Recrystallization of Compacted Gold Foil Specimens D. D. SCHLOYER, P. G. WINCHELL, R. W. PHILLIPS, and M. R. LUND Indiana University School of Dentistry, Indianapolis, Indiana 46202, USA
Gold foil specimens were compacted by a technique commonly used in preparing dental restorations. Their recrystallization behavior for temperatures between 100 and 300 C was followed by the associated decrease in hardness, discontinuous increase in g7ain size, and disappearance of X-ray line broadening. The empirical dependnece of time, t, (in minutes) for 50% recrystallization on annealing temperature (in K), T, is approximately log10 t - - 12.3 + 6.5 X 1,000/T, indicating a recrystallization time areater than 100 years. Although direct gold restorations properly placed give good service, the retention of suit-able hardness has been questioned.' Because of the malleability required in gold foil and because of the compaction process, it has been theorized that the recrystallization temperature might be decreased to below that which has been detcrmined for high purity bulk gold. The present woork is an experimental laboratorv study of the recrystallization of specimens from gold fails cornpaCted to approximate direct gold restorations. Such a study involves experimental problems caused by the presence of numerous voids in the specimens and the inhomogeneity of the deformation. The first of these problems, coupled with the difficulty of etching gold, has evidently discouraged research on the metallographic structures of gold foil specimens. The second, coupled with the higi impurity sensitivity of recrystallization kinetics, makes prediction of recry-stallization beha-vior uincertain; hence an experimental determination of those kinetics is desirable. Several studies of annealing of bulk gold This investigation was supported by the US Navy, Indiana University, and Purdue University. Portions of this article were taken from a thesis by Dr. Schloyer in partial fulfillment of the MSD degree. Received for publication April 8, 1976. Accepted for publication June 14, 1976. * No. 4 noncohesive gold foil from Williams Gold Refinery Co., Inc., which is claimed to be "without detectable impurity." t McShirley Electromallet intensity 4, frequency 1,800 minute-'.
486
and thin goldl foils have been reported. Using hardness as an indicator, Rose2 reported that "pure" gold recrystallizes at 150 C in 30 mintttes and that gold with 0.05 wt% silver requires 225 C. A study of electron diffraction from a polished layer of gold3 derived a particle size of 100 A at room temperature. In a modern X-ray diffraction study of 99.95% gold fillings made at -130 C, Wagner4 showed that almost all faulting and measurable residual elastic strain vanish during annealing below room temperature, but a particle size of 300 to 400 A remains. In the present work, the hardness, microstructure, and X-ray line broadeniing are reported for specimens that approximate direct gold restorations. Materials and Methods The manufacture of specimens was designed to approximate a technique commonly used for direct gold restorations. Sheets of gold foil* 1.3 micrometers (gtm) thick were cut into 64ths, rolled into pellets, degassed in an alcohol flame, and compacted at 21 C in a 2X2X15/4mm stainless steel die or in a lox1lOX'/2-mrn poly (methylmethacrylate) die. Compaction was done using a hand-operated electromechanical hammert with a straight handpiece and a 0.5-mm2 rectangular condenser point. The smaller specimens were compacted in about 45 minutes with a backing of mat foil and were given isothermal anneals and used for microhardness measurements or metallography. The larger specimens were compacted in about 300 ninutes with a backing of mat foil and were given isochronal anneals and used for X-ray diffraction and microhardness measurements. Heat treatment techniques involved repeated heating of the same sample so that sample-to-sample variation was cliininated and sample requirements were reduced. Heating was done in liquid baths of water, oil, or (molten) salt. The bath temperature was controlled at + 1 C in isothermal heat treatments
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Vol. 56 No. 5
COMPACTED GOLD FOIL SPECIMENS
uised for measurement of recrystallization kinetics. The total time at temperature is reported as the annealing time. The one-hour isochronal treatments used for X-ray line broadening were carried out in order of increasing temperature; the repoited temperature is that of the latest treatment. Metallographic sections were prepared by mechanical polishing followved by electropolishing and chemical etching. That the polishing technique produced surfaces with little distortion is clearly showsn by the electron-channeling contrast micrographs that were obtained from recrystallized specimens. (Since this mechanism of contrast formation in the scanning electron microscope relies on channeling of electrons along open lattice directions inside the gold crystals, distorted crystals are easily recognized.5) Also, the electropolishing technique did not excessively widen the occasional interfoil voids present in the sample. The mechanical polishing sequence consisted of using waterlubricated SiC of 320, 400, and 600 grit followed by 6-,um diamond witlh a standard oil lubricant on nylon cloth and a water suspension of 0.05 !umAl2O3 on a higher nap cloth.+ The modified Cr,03, C2H40H electropolish described by Glen and Raley6 was used at 10 v for 15 minutes at room temperature. Electropolished samples were rinsed in glacial acetic acid and then in ethanol. Etching was carried out on clean dry samples by immersion for 20 seconds in a fresh solution of 50 vol% HCl, 25 vol% HNO, and 25 vol% glycerine.7 This etch grooves the grain boundaries in recrystallized samples and produces a molded structure in the unrecrvstallized samples. Microscopic observations were made by three techniques. Etched specimens were observed by standard reflected light microscopy at X 1,280 or by a scanning electron microscope using a seconidarv electron image. Electropolished samples were observed bv electron-channeling contrast in the scanning electron microscope. Such contrast requires a small beam t Microcloth, A. Buehler, Ltd., Evanston, I1. * Knoop indenter, 300-gm load, Tukon Model MO, Wilson Manufacturing Co., Bridgeport, Ct. t Siemans type F with incident beam; soller slit, 1/8 receiver slit. Peaks were continuously chart recorded at 0.5° 20 per minute. The irradiated area was smaller than the sample. § Ortec, Inc., Oak Ridge, Tn. ** Kristalloflex 4, Siemans America, Iselin, NJ. ¶ Here residual strain means a retained change in interatomic distance as contrasted to plastic strain which is obtained by replacing one atom by another via slip of an atomic plane by a lattice translation.
487
divergence angle and the resulting beam size provides clear resolution at lower than about X 1,000, but for comparison with optical micrographs a standard magnification of X 1,280 was used. Standard microhardness measurements* were made on polished and burnished surfaces. The surface was prepared by mechanical metallographic polishing and then heavily burnished by hand using a Spratley carver. The change in hardness was found to be the most consistent measure of the extent of recrystallization. The breadth of X-ray diffraction peaks was measured on peak profiles recorded on a diffractometer. The powder diffractometert was equipped with a fine-focus copper tube maintained at 40 kv by a stable X-ray generator.** Diffracted radiation passed through ,B nickel filler and was registered by a scintillation counting system.§ The I 1 1 and 222 peaks were recorded and K01, and K.2 components were partiallv resolved. The K02 component was removed using the Rachinger correction.8 The peak breadth was measured as the breadth in degrees 20 at half maximum height above backgrotind. To evaluate the broadening resulting from the instrument, the gold standard supplied with the diffractometer was run. The breadth of its peak was measured, and the square of the breadth of the sample peak less the square of the breadth of the standard peak was taken as the square of the breadth that the peak would have exhibited if instrumental broadening had been absent. This technique is exact only when the X-ray peak is a Gaussian shape and when the standard sample contains no internal sources of broadening. The broadening of the Il l and 222 peaks should be the same if broadening is due to small particle size (or extensive faulting), but the broadening of the 222 should be significantly larger than that of the 111 if a distribution of residual strains¶f is retained in the deformed samples. Methods of separating the two contribuiions are availableI but will not be presented because (to anticipate the results) broadening of 'Lhe 222 peaks was not observed to exceed broadening of the 111 peaks and, thus, measurable broadening resulting from strain was not observed. Results The microstructire of the foil sheet after passing through the alcohol flame and before compaction was approximated by annealing a
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FT
AL
J Dent Res Maj, 1977 the mU-iioscopic techniques used (optical ele(trons microscopy of etclied structLore's and hanneling contrast electron microsc opy of polished samples). In soiime pUaces the foil sheet contours w-exre still v isilelc. II wx idely s( attered spots, large (raiins approximiatin'g the sizc shown in Figur e 1 wvere present and appeareci to be free of strain markiings. Around these spots poorslv bonded foils wx ere ex dent and contained the very fine microstructure aforenmeintioned. The foil colitours inear suich spots have previously been observed and the .spots lbave been attributed to the local fuisinig of foils in the alcohol flame.10 The presenit observation of the large grains they contain reinforces the vieNv that they have been overheated, and siInce these grains are not greatly deformied, inidicates that deformnation is nlot coiImonly accomplished in and near such spots Ly
sc anning
c
4*
...
FIc 1. Microstructure of no. 4 gold foil sheet annealed five seconds at 621 C (1,150 F). Foil is 1.3 ILm thick. Optical micrograph; electropolished and etched (X 1,289).
foil sheet for fixe secoInds at 620 C9 in an air furnace. A ty pihal portion of the foil surface is shown in Figure 1. Since No. 4 foil sheet is 1.3 tin thick, the giains shown are pancake shaped, with diamiieter-thickness ratios of the order of 10. Thlle clnipacted foils werc xw ell bonided cxcept for widely scattered spots. The inost ommnM Mi rostruct ire xvas toe, fine to be resoxlv d c
dturinig compaction.
The fineness of the miicrostrticture 's also revealed by the broadening,- of thie 1 I I and 222 X-rax diffraction lines from nas-coompacted specimlens. The broadening show n by these peaks svas 0.15 and 0.19° 20, respectivelv. According to the S herrer equation,8 the bi oadeningr, B, is related
the
to
O/X, where
particle
size,
L, by
s
i
90 z
80
70
'60
50
H f-
-
i __1_ 1_
.01
.1
__-
.
cos
L
is thle Bragg ano-le and X is the wavetlength cf the X-ray-s. Accoiding to this relationI and the broadening measurenic-nts, IL 580 A for the I 1 1 and 620 A for the 222. The near equAlity of these two figniurs shoiN that s,train rosdlening is present. This concluision a
1___
2
10 100 Time tmin)
1_
2000
10,000
100,000
FIG 2.- Hardness sariation with time for conipacted gold foil specimens annealed at temperatures shown. Few points determined by quantitative microscopy are included assuming linear variation of hardness with fraction recrystallized. Initial hardness of each sample is indicated by left-pointing airow. As-recrystal]ized hardness is constant. Open and solid circles, hardness; X. imicrostructure plotted at %c Rx 100 (H-H) /(Hi-Hf). Downloaded from jdr.sagepub.com at TU Muenchen on July 1, 2015 For personal use only. No other uses without permission.
no
COMPACTED GOLD FOIL SPECIMENS
Yol. 56 No. 5
(min) lox
Recrystallized
e :H-
489
Century
ttNg Decade
Year
Month
compacted Hordn ess' 82 KHN 25tDoy
103
Hour
Minute
16
20o
2.4
2.8
1000 /T
3.2
3.6
(
K-1)
FIG 3. Time for 50/ recrystallization vs reciprocal absoltute ternperature. Microstructure and hardness of compacted and recrystallized direct filling gold foil specimens are indicated in their appropriate time-temperature clomains.
followss because strain broadens the 222 twilce as inuclh as the Ill antI hence -vvould result via the Scherrer e(quLation, in a fictitiouisly low particle size for the 222. The X-ray parlicle size of 600 A (0.06 gmi ) aitii the absence of strains is in line witPh previouis results bv Wag,ner4 oll cold-filed Vold annealed at room temperature. He also foun-id strain broadening Xas comnpletely retovererl at roomii temperature and slhe pa ticle size" xx.as Guli) two thirds of that reported here. T>he diffe en rte irs particle size is niot ulne\l)eeted in viev of tile imorc violent defor mation used bs)
WVagner.
During anne alin, for one hlour at temiiperatuLres at or los er than 150 C (x hich is before re-
cryqtallization begins ), the particle size increased bv about 15% as indicated by linebroadening ineasuireImienlts of the 111 and 222 peaks. After recrystallization was,s romplete, foi exCample, after one houir at 250 C, no linle l)roadlening was observed. Anneiallinig the compactec foil specimens at suffic ientlv Iigh temperature.s or for sunfficentli long timees i-cstilted in the appearance of large rec r-stalliized grains in, the fite ( old- workedl sIirostMuctUre. This chanige ini microstrLuCture
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490
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SCHLOYER ET AIL
ssvas at" otlipaniht iby a decrease in average lhardness. l'Aperimeutal. results of the fraction- recrystallzizcl as ol)servedl mnetallographically anid of the change in hardiness arc piesented together.
The variations of hardness xsith annealing tiine at variotim annealingl temperplatures are shoss n as the points in Fig(lure 2. 1bh rosses are) quantitatis e icetallographit neiasuremei-is of tlhe fr-actioni rcci ystallized. They has e been Plotted oni the hlirdness ordinate scale using the aipproximate linear relation intlulcded in the fieire. Thle scatter in the dctla represents a stcatter in restilts fromii different aieas of the specimiieni. rol)alls]- resultinlg nhfl01 the inholimogeeitx of cleff o-In t imoi. Th11e currves seen in Ii Fiure 3 represenat 1rogre-ss of retcry stallization at thei various tel)peratiures at whiich meni-nenuirents vrme maIdc. Au1 appiroximaite c orr elition of sinch cuoves c an usiatlly be madIe based on the alsuuiption that recrSt'staliiltiotlbellaves as a thelma'llsv attixvatedt Th is leas to 1th Anr I n i Ius a t ion ji)ro (ess.
De-nt Res
Afay
1977
that the process inxvolved is not a single. actisated process and the actixation enertso obtained must he re,arded as an empirical tonstanlt.12 Figulire 3 shoss s the appiropriate plot and also shoss s ty pital recrystallizedcl and coldw orked miurostriictures. The linie thlat best fits the expec-i imeuital points is logj,, t-- 12.3 + 6.5 X 1,000 / (whic h yiekls an etl t?iP itcal atctix-ation- energy of 1 25 KJ/niiole ['30 Kcal/imole ). Extrapofation of this lin tft 37 C is indicated in1 theI fillgre. The tise of s log sca1le fLo timiie andl a 1,000 /T sc ale for tfem)per,ttiute is required to obtain a linear telationslhlip). but oni tie opotsitesidecs of the fiiurt miore commIIlonit utits; aire inch:1ate cl n11 nnllinear scales. The mic rostructore of the tonlIXl ted(l specimnen shoss-s the fine structure trpitcal of nearly all regions of thle uannealed spet imiiens. The ctmheri1 eroluolls atle in the ftused blobs priesituslv dcestctibed anti
ae an
inclesital.'le r1suilt of
piocessing.) Fultheilmort, ans sattiple slibjict to an annealingy treatment to the iright tof thle 5-)()" rcicsstall i7atic nx lint' is mtstly compicrlsed f thIis struttictte. 1Fhe u( cosfiti1t Ine11 Ii o f a ictc cIx .talMli7ed spe itimon is also shoewsn. 1 he ot att, a1t aloe- aintImans- strxaight andl steppedI lbiituldaties, wshithlare eharacteristit of cross Ith twin liouncarlaics, are pireenft. A\ ty pical regiotn is L )-dccl R in the ret y-stllizecld stint r' tm. A\y tfento
rate
-
A c-Q/J
l
inl 5 hit lh t Is the tiu' for- half tcoiplIction of ret (Istsaliation; .1, is a conIIstant; Q; the em(11-1 pirinal "activation energy"; R, the gas conslant; and '1', the absolute tetilp)ertitil'. Applicatitn. iif tlii reIalolla.o toric ix stalli,'atiou kinetic s (ot(iittit0 ittild is baiseci oil it.s cutipitritild si ccess for spec ifii alloys. Thle imiipiirits s5tnsitiv ity(o the pritreIwter obatliutldildE ates
Fic; . left, channeling contrast scanningr electron micrograph of as-compacted gold foil specinien. Electropolished and unetched (x 1,280). Right, challnelinig contrast scannxing electron micrograph of compactecd gold foil specinmeni annealed two iminutes at 250 C. Some
mtint cotresponcling to a Ipoitit ito the lift tf the. )-e r>] a 5s llittd ]illn has itistlxt tiiS s,ttsi 'atictittee. 1 he tallsitioli ftottltoes1 suit ture tfi the otlter i's sucllden in cat h1 lotcalitv f the speSPimen but thle tilux fcir srxneaicliin, of ret tlstillizi,t-
uinrecrystallized grains (cdotted fine struCture) remnain btlit imost of sample is occupied by large recrvstallized grains with growth twins. Their unifon r contrast is good inclication of lowcdistortion near their surface. Electropolished and Lunetched ( X 1,280).
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COMPACTED GOLD FOIL SPECIMENS
Vol. 56 No. 5
tion throughout the specimen is about 100 times the time required for its initiation (Fig 2). Within about a factor of ten of the time for 50% recrvstallization both as-compacted and recrvstallized regions exist in the same sample. Actually, the so-called recrystallized structure presented contains a small region that has the as-compacted appearance; it is labeled "C" and the treatment of this specimen is in the transition region, four minutes at 250 C. In the tran-sition region, the average hardness changes from the value characteristic of as-compacted gold, about 80 Knoop hardness number (KHN), to 50 KHN, the value characteristic of the recrystallized structure. In order to further show the microstructural change accompanying recrystallization, electron-clhanineling contrast5 scanning electron microscope micrographs of electropolished samples before and after recrystallization are shown in Figure 4. This technique reveals by the uniformity of contrast the strainfree recrystallized grains. Its spatial resolution is too small to show the fine structure of the cold-worked samples. Discussion The outgassed gold foil sheets contain pancake-shaped grains with a 10 jum diameter -and a thickness equal to that of the foil, 1 ,um. These grains are cold welded together during compaction. The foil sheets contain occasional regions that were accidentally fused during outgassing'0 and these regions contain large ball-shaped grains. Compaction does not greatly deform these grains and the foil sheets neighboring these regions are poorly welded during compaction. Except for these regions, compaction results in formation of rather fine cells or subgrains with a particle size of about 600 A (0.06 ,um). These cells have a level of residual -elastic strain that is too low to cause X-ray line broadening and, according to previous -work,4 have few if any stacking faults. The particle size alone wvill account for all the hardness observed in the as-compacted gold. The strength a that results from grain size, or cell size, 1, usually obeys the Hall-Petch equation: 13,14 a=
k
aO +-
-where aO is the vield strength of a very large grained polycrystal; k/,u a constant that has the -value 8X 10-5 mm/2 for both silver and copper; and A, the shear modulus that is about 4x 106 _psi (2.8X104 N/mm2). For a very rough esti-
491
mate of the grain size strengthening, we assume that KHN BHN _ (0.002) a(psi)15 and we take a___ 0, as suggested by the yield strength of single crvstals of gold.16 The KHN so calculated (80) is fortuitously close to the observed hardness. The agreement indicates that the cell size can account for the observed hardness of as-compacted gold. During recovery, that is, during annealing below the recrystallization range, a small increase in cell size was observed and a slight decrease in hardness, but from a practical viewpoint the hardness variation from sample to sample is more significant than is the effect of recovery. Durilng recrystallization, the large grains of gold appear in some regions of the sample. They grow across old foil sheet interfaces as well as in the plane of the foil sheets and end up as ball-shaped crystals containing numerous annealing twins. The recrystallized samples have no measurable X-ray line broadening. The recrystallization kinetics behave as usually observed for metals, i.e., they exhibit sigmoidal fraction recrystallized curves as seen in Figure 2 and dependence of recrystallization time on recrystallization temperature (Fig 3). The recrystallization is less rapid than that reported for pure bulk gold.2 Perhaps this is due to the lower amount of deformation introduced in hand-compacted gold foils as compared to the 96.7% reduction by rolling reported by Rose.2 Another somewhat unusual feature is the increase in grain size obtained on crystallization. This may be produced by the recombination of two effects: the thin initial grains and the small deformations introduced by compaction. The beaten gold is annealed (outgassed) to produce thin grains in thin foil sheets. Welding of these sheets by compaction probably does not involve large deformation. The crystallized grain size is consequently"" 2 rather large compared to the original deformed grain thickness and, as is always the case, is much larger than the cell or subgrain size measured by X-ray line broadening. Finally, and or most significance, the recrystallization process will not occur at 37 C in the material tested within the lifetime of a restoration; nor will occasional increases in mouth temperature alter this conclusion. Indeed, a week at 100 C is 1 equired to initiate softening and ten months is required to produce 50% recrystallization. Thus, recrystallization and associated softening are probably not a clinically -
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492
SCHLOYER ET AL
significant process in direct
filling gold restora-
tions.
Conclusions Laboratory experiments on direct filling gold specimens compacted as if they were restorations show that as-compacted gold foil specimens made from alcohol-flame-outgassed foil sheets are composed of pancake-shaped crystals extending through a foil sheet 1.3 Am thick) and having an extent in the foil plane of the order of ten times the foil thickness. These grains are deformed during compaction, however, according to X-ray line broadening, they have little residual elastic strain (or stacking faults) but have a cell size of about 0.06,m. Recrvstallization results in a drop in hardness by about 30 KHN and an abrupt increase in grain size. The erains become ball shaped and contain several gold foil sheets. Fifty percent of the specimen is recrystallized after annealing for a time of t minutes which varies with the absolute temperature T (K) according to log1ot ;=- 12.3 + 6.5 X 1,000/T for 573 < T . 373. Fifty percent of recrystallization is expected to require about 900 years at 37 C, but it would probably start at about one tenth of this time. Loss of hardness of direct filling gold restorations by recrystallization is not expected to become a clinical problem under ordinary circumstances. The material was supplied by the Williams Gold Inc.
Refining Co.,
References 1. PHILLIPs, R.W.: Science of Dental Materials, 7th ed, Philadelphia: W. M. Saunders
Co., 1973, p 379. 2. RoSE, T.K.: On the Annealing of Gold, J
Inst Metals 10: 150, 1913.
3. KURIYAMA, M.; KOHRA, K.; and TAKAGI, S.: Electron Diffraction Study on Polished
J Dent
Res
May 1977
Layers of Gold: I, J Phys Soc Jpn 12:151-
156, 1957. 4. WAGNER, C.N.J.: Stapelfehler in Gold nach giner Kaltverformung bei tidfer Temperatur, Z Metallkde 51:259, 1960. 5. Joy, D.C.; NEWBURY, D.E.; and HAZZLEDINE, P.M.: Anomalous Crystallographic Contest in Rolled and Annealed Specimens, in JOHAR, O.M., and CORVIN, I (eds): Proceedings of Fifth Annual Scanning Electron Microscope Symposium, 1972, pp 98103. 6. GLEN, R.C., and RALEY, J.C.: Improved Procedure for Thinning Metallic Specimens for Transmission Microscopy, ASTM Spec Tech Pub 339:60, 1962. 7. REDPATH, D.L., and JOSHI, K.C.: Metal-lographic Preparation of Aluminum and Gold, Microstructures 2:21, 1971. 8. WARREN, B.E.: X-Ray Diffraction, Reading, Mass: Addison-Wesley Publishing Co., 1969, pp 256-275. 9. HOLLENBACK, G.M., and COLLARD, E.W.: An Evaluation of the Physical Properties of Cohesive Gold, J S Calif Dent Assoc 29:280, 1961. 10. HODSON, J.T., and STIBBS, G.D.: Structural Density of Compact Gold Foil and Mat. Gold, J Dent Res 41:339, 1959. 11. CAHN, R.W.: Physical Metallurgy, 2nd ed, Amsterdam: North Holland Publishing Co., 1970, pp 1129-1197. 12. REED-HILL, R.E.: Physical Metallurgy Principles, 2nd ed, New York: Van Nostrand Co., 1973, pp 286-288. 13. ARMSTRONG, R.; CODD, I.; DOUTHWAITE, R.M.; and PETCH, N.J.: The Plastic Deformation of Polycrystalline Aggregates, Phil 7:45-58, 1962. 14. THOMPSON, A.A.W.: Yielding in Nickel as. a Function of Grain or Cell Size, Acta Met 23:1337-1342, 1975. 15. DIETER, G.E.: Mechanical Metallurgy, New York: McGraw-Hill Book Co., Inc., 1961, p 288. 16. HONEYCOMBE, R.K.W.: The Plastic Deformation of Metals, New York: St. Martins. Press, 1968, p 12.
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Gold foil specimens were compacted by a technique commonly used in preparing dental restorations. Their recrystallization behavior for temperatures between 100 and 300 C was followed by the associated decrease in hardness, discontinuous increase in g7ain size, and disappearance of X-ray line broadening. The empirical dependnece of time, t, (in minutes) for 50% recrystallization on annealing temperature (in K), T, is approximately log10 t - - 12.3 + 6.5 X 1,000/T, indicating a recrystallization time areater than 100 years. Although direct gold restorations properly placed give good service, the retention of suit-able hardness has been questioned.' Because of the malleability required in gold foil and because of the compaction process, it has been theorized that the recrystallization temperature might be decreased to below that which has been detcrmined for high purity bulk gold. The present woork is an experimental laboratorv study of the recrystallization of specimens from gold fails cornpaCted to approximate direct gold restorations. Such a study involves experimental problems caused by the presence of numerous voids in the specimens and the inhomogeneity of the deformation. The first of these problems, coupled with the difficulty of etching gold, has evidently discouraged research on the metallographic structures of gold foil specimens. The second, coupled with the higi impurity sensitivity of recrystallization kinetics, makes prediction of recry-stallization beha-vior uincertain; hence an experimental determination of those kinetics is desirable. Several studies of annealing of bulk gold This investigation was supported by the US Navy, Indiana University, and Purdue University. Portions of this article were taken from a thesis by Dr. Schloyer in partial fulfillment of the MSD degree. Received for publication April 8, 1976. Accepted for publication June 14, 1976. * No. 4 noncohesive gold foil from Williams Gold Refinery Co., Inc., which is claimed to be "without detectable impurity." t McShirley Electromallet intensity 4, frequency 1,800 minute-'.
486
and thin goldl foils have been reported. Using hardness as an indicator, Rose2 reported that "pure" gold recrystallizes at 150 C in 30 mintttes and that gold with 0.05 wt% silver requires 225 C. A study of electron diffraction from a polished layer of gold3 derived a particle size of 100 A at room temperature. In a modern X-ray diffraction study of 99.95% gold fillings made at -130 C, Wagner4 showed that almost all faulting and measurable residual elastic strain vanish during annealing below room temperature, but a particle size of 300 to 400 A remains. In the present work, the hardness, microstructure, and X-ray line broadeniing are reported for specimens that approximate direct gold restorations. Materials and Methods The manufacture of specimens was designed to approximate a technique commonly used for direct gold restorations. Sheets of gold foil* 1.3 micrometers (gtm) thick were cut into 64ths, rolled into pellets, degassed in an alcohol flame, and compacted at 21 C in a 2X2X15/4mm stainless steel die or in a lox1lOX'/2-mrn poly (methylmethacrylate) die. Compaction was done using a hand-operated electromechanical hammert with a straight handpiece and a 0.5-mm2 rectangular condenser point. The smaller specimens were compacted in about 45 minutes with a backing of mat foil and were given isothermal anneals and used for microhardness measurements or metallography. The larger specimens were compacted in about 300 ninutes with a backing of mat foil and were given isochronal anneals and used for X-ray diffraction and microhardness measurements. Heat treatment techniques involved repeated heating of the same sample so that sample-to-sample variation was cliininated and sample requirements were reduced. Heating was done in liquid baths of water, oil, or (molten) salt. The bath temperature was controlled at + 1 C in isothermal heat treatments
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Vol. 56 No. 5
COMPACTED GOLD FOIL SPECIMENS
uised for measurement of recrystallization kinetics. The total time at temperature is reported as the annealing time. The one-hour isochronal treatments used for X-ray line broadening were carried out in order of increasing temperature; the repoited temperature is that of the latest treatment. Metallographic sections were prepared by mechanical polishing followved by electropolishing and chemical etching. That the polishing technique produced surfaces with little distortion is clearly showsn by the electron-channeling contrast micrographs that were obtained from recrystallized specimens. (Since this mechanism of contrast formation in the scanning electron microscope relies on channeling of electrons along open lattice directions inside the gold crystals, distorted crystals are easily recognized.5) Also, the electropolishing technique did not excessively widen the occasional interfoil voids present in the sample. The mechanical polishing sequence consisted of using waterlubricated SiC of 320, 400, and 600 grit followed by 6-,um diamond witlh a standard oil lubricant on nylon cloth and a water suspension of 0.05 !umAl2O3 on a higher nap cloth.+ The modified Cr,03, C2H40H electropolish described by Glen and Raley6 was used at 10 v for 15 minutes at room temperature. Electropolished samples were rinsed in glacial acetic acid and then in ethanol. Etching was carried out on clean dry samples by immersion for 20 seconds in a fresh solution of 50 vol% HCl, 25 vol% HNO, and 25 vol% glycerine.7 This etch grooves the grain boundaries in recrystallized samples and produces a molded structure in the unrecrvstallized samples. Microscopic observations were made by three techniques. Etched specimens were observed by standard reflected light microscopy at X 1,280 or by a scanning electron microscope using a seconidarv electron image. Electropolished samples were observed bv electron-channeling contrast in the scanning electron microscope. Such contrast requires a small beam t Microcloth, A. Buehler, Ltd., Evanston, I1. * Knoop indenter, 300-gm load, Tukon Model MO, Wilson Manufacturing Co., Bridgeport, Ct. t Siemans type F with incident beam; soller slit, 1/8 receiver slit. Peaks were continuously chart recorded at 0.5° 20 per minute. The irradiated area was smaller than the sample. § Ortec, Inc., Oak Ridge, Tn. ** Kristalloflex 4, Siemans America, Iselin, NJ. ¶ Here residual strain means a retained change in interatomic distance as contrasted to plastic strain which is obtained by replacing one atom by another via slip of an atomic plane by a lattice translation.
487
divergence angle and the resulting beam size provides clear resolution at lower than about X 1,000, but for comparison with optical micrographs a standard magnification of X 1,280 was used. Standard microhardness measurements* were made on polished and burnished surfaces. The surface was prepared by mechanical metallographic polishing and then heavily burnished by hand using a Spratley carver. The change in hardness was found to be the most consistent measure of the extent of recrystallization. The breadth of X-ray diffraction peaks was measured on peak profiles recorded on a diffractometer. The powder diffractometert was equipped with a fine-focus copper tube maintained at 40 kv by a stable X-ray generator.** Diffracted radiation passed through ,B nickel filler and was registered by a scintillation counting system.§ The I 1 1 and 222 peaks were recorded and K01, and K.2 components were partiallv resolved. The K02 component was removed using the Rachinger correction.8 The peak breadth was measured as the breadth in degrees 20 at half maximum height above backgrotind. To evaluate the broadening resulting from the instrument, the gold standard supplied with the diffractometer was run. The breadth of its peak was measured, and the square of the breadth of the sample peak less the square of the breadth of the standard peak was taken as the square of the breadth that the peak would have exhibited if instrumental broadening had been absent. This technique is exact only when the X-ray peak is a Gaussian shape and when the standard sample contains no internal sources of broadening. The broadening of the Il l and 222 peaks should be the same if broadening is due to small particle size (or extensive faulting), but the broadening of the 222 should be significantly larger than that of the 111 if a distribution of residual strains¶f is retained in the deformed samples. Methods of separating the two contribuiions are availableI but will not be presented because (to anticipate the results) broadening of 'Lhe 222 peaks was not observed to exceed broadening of the 111 peaks and, thus, measurable broadening resulting from strain was not observed. Results The microstructire of the foil sheet after passing through the alcohol flame and before compaction was approximated by annealing a
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488
SCHLOYER
FT
AL
J Dent Res Maj, 1977 the mU-iioscopic techniques used (optical ele(trons microscopy of etclied structLore's and hanneling contrast electron microsc opy of polished samples). In soiime pUaces the foil sheet contours w-exre still v isilelc. II wx idely s( attered spots, large (raiins approximiatin'g the sizc shown in Figur e 1 wvere present and appeareci to be free of strain markiings. Around these spots poorslv bonded foils wx ere ex dent and contained the very fine microstructure aforenmeintioned. The foil colitours inear suich spots have previously been observed and the .spots lbave been attributed to the local fuisinig of foils in the alcohol flame.10 The presenit observation of the large grains they contain reinforces the vieNv that they have been overheated, and siInce these grains are not greatly deformied, inidicates that deformnation is nlot coiImonly accomplished in and near such spots Ly
sc anning
c
4*
...
FIc 1. Microstructure of no. 4 gold foil sheet annealed five seconds at 621 C (1,150 F). Foil is 1.3 ILm thick. Optical micrograph; electropolished and etched (X 1,289).
foil sheet for fixe secoInds at 620 C9 in an air furnace. A ty pihal portion of the foil surface is shown in Figure 1. Since No. 4 foil sheet is 1.3 tin thick, the giains shown are pancake shaped, with diamiieter-thickness ratios of the order of 10. Thlle clnipacted foils werc xw ell bonided cxcept for widely scattered spots. The inost ommnM Mi rostruct ire xvas toe, fine to be resoxlv d c
dturinig compaction.
The fineness of the miicrostrticture 's also revealed by the broadening,- of thie 1 I I and 222 X-rax diffraction lines from nas-coompacted specimlens. The broadening show n by these peaks svas 0.15 and 0.19° 20, respectivelv. According to the S herrer equation,8 the bi oadeningr, B, is related
the
to
O/X, where
particle
size,
L, by
s
i
90 z
80
70
'60
50
H f-
-
i __1_ 1_
.01
.1
__-
.
cos
L
is thle Bragg ano-le and X is the wavetlength cf the X-ray-s. Accoiding to this relationI and the broadening measurenic-nts, IL 580 A for the I 1 1 and 620 A for the 222. The near equAlity of these two figniurs shoiN that s,train rosdlening is present. This concluision a
1___
2
10 100 Time tmin)
1_
2000
10,000
100,000
FIG 2.- Hardness sariation with time for conipacted gold foil specimens annealed at temperatures shown. Few points determined by quantitative microscopy are included assuming linear variation of hardness with fraction recrystallized. Initial hardness of each sample is indicated by left-pointing airow. As-recrystal]ized hardness is constant. Open and solid circles, hardness; X. imicrostructure plotted at %c Rx 100 (H-H) /(Hi-Hf). Downloaded from jdr.sagepub.com at TU Muenchen on July 1, 2015 For personal use only. No other uses without permission.
no
COMPACTED GOLD FOIL SPECIMENS
Yol. 56 No. 5
(min) lox
Recrystallized
e :H-
489
Century
ttNg Decade
Year
Month
compacted Hordn ess' 82 KHN 25tDoy
103
Hour
Minute
16
20o
2.4
2.8
1000 /T
3.2
3.6
(
K-1)
FIG 3. Time for 50/ recrystallization vs reciprocal absoltute ternperature. Microstructure and hardness of compacted and recrystallized direct filling gold foil specimens are indicated in their appropriate time-temperature clomains.
followss because strain broadens the 222 twilce as inuclh as the Ill antI hence -vvould result via the Scherrer e(quLation, in a fictitiouisly low particle size for the 222. The X-ray parlicle size of 600 A (0.06 gmi ) aitii the absence of strains is in line witPh previouis results bv Wag,ner4 oll cold-filed Vold annealed at room temperature. He also foun-id strain broadening Xas comnpletely retovererl at roomii temperature and slhe pa ticle size" xx.as Guli) two thirds of that reported here. T>he diffe en rte irs particle size is niot ulne\l)eeted in viev of tile imorc violent defor mation used bs)
WVagner.
During anne alin, for one hlour at temiiperatuLres at or los er than 150 C (x hich is before re-
cryqtallization begins ), the particle size increased bv about 15% as indicated by linebroadening ineasuireImienlts of the 111 and 222 peaks. After recrystallization was,s romplete, foi exCample, after one houir at 250 C, no linle l)roadlening was observed. Anneiallinig the compactec foil specimens at suffic ientlv Iigh temperature.s or for sunfficentli long timees i-cstilted in the appearance of large rec r-stalliized grains in, the fite ( old- workedl sIirostMuctUre. This chanige ini microstrLuCture
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490
J
SCHLOYER ET AIL
ssvas at" otlipaniht iby a decrease in average lhardness. l'Aperimeutal. results of the fraction- recrystallzizcl as ol)servedl mnetallographically anid of the change in hardiness arc piesented together.
The variations of hardness xsith annealing tiine at variotim annealingl temperplatures are shoss n as the points in Fig(lure 2. 1bh rosses are) quantitatis e icetallographit neiasuremei-is of tlhe fr-actioni rcci ystallized. They has e been Plotted oni the hlirdness ordinate scale using the aipproximate linear relation intlulcded in the fieire. Thle scatter in the dctla represents a stcatter in restilts fromii different aieas of the specimiieni. rol)alls]- resultinlg nhfl01 the inholimogeeitx of cleff o-In t imoi. Th11e currves seen in Ii Fiure 3 represenat 1rogre-ss of retcry stallization at thei various tel)peratiures at whiich meni-nenuirents vrme maIdc. Au1 appiroximaite c orr elition of sinch cuoves c an usiatlly be madIe based on the alsuuiption that recrSt'staliiltiotlbellaves as a thelma'llsv attixvatedt Th is leas to 1th Anr I n i Ius a t ion ji)ro (ess.
De-nt Res
Afay
1977
that the process inxvolved is not a single. actisated process and the actixation enertso obtained must he re,arded as an empirical tonstanlt.12 Figulire 3 shoss s the appiropriate plot and also shoss s ty pital recrystallizedcl and coldw orked miurostriictures. The linie thlat best fits the expec-i imeuital points is logj,, t-- 12.3 + 6.5 X 1,000 / (whic h yiekls an etl t?iP itcal atctix-ation- energy of 1 25 KJ/niiole ['30 Kcal/imole ). Extrapofation of this lin tft 37 C is indicated in1 theI fillgre. The tise of s log sca1le fLo timiie andl a 1,000 /T sc ale for tfem)per,ttiute is required to obtain a linear telationslhlip). but oni tie opotsitesidecs of the fiiurt miore commIIlonit utits; aire inch:1ate cl n11 nnllinear scales. The mic rostructore of the tonlIXl ted(l specimnen shoss-s the fine structure trpitcal of nearly all regions of thle uannealed spet imiiens. The ctmheri1 eroluolls atle in the ftused blobs priesituslv dcestctibed anti
ae an
inclesital.'le r1suilt of
piocessing.) Fultheilmort, ans sattiple slibjict to an annealingy treatment to the iright tof thle 5-)()" rcicsstall i7atic nx lint' is mtstly compicrlsed f thIis struttictte. 1Fhe u( cosfiti1t Ine11 Ii o f a ictc cIx .talMli7ed spe itimon is also shoewsn. 1 he ot att, a1t aloe- aintImans- strxaight andl steppedI lbiituldaties, wshithlare eharacteristit of cross Ith twin liouncarlaics, are pireenft. A\ ty pical regiotn is L )-dccl R in the ret y-stllizecld stint r' tm. A\y tfento
rate
-
A c-Q/J
l
inl 5 hit lh t Is the tiu' for- half tcoiplIction of ret (Istsaliation; .1, is a conIIstant; Q; the em(11-1 pirinal "activation energy"; R, the gas conslant; and '1', the absolute tetilp)ertitil'. Applicatitn. iif tlii reIalolla.o toric ix stalli,'atiou kinetic s (ot(iittit0 ittild is baiseci oil it.s cutipitritild si ccess for spec ifii alloys. Thle imiipiirits s5tnsitiv ity(o the pritreIwter obatliutldildE ates
Fic; . left, channeling contrast scanningr electron micrograph of as-compacted gold foil specinien. Electropolished and unetched (x 1,280). Right, challnelinig contrast scannxing electron micrograph of compactecd gold foil specinmeni annealed two iminutes at 250 C. Some
mtint cotresponcling to a Ipoitit ito the lift tf the. )-e r>] a 5s llittd ]illn has itistlxt tiiS s,ttsi 'atictittee. 1 he tallsitioli ftottltoes1 suit ture tfi the otlter i's sucllden in cat h1 lotcalitv f the speSPimen but thle tilux fcir srxneaicliin, of ret tlstillizi,t-
uinrecrystallized grains (cdotted fine struCture) remnain btlit imost of sample is occupied by large recrvstallized grains with growth twins. Their unifon r contrast is good inclication of lowcdistortion near their surface. Electropolished and Lunetched ( X 1,280).
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COMPACTED GOLD FOIL SPECIMENS
Vol. 56 No. 5
tion throughout the specimen is about 100 times the time required for its initiation (Fig 2). Within about a factor of ten of the time for 50% recrvstallization both as-compacted and recrvstallized regions exist in the same sample. Actually, the so-called recrystallized structure presented contains a small region that has the as-compacted appearance; it is labeled "C" and the treatment of this specimen is in the transition region, four minutes at 250 C. In the tran-sition region, the average hardness changes from the value characteristic of as-compacted gold, about 80 Knoop hardness number (KHN), to 50 KHN, the value characteristic of the recrystallized structure. In order to further show the microstructural change accompanying recrystallization, electron-clhanineling contrast5 scanning electron microscope micrographs of electropolished samples before and after recrystallization are shown in Figure 4. This technique reveals by the uniformity of contrast the strainfree recrystallized grains. Its spatial resolution is too small to show the fine structure of the cold-worked samples. Discussion The outgassed gold foil sheets contain pancake-shaped grains with a 10 jum diameter -and a thickness equal to that of the foil, 1 ,um. These grains are cold welded together during compaction. The foil sheets contain occasional regions that were accidentally fused during outgassing'0 and these regions contain large ball-shaped grains. Compaction does not greatly deform these grains and the foil sheets neighboring these regions are poorly welded during compaction. Except for these regions, compaction results in formation of rather fine cells or subgrains with a particle size of about 600 A (0.06 ,um). These cells have a level of residual -elastic strain that is too low to cause X-ray line broadening and, according to previous -work,4 have few if any stacking faults. The particle size alone wvill account for all the hardness observed in the as-compacted gold. The strength a that results from grain size, or cell size, 1, usually obeys the Hall-Petch equation: 13,14 a=
k
aO +-
-where aO is the vield strength of a very large grained polycrystal; k/,u a constant that has the -value 8X 10-5 mm/2 for both silver and copper; and A, the shear modulus that is about 4x 106 _psi (2.8X104 N/mm2). For a very rough esti-
491
mate of the grain size strengthening, we assume that KHN BHN _ (0.002) a(psi)15 and we take a___ 0, as suggested by the yield strength of single crvstals of gold.16 The KHN so calculated (80) is fortuitously close to the observed hardness. The agreement indicates that the cell size can account for the observed hardness of as-compacted gold. During recovery, that is, during annealing below the recrystallization range, a small increase in cell size was observed and a slight decrease in hardness, but from a practical viewpoint the hardness variation from sample to sample is more significant than is the effect of recovery. Durilng recrystallization, the large grains of gold appear in some regions of the sample. They grow across old foil sheet interfaces as well as in the plane of the foil sheets and end up as ball-shaped crystals containing numerous annealing twins. The recrystallized samples have no measurable X-ray line broadening. The recrystallization kinetics behave as usually observed for metals, i.e., they exhibit sigmoidal fraction recrystallized curves as seen in Figure 2 and dependence of recrystallization time on recrystallization temperature (Fig 3). The recrystallization is less rapid than that reported for pure bulk gold.2 Perhaps this is due to the lower amount of deformation introduced in hand-compacted gold foils as compared to the 96.7% reduction by rolling reported by Rose.2 Another somewhat unusual feature is the increase in grain size obtained on crystallization. This may be produced by the recombination of two effects: the thin initial grains and the small deformations introduced by compaction. The beaten gold is annealed (outgassed) to produce thin grains in thin foil sheets. Welding of these sheets by compaction probably does not involve large deformation. The crystallized grain size is consequently"" 2 rather large compared to the original deformed grain thickness and, as is always the case, is much larger than the cell or subgrain size measured by X-ray line broadening. Finally, and or most significance, the recrystallization process will not occur at 37 C in the material tested within the lifetime of a restoration; nor will occasional increases in mouth temperature alter this conclusion. Indeed, a week at 100 C is 1 equired to initiate softening and ten months is required to produce 50% recrystallization. Thus, recrystallization and associated softening are probably not a clinically -
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492
SCHLOYER ET AL
significant process in direct
filling gold restora-
tions.
Conclusions Laboratory experiments on direct filling gold specimens compacted as if they were restorations show that as-compacted gold foil specimens made from alcohol-flame-outgassed foil sheets are composed of pancake-shaped crystals extending through a foil sheet 1.3 Am thick) and having an extent in the foil plane of the order of ten times the foil thickness. These grains are deformed during compaction, however, according to X-ray line broadening, they have little residual elastic strain (or stacking faults) but have a cell size of about 0.06,m. Recrvstallization results in a drop in hardness by about 30 KHN and an abrupt increase in grain size. The erains become ball shaped and contain several gold foil sheets. Fifty percent of the specimen is recrystallized after annealing for a time of t minutes which varies with the absolute temperature T (K) according to log1ot ;=- 12.3 + 6.5 X 1,000/T for 573 < T . 373. Fifty percent of recrystallization is expected to require about 900 years at 37 C, but it would probably start at about one tenth of this time. Loss of hardness of direct filling gold restorations by recrystallization is not expected to become a clinical problem under ordinary circumstances. The material was supplied by the Williams Gold Inc.
Refining Co.,
References 1. PHILLIPs, R.W.: Science of Dental Materials, 7th ed, Philadelphia: W. M. Saunders
Co., 1973, p 379. 2. RoSE, T.K.: On the Annealing of Gold, J
Inst Metals 10: 150, 1913.
3. KURIYAMA, M.; KOHRA, K.; and TAKAGI, S.: Electron Diffraction Study on Polished
J Dent
Res
May 1977
Layers of Gold: I, J Phys Soc Jpn 12:151-
156, 1957. 4. WAGNER, C.N.J.: Stapelfehler in Gold nach giner Kaltverformung bei tidfer Temperatur, Z Metallkde 51:259, 1960. 5. Joy, D.C.; NEWBURY, D.E.; and HAZZLEDINE, P.M.: Anomalous Crystallographic Contest in Rolled and Annealed Specimens, in JOHAR, O.M., and CORVIN, I (eds): Proceedings of Fifth Annual Scanning Electron Microscope Symposium, 1972, pp 98103. 6. GLEN, R.C., and RALEY, J.C.: Improved Procedure for Thinning Metallic Specimens for Transmission Microscopy, ASTM Spec Tech Pub 339:60, 1962. 7. REDPATH, D.L., and JOSHI, K.C.: Metal-lographic Preparation of Aluminum and Gold, Microstructures 2:21, 1971. 8. WARREN, B.E.: X-Ray Diffraction, Reading, Mass: Addison-Wesley Publishing Co., 1969, pp 256-275. 9. HOLLENBACK, G.M., and COLLARD, E.W.: An Evaluation of the Physical Properties of Cohesive Gold, J S Calif Dent Assoc 29:280, 1961. 10. HODSON, J.T., and STIBBS, G.D.: Structural Density of Compact Gold Foil and Mat. Gold, J Dent Res 41:339, 1959. 11. CAHN, R.W.: Physical Metallurgy, 2nd ed, Amsterdam: North Holland Publishing Co., 1970, pp 1129-1197. 12. REED-HILL, R.E.: Physical Metallurgy Principles, 2nd ed, New York: Van Nostrand Co., 1973, pp 286-288. 13. ARMSTRONG, R.; CODD, I.; DOUTHWAITE, R.M.; and PETCH, N.J.: The Plastic Deformation of Polycrystalline Aggregates, Phil 7:45-58, 1962. 14. THOMPSON, A.A.W.: Yielding in Nickel as. a Function of Grain or Cell Size, Acta Met 23:1337-1342, 1975. 15. DIETER, G.E.: Mechanical Metallurgy, New York: McGraw-Hill Book Co., Inc., 1961, p 288. 16. HONEYCOMBE, R.K.W.: The Plastic Deformation of Metals, New York: St. Martins. Press, 1968, p 12.
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