The theoretical aspect of the directional solidification :is briefly
described. Two different experimental techniques were used for
the production of directionally solidified high refractory oxide and
carbide eutectics. Microstructure, crystallography, mechanical properties
and mechanism of failure were investigated. Some of the
inves·tLgated eutectics retain considerable strength at elevated temperature,
whereas the majority of alloys and sintered oxide bodies
show rapid loss in strength. Eutectic in the system B4C-SiC shows
excepti.onally high wear r~sisfance

V. S. Stubican

English

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CROATICA CHEMICA ACTA CCACAA 48 (4) 423-430 (1976) CCA-948 YU ISSN 0011-1643 548.40 Conference Paper Crystallization of Eutectic Microstructures from Oxide and Carbide Melts V. S. Stubican Department of Material Sciences, The Pennsylvania State University, University Park, PA 16802, USA The theoretical aspect of the directional solidification :is briefly described. Two different experimental techniques were used for the production of directionally solidified high refractory oxide and carbide eutectics. Microstructure, crystallography, mechanical pro-perties and mechanism of failure were investigated. Some of the inves·tLgated eutectics retain considerable strength at elevated tem-perature, whereas the majority of alloys and sintered oxide bodies show rapid loss in strength. Eutectic in the system B4C-SiC shows excepti.onally high wear r~sisfance. · SOLIDIFICATION OF EUTECTIC MELTS When a melt of exactly the eutectic composition is solidified, two solid phases (in a binary system) form at the liquidus temperature and all of the liquid disappea•rs before the temperature decreases further. It was already known since the 1920's1 that by controlling solidifica·tion (removing heat pre-ferentially fr.om one direction) a microstructure can be produced which con-sists of a matrix :phase withim which is dispersed an ali~ned second phase. The second phase may be in the form of needles, rods or two phases can occur as alternate lamellae. The phases can be thought of as one s.i:ngle. crystal imbedded in a maitrix of another single crystal. It is now well established that the two phases grow simultaneously2 so that their common interfaces would be perpendicular to the solid-liquid inter-face. Each lamella has its own isolid-liquid interface. It follows tha1t the liquid in front of each lamella becomes enriched ·in the major component of the neighboring lamellae as shown in Figure 1 and that transverse diffus~on of both components must take place. It has been observed and to a cer.taim degree ex;plained foeoretically3, that the planar front growth is necessary to obtain normal eutectic microstructures. The oondition for the breakdown of the planar interface is given by4 G< ~Tf. R D (1) where G is the temperature gradient, D is diffusion coefficient in the l.iquid, R is the growth rate and ~Tf is the freezing range of the mixture. To obtain the planair front grnwth and to produce well-alligned eutectic microstructures, it is then necessary to use steep temperature gradients, slow growth rates and very pure starting materJals. Non-pla111ar liquid-solid interface results in a 424 B-rich phase A-rich phase V. S. STUBICAN .+A-rich liquid \\ Jj ...,.. B-rich liquid Figure 1. Diffusion paths for the g rowth of a lamellar eutectic. Ref. 3. colony microstructure in which the interface brea1"s down into regions of high curvature separated by higher concentration of impurities. It has been shown experimentally and has been explained .theoretically5 that in the eutectic microstructures the distance between the lamellae or rods (Ii.) depends ·on the irate. of so1idifica•tion (R) and that the relationship is (2) Most eutectic systems show a strong preference for a rod fo!'lffi or lamellae form of microstructure. This behavior is dependent on differences in the total solid-solid inte.rfacial energy, which in turn .iJS partly dependent on the relative volumes ·of the two eutectic phases. It is easy to derive on geometr~cal grounds6 that if the relative volume fraction (VF) of two phases is less than 0.28 minor phase will grow in the form of rnds. Lamellar type ·of micros.tructures can ·be expected i f VF> 0.28. On Figure 2 rod to lamellar morpho1ogy cha111ge in terms of interfacial area/unit volume change for several metallic eutectics is shown. Preferred cry:stallographic orientations are often formed during controlled solidification of eutectic melts. It seems that in most cases the •preferred orientahons which develop are associated with low va1ues of the interfacial energ·ies and with the similarity of a·tomic densities of the ill1terfacial planes7• It has been postulated8 that to obtain nonfaceted .growth*, a factor a should be less than 2. Factor a was defined as: * Normal eutectic microsbructures contaill1 nonfaceted phases. CRYSTALLIZATION OF EUTECTIC MICROSTRUCTURES NiAI- Cr l Sb-MnSb Cu-Cr 0 0.08 0.2 0.28~ 0.4 0.6 Pb-Sn Al-Zn Fe,P•Fe 0 .8 Volume Fraction (V) 1.0 425 Figure 2. Rod to lamellar morphology change in terms of interfacial area/unit volume. Ref. o. (3} where Q is a crystallographic factor always less than or equal .to one, ~SF is the entropy ·Of melting and R is the gais constant. Most metals and organic· materials haive lrow entropies of melting so that the condition a< 2 is easily met. Refractory oxides and carbides have high entropies .of fusion and sh{)<Uld show faceted growth. However, this ~s not the case and normal eutectic microstructures can be abtfilned in most refractory oxide systems. EXPERIMENTAL METHODS Two experimental methods were used to solidify oxide-oxide and carbide-carbide eutectics. There are (i) ; a modified Bridgman-Starkbarger method9,10, and (ii), a floating molten zone technique11• The first technique requires a furnace similar to that shown on Figure 3. Ingots are directionally solidiified by lower1ng the loaded crucible, suspended from the upper travelilng head of the furnace, by an Mo wire through the susceptor at the desired solidification rate. Because of the difficulty of measuring the actual growth rate of the ingnt, it is assumed that the rate of growth is equal to the lowering rate of the crucible, i. e. the soltdification rate. The lowering rate of the crucible is usually 0.5-30 cm/hr and the furnace is capable of reaching temperatures up to - 2500 °c. In our experiments the melting temperatures were = 50 °c above the eutectic liquidus. The thermal gradient at the eutectic liquidus temperature, determined by a W-Re thermocouple, was = 200 °C/cm. All melting and solidification procedures were conducted 1n a•rgon. With the floating zone technique temperature gradients of 500--1500 °C/cm can be obtained. The actual heathng of the pres.ilntered road Ls accomplished by radiation from a small carbon rung susceptor and the molten zone is only 3-4 mm thick. 426 V. S. STUBICAN ------MOLYBDENUM WIRE------ CRUCIBLE ~---GRAPHITE SUSCEPTOR ---ZIRCONIA FELT INSULATION FUSED SILICA TUBE INDUCTION COIL ZIRCONIA SUPPORTS Figure 3. Interior layout of solidification furnace. Ref. 13. The resultiing ingots can be sectioned and used for optical and x-ray in-vestigations. With both techniques it is possible to obtain large ingots which can be used for the investigations of mechanical, electrical, optical, magnetic or thermal properties. REFRACTORY OXIDE AND CARBIDE EUTECTICS Microstructure The fo11owing refraotoTy oxide eutectics were di.rectionally solidified: Al6Si20 13 (mullite)-Al20/0 , MgAl20 4-Al20 312, MgO-MgAl20 413, Zr02-Mg014, Ti02- Mg015, Zr02- CaZr0310•15, Zr02-SrZr0315, CaO-Mg010•15, Al20 3-Zr0216, Al20 3-Y20 39• Only one carbide system SiC-B4C was direct~onally solidified. Normal eutectic microstructures, lamellar, rod OT fibrous type could be prn-duced with most high melting oxide eutectics by using directional solidifi-cation. For example, the eutectic between MgO and MgA120 4 is an attr.active candidate for directional solidification; the phase diagram is shown in Figure 4. The eutectic is at 45 wt ()/o MgO and 55 wt O/o M20 3 and has a liquidus temperature of 1995 °C. This composition yields~ 14 vol. ·O/o MgO, .so that on Croat. Chem. Acta 48 (4) 1976 [To face page 427] Figure 5. Microstructure of ingots solidified a t 1 cm/hr . A. MgO-MgAJ,O, eutectic, transverse section. B. Zr02-MgO eutectic transverse section. C. Zr02-SrZr03 eutectic, long itudinal section. Ref. 13 and 15 . Croat. Chem. Acta 48 (4) 1976 [To face page 427] Figure 6. A. Lamellar Zr02-SrZrOa e u tect ic, quenched to show isot h ermal solid-liquid interface B . Zr02-SrZrOa eutectic solidified at 6 cm 'hr. Colony microstructure . Ref. 15. CRYSTALLIZATION OF EUTECTIC MICROSTRUCTURES w !3 2300 ~ 2200 w a:: (/) 8:' 2100 .5 ::;: c ~ 2000 ii' 1900 1800 1700 If PERICLASE LIQUID PERICLASE • SPINEL LIQUID SPINEL •LIQUID -2135 160~ 10 20 30 40 WEIGHT% Figure 4. The system MgO-MgAl204. Ref. 18. LIQUID 427 directional solidificatron the eutectic might be expected to yield a composite · consisting of MgA120 4 (spinel) matrix reinforced with MgO whiskers. Figure 5A shows transverse section of an iingot solidified at 0.9 cm/hr where no colony microstructure is present. Similarly, colony free microstructures shown on Figure 5B and C could be oibtaiined if Zr02-SrZr03 and Zr02-MgO eutectic . me1ts we.re directionally solidified using relatively pure oxides and rates of solidification less than 2 cm/hr. In these ca:ses a planar front .growth was achieved as shown in the Figure 6A. Higher solidification rates always produced colony type microstructures (Figure 6B). ln systems which show stirong tendency towards metastability, either glass or metastable pha,se for-mation, the :solidifica.tion process may in:ot follow the equilibrium (f>hase diagram. The metastable behavJior prevents the formation of the normal eutectic mi-cro.structures as e. g. in the system Al20 3-Si:02 and MgAl20 ,-Al20 3 10• With high temperature ox,ide eutectics the spacing {A.) between rods or lamellae and the solidification •rate (R) are related by the well-known for-mula A a R-'h. The crystallo:graphic relationships of a number of the oxide-oxide eute-ctics have been investigated using Laue and Buerger p.recessioili x-ray techni-ques. Generally we find that if both phases are cubic, the growth direction of both phases is the [111] direction with all (hkl) planes parallel. The lattice mismatch is accomodated with a dislocation network at the i:nterface. Deter-mination of foe crystallographic relationship in more complex systems is in progress15• Mechanical Properties There are several factors which could improve mechanical properties of ceramic materials with eutectic microstructures over that obtamed .by con-ventional sintering ·or hot pressing. Some of these factors a·re: (i) high strength at the high temperature of the mrnor phase in the whisker form (ii) stability of microstructure at high temperature (iii) the matrix phase may be placed i<n 428 V . S. STUBICAN compression upon cooling, which will increase the strength of eutectic com-po·site. Fracture strength was studied at temperatures up to 1600 °C with ingots solidified at different rates. The obtained results showed that the strength of these composites varied little with the temperature up to 1600 °c and was insensitive to the. rate of solidif.ication. Comparison of the flexural strength of ceramic eutectics with hot pressed Alz03 , Si3N4 and a Ni-alloy (Figure 7) 160 B-1900 140 ,., I 0 120 x (\I .E ' .Cl 100 :r: EUTECTIC I-(,!) Al20 3 -Zr02 (Y20:Y z 80 UJ ---l a:: I-{/) --_J ~ 60 a:: ::;) x UJ _J EUTECTIC LL. 40 CoO-MgO ~ ----20 -====--r--EUTECTIC MgO~MgAlz04 0 0 400 800 1200 1600 2000 TEMPERATURE (°C) Figure 7. Flexural strength of several eutectics compared with hot pressed AbOa(HP), SiaN.CHP) and Ni-alloy B - 1900. Ref. 11. shows that ceramic eutectics may be some of the strongest known materials at high temperature11• A partioularly interesting area for further •research is determiin:ation of the fracture strength of high refractory eutectics at tempe-rature 1600-2000°C. Fracture surface energies measured at room temperature for MgO--MgA1204 eutectic and Zr02-CaZr03 eutectic averaged to 22 X 103 er.g/cm and 5 X 104 erg/cm respectively. At 1600 °c fracture surface energies increased to 45 X 103 erg/cm2 and 100 X 104 erg/cm respectively. The sudden increase Croat. Chem. Acta 48 (4) 1976 [To face page 428] Figure 8. Fracture surface of Zr02-MgO eutectic at room t emperature. (etched with HCL). SEM photomicrog raph. Ref. 15. CRYSTALLIZATION OF EUTECTIC MICROSTRUCTURES 429 between 1000-1600 °c may be explained by the absorption of energy in deforming more ·plastic matrix and in the shear process in pulling fibers out of the matrix. Topography of fractured surfaces was investigated with ingots fractured at different temperatures by using SEM. The effect of the fractured surface was found to be orientahon dependent. In several cases the noticeable inter-action betwe,en the fracture path iam.d the microstructure was observed. Figure 8 shows deviation of crack by MgO in Zr02-MgO eutectic ingot which was fractured at room tempera.ture. With specimes broken at high temperatures 1200-1500 °c, which com.tained ductile matrix e.g. CaZr03 , the minor phase Zr02 was partially extracted from the ductile matrix11 • Wear res~stance was measured17 wJth SiC-B, C solidified ingots of eutectic composition. The minimum wear was found with eutectic composition at a solidificatiion rate o f apprnximately 9 omihr. The wear of the ingot of eutectic composi.tion solidified at 9 cm/hr was considerably less than the wear ·of solidified B, C or hot presed SiC (Figure 9). (/) (/) 0 ..J 4 a: 2 <1 w " - x- B4 C -•-HOT PRESSED SiC -c- 70 wto B4C - 30 w/o SiC (R 0 8.9cm/hr) ~ ~ ~ !~6.~~~._-______l.-~_-::__-::__- _L_~~~~~~"-"~ 10 20 30 40 50 60 70 TIME (minutes) Figure 9. wear loss of B , C-SiC e u tectic compared with that of B ,c a nd SiC. R ef. 17. Acknowledgments: This work was supported by the United States Army Research Office, Durham, North Carolina, under Grant No. 8115-MC. REFERENCES 1. G . Tamm an n , Textbook of Metallography, Chemical Cata log Co., New York, 1925, ;p. 182. 2. W . St r au ma n is and N . B r a a ks, Z. Physik. Chem., 29 (1935) 30. 3. B. Ch al mer s, Principles of Solidification, John Wiley and Sons, Inc., New York, 1964, p. 194. 4. F . R. Mo 11 a rd and M . C. Flemings, Trans . Met. Soc. AIME, 239 (1967) 526. 5. W . A. Ti 11 er, Liquid Metals a.nd So!.idification, American Society for Metals, Cleveland, 1958, p. 276. 6. L . M . Hog a ;n, R. W . Kraft and I. D. L e mke y, ~n Advances in Materials Research, Vol. 5, edited by H. Herman, W eley - Interscieince, New York, 1971, p . 83. 7. R. W. Kraft, Trans . Met. Soc. AIME, 221 (1961) 704. 8. J. D . Hunt and K. A . Jackson, Trans. M et. Soc. AIME 236 (1966). 9. D . Vi e ch nick i and F . S chm id, J . Mater. Sci. 4 (1969) 84. 430 V. S . STUBICAN 10. F. L. Kenn a rd, R. C. B r a d t, and V. S. Stub i can, in Reactivity of Solids, Proceedings of the 7th International Symposium on the Reactivity of Solids, Bristol. Edited by J . S. Ainderson, H. W. Roberts and F. S. Stone. Chapman and Hill, London 1972, p. 580. 11. C. Hu 1 s e and J. Batt, Final Report to ONR Project No. 032-516/9-28-72, May 1974. 12. D. Vie ch n i ck i, F. Sch mi d and J. W. Mc Ca u 1 e y, J. Amer. Ceram. Soc. 57 (1974)' 47. 13. F . L. Kenn a rd, R. C. B r a d t, and V. S. Stub i can, J. Amer. Ceram. Soc. 57 (1974) 566. 14. F. L. Kenn.a rd, R. C. Br ad t, and V. S. Stub i can, J. Amer. Ceram. Soc. 57 (1974) 428. 15. W. Minford, R. C. Br a d t and V. S. Stub i c an, unpublished results. 16. F . Schmid and D. Vie ch nick i, J. Mater. Sci. 5 (1970) 470. 17. Ten - Der Hong, M. Sc. Thesis, The Pennsylvania State University, Univer-sity Park, PA (1974). 18. A. M. Alper, R. N. McNally, P . H. Rib be, and R. C. Doman, J. Amer. CeMm. Soc. 45 (1962) 263. SAZETAK Kristalizacija eutektickih mikrostruktura iz oksidnih karbidnih talina V. s. Stubiean Opisan je teorijski aspekt usmjerene ,solidifikacije. Koristene 1su dvije razlicite e~sperimentalne tehnike za pripremanje usmjereno solidificirainih visoko otpornih oksidnih i karbidnih eutektika. Istrazivana je mikrostruktura, kristalografija, me-hanicka svojstva i mehanizam promailaja (mechanism of failure) . Neki od istrazivanih eutektikuma zadrfavaju znatI11U otpornost na povisenim temperaturama, dok veciina slitina i sintrovanih oksidnih tijela pokazuju brz gubitak otpornosti. Eutektik u · sistemu B4C---SiC pokazuje izuzetno visoku i trajnu otpornost. DEPARTMENT OF MATERIAL SCiENCES THE PENNSYLVANIA STATE .UNIVERSITY UNIVERSITY PARK, PA 16802 USA 

Crystallization of Eutectic Microstructures from Oxide and Carbide Melts

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