens were cut IO provide: I microhardness specimen; 1 TEM specimen, and 2 archive sFhlms. 7 1 ~  [age “dog hone” style spwimcn \\-as tensile specimen. Figurc 6 is a fiactogaph of a typical tcnsile test specimen tested (3 total), which showed a dcsirable “dimpled rupture” (ductile) fi-acture. Table 3 lists the results of thc tensile and microhardness measurcnicnts taken. Figure 5. Close-up photo showing specimen location (typical). It can In- Seen that there was a rcduction in hardness in tlic mataial(44 KN to 32.75 KN). This would noimally indicate a reduction in tensile strength. This was not the case as the tensile strength appeared to increase approsimately ( 1 -2)%, which is within thc scatter band of the tensile test (using I spccimcdtcst). Note: h o o p  microhardness values arc deteimined by nieasming the size of thc unrecovered indentation and coniparing this value to forniulas or tables meeting establislicd standards. The Knoop indenter *... is a highly polished, rhombic-based pyramidal diamond that produces a diamond-shaped indentation with a ratio between long and shofi diagonals of about 7 to 1 .“ The Knoop indenter produces a depth of indentation approsimately onc-thirtieth its length. The dewloped Knoop hardness number (HK) is equal to the ratio of the applied load to tlie uru.eco\wed pro-jected area: Figure 6. Imv-magnification fi-actograph showing ductile t fracture face after tensile test (typical). HK = PIA = P/CL2 After irradiation, the area of masimum p~oton h e n c e  was determined using a photographic film technique. This area (of highest fluence) had tensde, microhardness, and TEM specimens ht from it. Figure 4 is a photograph of the 15-cm square disk in the electric-discharge machining (EDM) unit. Figure 5 provides a closeup of the specimen location on the aluminum sheet. where: P = applied load A = unrecovered projected area of indentation L = measured length of long diagonal C =constant for indenter relating area of indentation to the square of the long diagonal. a .( Figure 7 is a rcprcscntatioii of'thc load \u-sus hnrdncss cambinations for dctaniining riiiiiimum thicknesses of' sheet that can be tcstcd. The 100-g loadings used for ti1e.x experiments adequatcly covered thc range of material thicknesses esamined (0.127 mm - 0.508 mm). ?'EM photographs of the I100 aluminum irradiated with 2-3.5-GeV protons showed no micros~ctctual changcs in the metal, hut deiinite areas of cavily formation (Figures 8-9). MDD. I I ! I !! 1 I 0 0 1 2 7  mer moO05.ol I . .  . ._.. . Discussion I....,, ',I Figure 7. I(noop minimun-tliickiiess chart. liidicates h i d  and hardness combinations for dctcmiiiiiiig the miniiiium thickness ofshect or foil that can be tcstcd. Figure 8. Ca\.ities were visiblc in the 1100 AI inndiated hy 23.5-GeV protons (5S,OOOX). 'lht: I 100 aluminuni ivhich had hccn in-adiatcd with 23.5-CieV protons increased in tensile strength. This increase in tensile strength is espcctcd duc to the previously described displnccnicnt hardening nicchanisni. 1 he rcduction in niicrohardocss \dues frnni :in average \aluc of 33 KN (irr-adintcd) to an avcrngc iduc of 32.75 KN (uninadiatcd) canno1 easily be intapretcd. (NOTE: No iiisti-urnentation was availablc for tenipcratui-e niesuremcnts during these inndiations). One possible csplanation involves thc ai-cas of ca\.ity foniiation seen in the E M  studies. Previous studies [ 12) of  gas accuniulation in aluminum afier SOO-MeV proton irradiation5 indicated Uiat higher-encrgy proton iiradiations (800- McV versus 600-MeV) fonn l q e r  hubbles in tile irradiated materials. This increase iii hubble size \vas accompanied by a significant d~xxcasc in buhblc density ( - j O  to 1 ) \vith increasing proton encrgy. 1 here were no references a\Glahle in the lit~~atul-e \\.id1 inhimation on prototi imdiniions in the 1 .O+ GeV range, so only suppositions at this point x e  a\:ailahle until addition:il data arc developed. It should be kept in  mind, I. .. J however, that cavity size and growth can bc afTectcd by gas generated by the spallation process, and that the number of gas atoms generated per dpa increases almost licearly with proton energy in the Gev range. If one follows the logic that the grea .er the increase in energy, the larger the cavit~es fornied in the Me’l ranges, then the GeV energies could produce large enough cwities actually to &ed microhardness measurements Pyramid ir identer anomalies have been observed [SI in anncalcd metals &en a sinking in of the metal around the flat faces of the pqramid results in overestimating the hardness of the work piece. The use of an ultrasonic hardness testing sys~cm (rf suficientl~ sensitive) would decfively eliminate the effects of internal gas civities, and might be effective in determining if microhardness measurements are affected by gas bubble density in the 1 100 ahminum. Without question, additional wrk is necdcd in the GeV range of energies to resolve this apparent conwdiciim between tl e tensile strength and microhardness measurcnicnts, and will be investigated if additional beam time in thc AGS becomes av: ilable. Aclino\vledgmcnts * This work was perfoind c:iJcr the auspices of the U. S. Department of Encr-9 unCcr Contract No. DE-AC02- 76CHOOO 16. References 1. Green\ i .ood,  G.W., FoiLmm. A 1.E , and Ririmer, D E ,  “The Role of Vacancies and Dis1053~ions in the Nucleation and Growth of Gas Bubbles in In di3tcd Fissile Material”, J.of Nuclear Materials, 4 (1959). 705-323 2. Wechsler, M.S., Stubbins, J I.-., Soinma-, W.F., Fcrguson, P.D., and Farnum, E.H., “Sclcction and Qualification of Materials for the Transmutation of Waste Pro-icct”, Los Alamos Report, LA-UR-92-12 1 I ,  1992. 3. Graber,M.J., and Ransick, J.H., “ETR Damage Sunreillance Programs, Progress Report 1 ‘I,  January 27, 1 96 I .  4. Smgh,B.N.,Lohmann, W., Ribbens, A., anti Sommer, W.F., “Microstructural Changes in Commercial Aluminum Alloys Caused by Irradiation with 800-MeV Protons”, ASTM STP 955, Radiation Induced ChanPes in Microstiucture: 13th International Spnosium (Part 11, 1987,508-5 19. 5. hhmann, W., Ribbens, A., Somnier, W.F., and Singh, B.N., “Microstructure and Mechanical Properties of Medium Energy (600-800 MeV) Proton Irradiated Comme-cia1 Aluminum Alloys”, Radiation Effects, 1986, Vol. 101,223-299. h Caused by hadialion 800-MeV Protons“, ASTM STP 955, Radiation-Induced Chnnces in Microstructure: 1 3th International Svmposium, 1987,508-5 19. 7. Prentice-Hail, Inc., 198 1. An Atlas of metal Damage, Engel, L. and Klingle, H., 8. Metals Handhook. Ninth Edition. Volume 8. Mechanical Testing, American Society for Metals, Metals Park, OH 44703 ( 1985). 9. Victoria, M., Green, W.V., Singh, B.N., and Leffers, T., “Nucleation, Growth, and Distribution of Cavities in the Vicinity of Grain Boundaries in Aluminum Irradiated with 600-MeV Protons”, ASTM STP 955, Radiation-Induced Changes in Microstructure: 13th international Swnposium, 1987,233-24 1. 10. Paschoud, I:., Gotthardt, R., and Green, S.L.. “Stability Studies of Radiation Defects in Aluminum Introduced by 600- MeV Protons”, ASTM STP 955, Radiation-Induced Chanws in Microstnrctiu-e: 13th Inteinational Svrnnosium. 1987,478-489. 1 1. Gavillct, D., Grccn, W.V., Victoria, M., Gotthardt, R., and Mailin, J-L., “In-Situ Electron Microscopy Observation of Dislocation Motion in 600-MeV Proton 11-adiated Aluminum”, ASTM STP 955, Radiation-Induced Changes in Microstmcture: 13th International Simposiuni, 1987,490-497. 12. Sin& R.B., 14orscwel1, A., Sonmier, W.F., and Lohmann, W., “Gas Accumulation at Grain Boundaries During 800-MeV Proton Irradiation of Aluminum and Aluminum Alloys”, J. Of Nuclear Materials, 14 1 - 143, I 986,7 18-722. 6. Sin& B.N., Lohmann, W., Ribbens, A., anti Sommer, W.F., “Microstructural Changes in Commercial Aluminum Alloys 

Investigation of high-energy-proton effects in aluminum

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Investigation of high-energy-proton effects in aluminum

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