chemically induced grain boundary migration

MATS KftLERT and CART R. PCRDTt-*

Divisisn of Physical Metallurgy. Royal Instituie of Ttciinolo<>. S-la> 14 Stockholm 73. Swzden

.~~t~~t-~~F~rim~n~~~ evidence is prmxted. showing &at grain bctundar:i meti~n can be inductd by chsnping thr composition bv means grain boundarv T% experiments c3rricd out treating specimens pure iron an atmosphere zinc or specimens in atmos- phcre a low:r potential. The rrsemblzs the boundary- rr,lg;ation discontinuous precipitation probides an method of grain b%>l migration ior

The experimental can be to evaluate dirrctlv the bounda:>- chrfusi~ity mobility. The thus obtained several orders map&u& iarger th;z values for stationary It is that the boundaq difFusiisivir> B geati)- enhanced hv ordin . = boundary motion.

f&urn&--On piCSsnte unt: prew- ~spdrimeatal; du fait que 1‘0:~ pcut i;tire migrer un joint grains en changeant !a composition par dif5sion inr~rgranulaire. Ddns ccs rup&isnce~. on a trait; des tchantll- Ions de i;-r pur dans une atmosph+re de zinc 21 dzs ichantillons de i‘s: 2 t de zix dans unz armosphtre dont le potentisi de zinc Ait plus foible. LCl r&Am resszmblc i la mi=ration des joints dz grains dans I3 pr&ipit-tion discontinu 5 et rile fournir unz m&hods erpi:imec& pou: is&r la migration int~r~r3nulair~ en 1;ue dr st7n ttude.

On peut uti!isei Ies r&uitats exptrimentaux pour t5xIuer assez directsir .ent la ditfusivite et la mobiiitk des joints de hT3ins. Les valeurs de la dirfusivlti’ que l‘on obtxni ainsi ion: sunOrieurrs de plUSiClliS

vrdrcs dz grandeur :I celics qu’on a publiCrs Fuur les joints jtatlonna~rss. On en daduit que la ditfusivitt d’un joint cst fortement augment& par son d~placrm~nr.

Zu~mmenfas~n~-Es wrd,n experiment&t Hinueise vorgelegt da&i. dn3 clne Korl?Qr~nzbe~~,vrgun~ durch die .&d-rung der Zusamm~ns~tzun~ iibtr KorngrenzendXwon tinzrlritct lxerden k.wn. Zur Durchfiihrunp Jzr Experiment: wurden Proben atis r&em Eisen in sine: Zici--\tn~0sphlrc und Proben US Zink-Eisen in diner ArmospMrc mit kieinerem Zink-Potential behandelt. Die Real&on ;ihneit der Korngrenzbewzgung unter diskontinuier!icher Auscheidun~ und strllt rine experimentrile Mirthode dar. die KornFenzbeweguns rtiiir die Untersuchung zu isolieren.

Mit den exprrimenrzllen Ergebnissen lassen sich Korngrenzenditfusivit;it und bewgiichkeit ziemlich dirrkt auswerten. Die so crhaltenen Diffusiiitltzn sind urn mehrers GrC%enordnungen gr6Der ais die- jenigen. die fir station&r Korngrenzen angcpcben uerden. Xus dw Ernebnissen wrd sefolgrt. daB die Korngrentdi%tsivitPt durch Korngrenzbcwpung betriichtlich verstitr,t w;rJ.

The structure and the structure-sensitive proper&s

of moving solid-solid interfaces need not be the same

as for their static counterparts. Indeed, a treatment due to Cahn [l] suggests that a variation of migration mechanism with driving force is generally to be

expected. Previous experiments on moving grain boundaries

have focussed on such phenomena as the rate of absorption of run-in dislocations [2] or of nucleation at moving boundaries [3]. Although these observa- tions are of interest. they do not lend themselves to simple quantitative analysis. In the present contribu- tion. we show that it is possible to induce gain

* Psrmsncn; add:?ss: Department of Sf&xllurg> and Mawials Science. S125las:er L~niversiw -_ . Hamilton. Ontario. Canadn

boundary motion in response to grain boundary diffusion. For certain specimen geometries, an analysis

of the solute consentration pro&e left in th,= wake

of the moving boundary yields rather dire&l; the grain boundary ditkivity and mobility. The diRusi- vity obtained in this way can be compared uirh that

measured for static boundaries. We will show here that the two diffusivities differ by about four orders of magnitude. This, in turn. implies that major struc- tural changes occur when grain boundaries are set in motion.

Since the conditions of the present experiments simulate accurately a major component of the discontinuous precipitation reaction in iron-zinc. it should be interesting to sompare the newly measured di;fusi- vity and mobilit>- with the values obtained z&r the same quantities from Speich’s kinetic mzawrcments on discontinuous precipitation. This comparijon will bz bawd upon ~1 rs-tvaluation of his data.

??J HILLERT AXD PURDY: CHE4iIChLLY Il\iDCCED GRAIS BOL-SDXRY MIGR_+TION

Fig. 1. An optical micrograph illustrating the surface relief that accompanies the zincificaricn of an iron foil. heated in Zn vapour for 4 h at 6WC. Interference contrast. x 700.

EXPERIMEXTAL lonp evacuated silica capsule. one end of which pro-

trudcd from the furnace. In addition. the dezincification of the Fe-l 1.3 wt.“; zinc ahoy aas observed in

the heating stage of the electron microscope.

Most of the present results concern the zincification of pure iron. Thin discs. 3 mm in diameter, were elec-

tropolished to perforation, to yield wedge specimens with a maximum thickness of about 1CO~lm. These specimens were sealed in evacuated sihca capsules,

and recrystallized at 9OO’C. The specimens were then resealed in silica capsules. along with several grains

of turnings of Fe-11.3 wt.:‘; Zn alloy. heat treated

at temperatures ranging from 545 to 6OO’C, and

The zincified specimens were sectioned and examined unetched in an ARL SEMQ electron microprobe

at 15 kV, 10 nA. Iron and zinc u-ere detected by opposed spectrometers, and the specimens were oriented such that the X-rays were taken-off approxi-

mately parallel to the plane of the specimen surfaces.

water-quenched. In a few cases, discs of uniform (50 pm) thickness were treated as described above and RESULTS

subsequently electropolished for examination in a 1. Mrtdlography

JEOL 1OOOD electron microscope. In one case, the Figure 1 is a micrograph showing the surface relief dezincification of the Fe,1 1.3 wt.?; Zn alloy was stud- that occurs when the grain boundaries move in the ied by heating the recrystallized sheet material in a thin pure-iron spscimen exposed to zinc vapour at

Fig. 3. A section through an iron wedge. exposed to zinc vapour for 3 h at 58O’C. Etched in 3”. nital. The bands marked A. B and C are enriched in zinc to a level of about SO, by weigh;. x 36;

Fig. 3. A zinc enriched band in a specimen exposed to zinc vspour fx I h at 6oi)‘C. and subsequently thinned.

x 10000.

a temperature of about 6OO’C. The zinc potential was

such that a solid solution of zinc in ferritic iron would

result irom complete cquiiibration of the iron specimen with the zinc source. In order to invesiigate what

reaction had actually taken place, wedge specimens were sectioned and examined. As shown in Fig. 3.

the areas swept by grain boundaries, A. B and C. have etched more deeply and it was established by

electron probe microanalysis that they represent zinc-

enriched regions. The remaining areas are essentially pure ion. an observation that is consistent with the very small extrapolated rates of volume diffusion at these temperatures [a]. Evidently, the transport of zinc into the specimens was accompIished almost en-

tirely by boundary ditfusion coupled with grain

boundary sweeping.

.% similar specimen was eiectroiy-ticaily thinned and

examined in the high-voltage ekctron microscope

subsequent to the zincitkation treatment. The grain

boundary can now be seen at the upper left corner

of Fig. 3. It showed no internal structure and is prob-

ably an incoherent high-angle grain boundan. The original position of the grain boundary near the bot-

tom right of Fig. 3. is marked by a remnant wall of dislocations. Simi!ar behaviour was observed in all cases, Figure 1 shows such a wall at a higher magnifi-

cation. It was imaged in a number of two-beam dif- fraction conditions and on analysis it proved to bz composed of misfit compensating dislocations. pri- marily of edge character with Burgers vectors tying

within 30” of the interface plane. The particular waI1

in Fig. 4 was found to lit on a (211) plane and the dislocations to lie roughly parallel to the [Oil]. if201 and [lo?] directions. The density of dislocations cor- responds roughly to the misfit one can expect due to the effect of zinc on the lattice parameter of ferrirz. if there is a discontinuous change of the zinc content from zero to about l@‘,. This observation indicates that the effect of volume diffusion must have been

negligible, at least in the early stage x-hen the grain boundary started to move.

Disiocations were oitzn generated in the growing

grain. Figure 3 indicates that dislocations actualiy- form at the migrating boundary. It is not clear whether they are extensions from the dislocation u-ail.

Figure 5 shows a case with a much higher diska- tion density. It was taken from the thinnest r-gion

of a wedge foil after exposure to zinc vapour in vaxo at 5SWC with no further preparation. This spectrcen contained many small precipitate particles, prasum-

ably oxides at or near the surfaces. It is clear that the dislocations trail the boundary as it is forced

through the array of particles. The particles may be responsible for the high dislocation density- in this

case. It was often observed that a boundary moved faster

close to the surface than in the interior of the specimen. The effect became more pronounced as the

Fig. 1. X wall of dislocations icft at the ori_-nal position oi ;1 grain boundarq-. g = Oil. x 3KlOO

336 HILLERT AZD PTJRD\-: CHEMICALLY WDVCED GRAIS BOUNDARY IlfIGRATIOX

Fig. 5. Generation of dislocations at a grain boundary as it is forced through an array of particles in a thin foil

specimen dezincificd for I h at 5SO’C. x 15000.

thickness of the specimen was increased and for bulk specimens the boundary migration was confined to the surface region. .An example is given in Fig. 6. Similar behaviour was observed for the zincification of thick specimens of an Fe-O.5 wt.:, MO solid solution at 6OO’C.

It was generally observed that different parts of a grain boundary moved in different directions thus making each one of the two adjoining grains grow into the other at different places. As a result, all the original volume will eventually be swept by grain boundaries and thus transform into a recrystalfized high-zinc material. The reaction is demonstrated in Fig. 7 where the present grain boundaries have been attacked by an etch-pitting etchant. The grain boundary in the center of the micrograph is now S-shaped and the middle of it is situated at the original position but rotated LSCP. The application of oblique illumina- tion reveals the sharp change in zinc content at the original position of the grain boundary on both sides of the S.

Dezincification was observed to yield the same be-

Fig. 6. Section of a thick specimen. exposed to ziiic vapour for SO min at 600X. The grain boundary was initially per- pendicular to the surface. ft has moved to the teft. The metal particles at the surface are electrotytic copper intro- duced into the mounting material to prowde electrical con-

ductivity. x 1300.

Fig. 7. h section taken parallel to the surface of an iron specimex? exposed to zinc vapour for 1 h at SWC. Note that the present boundary position is marked by a row

of etch pits. x 900.

h&our and the morphofogy was very similar. As an example. Fig. S shows evidence of extensive boundary motion near the surface of a partially dezincified Fe-11.3 wt.?; Zn specimen.

Dezincification could also be studied directly in the hot stage of the high-voltage electron microscope either due to surface oxidation or evaporation of zinc. An obserrvation of grain bounda? motion in the Fe-17 it.“, Zn ailoy is reproduced in Fig. 9. Here the initial boundary position &A \vas straight. and the nonplanar morphology developed as migration proceeded at 4YO’C. In other cases the migrating boundaries remained nearty ptanar.

A number of electron microprobe step-scans were made of the zinc-enriched regions left by the moving boundaries in the zincification experiments carried out at about 6OWC. No significant variations in concentration were found from scans taken parallel to the specimen surfaces. Also, for the thinner regions of the foils, up to about 30 Elm, the concentration profiles were found to be flat from one surface to the other, for regions like those marked A. B and C on Fig. 2. Thicker regions of these foiis showed some concentration variation in the direction normal to the specimen surface.

Figure 10 shows a concenrration profile obtained for a boundary which has moved about 5 b[rn in 2 h

Fig. Y. X section raksn parallel :o the sud3cc of an Fe-1 I.2 wt.0, Zn specimen dezincitied for 2h at XWC. x 1300.

Fig. 9. High ~olragr electron microgrnph ~f in Fe-1 7 xt." , Zn foil t&en during dezincification in the hot stage at 4SO’C. The original position of the grain boundary; u-z such that the boundary vrrrisally bisected the micrograph

at SSO’C. Initially it went straight through the specimen which Was 45 jtm thick. It was still fairly straight

after the movement. \Ve consider this a typical case. Figure L I shows two other cases obssryed in the

same specimen. The! show a more unicorm conxn-

tration. for a someuhat thinner section of the foil. Although we have not managed to make the quan-

titative experiments completely reproducible the present results are good enough to justi& some conclusions and evaluations.

THEORETICiL TRE.UMEST

The migration of grain boundaries in the present experiments has great similarities with the migration

of the grain boundary in discontinuous precipitation, in particular at early stages where the precipitating phase has not yet aligned itself [YJ. That case was

first given a satisfactory treatment by Cahn [6]. Assuming that the reaction is governed by boundary diffusion and approximating the shape of the bound-

ary as planar. he found the following solution

s - 51 coshz, a.5 -= - . (1) xcr) - XL cash, (I 3

Fig. 10. Experimental concentration profile of zinc from one side of a thin specimen to the other in a z.incifird

region like A. B and C sho\vn in Fig. ‘1.

n-here c: = ?S’ k;.@d. .si is ihe jo!ute content & rhe

parent grain and x1> is the so!ute contzx of the gow-

ing lamella at i:s sides. z is the coordinate in the direction oi diffusion in the boundar). S is the :\-idth

of the lams!la. r is the velocit>- oi ths boun&rp. K

is the equilibrium distribution coe&knt of the solutZ

between the boundary and the grow&~ grain. Db is the di!Tusitity and 3 the width or the boundary. Szvzral attempts ha:e been made fo modi@ Cahn’s

treatment in order to take into account the asn- planar shape of t’ce boundary and the criisct of capillarity. The most detailed treatment has been pres-

ented by Sundquk [*] and is based upon txo equations. Using i as a measure of the distance along the curved boundary Sundquist t‘ormuiarcd :he foliowing

equation for the boundary dikion.

,-L

L is the value of ! in the middle of the specimen and ?’ is the migration rate of the boundary in the direction

of the normal. The balance of forces at the boundary

yields the second equation.

(3)

where 01 is the angle between the macroscopic growth direction and the local normal of the boundary. The last term is due to capillarity and it has a decisive

influence in the case of discontinuous precipitation. Howver. in the present case the width of the speci-

mzn is very much larger than the width of the Iamel- lae in discontinuous precipitation. As a consequence.

the values of d#,dl will normally be very much smaller. For the present purpose. this term will be neglected. M is the mobility of the boundary. It con- tains the intrinsic mobility of the boundary and in alloys it may also be strongly aITected by solute-drag effects. It has often been suggested that the mobility varies with the size of the driving force but it will now be treated as a constant. AG, is the driving force

acting on the boundary. In the present case it comes from the chemical Gibbs energy driving the reaction

o irom Zn enriched 1c1 grn wide /

+ from Zn enriched re;~n.~, ;& 5 i;rn wide.

-! 3 /

0 :o 23 30 .dis!ance urn .,

Fig. Ii. TLVO other extmples of concentration pro&es in zincificd specimens.

:3:! HILLERT .am PURDY: CHEMICALLY INDL’CED GRXf?i BOIJSDARY MIGILJTION

which can be evaluated from the following equation if the phase under consideration can be described as a sub-regular solution.

,@$R = 1 - Xi (1 - s,)[Rlln E -i IA i 38)

- (1 - s)‘] + 4B[it - .Y,)~

1 - (1 - .#] ). 1

(4)

There are several sinks for this chemical driving force [7. S] but they will not be considered in the present discussion.

Equations (2) and (3) were used in a series of calculations in order to simulate the present experiments in the Fe-Zn system where the r phase can be described with the following parameters A = 4384.8 + 6.2781 T J:mol, B = 4439.2 J/mol [9]. For the planar case equation (2) reduces to Cahn’s solution, equation (1). The boundary ~vas reasonably Hat in the case illustrated in Fig. 10 and the concentration profile was calculated from Cahn’s solution using 0 = 1.4, x1 = 0 and x0 = 0.06. The experimental data seem to be well represented by this curve, thus indicating that the reaction is governed by boundary diffusion. In combination with the experimental rate of migration, I; = 7 x 10-8cm!s, the a value yields KDbd = 3.2 x lo- ” cm3is at the experimental temperature which was 5SO’C. This value is much larger than values usually observed for boundary diffusion in iron and iron alloys, which are around lo- ” cm3/s [lo]. It should be emphasized that the concentration profiles presented in Fig. 11 would yield even larger values.

It is easy to apply equation (3) to the case presented in Fig. 1 since that boundary was fairly planar during its misation. The last term can thus be neglected and the rate of migration can be represented by an average which was 7 x lo-‘cm,‘s. According to equation (J), the driving force varies from 367 J/mol at the surface to 259 J’mol at the center of the specimen. The average is 2992 J/mol and inserted in equation (3) it yields a value of .tl = 2 x 10-l’ mJ!J,s. This is much lower than the mobilities observed in recrystallization and grain growth which have yielded values of lo- l2 m’,/J,s or higher at 5SO’C [ll].

Accepting the values of KDb6 and M obtained for the case shown in Fig. 1, a series of calculations were carried out from equations (2) and (3) for different widths of the specimen and without the simtilication of a constant rate of migration for the whole boundary. Instead. the local rate was evaluated from equation (3) using the local value of the driving force. The calculation was carried out under the simplifying

a:ssumption that the rate is independent of time. which should be true for short times when the boundary is still reasonably planar. The resulting concentration proliles are presented in Fig_ 12. For thin specimens the concentration is thus predicted to be fairly constant but for thick specimens the concentration will be much lower in the middIe. The shapes of the boundaries were calculated simultaneously but are not plotted because they gave almost identical curves to those shown in Fig. 17, the reason being that the migration distance is assumed to be proportional to the rate, the rate is proportional to the driving force in view of equation (3) and the driving force is approximately proportional to the Zn content. .L in view of equation (4). if the initial specimen is free of Zn. The shape of the migrating boundary. thus predicted for the case shown in Fig. 10. S/2 = 22.5 pm, is not quite as planar as the shape actually observed. However, the agreement may still be regarded as qualitatively correct. The lowest curve in Fig. 12 was obtained for an infinite thickness and it approaches zero asymptotically. For longer times. the boundary becomes increasingly more non-planar. The diffusion distance along the boundary down to a certain depth under the surface dll thus increase and the concentration as well as the migration rate will decrease with time at any given depth. One may thus conclude that there seems to be a qualitative agreement with the shape for a bulk specimen presented in Fig. 6.

In spite of the scatter in the experimental information obtained for boundary migration in the present study there is a qualitative agreement on a sufficient number of points to justify the conciusion that the rate of reaction is governed by boundary diffusion, that the driving force is supplied by the chemical

Oe 0 10 20 50

Distance, pm

Fig. 12. Concentration profilzs calculated for a series of specimen thickness. The specimen centers are marked by a small vertical line. The calculations were carried out with .ci = Z x lo-” m’, J. s and KD”6 = 3.2 Y to-” cm3/s.

HILLERT ASD PURDY: CHE_MICALLY INDUCED GRAIN BOUNDARY lLlIGR.~TION 339

Gibbs energy and that the mobility is fairly indepen-

dent of the size of the driving force. On the other

hand, the values of the boundary diffusivity and

mobility do not agree with values obtained for pure iron. This disagreement may be due to the presence of Zn in the present experiments and it should thus

be interesting to compare with values evaluated from information on discontinuous precipitation in the Fe-Zn system.

COMPARISON WITH DISCONTINUOUS PRECIPITATIOK

Speich [4] has made a detailed experimental study of discontinuous precipitation in the Fe-Zn system. He treated the data theoretically and evaluated the

diffusivity and the relation between the driving force and the rate of migration of the grain boundary. His diffusivity data yielded a value of KDb6 = -I x lo-” cm3js which is two orders of magnitude larger than values previously reported for iron and iron alloys but it is two orders of magnitude lower than

the values obtained from the present experiments. Speich found a cubic relation between the migra-

tion rate and the driving force. If applied to the ex-

periment presented in Fig. 1, such a law would predict a migration rate in the middle of the specimen which

is only a quarter of the migration rate close to the surface but the shape of the migrating boundary indi-

cated an almost constant rate. In order to test if the cubic law is actually the best description of Speich’s experimental data. they were now reevaluated. Speich’s evaluation of the chemical driving force has previously been criticized [IZ] and it was now cvalu- ated using equation (4). A difficulty was that Speich only measured the average composition of the growing I grain. He then assumed that the composition

at the sides of the x lamellae. x0, was the equilibrium

composition according to the phase diagram. There is no justification for such an assumption because the

edges of the adjoining f lamellae are expected to be

curved. Lacking experimental information on the cur- vature one might guess on any .x0 value between the

equilibrium value and the experimental average value

reported by Speich. As a consequence, Speich’s evaluation of the diffusivity should be regarded as an evaluation of a minimum value only. The closer x0 lies to the average .K. the higher will the resulting

diffusivity be. It is thus possible to obtain agreement between the very high diffusivity evaluated from the

present experiments and Speich’s experimental data by choosing an .vo value close to his experimental

average of 5. The mobility was now evaluated from Speich’s ex-

periments using both of the extreme x values. The results are presented in Table 1. AGs is the Gibbs

energy absorbed by surface energy in the interfaces between the lamellae. It was subtracted from the driv-

ing force before the mobility was evaluated. In pass-

ing, it is interesting to note that the driving force seems to be only a few times larger than the surface energy, a result which seems to be in a rather good agreement with Zener’s maximum growth rate hy- pothesis. An accurate comparison is impossible since the value of the specific surface energy between the lameliae is unknown. Speich used a value of 0.7 J/m’

and the same value was used here. The surprising result that the value of AG, evaluated for the Zn

rich alloy is not larger than the estimated surface energy may be removed by choosing a lower specific

surface energy. The mobility values obtained with the two extreme

.x0 values do not deviate much from each other. In

both cases there is a strong variation of the mobility with the alloy content but, contrary to Speich’s evaluation, this variation now does not seem to be related

to a variation in the driving force but rather to the

variation in alloy content. The mobility decreases with decreasing Zn content and an attempt to ex- trapolate these data to the conditions of the present experiments yielded a value of M between 10-i’ and

Table 1. Evaluation of experimental data by Speich on discontinuous precipitation in the Fe-Zn system

0.07 400 0 010 005 450 0.03 0.065 500 0.053 0.07

0.152 400 0.010 0.072 450 II 034 0.OY.l

500 0.053 0.095 550 0081 0.1 I

0235 400 O.KO 0.10’ 450 0.034 0.11

500 0.1353 0.125 '50 OOJI 0. I39 6Nl 0 131 0.I4.l

0.305 100 II.‘XO 0.12: 450 0 IV4 0. I39 500 0.053 0.15 550 OOJI 0.1’ 600 0 III 0 I65

2.8 5.2

I?

9.6

29 75

I70

48

380 II00 1600 1100

190

13w 2600 5400 56iN

I.7 6

6.4 34 6.9 26

4.5 16 2.5 Y

12 16 II 10

28 19 II

4

20

IO 50

200

19

II IO

IO 19

90 16

4Y.l 17

1033 I4

108) I4

10 3X 900

Ixnl Jo00

If II 6 4 9

5 3 30

340 HILLERT AND PCRDY: CHE4IICALLY INDUCED GRAIX BOUNDARY MIGRATION

IO-“’ m’/J,s to be compared with the value 2 x lo-” m’!J,s obtained from the present experiment and lo- I2 m’/J,s obtained for pure iron.

CONCLUSIONS

As compared to previous information for iron or iron-base alloys, the present experiments have yielded surprising values for the boundary diffusivity. It is four orders of magnitude larger than the values obtained from experiments with stationary grain boundaries. The discrepancy cannot be explained by errors in the experiments or evaluations. The explanation may be that the properties of a grain boundary is quite different when its is stationary and when it is moving.

It is not certain whether the diffusivity is different in the present experiments and in discontinuous precipitation in the same alloy system because the driving concentration gradient has not been measured in the latter case. On the other hand, there is a recent study of the concentration gradient in another case of discontinuous precipitation (131 which indicates that the conc&ntration gradient may be rather high. If that result can also be applied to the Fe-Zn system, the growth rates for discontinuous precipitation would yield a diffus~vity which is two orders of magnitude lower than the present result but it is still two orders of magnitude higher than the values obtained for stationary grain boundaries. It is thus difficult to avoid the conclusion that grain boundary diffusion is greatly enhanced by the movement of the boundary although the degree of enhancement seems to differ from one case to another.

The low values obtained for the mobility as compared to pure iron are not surprising and the differ- ence is most probably due to solute-drag effects. The fact that the present results are even lower than the values obtained from discontinuous precipitation is surprising and no explanation will be offered here. The fact that dislocations were sometimes observed to trail the migrating boundary does not seem to pro- vide any explanation because the dislocation energy is negiigible compared to the chemica1 force driving the boundary motion in these experiments.

It has been suggested that the grain boundary migrates in discontinuous precipitation because of a driving force that arises when the composition is changed by the action of grain boundary diffusion [S]. The present experiments confirm this mechanism and provides a technique for the direct study of it without the complications that occur in the complex process of discontinuous precipitation. This new experimental technique may be described as dis-

continuous zincification or dezincification and may yield valuable info~ation on the mechanism of discontinuous precipitation. Other applications suggest themselves. howkver, and we will brie& list some of these:

AS a probe for the structure of moving gain boundaries. the method appears to offer promise as a quantitative, if indirect, tool. We consider that the present results constitute the clearest evidence for the need to distinguish between the static and dynamic structures of grain boundaries, and one can imagine a series of similar experiments on boundaries of known misorientation. The remnant wall of epitaxial dislocations is also of interest; it is possible to prepare for study a planar epitaxial array on any prescribed crystallographic plane. The stability of these walls under conditions when volume diffusion is not negligible also invites investigation. It is possible to con- struct experiments for the study of dynamic boundary interactions with dispersed particles, or with dissolved solutes (although~ in the latter case. one can expect complications due to the presence of zinc) and it may prove possible to extend this type of study to include the dynamics of interphase boundaries.

We note that this process of disconrinuous zincification or dezincification may be of practical signifi- cance, as it provides a rapid method of changing the zinc concentration in iron. We expect that these same methods will be effective in other sysrems as well, provided that the rate of boundary diifusion is much greater than that of volume diffusion and that the mobile solute is capable of being supplied or removed via a gaseous or fluid medium.

Acknowledgements-This work was financially supported by the Swedish Board for Technical Development, and the National Research Council of Canada.

REFERENCES

1. J. W. Cahn. Acta 1Mer. 8, 554 (1960). 2. P. H. Pumphrey and H. Gleiter. Phil. Mag. 32, 881

(1973). 3. H. Westengen, Doctoral Thesis, Institut for Fysikalisk

Metallurgi. N.T.H., Trondheim (1977). 4. G. R. Spcich, Trans. A.I.M.E. 242. 1559 (1965). 5. M. Hillerr and R. Lagnebog, Afarer. Sci. 6. 208 (1971). 6. J. W. Cahn. Acta Met. 7. 1S [1959& 7. 3. E. Sundquist, Meta11. Trans. 4. 1919 (1973). 8. M. Hillert. Inst. Met. Proc. Int. Symp. on The Mechan-

ism of Please Transformations in Crystalline Solids. Manchester. July 1968. p. 231 (1969).

9. G. Kirchner, H. Harvis, K.-R. Moq\-ist and M. Hiltert, Arch. EisenlIiitrenw. 44, 227 (1973).

10. J. Fridberg L.-E. Tarndahl and M. HiUert, fernkonr. Ann. 153, 263 (1969).

il. M. Hillert. Merall. Trans. (A) 6, 5 (1975). 13. M. Hillert, Metali. Trans. 3, 2729 (1971). 13. D. Porter. Personal communication

chemically induced grain boundary migration

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