W. B. Jones, J. A. Van Den Avyle, W. B. Bauster and W. R. Wampler Sandia Laboratories Albuquerque, NM 87185

Transcription

1 ASSESSMENT OF POSTRON ANNlliiiATON AS A POTENTAL NON-DESTRUCTVE EXAMNATON TECHNQUE W. B. Jones, J. A. Van Den Avyle, W. B. Bauster and W. R. Wampler Sandia Laboratories Albuquerque, NM ABSTRACT The positron annihilation tehnique an provide a sensitive measure of defet density in metals. n this program the tehnique has been used to monitor defets generated during plasti deformation by old ork or fatigue yling. The primary goals have been 1) to assess the degree of sensitivity of the tehnique, 2) to orrelate positron annihilation readings ith observed mirostrutural hanges to better understand the physial basis for these readings, and 3) to determine orrelations beteen positron annihilation measurements and number of fatigue yles. Examination of fatigued samples by transmission eletron mirosopy indiates some orrelation beteen disloation density and positron annihilation lineshape parameter (determined by the Doppler broadening tehnique). Hoever, annealing studies of deformed samples indiate that positron annihilation response in 316 stainless steel is sensitive primarily to exess vaanies generated during the deformation and is less sensitive to disloation density. Data on deformed nikel sho sensitivity to both vaanies and disloations. n general, lineshape parameter values tend to ahieve a onstant level at approximately 1 per ent of fatigue life. NTRODUCTON Elevated temperature design for advaned nulear reator omponents must inorporate reep and fatigue and their interation. Current ASME Design Codes utilie the onept of damage aumulation to treat ombined reep-fatigue loadings. This approah assumes that bulk hanges our during the servie life that represent "damage". One question addressed by this study has been: s there a measure of the bulk hanges that our during servie that ould be orrelated to the aumulated "damage"? Fla detetion is not the purpose of suh a tehnique. Several andidate shemes have been surveyed ith positron annihilation appearing to have more promise than the others (1,2). Positron annihilation has been shon (3) to be sensitive to vaanies, vaany lusters and disloations indued by irradiation or deformation of metals. RESULTS AND DSCUSSON Positrons injeted into a metal annihilate ith eletrons emitting to gamma photons. The momentum of the annihilating eletron auses a Doppler shift in the energy of the emitted photons. The sensitivity of positron annihilation to vaany and disloation onentrations in rystals arises from differenes beteen eletron momentum distributions in the perfet lattie and at defets. Figure 1 shos a shemati draing of the apparatus used to measure the Doppler broadening of the annihilation line. Positrons are produed in the soure by the + deay of a naturally radioativive isotope. For this study, 22 Na as the isotope used. These positrons penetrate relatively short distanes into the metal sample (Fig. 2) here they annihilate ith eletrons ithin about 1 pse. Test speimens examined in this study ere setioned normal to the stress axis so that the material sampled by the positrons as originally at the interior of the gauge setion. The result of the Doppler broadening measurement is a distribution of gamma energies around the 511 kev line. The parameter hosen to haraterie this urve is the lineshape parameter, S. Details onerning this harateriation an be found in referene 1, hoever, it is important to note that the value of S depends on the density of disloations and vaanies. The larger the value of s, the higher is the density of vaanies and disloations. To initiate this study, a number of fatigue tests on type 316 stainless steel ere onduted at room temperature and the disloation density monitored for omparison ith lineshape data. The lineshape parameter, S, is plotted versus number of fatigue yles in Fig. 3 together ith transmission eletron mirographs of representative disloation substrutures. For these test onditions the disloation density and the lineshape parameter both inrease ith inreasing number of fatigue yles. Figure 4 shos the results of fatigue tests onduted under several different onditions. For eah set of onditions, the lineshape parameter folloed the disloation density hanges; hoever, omparison of the values of S and the disloation densities among the several onditions did not reflet a orrelation (Fig. 5). n addition, the observed hanges in S saturated at about 1% of life. As pointed out earlier, the positron annihilation response is knon to be sensitive to the presene of both disloations and vaanies. n order to distinguish the relative magnitude of eah ontribution, isohronal annealing as onduted on samples of old orked 316 stainless steel and pure nikel. Figure 6 shos the hanges in lineshape parameter indued by old ork for both materials. n the present study, a pure Ni speimen as old rolled to a 25% redution in thikness and isohronally annealed (Fig. 7). Together ith eletron mirosopy results, these data sho that after annealing the Ni to 6 K to remove the exess vaanies, the lineshape parameter has notieably dereased due to the loss of vaanies. Hoever, about 6% of the total initial inrease remains due to the disloations. Figure 8 shos that after annealing 316 stainless steel at 873 K, the lineshape parameter has nearly ompletely reovered hereas eletron mirosopy and mirohardness have shon that the disloation struture has not 115

2 hanged. From this e onlude that most of the initial response as due to the vaanies and only a very small fration ould be aused by the disloations in the 316 stainless steel. Figures 9 and 1 sho the annealing response of 316 stainless steel yled at room temperature and 866 K. As antiipated, the material fatigued at room temperature undergoes signifiant annealing of the lineshape parameter due to the large number of exess vaanies present after yling. n ontrast, yling at 866 K results in a minimal aumulation of exess vaanies and, aordingly, the annealing treatment produed very little hange in the lineshape parameter. CONCLUSONS This study has shon that positron annihilation sensitivity to disloation density must be established for eah alloy. Even hen suh sensitivity is present, the response may be dominated by the presene of exess vaanies. Transmission eletron mirosopy has shon that the density and distribution of disloations reahes a steady-state ondition in the first lo% of the fatigue life so that, even ith good disloation sensitivity, the positron annihilation response ould saturate at about lo% of total fatigue life. ACKNOWLEDGEMENTS Sandia Laboratories is au. S. Department of Energy faility. This artile as sponsored by the U. S. Nulear Regulatory Commission under ontrat DE-AC4-DP789. SOURCE POSTRON ANNHLAnON DOPPLER BROADENNG f SluiV l LN i \ SAMPLE. Figure -'-., 1 \ muv,. / \FROM Cu.,. \., CHAN{. NUMBER REFERENCES. B. Gauster, W. R. Wampler, W. B. Jones, and J. A. Van Den Avyle, Sandia Laboratories Report SAND , May J. A. Van Den Avyle, W. B. Jones, and J. H. Gieske, Sandia Laboratories Report SAND , July F. Coleman and A. E. Hughes, in Researh Tehniques in Nondestrutive Testing, R. S. Sharpe, ed., (Aademi Press, Ne York) Vol. 3, G. Dlubek,. Brummer, and E. Hensel, Phys. Stat. Sol. (a), 34, 737 (1976). W. Wysik and M. Feller-Kniepmeier, J. of. Nu. Mat., 69 and 7, 616 (1978). Depths of Penetration of Positrons in Copper (Values lor Steel ill be -1 to 2 Higher) e-loldi ng distane' 23.11/m.9 in maximum penetration.31 mm.12 in m. 6 in 1.42 mm. 56 in Depth in material at hih positron nux is lie (37'/ol of its value at the surfae. Figure

3 :: "" Cl... "" Cl. :J: Vl "" o.1t ":t.. 9'J., 294 K.1E=1.8'l6 N =37 S= FATGUE CYCLES ll. O!Hl Figure le <( <(.. <( ::J: ) :: / ( 6E=1.8" ;;.-- FALED o o/ FA TGUE CYCLES _.-- -L._ ' -6E=.5" t -6E=1.". -.O.E=O.S" + 1 MN TENSLE HOLD 1 1 POSTRON ANNHLATON RESPONSE OF FATGUED 316 STANLESS STEEL Figure K 11E=.8% N=47173 S=31.8 _,;:; 888K.1E=.5% N =ioooo $ 29.2 Figure

4 POSTRON ANNHLATON RESPONSE OF COLD WORKED 316 STANLESS STEEL AND NCKEL o ANNEALED 316 SS o AS RECEVED 316 SS o NCKEL (Diubek elal.) (4) 8 ll! ' 6 " S! REDUCTON N THCKNESS A.!!. (%) do Figure r--r ;_r---, r r-, :1: 4 39 :E p 25 3 vaany annealing.o / disloation annealing s Ni. 25% COLD WORKED.. QUENCHED /. Ni (Wyisk et. al.) (S) oo o - 1. t:j.p -!:J.po :;; : 31. :E ) t:; :E : :1: ) ss t1 COLD WORKED 25% oo oo l 316 ss COLD WORKED 75% Figure ANNEALNG TEMPERATURE (K) Figure

5 1. f ALt.6%.5. i ROOM.. TEMPERATURE FATGUED :E ROOM 1. FATGUED!1: A<t = 1.8%.. TEMPERATURE Figure 9. (/) <> 316 STANLESS STEEL FATGUED AT 866 K 1..1<:.5% t i5.5 t- < OoOOo oooo ooo <.... <. :r (/) ::J Figure