Annealing of deformation substructure in fatigued silver single crystals

ANXEALIXG OF DEFORMATION SUBSTRUCTURE F.\TIGUED SILVER SINGLE CRYST.4LS s. >I.

IX

L. sxxRI

StcDonnell Douglas Research Laboratories. St. Louis. \I0

63166. r.S..A

and

Departmat

oi Mt~allurp~ and SLirerials Science. L’ni\ktx Canada S15S IA-L

of Toronto. Toronto

Abstract-The annealing behavior of dislocation substructures and point-defect clusters produced b! fatigue deformation was studied in single crystals of silver. Crystals fatigued to saturation in alternate tension and compression at a constant plastic strain amplitude of 20.02 were annealed at direrent temperatures, and the resulting microstructural changes were studied by transmission electron microscopy: High-temperature annealing (> 5U.K) of the dislocation cell structure results in almost complete anmhllation of the dislocation cell walls with no indication of polygonization and recr>stallizatlox .Annealing at intermediate temperatures results in a partially annealed structure consisting of dislcxl II~Y tan+ formed by primary and secondary dislocations. several of which are held up at Lonxl- Colt:.:!! barncrs and point-defect clusters. A large number of dislocation dipoles and loops comprlslny tl:< dislocation cell Lvalls in fatigued specimens anneal out in the temperature range 30%5OO’C. Upon annealing, the point defects produced during fatigue deformation acquire various configurations tvhich Include Frank loops. stacking fault tetrahedra and prismatic loops. RCsumLOn a CtudiC dans des monocristaux d’argent Is recuit de sous-structures de dislocations :t d’amas de dtfauts ponctuels produits par des essais de fatigue. On a recuit 6 diverses temperatures des monocristaux fatiguts en traction et compression alterntes. I’amplitude de la dCformation plastiqL:_ &ant constante et &gale a k 0,02: on a etudie par microscopic tlectronique en transmission les changements correspondants de la microstructure. Un recuit B haute tcmpirature (>5OO’C) d’une structure de cellules de dislocations fait disparaitre presque complZtement les parois des cellules, sans qu’or: voiz de polygonisation ou de recristallisation. Un recuit 1 des temptratures intermediaires produlr une structure parriellement recuite: celle-ci consiste en des tchevaux de dislocations primaires et secondaires. dont plusieurs sont fixes par des barrieres de Lomer et Cotrrell et par des amas de dOfauts ponctuels. De nombreux dip3les et boucles de dislocations (dont les parois de cellules) disparaissent dans les echantillons fatiguts entre 300 et 5GQ’C. Au tours de recuit, les dtfauts ponctuels produi: au tours de I‘essai de fatigue prennent des configurations diverses, comme Ies boucles de Frank. ies tktraidres de dkfauts d’empilement et les boucles prismatiques.

Silbereinkristallen wurde das Ausheilverhalten von Versetzungssubstrukturen und Punktfehleragglomeraten, die im Ermldungsversuch erzeugt wurden. untersucht. Die Kristalle wurden bei konstanter Dehnungsamplitude von f 0,02 im Druck-Zug-Versuch ermiidet und daraumin bei verschiedenen Temperaturen ausgelagert; die entstandenen mikrostrukturellen Versnderungen wurden mittels Durchstrahlungselektronenmikroskopie untersucht. Die Hochtemperaturbehandlung I > 500 C) der Versetzungszellstruktur fiihrte zu einem fast vollstHndigen Abbau der Zzllwlnde ohne Anzeichen van Polygonisation und Rekristallisation. Auslagern bei mittleren Temperaturen fiihrte zu einer teilweise ausgeheilten Struktur bestehend aus Versetzungskniueln primzrer und sekundarzr Versetzungen. von denen manche an Lomer-Cottrell-Versetzungen und Punktdefektagglomeraten aufgshalten werden. Ein grol3er Teil der Versetzungsdipole und -schleifen, welche die VersetzungswCnds in ermiideten Proben ausmachen, heilen im Temperaturbereich von 3OC-SOO’Caus. Die wShrend des Ermiidungsversuches erzeugten Punktfehler nehmen nach der Auslagerung verschiedenartige Konfigurationen an, welche Franksche und prismatische Versetzungsringe und Stapelfehlertetraeder einschlieDen.

Zusammenfassung--An

IN-IXODUCTIOS The deformation substructure in the saturation stage of fatigued. face-centered-cubic (fcck metal single crystals consists of open bands of dislocations at small strain amplitudes which chanse into a closed dislocation cell structure at lsrpe strain amplitudes [IA]. Interspersed through th; dislocation structure is a

high density of point-defect clusters [CS]. In .i previous study [4] the development of dislocation substructure in silver single crystals during fatigue deformation over a wide range of strain amplitudes XIS reported and the dependence of saturation stress upon dislocation cell spacing and the point-defect cluster density was established. In the present nork.

12’1

“r‘lIw tkc c!sta.i!s oi ths subof annealing LL, produced during lbti$ue J:f;lrmaiion of silver single wstals ww ritudied by- ann~aliry the fatigue microstructure at diffiritnt temperatures and examining the resulting microstructural changes by transmission electron microscopy (TEM. The annealing experiments reporred in this paper were undertaken in order to study il\ the thermal stability of the fatigue-induced substructure. (21 the details of the dislocation cell structure and point-defect clusters. and (3) post-annealing defect configurations. the &xt

StIliCtUR

EXPERIAIEST.4L Single crystals of silver oriented

for sin$e

slip

(Schmid factor -- 0.5) wre fatigue tesed to saturation to a cumulative shear strain of about Y.0 in alternate tension and compression fatigue at a constant plastic strain amplitude of ~0.01. Slices -2 mm thick parallel to the primary slip piane were cut from the fatigued specimens bv spark machining. The slices were encapsulated in quartz tubes under vacuum and ivere annealed at various temperatures in the range NO-SOO’C. Thin slices for TEM were prepared from the annzalctd specimens by chemical thinning in dilute nitric acid and then electrolytic thinning in a 1.5”~ potassium cyanide soiution. ttt situ annealing esp~C ments were carried out by heating the thin foils prepared from fatigued specimens in the heating stage

Fig. I. Transmission electron micrographs of specimens faatipued to saturation (a) and annAed 4WC ior I ,2h 10).Primary slip plane = (11L1./yp = *OX.

at

RESL’LTS In Fig. i tltr genrral dislocAx3 ~r~ln~ernent in a yxcimetn i%ti~ucd to saturation is compared with that in a specimen given post-fatigue annealing treatment. The dislocation structure in the fatigued specimens (Fig. 131
The temperature dependence of anneahng of i;itigue microstructure was studied b> esamining the dislocat&n structure: of the specimens antwaled in 54k zs

127-i

SASTRY

AYD RAM.X3~.4~fI:

Alr;NE;\LING

well as that of those amxaled in the heating stage in the electron microscope. Figure 2 is a sequence of micrographs oithe same region of a fatigued specimen heated in the microscope in the temperature range I@--5OOC. A significant reduction in dislocation density and point-defect density both in cell wails and in cell interiors occurs in the temperature range 330-XWC. This is the temperature range predicted for rapid annihilation of dislocation loops in silver from calculations based upon annealing kinetics of dislocation loops in other metals [7]. From a study of dislocation-loop annealing kinetics in copper and nickel, Segall, rt a[. [7], predicted that the temperature at which significant annealing of dislocation loops of the size >30 nm in silver is -610 K. The observed temperature at which rapid annealing of the dislocation substructure takes place in silver is in good agreement with the predicted value. As the temperature of annealing is increased above 300°C joggy dislocations become straight by climb, as at A, and nei&boring defect clusters coalesce to form larger defect configurations. After high-temperature annealing (Fig. 3d). the dislocation cell boundaries disappear

OF

FriTIGCED

SILVER

SISGLE

CRYSTALS

almost completely. X few secondary dislocations pinned by point-defect clusters and threading through the cell boundaries still persist. So recrystallization was observed in either thin foils or bulk specimens.

The details of dislocation configurations resulting from annealing treatments were studied by standard contrast experiments under ‘two-beam’ conditions. Contrast experiments coupled with the ‘line of no contrast’ criterion of Thomas and Bell [lS] and the ‘black-white vector orientation‘ criterion of Wiikens [9] and Riile [lo] were used to characterize and identify small-defect configurations. Fatigued specimens annealed at SOO’C for 1 h were selected for this study since after such treatment, the density of primary dislocations and dislocation dipoles decreased considerably, facilitating the ease of analysis of the remaining dislocations and defects. Also after this treatment defect coalescence results in large recognizable defect configurations, and annihilation of large defects such as stacking-fault tetrahedra does not readily occur.

Fig. 3. Defect structure in fatigued specimens annealed at 5OO’C for I hr, imaged under different vectors. Primary slip plane = (111). (a) g = (202). (b) g = (?I!O), (c) g = (033. (d) g = (I! 1).

g

Figures ?rai+dt are micrographs of the same region (as at C in Fig. ;jalsoresuits from annealing of fatiof a spsimcn imaged under different y vectors. Some gued silver single crystals. primary dislocations (b = [iOl]) subsist. even after Finally attention is drawn to the effect of the dekiannealing at 5GO’C. These primac dislocations are ation parameter s upon the visibility and contrast of held up at either secondary dislocations or pointdefect clusters. Many defects Visible when 5 is zero defect clusters. A major portion of the total disloca(Fig. 43) are not visible when observed under large tion density belongs to secondary systems, as seen deviations from exact Bra-z diffracting conditions in Fig. 2t,dl where primary dislocations are out of con(Fig. -lb). This observation is consistent with the trast. These secondary disiocations are heavily jogged results of Thomas and Bell [SJ (viz.. for s = 0. defects producing strain-field contrast are risible through the and are held up at point-defect clusters. Dislocation foii thickness whereas for s f 0, kisibility is confined tangles, as at B, contain dislocations having diffsrent Burgers vectors and Cottrell-Lomer locks. A signifi- to either the top or bottom surface of the foiI1. Some cant fraction of point defects showing black-white of the defect clusters which appear either as black _contrast for y = [Ii I] (Fig. 3d) is out of contrast for spots or black-white lobes for s = 0 exhibit a double$7= z (Figs. 3a. b. and c). These defects, there- line contrast when s > 0. fore. are small dislocation loops with h = a. 3 (I I1 j. Defects such as C. which exhibit double-arc contrast DISCUSSION with line of no contrast parallel to s:Yzmn:ettical defect clusters. and stacking-fauit 1, detaited analysis of the point defects presented in tetrahzdra. :I significant number of prismatic loops

SASTRY ANDR.-tM?SWAMI:

ANNEALIXG OF FATIGCED SILVER SISGLE

CR’iSTALS

Fig. 4. Conventional dark-field (a) and weak-beam dark-field /b) micro_eraphs of defect con+xrations Primary fIip plane = (lit); g = tili).

the prstixis -ticdon reveals t52 folloxin~ important f2atures: +!ic deiormation gix5 rise to a preattx concentration of point dcf=ts than undircctional deformation: ths di&rtnt defect configurations resulting from cyclic deformation consists of stacking-fault tetrahedra: Frank !oops, prismatic loops, and sphzrical clusters; and the various defect configurations act as thrrmally activated barriers to dislocation motion. Esamples of dislocations being held up at szvcral types of point-def2ct clusters can b2 ~22x1in Figs. 3 and 1. Thus. thz saturation str2ss in strain-amplituds controlled-fatiguz deformation is som2 form of superposition of wsss neccsssary to w2rcome these pointdef2ct cluster barriers and other contributions to saturation stress (Liz.. the Frank-RsJd stress of th2 screw dislocations bowing out bztiveen the cell ~~~11sand the interaction stress bsttveen mobile primary dislocations and other mobile primary and srcondary dislocations). At small strain amplitudes. the point-drf2ct clustctrs apprar to be more important as obstacles to dislocation motion. since a weli-definsd celi structure is not develop2d at small strain amplitude. and thus thr Frank-Read stress for screw dislocations bowing between the cell \!a& is not u2il defined at small ;cP. As the strain amplitude is incrsased. a cell structure with dscreasing cell-wall spacing develops [3.4] ; hence, in the high-strain amplitude region. the FrankRead stress uill be mor2 dominant. .A possiblr mzchanism for ths formation of stacking-fault tetrahsdra by dissociation of Frank dislocation loops has bsen given by Siicov and Hirsch [13]. Stacking-fault tztrahedra hsxs bszn observed in d:formsd fee mztals[!6] and more recently in fatigued copper [I:]. .A largz number of stacking-fault tetrahedra u’sre also observed in fatigued silver single crystals (for esample. see Fig. la). These obssrvations suggest that the formation of stacking-fault tetrahedra is a process which occurs readily during plastic defor-

mation of fee metAs of low and intermsdiate stacking-fault snergy. The stxkiqfault tetrahedra have been shown to bc sstrem2Iy stable at high temperatures. This is further confirmsd by ths present study which has shown that stacking-fault tetrahedra persist 2ven aft2r high-tzmpernture annealing and that some of the other point-d2ixt clusters take up the configurations of stacking-fault tetrahcdra upon high-tsmperature ann2alinp.

Th2 diRerent types oi dislocation and d&x COGfigurations resulting from annealins of fat&x microstructuxs uerr studisd in silver sing12 cr>srJis b\ TEbf using ‘difFraction-contrast’ and .strain-&id-contrast’ critsria. High-temperature annsaliny result4 in complztr annihilation of dislocation cc11 M.alls with no indication of recrystallization. Int2rmediat+tempsraturs amxaliny resultsd in dislocatifin tan$es consisting of primary and secondary dislocations and Cortr2ll-Lomer locks. .A high density of dislocation IOOPSwith (1 3 :~I I1 ;> Burgers vtxiors. stacking-fault tztrahedra. and prismatic loops )\2r: obslrrv2d in thz annealed qxcimens. Th2 results Isnd furth2r support to th2 earlier obssrvations of thr rolr of forest dislocations and point-drfect clusters in fatig~c deform:~tlon.

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14. T. Broom

and

166-179: (.A) 251, iSi199

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