Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

7/17/2019 Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

http://slidepdf.com/reader/full/abnormal-grain-growth-in-alumina-with-anorthite-liquid-and-the-effect-of-mgo 1/9

Abnormal Grain Growth in Alumina with Anorthite Liquid and theEffect of MgO Addition

Chan Woo Park † and Duk Yong Yoon*

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,

Daejeon 305-701, Korea

Abnormal grain growth (AGG) in alumina with anorthiteliquid has been observed with varying anorthite and MgOcontents, at 1620°C. When only anorthite is added to form aliquid matrix, the grain–liquid interfaces have either flat orhill-and-valley shapes indicating atomically flat (singular)structures. The large grains grow at accelerated rates toproduce AGG structures with large grains elongated alongtheir basal planes. This is consistent with the slow growth atlow driving forces and accelerated growth above a criticaldriving force predicted by the two-dimensional nucleationtheory of surface steps. With increasing temperature, the AGG

rate increases. The number density of the abnormally largegrains increases with increasing anorthite content. The addi-tion of MgO causes some grain–liquid interfaces to becomecurved and hence atomically rough. The grains also becomenearly equiaxed. With increasing MgO content the numberdensity of the abnormally large grains increases until the graingrowth resembles normal growth. This result is qualitativelyconsistent with the decreasing surface step free energy associ-ated with partial interface roughening transition.

I. Introduction

IN MOST alumina products, there are small amounts of liquidphase with compositions close to anorthite.1–4 The grains often

grow abnormally during the sintering treatment.1–4

Some contactareas between the neighboring grains are separated by liquid filmswith grain surfaces of hill-and-valley structures2,5–7 while othercontact regions between the grains are separated by grain bound-aries.8 Such hill-and-valley grain surfaces and grain boundariesindicate that there are deep cusps in the radial plot of the surfaceor grain boundary energy relative to the boundary normal direc-tion.7 In this paper the interfaces between grains and the liquidmatrix are called grain surfaces.

In normal growth of grains with a liquid matrix phase, the grainsize distribution (normalized to the average size) is relativelynarrow and remains invariant during heat treatment for variousperiods.9–11 The grains are usually spherical or equiaxed withrounded shapes because of their rough surface structure.12 Inabnormal grain growth (AGG), some grains grow to large sizes

relative to the matrix grains, which grow very little. As the largegrains grow into the regions of the fine matrix grains, they impingeon each other. When only the impinged large grains remain, the

normalized grain size distribution is usually narrow. AGG is thus

characterized by changing (normalized) grain size distribution (or

equivalently, the lack of self-similarity in the grain microstructure)

during the heat treatment. It has been suggested by Yoon et al.13

that because the AGG behavior can gradually change to normal

growth with temperature or composition, the two grain growth

modes are indistinguishable, and hence any deviation from normal

growth can be described as AGG. Because the mechanism of AGG

is closely related to the anisotropic surface energy, the grains

growing abnormally often develop elongated shapes, especially in

the hexagonal crystal systems.It was recently proposed that the polyhedral grains dispersed in

a liquid matrix coarsen abnormally because the grains grow by

two-dimensional nucleation of steps if they are free of defects.12,13

If they have defects such as dislocations, twins, and contact edges,

the grains can still coarsen abnormally, because the variation of the

growth velocity with the driving force arising from the capillarity

can still be nonlinear. If the amount of the liquid matrix phase is

quite small, as in most of the alumina systems, the grain surfaces

at the intergranular liquid films and grain triple junctions can have

singular planes and hence they may grow by two-dimensional

nucleation of steps. The first goal of this work is to observe the

dependence of AGG in alumina containing anorthite liquid on

temperature and anorthite content, and analyze the results in terms

of the two-dimensional nucleation theory.

The second goal of this work is to examine the well-known

effect of MgO on grain growth in alumina.14–18 Earlier, we have

shown that MgO addition to alumina with small amounts of CaO

and SiO2 causes grain boundary roughening and hence the trans-

formation from AGG to normal growth behavior.19 In this work,

we observed the effect of MgO addition on grain growth in

alumina containing anorthite liquid and analyzed the results in

terms of the grain surface roughening transition. Because the

surface roughening implies the decrease of the surface step free

energy, the effect of MgO addition is closely related to the step

nucleation mechanism, which is used to explain AGG with

anorthite liquid without MgO. As noted earlier, the polyhedral

grains in liquid matrix with singular surfaces coarsen abnormally,

but if the grain surface becomes rough and hence the shape

becomes spherical at high temperatures or with an additive, thegrains grow normally controlled by diffusion in the liquid ma-

trix.12,13 Such an effect of the interface roughening transition on

grain coarsening has been observed in NbC–Fe with both temper-

ature increase and B addition.20,21 Since the initial proposal by

Burton, Cabrera, and Frank,22 the surface roughening transition

has been extensively studied both theoretically23–27 and experi-

mentally.28–30 The effect of the crystal surface structure, which

can be either atomically flat or rough, on crystal growth from

supersaturated melt or vapor has also been studied both theoreti-

cally31,32 and experimentally.30,33,34 In this work we will attempt

to apply these theories to the grain growth in the alumina–anorthite

liquid–MgO system by observing the growth behavior and the

shape of the grain surfaces at the junctions between two and three

grains.

L. C. Klein—contributing editor

Manuscript No. 188441. Received June 26, 2000; approved March 8, 2002.Supported by the Korea Ministry of Science and Technology through the National

Research Laboratory (NRL) Program and by the Korea Ministry of Education throughthe Brain Korea 21 (BK21) Program.

*Member, American Ceramic Society.†Present address: Wireless Communication Devices Department, Basic Research

Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Korea.

J. Am. Ceram. Soc., 85 [6] 1585–93 (2002)

1585

journal



II. Experimental Procedure

In this paper all additive contents will be specified by weightpercent. The amount of CaO2SiO2 added will be designated bythe equivalent weight percent, which is equal to the weightpercent of anorthite formed with Al2O3. Thus 1 equiv wt% of anorthite is equal to 0.633 wt% of CaO2SiO2. The anorthitecontent was varied between 0.05% and 1%, and the MgOcontent was also varied. For the sources of SiO2, CaO, andMgO, Si(OC2H5)4 (99.999%), Ca(NO3)2 xH2O (99.99%),

and Mg(NO3)26H2O (99.9%) were used. The alumina pow-der was mixed with the dopants in ethyl alcohol and dried at70°C. The doped powder was pressed into cylindrical compactsand then isostatically at 100 MPa. The compacts were heated to950°C for 1 h to convert the dopant chemicals to oxides and toremove ethyl alcohol. The compacts were then sintered andheat-treated mostly at 1620°C in air in a high-purity aluminatube furnace. The heating and cooling rates for the sinteringtreatment were 150°C/min and 300°C/min, respectively. Thepolished surfaces of the sintered specimens were thermallyetched at temperatures about 30°C lower than the sinteringtemperatures for 30 min for examinations under optical andscanning electron microscopes (SEM; Model 515, Philips,Holland). The specimens for transmission electron microscopy(TEM; JEM-3010, JEOL, Tokyo, Japan) were thinned by

mechanical grinding and ion-beam milling. The equivalentsphere radii of the grains were measured by tracing on opticalmicrographs using a digitizer connected to a personal computer.

III. Results and Discussion

Because the major experimental variables in this work were theanorthite content, the heat-treatment temperature, and time, eachspecimen (without MgO) will be designated by these variables;thus, for example, 0.15%/1620°C/12 h for the specimen with0.15% anorthite heat-treated at 1620°C for 12 h. The specimenseries with the same anorthite content heat-treated at the sametemperature for different periods will be designated by its anorthitecontent and heat-treatment temperature; thus 0.15%/1620°C for

the series with 0.15% anorthite heat-treated at 1620°C for variousperiods.

When alumina was sintered at 1620°C without adding anyanorthite, normal growth was observed as shown in Fig. 1. Thisresult agreed with previous observations.16,17,19 As exhibited inFig. 2 for the 0.15%/1620°C series, all specimens with anorthiteshowed AGG. After sintering for 5 min the specimen was quitedense with only a few pores remaining and some large abnormalgrains appeared as shown in Fig. 2(a). The measured distributionsof the equivalent sphere sizes of the grains are shown in Fig. 3.After 10 min (Fig. 2(b)), more large grains appeared with typicalelongated shapes as also observed previously3,18 when the impu-rity content (usually containing some SiO2) was relatively high.After 20 min (Fig. 2(c)), most of the large grains impinged on eachother, but some fine grains still remained. After sintering for 1 h

(Fig. 2(d)), most of the large grains impinged on each other andthey grew slowly on further sintering for 6 h (Fig. 2(e)) and 12 h(Fig. 2(f)). These microstructures and grain distributions showed atypical AGG pattern.

The TEM observations showed liquid pockets at all of about 30randomly selected grain triple junctions as exhibited in Fig. 4(a).All grain surfaces at all triple junctions examined were either flator faceted with sharp edges. Although the intergranular regionswere not extensively examined, some intergranular liquid filmsshowed flat grain surfaces and hill-and-valley structures as exhib-ited in Fig. 4(b). The flat grain surface on the upper side of Fig.4(b) might be a basal plane of that grain. These flat and facetedgrain surface shapes at the triple junctions and hill-and-valleyintergranular liquid films are consistent with a polyhedral equilib-rium shape of alumina in anorthite liquid. When the anorthite

contents are relatively low, as in our specimens, portions of theequilibrium surfaces are expected to be exposed at the triple

junctions, and the grain surfaces at the intergranular regions willhave either flat or hill-and-valley shapes as shown by Kim et al.5

and Blendell et al.7

When grain surfaces and grain boundaries are singular withorientations corresponding to the cusps in the polar plots of theinterface energy against the normal directions, they can move eitherby two-dimensional nucleation of steps or on existing steps producedby dislocations.22 For our qualitative analysis we assume the two-dimensional nucleation mechanism. Because the movement of theliquid pockets at the triple junctions can control the overall graincoarsening kinetics, the two-dimensional nucleation process cancontrol the grain coarsening in defect-free materials with a liquidphase. The predominant dependence of the two-dimensional nucle-ation rate R on experimental variables is given by

R D exp(G*/ kT ) (1)

where D is approximately the diffusivity in the liquid and G* isthe activation energy for step nucleation given by

G* V ms

2T

hg (2)

where V m is the molar volume of the solid phase, s(T ) the surfacestep free energy per unit length, h the height of the step, and g thedriving force. The driving force for the growth g(r 1) of a grain of size r 1 may be expressed as

gr 1 2V m1

r

1

r 1 (3)

where is the interfacial energy and r is the size of the grain which

neither grows nor shrinks. According to Eq. (1), R is small forsmall values of g and increases rapidly as g exceeds a certainvalue. If the grain size distribution is relatively narrow and r islarge, g(r 1) will be small even for the largest grains. Then thegrains will undergo slow coarsening limited by the low rate of stepnucleation. We proposed13 to call such a process “stagnantgrowth” to distinguish it from normal grain growth.

The observed AGG behavior shown in Fig. 2 appears to bequalitatively consistent with this two-dimensional nucleation of steps as proposed earlier.12,13 The small matrix grains undergoslow stagnant growth, and apparently some large grains, probablywith dislocations, grow rapidly to form the abnormally largegrains.

The observed dependence of AGG on heat-treatment tempera-ture is shown in Fig. 5. The compacts with 0.05% anorthite were

first sintered at 1500°C for 12 h to obtain almost fully densestructures with nearly uniform grain sizes. They were heat-treated

Fig. 1. SEM microstructure of pure alumina sintered at 1620°C for 12 h.

1586 Journal of the American Ceramic Society—Park and Yoon Vol. 85, No. 6



again for 12 h at higher temperatures to observe the grain growthbehavior. At 1520°C, which is below the eutectic temperature(1553°C)35 of anorthite, there was some grain growth with a fewgrains apparently beginning to grow abnormally as shown in Fig.5(a). At 1560°C some large grains appeared as shown in Fig. 5(b),and at 1580°C the large grains became elongated probably alongtheir basal planes as shown in Fig. 5(c). At 1600°C all of the largegrains impinged on each other as shown in Fig. 5(d). The series of microstructures at increasing temperatures in Fig. 5 shows that therates of formation of abnormal grains and their growth increasewith temperature as expected from the temperature dependences of

D and the exponential term in Eq. (1). The step free energy s(T )is not expected to decrease substantially in this relatively smalltemperature range.

Figure 6 shows a striking dependence of AGG behavior onanorthite content at 1620°C. These microstructures obtained

after heat-treating for 12 h showed the abnormal grains whichimpinged on each other. The number of the abnormal grainsincreased and hence their average size after the impingementdecreased with increasing liquid content as shown in Fig. 6.This observation appears to be consistent with kinetic rough-ening of the abnormal grains during their growth. When acrystal with a singular surface grows by nucleation of stepsunder high driving forces, the step nucleation rate can be sohigh that the singular surface can be kinetically rough asproposed earlier.36 Such a surface is predicted to grow contin-uously with its rate (in a multicomponent system) determinedby diffusion flux. The growth rates of the abnormal grains canhence be determined by diffusion in the liquid matrix andtherefore depend on the liquid volume fraction. If the grain

growth is limited by the movement of the triple junctions, itsrate will increase with decreasing size of the liquid pocket at the

Fig. 2. Optical microstructures of the specimens with 0.15% anorthite heat-treated at 1620°C for (a) 5 min, (b) 10 min, (c) 20 min, (d) 1 h, (e) 6 h, and(f) 12 h.

June 2002 Abnormal Grain Growth in Alumina 1587



triple junctions as in other systems which undergo diffusion-controlled Ostwald ripening.9–11 The growth of the fine matrixgrains, however, will still be limited by the rate of two-dimensional nucleation and hence independent of liquid con-tent. Because with lower liquid content, the growth rates of thelarge grains relative to the smaller ones will be higher, fewergrains will grow to larger sizes as observed in Fig. 5. Thegrowth behavior of the system of grains of various sizes is fairlycomplex and needs to be further examined by numericalanalysis and other methods. The previous observations of AGGin alumina during infiltration with anorthite liquid by Lee et

al.37 showed a similar dependence on liquid volume fractionand can also be explained by diffusion-controlled growth of theabnormal grains.

In the second part of this study, the effect of MgO addition (to

the powder mixtures) was investigated in the specimens withvarying anorthite contents. In Fig. 2 was shown a typical AGG

behavior in the 0.15%/1620°C series exhibiting large grainselongated along the basal planes. When 0.3% MgO was added tothis specimen, the grains were nearly equiaxed as shown in Fig. 7.As shown in Fig. 8, the grain size distribution became wider withincreasing sintering time, indicating still an AGG behavior, but notas pronounced as those for the specimens without any MgO shownin Figs. 2 and 5. But with MgO the number density of the abnormalgrains was much larger and therefore their sizes after theirimpingement were smaller than those of the specimen without anyMgO, as can be seen by comparing Figs. 2(d,e,f) and 3 to Figs. 7and 8. When the MgO addition was increased to 1%, the abnormalgrains were more numerous and their sizes even smaller as shownin Fig. 9. But again AGG appeared to occur as indicated by thewidening grain size distribution as shown in Fig. 10. When 0.5%MgO was added, the grain structures were intermediate between

those with 0.3% (Fig. 7) and 1% MgO (Fig. 9). When a maximumamount of 2% MgO was added, the grain evolution appeared to be

Fig. 3. Measured equivalent sphere grain sizes of the specimens shown in Fig. 2.

Fig. 4. TEM microstructures of (a) a liquid pocket at a triple grain junction and (b) an intergranular liquid film in a specimen with 1% anorthite heat-treatedat 1620°C for 10 min.

1588 Journal of the American Ceramic Society — Park and Yoon Vol. 85, No. 6



Fig. 5. Optical microstructures of the specimen with 0.05% anorthite heat-treated at 1500°C for 12 h and further at (a) 1520°, (b) 1560°, (c) 1580°, and(d) 1600°C for 12 h.

Fig. 6. Optical microstructures of the specimens with (a) 0.05%, (b) 0.15%, (c) 0.25%, and (d) 1% anorthite heat-treated at 1620°C for 12 h.




identical to those with 1% MgO (Fig. 9). We observed similareffects of MgO addition when 0.1% MgO was added with 0.1%anorthite, 0.03% MgO with 0.05% anorthite, and 2% MgO with1% anorthite.

The TEM observations of the liquid pockets at grain triple junctions of the 1%/1620°C/10 min specimen with 2% MgO

revealed some curved grain surfaces as shown in Fig. 11(a). Butsome grain surfaces still appeared to be flat at the triple junctions.A triple junction in a specimen with 0.1% anorthite and 0.1% MgOalso showed a curved grain surface meeting with a flat segmentwith apparently a discontinuous change of the slope. Out of about40 triple junctions examined in the specimens with MgO, 4 of them showed curved grain surfaces as shown in Fig. 11(a), whilethe rest still showed only faceted or flat grain surfaces. A liquidfilm in the specimen with 1% anorthite and 2% MgO showed acurved grain surface on one side and a hill-and-valley grain surfaceon the other as shown in Fig. 11(b). In contrast, the specimenswithout any MgO always showed either flat or hill-and-valleygrain surfaces at the liquid pockets and films as shown in Fig. 4.

These results show that when MgO is added, the corners, edges,and possibly some flat (singular) surfaces (in the equilibrium shape

of the alumina grains) become rounded with atomically roughstructures, while some low index surfaces, probably those with

very low surface energies like the basal planes, still remain

singular. The surface normal appeared to change discontinuously

from a curved surface segment to a flat region when observed at a

triple junction as described earlier, and such a discontinuous

surface slope change is consistent with the hill-and-valley structure

of a grain surface shown in Fig. 11(b) between two grains. If some

grain surface segments are singular, they will move by two-

dimensional nucleation of steps. Because the movement of the

singular surface segments is likely to control grain coarsening,

AGG can still occur even with partial roughening of some surface

segments. But with the roughening at the corners and edges, the

areas of the surfaces that still remain singular will decrease. This

means that the step free energies of those singular surfaces also

decreased approximately in proportion to their decreasing ra-

dii.27,38 In phenomenological theories,39,40 the surface roughening

induced by additives was attributed to the decrease of step free

energy caused by segregation at the edges of the steps. The MgO

segregation at the nonbasal planes apparently decreased their

surface energies sufficiently to make the grains nearly equiaxed. It

is possible that the effect of MgO is that of nullifying the effects

of CaO and SiO2, which appear to increase the anisotropy of theinterfacial energy and produce singular interfaces.

Fig. 7. Optical microstructures of the specimens with 0.15% anorthite and 0.3% MgO heat-treated at 1620°C for (a) 1, (b) 6, and (c) 12 h.





If the step free energies of these remaining singular surfaces arereduced by MgO addition, the activation energy for step nucleationis reduced in accordance with Eq. (2). Therefore, for a certain grainsize distribution, the number of grains which can grow rapidly isexpected to increase when MgO is added. The increased number

density of the abnormal grains with MgO addition shown in Figs.7 and 9 is thus consistent with the decrease of step free energy,which is indicated by the curved grain surfaces. As noted earlier,further increase of the MgO content did not cause any change of the grain growth behavior, indicating that there is a limit to thedecrease of the step free energy with increasing MgO content. Itthus appears to be impossible to roughen all alumina surfaces incontact with anorthite liquid by adding more MgO. If all aluminasurfaces become rough, all grains can grow continuously andhence normal growth will occur. When 1% MgO was added with0.15% anorthite, the step free energy apparently became so smallthat most of the large grains could grow continuously, closelyresembling the normal growth as shown in Fig. 9. As proposedearlier by Yoon et al.,13 AGG behavior can approach normalgrowth, if the number density of the abnormal grains is increased

by the reduction of step free energy, which can be induced eitherby temperature increase or by an additive like MgO in alumina.

As shown in Fig. 6, the number density of the abnormal grainssubstantially increased with increasing anorthite liquid content.This can arise from the diffusion-controlled growth of the abnor-mal grains as proposed earlier in this paper. If there is an increaseof the liquid volume fraction with MgO, it can likewise increase

the number density of the abnormal grains. But it was indicatedpreviously4,41 that the MgO addition can actually decrease theamount of SiO2-rich liquid because of increased solubility inalumina. Changes of liquid diffusivity and grain–liquid interfaceenergy with MgO addition can also alter the AGG behavior, butsuch a drastic change of the AGG behavior from Figs. 2(d), (e),and (f) to Figs. 9(a), (b), and (c) appears to occur predominantlyfrom the partial grain surface roughening.

The surface roughening and the suppression of AGG induced byMgO addition in alumina is analogous to similar changes inNbC–Fe induced by temperature increase or B addition.21 But inNbC–Fe the grain shape changes from cubic to spherical by theroughening transitions could be readily observed because of muchlarger liquid volume fraction (on the order of 20%) and theinterface roughening was so complete that the grains became

spherical and the grain coarsening completely normal.21 It shouldalso be noted that the MgO addition caused both grain surfaces in

Fig. 9. Optical microstructures of the specimens with 0.15% anorthite and 1% MgO heat-treated at 1620°C for (a) 1, (b) 6, and (c) 12 h.





contact with vapor and grain boundaries (which were madesingular by CaO and SiO2) to become completely rough.19,42 The

roughening effect of MgO thus appears to be quite general for all

types of alumina interfaces.

IV. Conclusions

By observing the shapes of the grain surfaces at grain triple

juncti ons and intergranu lar liquid films , it was possi ble to relat e

the interface atomic structure to grain growth behavior in

alumina with anorthite liquid. If the singular interfaces move by

step nucleation, AGG can occur because only a few large grains

can grow rapidly by kinetic roughening of their interfaces. The

striking dependence of AGG behavior on the anorthite contentis consistent with the possibility that the rapid growth of abnor-

mally large grains is controlled by diffusion in liquid matrix. The

grain–liquid interfaces became partially rough by MgO addition

and observed increase of the number density of the abnormal

grains with MgO content is consistent with decrease of the step

free energy. All observations can thus be qualitatively explained

by the step nucleation theory. We are currently carrying out a

detailed numerical analysis of the model to compare with these

observations.

It is possible that dislocations and other surface defects can

influence the growth behavior and particularly the stagnant growth

at relatively low driving forces. But the relationship between the

growth rate and the driving force will still be nonlinear and hence

AGG can occur if the interfaces are singular.

In this work it was shown that AGG in alumina with anorthite

liquid can be nearly suppressed by adding MgO. Earlier, we have

shown that AGG in alumina doped with CaO and SiO 2 at relatively

low concentrations to avoid liquid formation could be completely

suppressed by adding MgO.19 With or without a liquid phase the

suppression of AGG was ironically achieved by enhancing the rate

of forming large grains. This work is another example which

showed that there is a gradual change from AGG to normal

growth, which may be regarded as the limit of AGG where all

grains can continuously grow with no energy barrier to step

nucleation. Another general strategy for suppressing AGG in

practice can be to prolong the stagnant growth period by decreas-

ing the heat-treatment temperature or using additives to increase

the step free energy. Indefinite stagnant growth might also be

achieved by increasing the initial average grain size above acritical value.

References

1S. C. Hansen and D. S. Phillips, “Grain Boundary Microstructures in a Liquid-

Phase Sintered Alumina (-Al2O3),” Philos. Mag. A, 47, 209–34 (1983).2C. A. Powell-Dogan and A. H. Heuer, “Microstructure of 96% Alumina Ceramics:

I, Characterization of the As-Sintered Materials,” J. Am. Ceram. Soc., 73 [12]

3670–76 (1990).3C. A. Bateman, S. J. Bennison, and M. P. Harmer, “Mechanism for the Role of

Magnesia in the Sintering of Alumina Containing Small Amounts of a Liquid Phase,”

J. Am. Ceram. Soc., 72 [7] 1241–44 (1989).4C. A. Handwerker, P. A. Morris, and R. L. Coble, “Effects of Chemical

Inhomogeneities on Grain Growth and Microstructure in Al2O3,” J. Am. Ceram. Soc.,

72 [1] 130–36 (1989).5D.-Y. Kim, S. M. Wiederhorn, B. J. Hockey, C. A. Handwerker, and J. E.

Blendell, “Stability and Surface Energies of Wetted Grain Boundaries in Aluminum

Oxide,” J. Am. Ceram. Soc., 77 [2] 444–53 (1994).6T. M. Shaw and P. R. Duncombe, “Forces between Aluminum Oxide Grains in a

Silicate Melt and Their Effect on Grain Boundary Wetting,” J. Am. Ceram. Soc., 74

[10] 2495–505 (1991).7J. E. Blendell, W. C. Carter, and C. A. Handwerker, “Faceting and Wetting

Transitions of Anisotropic Interfaces and Grain Boundaries,” J. Am. Ceram. Soc., 82

[7] 1889–900 (1999).8K. J. Morrissey and C. B. Carter, “Faceted Grain Boundaries in Al2O3,” J. Am.

Ceram. Soc., 67 [4] 292–301 (1984).9T.-K. Kang and D. N. Yoon, “Coarsening of Tungsten Grains in Liquid

Nickel–Tungsten Matrix,” Metall. Trans. A, 9A, 433–38 (1978).10S. S. Kang and D. N. Yoon, “Kinetics of Grain Coarsening during Sintering of

Co–Cu and Fe–Cu Alloys with Low Liquid Contents,” Metall. Trans. A, 13A,

1405–11 (1982).11S. S. Kim and D. N. Yoon, “Coarsening of Mo Grains in the Molten Ni–Fe

Matrix of a Low Volume Fraction,” Acta Metall., 33, 281–86 (1985).12Y. J. Park, N. M. Hwang, and D. Y. Yoon, “Abnormal Growth of Faceted (WC)

Grains in a (Co) Liquid Matrix,” Metall. Mater. Trans. A, 27A, 2809–19 (1996).13D. Y. Yoon, C. W. Park, and J. B. Koo, “The Step Growth Hypothesis for

Abnormal Grain Growth”; pp. 3–21 in Ceramic Interfaces 2. Edited by H.-I. Yoo and

S.-J. L. Kang. Institute of Materials, London, U.K., 2001.14

R. L. Coble, “Transparent Alumina and Method of Preparation,” U.S. Pat. No.3 026 210, March 20, 1962.15S. J. Bennison and M. P. Harmer, “Effect of MgO Solute on the Kinetics of Grain

Growth in Al2O3,” J. Am. Ceram. Soc., 66 [5] C-90–C-92 (1983).16S. J. Bennison and M. P. Harmer, “Grain-Growth Kinetics for Alumina in the

Absence of a Liquid Phase,” J. Am. Ceram. Soc., 68 [1] C-22–C-24 (1985).17S. I. Bae and S. Baik, “Critical Concentration of MgO for the Prevention of

Abnormal Grain Growth in Alumina,” J. Am. Ceram. Soc., 77 [10] 2499 –504 (1994).18H. Song and R. L. Coble, “Origin and Growth Kinetics of Platelike Abnormal

Grains in Liquid-Phase-Sintered Alumina,” J. Am. Ceram. Soc., 73 [7] 2077–85

(1990).19C. W. Park and D. Y. Yoon, “Effects of SiO2, CaO, and MgO Additions on the

Grain Growth of Alumina,” J. Am. Ceram. Soc., 83 [10] 2605–609 (2000).20S. Sarian and H. W. Weart, “Factors Affecting the Morphology of an Array of

Solid Particles in a Liquid Matrix,” Trans. Metall. Soc. AIME , 233, 1990–94 (1965).21J. Y. Jun, “Grain Shape and Abnormal Grain Growth in NbC–Fe”; M.S. Thesis.

Department of Inorganic Materials Engineering, Seoul National University, Seoul,

Korea, 1994.22W. K. Burton, N. Cabrera, and F. C. Frank, “The Growth of Crystals and the

Equilibrium Structure of Their Surfaces,” Philos. Trans. R. Soc., London, Sect. A,

A243, 299–358 (1951).

Fig. 11. TEM microstructures of (a) a liquid pocket at a triple grain junction and (b) an intergranular liquid film in a specimen with 1% anorthite and 2%MgO heat-treated at 1620°C for 10 min.




23J. D. Weeks, “The Roughening Transition”; pp. 293–317 in Ordering in Strongly

Fluctuating Condensed Matter Systems. Edited by T. Riste. Plenum, New York, 1980.24J. M. Kosterlitz, “The Critical Properties of the Two-Dimensional XY Model, ” J.

Phys. C: Solid State Phys., 7, 1046 – 60 (1974).25J. M. Kosterlitz and D. J. Thouless, “Ordering, Metastability and Phase

Transitions in Two-Dimensional Systems,” J. Phys. C: Solid State Phys. , 6, 1181–203

(1973).26H. van Beijeren, “Exactly Solvable Model for the Roughening Transition of a

Crystal Surface,” Phys. Rev. Lett., 38, 993–96 (1977).27H. van Beijeren and I. Nolden, “The Roughening Transition”; pp. 259 –300 in

Structure and Dynamics of Surfaces II: Phenomena, Models, and Methods. Edited byW. Schommers and P. von Blanckenhagen. Springer-Verlag, Berlin, Germany, 1987.

28

T. Engel, “Experimental Aspects of Surface Roughening”; pp. 407–28 inChemistry and Physics of Solid Surfaces VII . Edited by R. Vanselow and R. F. Howe.

Springer-Verlag, Berlin, Germany, 1988.29J. E. Avron, L. S. Balfour, C. G. Kuper, J. Landau, S. G. Lipson, and L. S.

Schulman, “Roughening Transition in the 4He Solid–Superfluid Interface,” Phys. Rev.

Lett., 45, 814 –17 (1980).30P. E. Wolf, F. Gallet, S . Balibar, E. Rolley, and P. Nozieres, “Crystal Growth and

Crystal Curvature near Roughening Transitions in HCP 4He,” J. Phys. (Paris), 46,

1987–2007 (1985).31J. D. Weeks and G. H. Gilmer, “Dynamics of Crystal Growth”; pp.157–228 in

Advances in Chemical Physics, Vol. 40. Edited by I. Prigogine and S. A. Rice. Wiley,New York, 1979.

32S. T. Chui and J. D. Weeks, “Dynamics of the Roughening Transition,” Phys.

Rev. Lett., 40, 733–36 (1978).

33S. D. Peteves and R. Abbaschian, “Growth Kinetics of Solid–Liquid Ga

Interfaces: Part I. Experimental,” Metall. Trans. A, 22A, 1259 –70 (1991).34X.-Y. Liu, P. van Hoof, and P. Bennema, “Surface Roughening of Normal

Alkane Crystals: Solvent Dependent Critical Behavior,” Phys. Rev. Lett., 71, 109 –12

(1993).35E. M. Levin, C. R. Robbins, and H. F., McMurdie; pp. 219 –20 in Phase

Diagrams for Ceramists, Vol. 1. Edited by M. K. Reser. American Ceramic Society,

Columbus, OH, 1964.36W. van Saarloos and G. H. Gilmer, “Dynamical Properties of Long-Wavelength

Interface Fluctuations during Nucleation-Dominated Crystal Growth,” Phys. Rev. B,

33, 4927–35 (1986).37S.-H. Lee, D.-Y. Kim, and N. M., Hwang; private communication, 2000.38M. Wortis, “Equilibrium Crystal Shapes and Interfacial Phase Transitions”; pp.

367– 405 in Chemistry and Physics of Solid Surfaces VII . Edited by R. Vanselow and

R. F. Howe. Springer-Verlag, Berlin, Germany, 1988.39N. A. Gjostein, “Adsorption and Surface Energy (I): The Effect of Adsorption on

the -plot,” Acta Metall., 11, 957– 67 (1963).40P. G. Shewmon and W. M. Robertson, “Variation of Surface Tension with

Orientation”; pp.67–98 in Metal Surfaces: Structure, Energetics, and Kinetics.

American Society for Metals, Metals Park, OH, 1963.41K. L. Gavrilov, S. J. Bennison, K. R. Mikeska, and R. Levi-Setti, “Grain

Boundary Chemistry of Alumina by High-Resolution Imaging SIMS,” Acta Mater.,

47, 4031–39 (1999).42C. W. Park and D. Y. Yoon, “Effect of Magnesium Oxide on Faceted Vicinal

(0001) Surface of Aluminum Oxide,” J. Am. Ceram. Soc., 84 [2] 456 –58 (2001).


Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Documents

Transcript of Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition