OAK RIDGE 25URC TANDEM ACCELERATOR* 1999 SNEAP LAB REPORT

Transcription

1 RIB PROJECT: OAK RIDGE 25URC TANDEM ACCELERATOR 999 SNEAP LAB REPORT M. J. Megs, D. L. Haynes, and R. C. Juras Oak Rdge Natonal Laboratory P. O. Box 2008, Oak Rdge, Tennessee 3783! A major mlestone was reached n the end of October 998, when an expermentallyuseful 7F beam (tjz=64.5 s) of 3x04 ons per second (ps) was delvered to expermenters at the astrophyscs endstaton. A beam of -8 PA of 44 MeV deuterons ncdent on a target of thn fbers of HfOz, together wth an alumnum vapor feed, produced a beam of A]7F+ons whch, after frst-stage mass analyss and breakup/charge exchange n Cs vapor, yelded 4x07 ps F- off the RIB platfom. The 7Fproduced for ths frst useful experment had only slght contamnaton of 70 (<%), but the breahp of the molecular AIF ntroduced a large energy spread and the large emttance of the beam probabiy resulted n some transmsson loss through the tandem. A new negatve surface onzaton source was nstalled and was used for most of the 9 hours of F producton n FY 999. Ths source has been used to delver 2x05 ps of F (wth -3 PA ncdent deuterons) but t can be hghly contamnated wth more than ten 70 for every F. A pure 7F beam of ths magntude has been obtaned by usng post-strppng to produce fully strpped fluorne ons wth greater effcency beng obtaned for the hgher accelerated beam energes. Ion source development contnues both at the UNISOR faclty and at two on source test stands. The next prorty for RIB development s the producton of a 5GN beam. Because of the long lfetme and the hghly refractory nature of N, the beam wll not be produced by conventonal ISOL technques, but rather n a mult-sample sputter source, n whch a target can be bombarded wth Oak Rdge Isochronous Cyclotron (ORIC) beam for a perod comparable wth ts half-lfe, and then rotated to a poston where the actvty can be exposed to a sputter beam for producton of a 5GNnegatve-on beam. Ths batch mode source s desgned so that one producton target can be exposed to ORIC beams whle another s smultaneously beng sputtered. The batch mode source has produced stable beam on the RIB njector platform. Other development actvtes are prmarly focused on a uranum carbde/retculated vtreous carbon fber target for producton of neutron-rch beams by fsson and a lqud germanum target system for producton of proton-rch sotopes of As, Ga, and Se. OPERATION: Durng FY 999, the 25URC provded approxmately 2800 hours of beam on target for the expermental program and RIB development, No new beams were Oak Rdge Natonal Laboratory, managed by Lockheed Martn Energy Research Corp. for the U.S. Department of Energy under contract number DE-AC05-960R22464.

2 u w accelerated ths year but we dd accelerate Be whch we had not done for a long tme. In the present ESH atmosphere, ths run generated qute a lot of cleanng and montorng of the on source before t was returned to routne operaton. Operaton for the expermental program was at termnal potentals from 3. MV to 2.3 MV. Twelve tank openngs were requred durng the year, two scheduled for general mantenance and ten unscheduled. I?our of the unscheduled openngs were just after a regular mantenance openng and were caused by persstent, small leaks n the termnal vacuum system. These leaks were fnally found by changng our leak huntng procedure to provde more pumpng speed. The leak detector was placed drectly after a turbo pump whch was pumpng on the system. Ths extra pumpng speed allowed the response tme of the leak detector to be greatly mproved n order to pnpont these small leaks. The other unscheduled openngs were caused by electronc falures, actuator mechancal falures, and one due to broken shortng rods. Durng the year, several mprovements were made to ncrease relablty and the 50 kv quadruple n our low energy tube was repared. A new stable-on-source bas supply was nstalled to replace the old 550 kv supply whch had been prone to frequent transstor falures. The new supply, a Glassman 400 kv, has performed very well and the rpple s actually less, whch should mprove beam stablty. A purchase of 8,700 pounds of SF,5was made, whch should ncrease the voltage level that can be obtaned on the termnal. ORIC has had many mprovements also, whch nclude replacng ancent power supples and upgrades n controls The control system has agan been changed. The VISTA system was plagued by crashes (generally n the mddle of the nght) and has just been replaced by an EPICS control system. Beam tme was agan only mnmally mpacted by the change. A beam has been completely tuned wth the new EPICS system, but more tme wll be needed to get all the knks out. Ths quck changeover was made possble by dong the majorty of debuggng durng mantenance perods. The EPICS system has been runnng for several weeks wth no crashes. FUTURE PLANS: A turbo pump wth controller, purchased from NEC, wll be nstalled n the next scheduled tank openng to mplement a recrculatng gas strpper. The focus for t%tureplans s to ncrease the transmsson of radoactve on beams.

3 <., Results and Dscusson Mcrostructure Development durng Contnuous Coolng Transmsson electron mcrographs of the samples subjected to contnuous coolng are compared n Fg.. The mcrographs show that wth an ncrease n coolng rate, the number densty of y precptates ncreased and the sze of y precptates decreased. Ths result s consstent wth that of MacKay and Ebert [0], n whch the y precptate sze decreased m changng the coolng condtons from forced ar coolng to ol quenchng n N - Al - Ta - Mo alloys. In the present work, n addton to an ncrease n the number densty of y precptates, ther shape changed wth ncreasng coolng rate. The mcrographs n Fg. show cubodal y precptates at slow coolng rates and ther shape progressvely becomes rregular and approaches a sphercal morphology n the waterquenched sample. Analyss of the mcrostructure cooled at 0.7 Cs- showed that the sze of the y precptates s larger than that of base metal. In ths condton, the y- precptates exhbted morphology that s consstent wth the coarsenng stage. Hgh-magnfcaton a) b) :C) d) Fg. Transmsson electron mcrographs of CM247D5 samples subjected to dfferent heat treatments maged n [00] zone axs (a) as receved, (b) 0.7 Cs-, (c) Cs l, (d) 0 Cs, (e) 75 CS-, and (/) water quenched. ) Babu et. al. 2 of 8

4 0.0 6?? ~ , c/s -3.0XIO-2 I I I I I Temperature, ( C) Fg. 2 Comparson of measured relatve radus change for CM247DS samples cooled at dfferent rates compared wth that of pure nckel sample [4]. mcrographs of samples cooled at Cs, revealed small amount of fne secondary y precptates. In addton to the above mcrostructural changes, the dlataton characterstcs were dfferent as shown n Fg. 2 [4]. In general, the dlatometry measurements are senstve to volume change and the coeffcent of thermal expanson changes due to phase transformatons. The observed change n Fg. 2 s speculated to be due to the y formaton [4, ] at dfferent undercookng. An nterestng observaton n ths work s that even under rapd coolng condtons mposed by water quenchng, the precptaton of y precptates could not be avoded. Ths result s consstent wth the work of Wendt and Haasen [2] who observed the completon of y decomposton to y precptate n a N - 4 at. YO A bnary alloy durng water quenchng. Wth APFIM concentraton profles, Wendt and Haasen concluded that n a N 4 at. YO Al bnary alloy, the decomposton occurs by a classcal nucleaton and growth mechansm. The observaton of ncreased y precptate number densty wth ncreased coolng rate (see Fg. ) suggests that the decomposton of y phase occurs by nucleaton and growth mechansm n the current alloy. However, as ponted out by Wendt and Haasen, the decomposton mechansm can only be understood by measurng the composton of the y and y phases, Also, t wll be nterestng to analyze the parttonng characterstcs of heavy elements such as tungsten durng such phase transformatons. Atom Probe Feld km Mcroscopy of Water- Quenched Samples: The concentraton profles of Cr, Co Al and W across y and y phases from the water-quenched sample are shown n Fg. 3. The concentraton profles revealed the followng characterstcs. There were m large concentraton gradents of ether Al or Cr wthn the y phase. Ths result s n agreement wth the work of Wendt and Haasen [2] n whch they observed y phase wth an equlbrum alumnum concentraton. However, n the present work, anomalous Co-enrched regons were observed wthn the y phase. Also, the concentraton of W n y phase should be hgher than th a t of ~ phase; however, the measured concentraton profles [see Fg. 3(b)] show a complcated behavor. Tungsten was found n hgher concentraton levels n both phases. Ths s attrbuted to rapd weld coolng condtons, whch does not allow for achevng equlbrum. However, the mechansm of extensve Cr, Al and Co-parttonng wthout the assocated W-parttonng remans to be understood. The APFIM results also showed concentraton gradents wthn the y phase. As ponted out n the earler work [3], the above concentraton gradents w affect the lattce msft between y and y phases. The above results support the nucleaton and growth mechansm proposed by Wendt and Haasen [2] even n multcomponent nckel-base superalloy. However, t s ntrgung that ths decomposton occurs wth dffuson of Cr, Co and Al even at rapd coolng condtons mposed by water quenchng. Although, APFIM analyss provded crucal nformaton of the y and y phase, stll three-dmensonal dstrbuton of these elements around a y precptate s needed to understand the phase transformaton mechansms. Further work s underway to nvestgate these parttonng characterstcs wth the use of atom probe tomography [3, 4]. Babu et al. 3 of 8

5 .,,. Y Yj co /! 40 Lo ---- v sm Dstance (rm) (a) Dstance (rm)./ Fg. 3 APFIM concentraton depth profles of Cr, Co and Al across y and y phases from the water quenched sample. The concentraton profles wtan the y regon (crcled) shows Coenrched regon and (b) shows concentraton profle for tungsten. (b) Babu et al. 4 of 8

6 4 :.- 2 # : :., ;...,.., ,.,..,,.8? #,,,, y -. r decomposton,. :. # / -.-.-, 4 :.,, range, -.., Lqud # 0.2 I Temperature Fg. 4 Comparson of predcted equlbrum fracton of varous phases as a functon of temperature wth DTA data obtaned whle heatng at a rate of O.7 CS-. The calculatons also ndcated formaton of small amount of MXCAand TC; however, they are not shown n the plot. ( C) Thermodynamc and Knetc Descrpton of Phase Evoluton: To evaluate the above mentoned mcrostructure development, thermodynamc and knetc calculatons were performed wth ThermoCalc [7J and DcTra software [9], respectvely. The frst set of calculatons consdered the equlbrum fracton of varous phases as a functon of temperature for the CM247DS alloy. The phases consdered n the equlbrum calculatons were NJJb, TC based carbontrde, y (FCC), y (L2-ordered), a, 8, MC, laves, MmC6, MTC~, and lqud. The predcted phase fracton was compared wth dfferental thermal analyss (DTA) data n Fg. 4. The predctons ndcate that the predomnant mcrostructure n ths alloy, at low temperature, s a mxture of y and y phases. However, as the temperature ncreases, the y fracton decreases and at 093 C, the alloy s expected to be mostly y phase. However, the DTA data shows that the y SOIVUS temperature range extends from 000 C to 279 C. Ths dfference s attrbuted to the knetc constrants, ncludng the effect of Ta and W, whch was gnored n the calculatons. The next set of calculatons consdered the soldfcaton of CM247DS alloy. Such calculatons would allow the estmaton of the freezng range of these alloys. Ths, n-turn, could be used to estmate the hot crackng tendency of these alloys. As a frst step, the calculatons consdered the soldfcaton condtons assumng Schel addtvty law. The sold fractons were determned for an alloy of N -9.2 at. Y. Cr % Co - 2. XOAl - l.o%t as a functon of temperature (see Fg. 5). In the above calculaton, the dffuson n sold s gnored and the mxng n lqud s consdered complete. In the next step, the soldfcaton smulaton consdered dffuson-controlled growth of y phase nto lqud as a functon of coolng rate. In these calculatons, two condtons were consdered one wth half dendrte arm spacng (HDAS) of lpm and another one wth 0pm. The geometry of the smulaton s shown n Fg. 5. The Babu et al. 5 of 8

7 ,... lqud composton was taken as N -9.2 at.?. Cr ?. Co Al - l. O%T. The smulatons consdered coolng from 427 C at a rate of and 00 CS-l. The calculatons descrbe dffuson n both lqud and Y phases. The above predctons are compared n Fg. 5. The results show that the freezng range calculated assumng Schel addtvty law s larger than calculated by equlbrum assumpton and dffuson controlled growth calculatons. In specfc, allowng for dffuson and coolng at Cs-l or 00 CS-l, the freezng range ncreased margnally from the equlbrum estmaton wth a dendrte arm spacng of 2P. Wth a large dendrte arm spacng of 20pm, the freezng range ncreased for both coolng rates, but stll the freezng range was less than that predcted by Schel addtvty law. Ths shows the dffuson s very rapd n these alloys and the freezng range may not be as large as predcted by. Schel addtvty law. However, note that these calculatons dd not consder the effect of Ta and W. Fnally, the decomposton of y phase through dffuson controlled growth of y phase was smulated. The geometry of the sample s shown n Fg. 6. An nterpartcle spacng of 20 r-an was assumed. Ths smulaton was performed to evaluate the APFIM composton profles obtaned from water-quenched sample. Smulaton was performed for an alloy composton of N -9.2 at. Y. Cr Co Al -.O%T whle t cools from 09O C to 888 C at a rate of 500 Cs-. Further growth of y phase nto y phase below Equlbrum 0.5x Dendrte Arm Spacng = 0 or pm e ) Temperature ( C) Fg. 5 Comparson of predcted sold fracton varaton wth temperature usng equlbrum calculatons, schel addtvty rule, dffuson controlled growth at l CS-l and 00 Cs-l. The rght sde of the dagram shows the geometry used n the smulaton. c o.- 6(m It > 0.5 x Inter Partcle Spacng= 0 nm - P , I I I I I Temperature ( C) Fg. 6 Predcted varaton of y volume fracton wth tme as the alloy cools from 090 C at a lnear rate of 500 Cs-. The rght sde of the dagram shows the geometry used n the smulaton. Babu et al. 6 of 8

8 4.< 888 C was gnored due to very low nterface veloctes n the smulaton and numercal nstabltes. The calculated volume fracton of y wth temperature for ths condton s shown n Fg. 6. The results show rapd growth of ~ nto y phase below 09O C. The composton profles at the end of smulaton at 888 C are shown n Fg. 7. The predcted concentraton profles qualtatvely agree wth the observed concentraton gradents n y phase (see Fg. 3). However, the magntudes of gradents predcted by smulatons were smaller than the measured ones. For example, the predcted Cr concentratons range from 2 to 0 at. Y. n the y phase. In the case of measured concentratons, they can vary as hgh as 30 at. % and reduces to 2 at. 70. Moreover, the smulatons cannot reproduce the anomalous Co-enrchment wthn y phase. It s_ mportant to_ note. that the_. smulatons assumed no dffuson w&-tn y phase and ths assumpton leads to buldup of small Cr concentraton profles wthn y phase. However, the expermental concentraton profles do not exhbt such profles. In addton, the calculatons do not consder the nucleaton of the y and therefore, the effect of subsequcmt ndependent nucleaton of y m the orgnal growth of y phase needs to be evaluated n future work. Summary and Conclusons The y-y mcrostructure evoluton n a CM247DS alloy durng contnuous coolng was nvestgated wt h TEM and APFIM. TEM revealed that the number densty of y precptates ncreased and the sze of y precptates decreased wth an ncrease n coolng rate. Wth an ncrease n coolng rate, the shape of the y precptate became non-cubodal and approached sphercal shape. The decomposton of y was complete even n waterquenchng condton. APFIM of water-quenched samples showed concentraton gra-dents w-lthn the y phase. An anomalous Co-enrched regon was detected wthn the y regon. Thermodynamc and knetc models were appled -0; \ I, t 5. o. 40 ; o ,, f ~ c El lo o- I I I Dstance Fg. 7 Predcted concentraton profles of Cr, Co and Al at 888 C after coolng from 09O C at a rate of 500 C. } n Al (rm) Babu et al. 7 of 8

9 to descrbe the weld mcrostructure development n N - base superalloy welds. The calculatons estmated the freezng range, as well as, the sold state decomposton of y phase nto y phase. Dffuson controlled growth calculatons showed the freezng range n these alloys may not be large as predcted by the Schel addtvty law due to extensve dffuson n lqud and sold. In addton, the dffuson-controlled growth of y phase nto y phase at a rate of 500 Cs- was smulated and the smulaton qualtatvely supported the measured concentraton gradents wthn y phase, but the predcted magntudes were smaller than that of measured ones.. R. W. Cahn, P. A. Semers, J. E. Geger, and P. Bardhan, Acts MetaU,, 35, , (987) 2. H. Wendt and P. Haasen, Acfa Metal/., 3, , (983). 3. M. K. Mller, J. of Mcros., 86, -6, S. S. Babu, S. A. Davd, J. M. Vtek, and M. K. Mller, to be presented at the TMS symposum on Advanced Technologes for Superalloy affordabity, Nashvlle, Acknowledgments The research was sponsored by the Dvson of Materals Scences, U.S. Department of Energy, under contract DE-AC05-960R22464 wth Lockheed Martn Energy Research Corporaton. The authors acknowledge the techncal assstance provded by J. W. Jones, R. W. Reed and K. F. Russell. Ths research was conducted utlzng the Shared Research Equpment (SHaRe) User Program facltes at Oak Rdge Natonal Laboratory. References. S. A. Davd, J. M. Vtek, S. S. Babu, L. A. Boatner, and R. W. Reed, Sc. Tech. Weld. Jonng, 2, (997). 2. S. S. Babu, S. A. Davd, and J. M. Vtek, Appl. Surf. Sc., 94/95, (995). 3. S. S. Babu, S. A. Davd, J. M. Vtek, and M. K. Mller, J. de Physque IV, 6, (996). 4. S. S. Babu, S. A. Davd, J. M. Vtek, and M. K. Mller, Proceedngs, Trends n Weldng Research, ed. Vtek et al., ASM Internatonal, Metals Park, OH, 999, S. S. Babu, S, A. Davd, J. M. Vtek, and M. K. Mller, Mcroscopy and Mcroanalyss, 4, 94-95, (998). 6. M: K, Mller, J. de Physque, 47, , (986). 7. B. Sundman, B. Jansson, and J. O. Andersson, Ca Iphad, 9, 53-90, (985). 8. N. Saunders, N-data nformaton;, Thermotech ltd., Surrey Technology Center, Gudford, Surrey, GU2 5YG, UK, J. Agren, S] Internatonal, 32, , (992) 0. R. A. MacKay and L. J. Ebert, Proceedngs, Ffth nternatonal conference on Superalloy, ed. M. Gell et al., AIME, New York, NY, 984, Babu et al. 8 of 8