ISIJ International, Vol. 40 (2000), Supplement, pp S164-SI 68

Grain Refinement in aby Multiple Deformation

304at o

Type.5 T~

Stainless Steel Caused

AndreyBELYAKOV')TakuSAKAIand Hiromi MIURA

Departmentof Mechanical Engineering and Intelligent Systems,The University of Electro-Communications, Chofu, Tokyo182-8585, Japan1)On leave from the Institute for Metals Superplasticity Problems, Ufa 450001

,Russia.

Structure evolution and deformation behavior of a 304 type austenitic stainless steel were studied in

multiple compression at a temperature of 873 K(0.5 T~) under a strain rate of about 10-3 s~1 The integrating

flow curve shows a maximumat moderate strains around 15 followed by a minor strain softening at high

cumulative strains above 3 The structural changes taking place during deformation can be characterized bythe evolution of elongated subgrains with their low-to-middle angle boundaries as dense dislocation walls at

Iow to moderate strains. These subgrains becomemore equiaxed and the subboundary misorientations

gradually increase with increase in strain. The volume fraction of the highly misoriented substructuresubstantially increases upon multiple deformation to high cumulative strains above 3, finally leading to the

development of a fine grained microstructure with an average grain size of about 300 nm. Themechanismsof

such structural development as well as the relationship between microstructures and deformation behavior arediscussed in details.

KEYWORDS:austenitic stainless steel; multiple warmcompression; geometncally necessary subboundaries;continuous dynamic recrystallization; strain-induced grain refinement.

1. IntroductionProduction of fine-grained steels and other materials

have somehigh advantages for the improved mechanicalproperties. Since the ne\v fine-gr'ained nlicrostructures canbe developed by plastic deformation and recr}. 'stallization,

the studies on grain refmement tmder thermomechanicalprocessing are of specific praclic,al importance. One of

most interesting mechanismsof structural changes is

dynamic recrv_ stallization (DRX), \vhich can lead directly

to new nnicrostructure formalion controlled under hotworkingl,2). Thecharacteristics of DRX,i.e. dynamicgrain

size evolved, effect of initial microstructure, etc. havebeen fairly clarified for hot deformation. It is generallv.

agreed th considerable refinement of microstructure canbe achieved by decrease in deformation temperature orincrease in strain rate. On the other hand, questions

concerning the structural changes taking place at lrigh

strains are ofien complicated bv_ somelimited workabilit~.,

of most nletallic m'aterials at lo\v-to-moderate

temperatures. RecentlV_,

several techniques including

multiple forging have beenproposed to provide very. highplastic strains3~6)

The aims of the present work are to study the

deformation behavior and the structural changes taking

place during severe warnl straining for a 304 typeaustenitic stainless steel. The s,~mples lvere deformed to

high strains above 6 in multiple intemrpted compression

C 2000 ISIJ S164

at 873 K (0.5 T~), wlrich is considered as a kind ofmultiple forging. The dynamic mechanismsof newgrain

evolution as well as the effect of microstructures

developed on the mechanical properties are discussed in

details.

2. Exl]erimental ProcedurcA 304 IV_pe austenitic stainless steel (C:0.058, Si:O.7,

Mn:0.95, P:0.029. S:O.O08, Ni:8.35. Cr:18.09. C.u:0.1_5,

Mo:O.13. all in masso/o, ',md the balance Fe) with an initial

grain size of about 25 umwas used as the samples for

nmltiple compressions. The samples w'ere machined in arectangular shape with the starting dimension of9.8:8.0:6.5 mm.Suchratio of about 1.)~: 1.22: 1.0 waskept

as a constant through subsequent compression to a strain

of 0.4 in each pass. Multiple compression tests werecarried out with consequent changing of the loadingdirection in 90' through three of mutuall), perpendicular

axes \vith pass-to-pass. The samples were compressedin

vacuumwith a powder of boron nitride 'as a lubricant at

873 K (0.5 T**,) under 'a strain rate of about l0-3 s~1. Thedeformed samples were quenched in water, slightly

ground to right-angle shape, 'and then reheated to 873 K~vithin 0.6 to 0.8 ks in each deformation pass.Metallograpluc analyses were carried out using an optical

microscope and a JEM-2000FXtransmission electron

microscope on the sections parallel to the compression

ISIJ International, Vol. 40 (2000) Supplement

axis. Misorientations of strain-induced (sub)grain

boundaries were studied using a conventional Kikuchi-

line teclmique with accuracy within one degree. The total

numberof boundaries was 80 ~ 20 at each strain level

studied.

3. ResultsThe integrating flow curve plotted over sixteen

compression passes in Fig. I is similar in appearance to

that controlled by dynamic recovery; namely, stresses

rapidly increase in about three times bv_ first compression

and approach a saturation value bv_ subsequent repeated

deformation. Strain hardening taking place in each pass

becomes quite small after third compression. It is

remarkable in Fig.1 that the yield stresses at reloading are

quite different from the one at first pass, i.e. for anannealed state, and, in contrast, roughlV. similar to the

stresses immediately before unloading afier second pass.

Suchbehavior maysuggest that any static aunealing effect

hardly took place for a period of reheating time under

multiple tests at 0.5 Tm'

~~~

t)

(D(/)

UJO(

H(1)

1ooo304 Stainless Steel T= 873K

800 ~= 8x l0~4 s~1

600

400

2oo

o

CUMULATIVESTRAIN,* 8'

Fig. I True stress -cumulative strain curves t~or 304

stainless steel under multiple compression at 873 Kandat a strain rate ot~ about IO-3 s~l

Typical microstructures evolved under warmmultiple

defonnation to various strains are shown in Fig.2.

Structural changes can be ch•aracterized bv_ the

development of curl) • microstructure composed of

frequently serrated grain boundaries at moderate strains

above I following pancaked grains evolved by early

deformation. With furdler defonnation to high strains, the

microstructural analysis becomes more complicated

because of high density of irreglilar grain boundaries aswell as defonnation bands. It should be especially noted in

Fig.2c that manyvery fine grains ~vith size of below I umcan be recognized to be evolved b~_, severe warnldeformation.

,--,10 um

Fig. 2 Typical microstructures developed in 304 stainless

steel under warmmultiple detbnnation to various

cumulative strains (a) ~8= 0.4. (b) ~c = I .6 and (c)

~8= 3.2.

S165

Fig. 3 Cell substructure with high densitV.

rich dislocation layers near anboundaries (GB) at ~e= 0.4

,--.-,1um

dislocations in

initial grain

C 2000 ISIJ

ISIJ International, Vol. 40 (2000), Supplement

Fig. 4

'-10.2 um

Strain-induced subboundaries developed in vicinity

of a junction of grain boundaries (GB) in 304stamless steel during wannmultiple deformation to

~c = 0.8. Thenumbersindic the misorientation in

degrees. Note, the misorientation between points

lettered bv AandB\vas about 35~.

,-,0.5 um

Fig. 5

@2000

0.2 um

Typical substnlctures developed in 304 stairiless

steel defonned to ~ 8 = 32. The secondary phaseprecipitations marked by. arrow heads mayroughlyindicate position of hrltial **,rain boundaries.

ISIJ S166

Fig.3 presents a typical substruclure evolved at 'an

earlV_ stage of deformation corresponding to rapid ~vork

hardening. The first compression to a strain of 0.4 Ie',rds to

the evolution of high density dislocations whuch areroughly homogeneouslV. arranged in rich dislocation

layers. The misorientations across such dislocalion

subboundaries are about 1-3~, as maybe suggested bysingle spots diffraction pattern obtained from selected areaof 6umin diameter. Further deformation brings about the

evolution of dense dislocation walls (DDWs)\vith low-to-

middle angle misorienlations. It can be clearly seen in

Fig.4 that the subboundaries lvith mediumto high anglemisorientations are frequently formed near the initial grain

corner. Ananother important point to be noted in Fig 4 is

lvigh lattice curvatures and/or rotations developed at the

grain corner under wanu multiple deformation. Themisorientation betweenpoints lettered as "A" and "B" wasabout 3-~~~. This suggests that the grain boundary junctions

and ledges should be considered as preferable potential

nucleation siles for strain-induced high angle grainboundaries.

A typical microstructure evolved at 8 = 3.2 is

represented in Fig.5. Further multiple deformalion to

cumulative strain above 3 Ieads to full development ofwell defined subgrains with middle-to-lvigh angleboundaries tllfoughout the initial grain interiors. Thesecond phase precipitations. mainly a-phase indic byarrovv heads in Fig.5, may rouglrlv. indicate the original

position of initial grain boundaries, however, the latters

cannot be e\actly recognized becatuse of the developmentof many fine grains with high-angle boundaries. It is

clearly seen in the enlarged lower picture that sonle ofsuch strain-induced fine grains and/or subgr containquite few dislocations and their boundaries give a sharpextinction contrast on TEMimage. The latter is t~.,pical ofconventional grain boundaries.

Upon further multiple compression to severe strains

these new fine grains becomemore equiaxed due lo

increase in the transverse size and their boundary.misorientations graduall), increase. The microstructures at

8 = 6.4 in Fig.6 can be generally described as an equi

grained one with high angle misorientations. The averagegrain size is abotut 0.3 um. It should also be noted in Fig.6that the secondary precipitations ',rre r,andomly ' distributed

in (sub)grain interiors. This suggests that grain boundary.sliding and its local minor migr'ation can take place notonly at original grain boundaries bLut also nelv str

induced (sub)gr ones.

4. Discussion

An analVsis of the results mentioned above re\,'e'aled

that the new fine grain fonnation is a consequence of

somecontinuous reactions on substructural level c'aused

by lvarm multiple defonnation. Earl~_, deformation bringsabout high density dislocations followed by the evolutionof strain-induced dislocation subboundaries as DDWSwith low-to-middle angle misorientations at moderatestrains. Gradual rise of the misorientations betweensubgrains, fmally, Ieads to the flrll e\'olution of anewfme-

ISIJ International, Vol. 40 (2000), Supplement

0.2 um

0.2 um

Fig. 6 TEMnricrographs of t"me-grained microstructures

evolved in 304 stamless steel det~ormedto ~8= 6.4

grained stnlcture at severe strains assisted by(sub)boundary sliding and recovery. Let us consider the

mechanismsof neTv fine grain formation in somemoredetails,

It is knovvn7,8) that severe plastic deformation c',m

break up the initial grains into domains with different

combinations of operative slip systems and, as ageometrical requirement, Ieads to lhe evolution ofgeometricallV. necessary_ boundaries (GNBS) betweendifferent domains. Multiple compression with changing of

the loading directions can accelerate the evolution of

manv_GNBSmutuall~. , crossed due to promotion of the

variations in slip systems operating at each deformation

pass.

By the way, DDWS\vith relatively lrigh

misorientations appears at grain boundary junctions

moderate strains above 1.0 and mavalso be considered as

GNBs, as sllOwn in Fig.4. SLlcll GNBSare frequenth.,

evolved near grain boundaries and grain corners to

accommodatestr'ain gr'adients and/or lattice rotations

(Fig.4). The latters may be caused bv. usual strain

incompatibilities as well as grain boLmdar)' sliding. SLlcll

elastic stresses (and local lattice rotations) evolved at

grain boundary_ junctions and ledges can be illustrated bysimplified schemesin Fig,7. The sliding or shearing alongboundaries markedbv. "A" and "B" in Fig.7a should lead

to the generation of misfit (i.e. virtual) dislocations at the

boundary junction because of geometrical differences in

the Burgers vectors oper'ating 'at different boundaries. Theaccumulation of such misfit dislocations can locally create

lvigh elaslic lattice distortions which maybe simitar to

those produced by a super dislocation. The high latlice

distortion can be also developed al a boundary ledge byusual intrinsic slip because of a difference in

misorientation increments at different facets (Fig. 7b). Themisfit dislocations at "B" facet in Fig.7b may result in

local boundary sliding, while those at "A" facet shouldadditionally lead to local lattice rotation, resulting in the

evolution of high internal stresses at boundary kinks. Then

new GNBSmay be developed by secondary slips to

decrease the elastic stresses9).

(a)

--~lA I JLJ_bi

grain boundarysliding

B/~~,~,~b2

'~ bi

b~//~:2

tmisfit dislocation

(b) Bb ~~ I b1

~/:/2 b\L~~~ ~~~ b'

f/\'Jlb

~L~Ll ;"' !" ~"'i_:

"!'

~L~F/1r4'~~ '

f~~" GNBAl secondary snP

Fig. 7 Long range misi'~rt stresses evolved at boundaryjunction caused b,~' (a) grain boundary_ sliding and (b)

usual intrinsic slip

Further straining can cause the numberof GNBSandtheir misorientations to increase. These subboundarymisorientations rise due to the increase in the numberoflattice dislocations evolved b), usual intrinsic slip and the

absorption of accommodation (or grain boundary)dislocalions th•at come from str'ain incompatibility ofsubgrains. Strain dependenceof the de\,'elopment of the

slrain-induced subboundaries. holvever. is still underdiscussionslo). The average misorient•ation of strain-

induced subboundaries for the present 304 stainless steel

linearlV increases with low-to-moderate strain,

approaclilng a value of conventional grain boundaries at

strains above 3_

, as shownin Fig.8. In the slrain range of0.5 to 3.O, the average misorientation (e) c beapproximated b) , a linear function of cumulative strain

(~c) as

e= 7(2:c- 0.2)

S167 @2000 ISIJ

[SIJ International, Vol. 40 (2000), Supplement

This result is quite similar to that reported for a pure

copper deformedat 0.35 T,,, 5)_ It should be noted in Fig.8

that a saturation value for the misorientations at highstrains is muchlo~t'er than the theoretical one of 40.7' Il)

This maybe the reasons wh~_, such strain-induced grain

structure should ahvays contain lo\v angle dislocation

subboundaries, which are dynamically evolved duringmultiple deformation.

It should be noted that the new grains developed at

high strains become more equiaxed as compared to

preceding subgrains at moderate strains. This mayresult

from local minor migration of strain-induced

subboundaries, which in turn should be affected byrecovery processes through the promotion of dislocation

rearrangement in subboundaries. Thephenomenonof ne\vgrain evolution described above is similar to continuousand/or rotation DRXtaking place in somematerials underhot deformation2,12) although the role of dynamic recoverybecomesless effective at a relatively lolv temperature.

o~zO 30-H

HzUJ

~ 20

Oco-~

LU IoO~UJ>

3O4 Stainless Steel

Cu

304

CUMULATIVESTRAIN,~g

Fig. 8 Variations in average misorientation for strain-

induced subboundaries with cumulative strain for

304 stainless steel and l~or Cu5)

of anewfine-grained structure with an average grain size

of about 300 mu.(3) The fine grain formation at severe high strains

can result from a kind of strain-induced continuousreactions taking place during deformation, that is

essentiallV. similar to continuous dynamicrecrystallization.

AcknowledgmentsThe authors acknowledgewith gratitude the financial

support received from the Recrystallization and TextureCommitteeof the lron and Steel Institute of Japan. Theyalso wish to thank the NKKCo. Ltd.

,for supplying the

test material. Oneof authors (A.B.) also would like to

express his hearty thanks to the Inoue Foundation for

Science and the University of Electro-Comrrlunicationsfor providing scientific fellowships.

REFERENCESl)T Sakai and J. J. Jonas: Acta A~ifetall., 32 (1984), 189,

2)F J. Humphreysand M. Hatherly: Recrystallization andRelated Anuealing Phenomena,PergamonPress. Oxford,

(1996), 363,

3) N. A. Smirnova, V. I. Levit. V. I Pilyugin, R. I. Kuznetsov,L. S. Davydovaand V A. Sazonova: Pllys. A•Iet. A(Ietallogr

,

61 (1986), 127,

4) M. Funikawa, Z Horita, MNemoto,R. Z. Valiev and T. GLangdon: Acta A•Iaterl

,44 (1996), 4619

5)A Belv. akov, WGao, H. Miura and T. Sakai: A(Ietall.

Trans.A, 29A(1998), 2957,

6) A. Belyakov, T. Sakai. H Miura ',md RKaibyshev: ISIJlnt.,

39 (1 999), 592.

7) D. Kuhlmaun-Wilsdorf and N. Hansen: Sc,ipta it•Ietall.

A(Iater., 25 (1991 ), 15578) D. A. HughesandN. Hansen:Acta A[ater,, 45 (1997), 38719) V. V. Rybin: Large Plastic Det~ormation and Destructure of

Metals, Metallurgy. Moscow.(IL)86), 61,

lO) A. S. Argon and P. Haasen:Acta A•Ietall. Mater., 41 (1993),

3289,

l I)J. K. Mackenzie: Bion,etrika, 45 (1 958), 22912) T. Sakai. X. Yangand H. Miur,a: A/atet'. Sci. Eng,. A234-236

(1997), 857,

5. Conclusions

Wanndeformation behavior and strain induced grainevolution in 304 type austenitic stainless steel werestudied in multipass compression at a temperature of 873

K (O,5 T~).

(1) The integrating flo~v curve increases to amaximwnat cumulative strains of about I.5 followed by aminor softening, Ieading to a steady-state-like flow at highstrains. Tlrls suggests that deformation can be controlled

mainly by dynamicrecovery.(2) The structural changes are characterized by the

evolution of elongated subgrains with their densedislocation boundaries at low to moderate str Thesubgrains becomenlore equiaxed and the misorientations

between them gradually increase with increase in

cumulative strain. finallV. Ieading to the full development

C 2000 ISIJ S168