Precipitation in an A1-0.212 wt.% Au alloy

PRECIPIT_ATION

IN AN Al-O.212 wt.‘, Au ALLOY R. SASE.-\RAS

CorporJte

Rcssarch Labomtorxs.

Var!sn Associates. 61 I Hansen Way. Palo Alto. CA 94303. C SA and

Department of IktAlurg~

end \latxlali

C. LllRD Swnce. C:ruverslt> oi Penns!l\arua

Philadslphta. PA 191’4 USA

Ahstract~~Ekctron mwoscope obserkatlons shoa that aging a supersaturated solid solution of AlkO.?lI?wt.“, ALIalloy produces a disiributlon of plate precipitates of the equilibrium phase q (Au;U,) on the rn,\trlx cube plants uithout the iormntmn or any metsstable phases (conwar! to prexlous report). Dk~~xt~ons m the matrix act ‘1s heterogeneous sites for the nucleation of these preclpitaxs. The large cohertnch strain ticld due to ;t mismatch of 4X0,, parallel to their broad t&es gibes nse to a large ‘tge-hardenine crtcct. 341sfit dislocations nucleated at longer ageing times to xcommod;tte the mismatch wducc th;s c15cct. RCsum&Dr~ obserbattons au microscope electromque ont montre que le v~edlissement d’une solu!:on solidc sursarurte de I’slhnge -\1-0.212”, .Au(en poidsl prodtut une rkpartitlon de prkipitCs en plaquc::es de la phase JSqukbrz t/ I.AuAI,) sur Ies plaus cublques de la matrice. sans qu’ll ) ait formation dc phase mklsrdble (contrulrcmmt h une publicarion ar&rieursL Les dislocations de la matrice se comportent wmme des sites pour la gcrmlnatlon h2tCrogine de ccs prklpltks Le champ de dkformxian dc coherence Important. produit par UII dcsaccord de 139; parallklement B lrur grande Iace, pro&t un etkt de durcissement structural imp+xtti. Des dlslocatlons d’mterface. qui apparaissent aprk des tamps dc vielll~ssemsnt plus longs. pour prc3idre en compte ce d&accord. reduisent cet effet. Zuwmmenfussung--Elsktronenmkroskoplsche Beobachtungen zelgen. da8 bei der Alterung e&r iibersattlgten festen L&tmg der AI-0212 Gew.“, Au-Legierung einc Verteilung plattenfdrmiger Ausscheidungen der Glelchgeblchtsphase rl 1.4uAlJ ohne Bildung irgendwelcher metastatnlen Phasen tim Gegensatz zu frhheren Arbeiten) auf den Wiirfelebenen der Matrix gebddet werden. Als heterogene Keime tiir dwse Ausscheldungen treten Versetzungen der Matrix auf. Das groBe Spannungsfeld. verbunden mit caner Fehlpassung van 4X, parallel zu den breiten Oberfichen der Ausscheidungen. berursacht einen grol3en .Alterungs-Verfestlgungseffekt. GrenzKlchenversetzungen. die bei Ilngeren Alterungszeiten zur Anpnssun g der Fehlpassung entstehen, verrmgern dlesen Verfestlgungseffekt.

1. ISTRODCCIION In 1965. bon Heimcndahl sho\\ed that the solid solubility of Au in Al is about 0.3 ~tt.O, at 640-C and decreases v.ith dccrcasing temperature [I. 31. Because of this. Al-Au allo) can bc age-hardened in rhc usual manner. Analgsmg the precipitation sequence in this shstrm b> transmission electron microscop) and S-ray techniques. he supgestcd the existence of a metastable precipitate r/’ that precedes the equilibrium phase t/ ( AuAl?) [il. He also observed a large age-hardening efTect m comparison uith the small solubilit> of Au in .A1 and attrlbutsd this to be part11 due to the homogeneous dl>trlbution of 11’ preclpitares. After examination of specimens aged after plastic deformation. he concluded that the homogeneous distribution 1s the result of the abxnce of prcfcrred nucleation of ry’ particles on d~slocar~ons. In the present paper. we report c\idence to show thx the precipitates bon Hcimendahl ldcntified as tf’ ar: actually 11 prcapitates during their earl> stages of Srouth and that the matrix dislocations do act as hsterogcneous sites for the nucleation of these precipitates. This uork formed

a part of a more general project to study the morpholog), interfacial structure and kinetics of Fowth of these precipitates and the effect of plastic deformation on the structural transformation [&73.

2. EXPERIMESTAL

PROCEDURE

Al-o.112 u.t.O0 Au alloy specimens cold rolled to about 0.12-O 15 mm thickness formed the starting material. Strips of this were solution treated at about 620’C for about 2 hr and quenched to room temperature prior to aging (quench-aging) or quenched directly to the agins temperature (isothermal aging). Thin foils were prepared by standard eiectropolishing techniques. Specimens cut from near the holes pro, duced during polishing were examined m a Philips EM 300 microscope equipped with a gomomercr stage. In some cases. after a short aging treatment in bulk to produce a distribution of precipitates in their early stages of transformation. the specimens uere subsequentI> aged in the hot stage oi rhe microscope to study the microstructural changes VI siru.

518

SANKARAN :\SD LXIRD:

PRECIPITATION IN AN Al-hu ALLOS

Fig. 1. Isothermally aged at 200 C for 20 hr

aging conditions. Also. the isothermal aging treatment reduces the number of nuclei per unit volume of the In agreement with the observations of van Heimenmatrix compared to the quench-aging treatment [S]. dahl [Z]. aging tht alloy at low temperatures for short Aowing the growth of the precipitate to proceed with durations produces a uniform distribution of platelets less interference from the diffusion fields of the neighi>ing on : lOOi, planes of the matrix. Fig. 1. The broad boring precipitates. Figures 3(a) and (bl show the infaces of these precipitates, being featureless. demonterfacial structure of a precipitate formed after this strate the coherent nature of these interfaces. The aging treatment and Figs. 3(c) and (d) show the LMoirC plates are initially circular, Fig. 2(a) and later octafringes produced due to these dislocations accommogonal with edges parallel to (lOO>, and ( 1lo), direc- dating the lattice mismatch between the precipitate tions. Fig. Z(b). Prolonged aging leads to square plates and the matrix. The similarity between Fig. Z(c) and ivith edges parallel to i lGO), and the broad faces conFigs. 3(a) and (b) and the evidence that the transfortaining a square network of misfit dislocations in edge mation from Fig. 2tb) to 2(c) is merely a change in orientation. Fig. 2(c) shows one of the two sets of interfacial structure suggest that all these precipitates dislocations, the second set being invisible due to belong to the same phase. namely q. &&.= 0 condition. These precipitates were identified The tf phase forms as a iine compound and has by van Heimendahl as I/‘. The transformation from an f’.c.c. structure (CaF, type) with (I = 6.COA. Fig. the coherent stage of Fig. Z(b) to the semicoherent 4(a). By the Bain re~~t~onship, this unit cell can aljo stage of Fig. t(c) has been directly verified by hot be described as a bodv centered tctragon with stage electron microscopy [a]. The results of the aging a= 3.243 A and c = 6.00*& Fig. 4(b).* When the Y/ treatments of von Heimen~~hl seem to suggest that phase forms as a plate on the matrix cube plane. the quench aging beyond 12 days at XO’C or 22 hr at edges of the b.c.t. unit cell match with the matrix 3OO’C produces the equilibrium phase rl. To verify cube edges. producing a lattice mismatch of 1.80, parthis. a specimen was i~thermally aged at 4OO’C for allel to the broad faces and 39”, normal to the broad 24 hr which is well beyond von Heimendahl’s extreme faces. Due to mismatch stresses. ‘CI’will be close to 4.04.~ initially and during grouth. \\ill approach * Tht similarity between the rl precipitates in hl-Au and 4.243 A. Since the structure of the edges of 4 plates tY in .-\I-Cu is ouitr” rcmarkablc. Preston rlO1 found thz unit 41 of u’ to de tetragonai with n = 5.7 .i and c = 5.S .A is not known. the r&t of mismatch stresses on ‘c’ and the atomic ~~r~~n~ern~ntas in Fig. +a). Silcock. :‘r cannot be estimated. in addition to the mismatch ai. ill] later modified this to a trtragonal ceil with stresses affecting the ‘~7’ and ‘c’ baiues. determination a = 1.0-i .i and c = 5.S .A and the atomic arrangement as of ‘c”from electron diffrxtion patterns is very inacin Fig. -t(b). In the madiiird unit crll. ‘(J’ vnrizs betwren curate. Since the precipitates form as plates about 4.04 and 4.095 A as the broad faces of N’lox cohcrcncv C-13. In the Preston unit c:ll. these values arc 5.i and *5.SAi 20 A thick even in the overaped condition. the diffraction spots from these will be elongated in the c-direcrrsprctiveiy. 3. RESCLTS AND DlSCl_XSlO~

:

space [9]. b\ a’nout 0.05 .A - I in reciprocal Bscaus; of this. using the (003 reflection. measured x;llue of ‘c.‘can be expected to var)- between 5.32 and -.(I7 .A. and more when the plates are thinner. Thus. it is invalid to conclude. \vith son Heimendzthl. that rlts obsericd interplxw spacings should be based on :he metastablc phase ~1’ with a tetragonal unit cell citl= 4.14.Aand c = 6.5 ‘4. and that the!- are quit? ditfsrent from those from the equilibrium phase q. -III the difiaction spots allowed in the b.c.t. lattice Ih - k f I = enm I with 12= 4.04-4.313 .A and L’= 5.22-7.07 .A (the range of values for c b&-g due x errors in measurement because of streaking) cart t‘c indcrzd IO the f.c.c. phase. rion

made in this stud! for the ii001 reflection from areas of the foil in the [otti], orientation containing onI\ precipitates of the type shown in Fig. Jbt. perpendicular to the electron bsam. Sctxted area diffraction patterns from these axas did not weal an! diffraction spot beti\e:en the main beam and the 13Y)j r&kction from the matrix. !Vhen the sActed ;~rr;l contains plates on habit planes parallel to the electron beam. however. these orientations can gi\c’ rise to reflections between the main beam and ths 1200) r&ction of the matrix as in Figs. jib) and ICI. When the sekcted area contains platzs on all the habit planes. the diffraction pxttcrn from a [OOl]x foil \viIl be a superposition of patterns in Figs. 5(a-ct. In addition to these reflections. double diffraction eKescd in Fig. 5(c). could give rise to a \\ hole set of sateliitt sputs Ltround each ol :he matrix spots. ft is not clear whether the I IOU)and L1Xt reflections \on Heimendahl obssrvcd .tw due to plates on

520

S.ANKAR.4S

AhD

LAIRD:

PRECIPITATIOS

IN ;\S .\I-Au ALLO1.

Fig 3. Isothermally nged at JOO’C for 24 hr. The insert in (a) shows 1w magnitication image of the precipitate and the area examined The dislocation spacin,o in (a) and (bt is about S4.A. fk MoirC fringe spacing for the (230) reflections is about 3s .A.

habit planes parallel to the beam or due to double diffraction effects. In addition to a homogeneous distribution of precipitate plates in the matrix. matrix dislocations act ils heterogeneous sites for their nucleation. Fig. 6. Since the matrix dislocations are usually of the type (ail) (1 lO),. only two of the three orientations were observed on any one dislocation. as in the case of 8’ plates in Al-Cu allo\-s [I?]. It seems likely that even the homogeneous distribution of q plates ‘actually nucleate on dislocation loops formed by the

I o / / 10 1 i

~

/

-_ i ! I i y-5I

(al i31 Fig. 4. la) F.c.c. unit ccl1 of ~1phiis_‘ I~I Equi*.Jent kr.c.t. unit c&f. Open points arc A! 3tORlS znd tiiisd points are

-lU ZttORlS.

-+t-

a

2%

r;=-

0 110

0 *

200

(a)

XXX X0X

xxx

.

b

condensation of vacancies retained during quenching from the solution-treatment temperature [la]. The wcancy loops will later annsal out. The different result obtained by van Heimendahl regarding the role of matrix dislocations durin_g nucleation ma> be due to the difference in the aging treatment. X qusnchaging treatment would produce a higher density of dislocation loops as a resuit of Lacancy condensation and the precipitation could occur almost twtirrlj on these loops. During isothermal aging. however. the matrix dislocations can compete with the lower density of loops for nucleation sites.

.fcLt~~rtl~,!~c,m~tlis---Thtsuork \\.u done at tht Laborator\

far Rzv~arsh on the Structure of Sfdtter. L-nlrer>it> ii Penns>l\ams Hhlch IS fundeci b> the Xxional Scirnce Founddtlvn {Grant No. FSF-GH-336331. &_lditional asr\sso
>lstdnce HJ%prvr~ded b) Vxim prepnratwt of the manuscript.

REFERESCES

1. bon Heim:ndrthl

St.. P/I!,< Sruc~c .S,iii& 10. K 131

(1965).

1 ton Helmendahl ht. Z. .\lir&X-. 58. ‘10 I 1967). 3: van H~lrn~nd~~hl51.. .&.t~i tlecr. 15. 1441 i 1967). 4. Sankaran R. and L,txd C.. Pint. .Urty. 29. li9 I 1974). 5. Snnkaran R. and Laird C.. 4cra Mrt. 22. 957 (1974).

6. ,%nkaran R. and Laird C.. .\l& Tran~ 5. 1795 (197-I). 7, Sanknran R. and Lnird C.. ,\lot. Sci. Enyrlg 15. 159 IL97-l). 1. CONCLUSIONS X?. 3. Aaronson H. I. and Laird C., TWIT. I US-.lJ.\JE. 1457 (i9SS). A supersaturated solution of Au in At decomposes 9, Hirsch P. 3.. Hottie A.. Xcholson R. 3. Pashle! D. to form the equilibrium ri (f.c.c. AuXi2) precipitates W. and Whelan hI. J.. EI~r,un Jlicroscop~ oj T/I/U C’~yuls. p. 98. Butteruorths. London i IY6Y). in the x-matrix, without forming an) metastable precipitates first. The 1 phase forms as plates 011 (loOi, 10.Pruston G. D. Phil. .Mn[7.26. $55 (133~ ii.Silcock J l&l.. Heal 1. 1. and Hard) H. K.. J. Iwt. planes of the matrix v+ith the orientation relationship Met& 82 239 (19%). (~~)=‘(liO)~ and [OOI], [QOl],. It is not known 12, Nicholson R. 3.. 1. Pfz.~. Rdtm 23. 524 (1969). whether GP zones form prior to the nucleation of 13. Njuten J. B. M.. .-lcta Ma II. I765 i i3h71. ‘7 precipitates. The large mismatch along directions

parallel to the broad faces of these plates gives rise to large strain fields around them and an appreciable ;lye hardening effect.

NOTE ADDED IX PROOF Comments by van H~imendahl on this art~cls and our

reply will be pubhshtd in fcriprti Met. 14 119%).