b IA-UR-95-116f)

We:

Author(s):

Subnitt6d 10

LosAlamosNAII[)N AI lAllollAlo FiY”

STRESS RELAXATION IN DISCONTINUOUSLY REINFORCEDCOMPOSITES

RECEIVEDHAY081995

0S11N. shi, ~. .1. Arscnault

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I aIIIIINatn.lm11’I

STRESS RELAXATION IN DISCONTINUOUSLY REINFORCED

(;OMPOSITES1

N. Shl

MS H805, LANSCE

Los Alamos National Lab.

Los Alamos, NM 87545

R. J. Arsonautt

Dept. of Matwials & Nuclear Engineering

University of MarylandCollegIa Park, MD 20742-2115

AOSTRACf

It IISS been obscnmd Ibat in dhmonthwously-reinforced At20J1’liAl composites thst as the

rcinkmemcnl size increases lhe m’ersge density O( dislocmimrs gmermed from the relmxmion

of Ihc lhcrrtml slrcsses increases, and Ibc corresponding Ihmrrd miidual rdrcsscs slighlly

decrease. Similm cb-nges msull wbcn Ibe rcinl’orccmcnl morpbtdogy cbangc!i from spheres

to short fi~rs 10 continuous fllamcnls. The cburgcs of dislocmlorr i’cnsily and Ihcrrrml residual

sIrc.sscs wilb respect to pmticle si?c me In contrast 10 Ihosc r,bscrvcd in Ibc SiC/AJ

countcqml. A previously developed slmplc model u.wd to explain Ibe SiC/Al IIaIm, wbicb WM

based on prismatic dislcatlon punching, suggcslcd that the dcnslly of the misfil disloaions

dccrcmtcs when tbc reinforcement size inuca.ws. In ibis Investigmlon, a simplr model is

proposed to cxpl~in tbr mnomdy in tbc develnpmenl of Ihcmml rcsidunl SUCS.USand Ihc

generation of misfit dlslocmlons M a thncllorr O( the ptrtlclc Azc and shnpc in A120#JiA.1

composites. As a result of n lack 0[ sufficient indc~ndenl.xllp. syslems in low symmclry

mstcrimls such ts NIAL plMIlc relwmdon of the thermal strc.ssceis ,sevcrcly constrained as

compared to fcc Al. As such, plm.k rclmxadwr rcquirm collnhorativc sllps in mnmggrcg-tc of

grains. This only rrccurs when the Icngtb wale ot’ the varying midll Ihrnntl stress flcld is

much larger Ihmi Ihc avcrngc @r size. Thw Is, IIJCmcrh~nlsm of plasIic rclnxtllorr lwromes

o~ratlvc whrn Ihc rcinfiwccmcnt size Incrca.ww

1 This resemch was suppnrted in parI hy the Otl’kc of Naval Rcscmh undm granl NWlt)14-

91-J- 1353. N, Shi would Ilkc 10 arkuowirdgc the support irmu Ihc U.S. Deparimt!ru ot’

Energy.

MASTER

lNTRODU~lON

It basbcen proposed tha[dislocaliona arc gcnerwed by relaxation of the thermal sIrcsses.

These [hcmnal strcs,scs arc developed during the cooling of a composite with a reinforcement

and matrix which have different cocfficicn[s of expansion (Arscnault, 1984. This concept has

been used in investigating several composile systems such as SiC/At, Si/Al, AJzOfliA.l and

Tiq/NiAl (Amcnault, lWI, Arscrmult and Fisher, 1983, Vogclsang, et al,, 1986)

several models have been developed [o predict Ihc density of dislocations generated and the

cffcctivc plastic strain (Amcnauh and Shi, 1986, Shi et al., 1992) due to the rcltxation of the

thermal sircascs. The prismatic punching model developed by Arscnault and Shi (1986) is

capabk of predicting how the dislocation density would change in a homogeneous matrix with

inctcasing the volume frac[ion and psrticlc six when the [hcnnal misfit arc compk[cly

released through dislocation generation, A%the volume fraclion increases the density of

dislocation increases, and as [he pafiiclc size increases at a constant particle volume fraction

the dislocation density decreases, These rcsulIs come from the fact thal the dislocation density

arc linearly related to Ihc total particle surface area. Experimentally it has hccn shown that in

Ihc case of SiC/Al [he changes in dislocation density wi[h particle size follow Ihc prcdic[ion

of ArscnaulbShi model (Arscnaulf and Shi, 1986) as shown in Fig.1.

Io’4 m.2

MML 200

g2 -

.g I20 V% SiCp/l 100 Al

8

51.5 ,., ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

‘a

8

aal . . ...,,,,,, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

&

%

2,= 0,5 .,, ,. ... . . . . . . . . . . . . . . . . .,, .,,,. ., .,..,, ,

i ~

b=

O*1 I 1 1 1

0 50 lrxl 159 200 2!KI

Particle size (D) ~m

Fig, I The Increase In average matrix dlslcrcatlondensity In 20 V% SIC/Al compoallein the as annealed (12 hrrr at 530”C), furnace cooled (12 hrs) condltlon as afunction of particle SIZO, The Increase In dislocation density (Ap) IS equat to

bm~sim - I)mablx, The malrlx Is annealed under the rtame rmrrdltlormns the

composltsm

.

t

In polycrystalline low symmetry materials such as NiAl, plaslic I1OWin a paflicular grain

is difficult due to the lack of a sufficient number of indcpcndcnt slip systems and [he

constraints by the neighboring grains. Therefore, in inttnnctallic matrix cumpositcs generation

of Ihcrmai misfit dislocations, may be affected by the slip constraints. In contrast 10

aluminum-based composites where a high mobile dislocation dcnsily is bcnelicial to [he

strengthening, a high dislocation density in irrlcrrnctallicbasedcompositesmay comributc 10Ihc improvement of ductility.

In Ibis investigation, thermal residualstresses(TRS) and the matrix dislocation density in

AfzOJ-reinforced NiAl were dclcnrrincd hy neutron diffraction and Transmissioncleclron

rnlcrmwopc(TEM), rcspcctivcly.The invcstiga[irrnwas conductedto uctcrminc the influence

of parliclc size and shape on Ihc plastic relaxation of Ihc thermal mistit stresses in [Lis

composite. The results were compmcd with those of SiC!Al composites. A complementary

FEM analysiswas conducted,and a simple model was cOnStNctcd 10explain the expctimcntal

results.

EXPERIMENTAL PROCEDURE

Composites of 20 V% AlzO, with five diffcrerr[ ,emforcement sizes and shapes were

produccd(tiqui-~ xetf parlic!cs with rliamctcm of 5, 75, 355 /(m, shorl fibers wi[h diamclcr of

lf)~m and average aspect ratio of 10, and coruinrrous filament Wilh dinmcter of 144 jm). Thecontinuous filament ~:OJ/Ni~ composites were prodtrccd by a powder cloth Icrhniquc,

whcmaa, the cqui-axed parliclcs were produced by mixing powders of AfJ03 and NiAt of the

same size, and then hot prc.aaing nt 169J K for 4 hrs al a pressure of 25 MPn. The short fiber

composite was produced by mixing 75 pm NIAI powder wilh the Al@3 shofi ti~m. followed

by the same hot prcaain~ procedure. After processing, all composites were anrrcalcd al 140t1°C

for 1 to 4 hours followed by [umacc-cool. Since Ihc compmilts (particle and shofi.fitur) were

producedby a similar PM pmccrfurcs, the as-proccs.wd microstructure in the mawix should

IM simil~r, i.e,, similar grain size, etc.

Ilc Iransmimiorrclccmonmicroscopywas pcrfonncd with a 200 KV ml 1 MV TEM. Tfrcdctalls of foil preparation nnd dnta analysis arc given clsewhcrc (Wsn~ ct al.]. The

nlcnsurcments rrf the TRS in Ihe matrix were perfonncd hy ncrrn’ondiffraction in Univcrsily

of Mlssourl Rcscnrch Remww with a munoclrromatic wave length O( 1,2 A, nnd2?0 NiAl

f3mgg peak was used to dctcmune matrix Iauice swains. The prncmlurm arc dcwribcd f’urthcr

by Sbl c! al. [SfIl et al,, 1Y93, Smith cr d., IW2)

In the FEM Invcstlgalion the A13AQUS FEM cofle WM USC4 mI~ nlr~frc~ w~wntinll IWO

dlmen~irmal axlsymrnctrlc unit cells containing spherical, short IIIIJ {wnlinuous tmvlindricnl

inclusion were made. The prmwturc is dcwrilrcd In more drtrnil hy Shi cl al. ( 1W2),

reinforcement increases, Ihe dislocation density increases (Fig.2b). The changes in dislocation

derrsi~ as function of the reinforcement are [be opposilt to ihose ob[ained for SiC/N

composites (Fig.1), in which [he diskwa[ion dcnsi[y decreases as [he SiC size increases.

MML370

~ 10’4 E

$...........

. . . . . . . . . . ,,, .

i

. . . . . . . . . . . . . .

. . ...1.... ., .,,,..

. . . . . . . . . . . . . . . . . . . . . . . . .

:10” ~,- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

Flg2a

. . . . . . . . . . . . . . . . . . . . .,,

,010 ~sphere fiber filament

The Increases In average matrix dislocation density In 20 V’% A120JNIAI

compoattas wtth particle shape (the sphere repremanta 355 ~m near. equlaxed

padlcles). The Increase In dislocation density (AII) Is equal to pmmPd,O -

P~.WIX,The matrix I@annealed under ttle same conditions RS the com~oslte

IJIZXJAIMEK

I%is report was prepared as an account O( work qmnsored by an agency of the United SmlCS

Govcrnmen?. Neither the Unid Stales Government nor any agency [hereof, nor any of their

employees, makes any warranty, exprw or implied, or assumes any Iegtil Iiabilily or respnsi-

ndily for Ihe accuracy, complelcne~, or usefulness of any information, appuralus, product, or

process disclosed, w represents that i[s usc would no[ inrringe privately ownd rights, Refer-ence herein 10 any specific commercial pr~ucl, prm~~, Ur ~miw by lr~dc name, lr~demark,manufacturer, or otherwise does not ncccuarily consl[tulc or imply its endorsement, rccom-

mendalion, or favorin~ by the United States Government or any agency [hereof, The views

&nd opinions of aulhors eaprd br~in dr) not n~~arllY SlaIe Or reflWI [hM of theUnited Slata Govcrnmen[ or any agency lherq,r.

E

c.-

Flg.2b

10’5

1410

10’3

10’2

10”

do

MML41O

I I I

I I II I II I II I II I II I I1 I II I IL J‘---- ----- ----- ---- ----- ----- .-L

I I II I II I II I I

!:I I

I I 1

I I

I I II I

[

----

ii

-- h------- 4 --------+ -----I I 1I II I1 I II I I

II II I II I I

F------ -r ------ -q ----- --- p---- ---

I I I

II I II I II I II I II I II I II I II I IL 4----- ---- ----- ---- ---._- L-------I I II I II I II I II I II I II I II I I

c) 100 200 300 400

Particle Size ( D ) w

Increase In average dislocation denelty In 20V% A120JNIAI composite with

particle elze,

TABLE 1

Thermal Resldr.mlStress In tho NIAI Matrix

Tensile .Wcss (MPa % 20 MPa)

A1203 Reinforamen[Axial Transveme

75* parricle

355 pr parlicle

short tllxr

cxmfinuoua tilament

144

105

280

mmprcwiveII (Saigal and Kupperman, 1901) I -“ 100

171

189

383

‘J8

<

Tsblc 1 Iisls [he Ihennal residual slrcsscs measured by neulron di[fracliort in [kc

composites. The axial and the transvc= tlireclions are defined as Ihc directions pnrallcl and

pcqmdicular to the hot pressingdircclion during processing.The axial l_RS dccrcascsas the

particle sim increaseswhile no clear mend ctin k de[cctcd in Ihc transverse direcliwt (changesarc witiin experimental cmor). Wi[h changes in reinforcement shape Table 1 indicates that

Ihcrc ia a significant mhrciion in [he matrix TRS as the morphology of the reinforcement

changes from Ilbrs 10 equi-axed particles. However, the matrix TRS is leas in Ihe composite

reinforced by conlinuons filaments.

We could not obtain sensible sims values for [he 5#m A1203 compcraile due 10 iron

comaminalion of [he NiAl, which had been ball milled 10 obtain 5 ~~mpwdcr. The TRS is

an.ktropic and the stress values from lransvc~ ditcclion is Iargcr in all cases. While # more

delailed invesliga[lon is king undctiakcn, we believe the following factors may have

contributed to the aniscmopy in TRS: (a) Ihe shorl fibers arc seen as a plnnnr anay

perpendicular 10 the compact ttircctirms. (b) for larger patliclc size (355 A), Iherc was also a

large dcgrcc of reinforcement cluslcriug along Ihe pcrpcndimtlar directions. Anisotropic elaalic

interaction Mwecrt grains In a IcxIutcd mamix CWIInl.somrrmibule 10 the ardsotropy 0[ the

avcrxge TRS,

Figure 3 shows Ihe changes In svcragc maidx effccllvc plastic strain wl’h reinforcement

morphology as deletmlncd by an FEM analysis. There Is about a factor of two incrca.w in

efkctivc plsstlc strain from spherical 10 continuous Illamenl rcinfo~cmcnt. If the cffcuivc

plsstlc slraht scalcn wilh rli!docaliongcncralicm,this resultis conlislcnt wilh lhe TEM rcriulL%.

DISCUSSION

From an initlsl conaidcrstlon of the tlala it is obvinus thal Iherc is a large diffcrcncc hctwccn

SiC/Al and 41zO@lAl comprmilcs, cspcclally in Ibc gcncrallon of dislocalimts. II Is flrsl

necessary 10 consider If Ihe Ihcmtal !urcsacs arc Mflcknl :0 prrxtuc? strcwcs that arc ahovr

the yicltl sltcsa of Ihc nm[rix (NIAI). An shown in Fig. 4 [he calculated Ihrnnsl SWCM(without

plastic rcltxalion) al all Icmpcraturea is Iargc: than [hc ylcld slrc~ !)( prdycryslalllnc NiA1.

Tbctcforc, dlsltwatlon gctiraliou ml mollon sbnuld owur.

n

m‘o

w

c.-td

8

a>---

~

t?

6

5

4

3

2

●●

●’8#*

●

4°●’

●’●*

●⑤●⑤

●’●

8°●

●°●°

●’●

●e

sphere fiber filament

Flg,3 Tho FEM-pradlcted averag~ dfactlw plasflc strain for different reinforcement

b

1,200

1,000

800

400

200

0

Cal, curve of TRS

of Al ~0 ~ /NiA1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.----, Exp. yield stress

of NiAl

●. ---

---~●

●●%

. . . ... . .. . . . . . . . . . . .. . . . .. .

b98\9

‘a●O

200 400 600 800 1,000 1,200 1,400 1,600 1,800

TEMPERATURE K

The experimentally meaeured yield strew of polycrystalllrw NIAJas a functionof feet temperature. The predicted matrix thermal stress without plastlc

relaxation,

MML411

particle

“?)

affected /

zone grains

~

particle

.—

(a)

(b)

Fig.5 Mismatch-affected zone In which high thermal stresses develop, (a)the

mismatch affected zone Is much larger than matrix gralrw (large particles); (b)

the mismatch affected zone Is much smaller than matrix grains (small

particles),

The mosl distinct difference in SiC/A.l and A120Y’NiAl are the changes in Ihe mismatchdislocation density with [he partic!e size as can be wen by comparing Figs. 1 and 3. The

increase in the dislocation density in lhe NiAl ma[rix with increasing pafiicle size cannot beexplained based on relaxationof thermal mismatchin a homogeneous medium. To understandthis anomalous trend in the change of dislocation density i[ is necessary to consider the slipcharacteristics in the two matrices.Al hasa fcc crystal structurewith 12different slip sys[emsof which 5 are independent slip systems, this allows an individual grain to slip independently.

In NiAl where the predominate slip system is c1OO>{ 110} there are only three independentslip syslems (Groves and Kelly, 1963) wi[h [he hard orienla[ion along <100> and the softorientations <110> and <111>. The difference in the cri[ical resolved shear stress between the“hard” and “soft” orienla[ions is atmut a factor of fourteen (Noebe et al., 1993). According toVon Mises (1928) however, slip in individual grains of a polycrystalline ma[erial withoutsacrificing intergranual deforrnaiion compatibility requires five independent slip syslems.Therefore, slip wi[hin a grain without crea[ing disconlinui[y in the NiAl matrix can occur only

when collaborative slip from neighlmring grains is activated. Such a de fomlation mode canbe facilitated when the misfitting themlal slress field e.lcompasses the pZkIeS over asignificant number of matrix grains. Figure 5 shows two extreme cases in which thereinforcement size is much larger (Tig. 5a) and smaller (Fig. 5b) than [he surrounding matrixgrain size. Due to the thermal mismatch stresses a misrna[ch-affected zone arises around each

particle within which a high thermal mistil s[ress develops. When the grain size is muchsmaller relative to the mismalch affected zone, i.e. large panicle size (Fig, 5a), criticalresolved shear strtss is exceeded in many grains. In some grains [he crystallographicorientations are aligned favorably for collaborative multigrain slip without tlcs[roying intergraincompatibility. Wirh a larger grain size relative to the mismatch affected zone, i.e. smallreinforcement size, grain alignment favorable for collaborative slip is statistically unlikely,

Therefore, collatmmtivc slip in an aggregate of grains is possible only when the size of thernismatch-affcctcd zone is much Iargcr [ban [hc average grain size (Fig. 5a), i.e., plasticrelaxation is more Iikcly with a larger avcmgc parliclc size. FEM modeling using a

polycrystalline aggregate model is needed ro quamilatively understand this phenomenon.

CONCLUSIONThc following conclusion can be drawn from [he experiment da[a and [hc modeling.

_ The dislocation density, due to the relaxation of the thermal srrcsses, increases as the A1203size increases in Al,03/NiAl composites. In contrasl, in [he case of SiC/A.l composites as SiCparticles size increa-ses the dislocation density decreases.● The distinct contrast in the dislocation density/particle rcla[ionship for [he two composi[cscan be explained by a simple model based on [he requirement of collabomlive slip Ixrwcengrains in polycrystalline NiA.1.● FE&f results have has shown tha[ shape charges from a sphere 10 cominuous filament

induce afmN a factor of P.vo increase in the effeclivc plas[ic s[rain, which has the same trendwith the changes in dislocation densi[y obtain from TEM.

● Neumon dlffrnction results indicate thrd the mutrix Ihemlnl residual stress in AIJO:/NiAlcomposites decreases as the purtlclc size increases.

REFERENCESArsenault, RJ. and Fisher, R. M., 1983, “Microstructure of Fiber and Particulate Metal

Mamix Composites”, Scripts Metal., Vol. 17, pp. 67-71.

Amcnault, R.J., 1984, ‘The !hengthenirrg of f3161 Aluminum by Fiber and Plmelet SiliconCarbide”, Murer. Sci Eng., Vol. 64, pp. 171-181.

&senault, RJ. and Shi, N., 1986, “Dislocation Genera[ion Due 10 Differences in

Coeffrcienrs of Thermal Expansion”, Mater. Sci. Eng., Vol. 81, pp. 175-187.

Arsenault, RJ., Wang L. and Feng, C. R., 1991, “S[reng[hening of Composites Due ToMicrostrucrural Changes in the MatriY”, itcm Me[al/., Vol., 39, pp.47-57.

Groves, G.W. and Kelly, A., 1%3, Phil Msg., Vol. 8, p. 877.Mises R. vorr, 1928, Z. angew. MatIi. Mech., Vol. 8, p.161.

Noe&, R. D., Bowman, R.R. and Nathal, M. V., 1993, “Physical and Mechanical Propertiesof the B2 Compound NiAl”, Inter. MIiI. Rev., Vol. 38, pp. 193-232.

Saigal, A. and Kupperman, D.S., 1991, Residual Themml Strains and Stresses in Nickel

AJuminide Matrix Composites”,Scripts MetalI., Vol. 25, pp. 2547-2552.

Shi, B., Wilner, B. and ilrsenaul~ RJ., 1992, “A FEM SIudy of [he Plaslic Defonnalion

Process of Whisker Reinforecd SiC/Al Composites”,Acts A4em/1.,Vol. 40, pp. 2641-2854.Shi, N., kserraul~ R,J., Krawilz, A.D. and Smi[h, L.F., 1993, “Deformation induced

Residual Stress Changes in SiC Whisker Reinforced 6061 Al Composites”, Mera//. Trans., Vol.

24Aj pp. 187-196.Smi[h, L. F., Kawitz, A, D., Clarke, P., Saimo[o, S., Soi, N. and Arserrault, RJ., 1992,

“Residual Sresses in Discontinuous Me[al Ma[rix Cbmposi[es”, A4uI.ScL& Eng,, Vol. A159,

pp. L13-L15.Vogelsang, M,, hrurult, RJ. and Fisher, R. M,, 1986, “h in situ HVEM study of

dislocation generation at al/sic imerfaces in melal matrix composites”, MefaL Tram, 17A379-389.

Wang, L., Bowman, R,R. and Amenault, RJ., “Dislocation in Corninrmus Filament

Reinforced W/NiAl and A.120@iAl timposiles”, Submittrd for publication.