STRUCTURE EVOLUTION OF THE AI203--ZrO2 ALLOYS UNDER NONEQUILIBR!UM

CONDITIONS

N. B. Zhekhanova, V. E. Gladkov, and A. A. Fotiev

UDC 666.672.11+666~672.52:541.12.017

AI203--ZrO2 alloys are widely used in the electronic, abrasive and refractory industries~ The alloys containing approximately 75%* A1203 and 25% ZrO2 (zirconic electrocorundum) are used as an abrasive material. The physical, mechanical, and operational characteristics of the alloys are determined by the dimensions of the structural constituents and their quantitative ratio which, in turn, depend on the cooling conditions of the melt during its solidification.

The Al2Os--ZrO2 system belongs to the eutectic-type systems [1-6]. The coordinates of the eutectic points (C E and T E) and the regions of existence of the solid solutions have not been established accurately in the phase diagram of the Al2Os--Zr02 system (1983~ <~. T E <~.i 2193~ 32% Zr02 .<~ C E < 55% Zr02). Primary corundum (~-Al2Os) crystals (grains) and the eutectic form the main structural constituents of the 75% Al20s + 25% ZrO2 hypoeutectic type alloys.

Eutectic solidification (crystallization) has been a subject of extensive investigation, particularly in view of the promising applications of the eutectic alloys as composite ma- terials. The studies on the metallic systems are concerned with the conventional aspects such as the identification of the phase leading to eutectic solidification, the structural anomalies, and the classification of the eutectic structures [7, 8]. It was established that depending on the degree of supercooling of the melt during its solidification, different types of eutectic structures can emerge from the same eutectic [8]. Consequently, by varying the cooling conditions of the melt, it is possible to obtain structurally different materials having identical chemical composition. However, there have been no systematic studies on the structural constituents (primary crystals and eutectic) of the AI~O3--Zr02 alloys and their general structure. Moreover, the regularities in the structure evolution of the alloys under nonequilibrium conditions have not been established.

*Here and elsewhere, chemical composition is expressed in terms of weight content.

12 J 9

Fig. i. Schematic of the structural fea- tures of the ingot: i) mold wall; 2) fine- grained chill zone; 3) zone of columnar grains; 4) zone of equiaxial (polyhedral) grains; 5) pores.

Institute of Chemistry, Ural Science Center, Academy of Sciences of the USSR. lated from Ogneupory, No. 5, pp. 20-22, May, 1986.

Trans-

268 0034-3102/86/0506-0268512.50 �9 1987 Plenum Publishing Corporation

TABLE I. Effect of the Cooling Conditions of the Melt on the Macro- structure of the AI=O3--ZrO= Alloys

Method of cooling the melt

rome/rap, kg/~

1:1 2:1

SMe/mp, ma/kg

0,005 0,009 0,155

Proportion of themacrostructural zones,

fine-grained zone of zone of chill zone . columnar equiaxial

grains gralns

I 30- -50 50--70 2 0- -10 88--98

20=-50 10--70 20--60

*Volumetric contents are indicated.

TABLE 2. Composition of the 75% AI=03 + 25% ZrO= Alloys Solidified under Nonequilib- rium Conditions

Method of cooling the melt

Struct. combos, of alloy, % pr'zmary a-Al~O~ crystals

41,8 48,4 59,6

eutectic

58,2 51,6 40,4

We studied the structure of the AI=03--Zr0= alloys as a function of the cooling conditions of the melt,

EXPERIMENTAL

In view of the high synthesis temperatures of the AI=O3--ZrO= alloys (>2100~ and the non- availability (absence) of reliable recording devices, it is not possible to determine the so- lidification rate and the degree of supercooling of the melt. In order to reveal the regular- ities of the structure evolution of the AI=O3--Zr02 alloys, the cooling conditions of the melt were varied. The ratio of the masses of the melt mp and the mold mMe , the volume of the so- lidifying ingot, and the contact area SMe of the melt with the cooled surface were the experi- mental variables. These parameters were varied under the following cooling conditions of the melt: i) in a metallic mold having a volume of 0.25 m3; 2) in a 0.25 m 3 mold by charging me- tallic balls into the melt [9]; and 3) in 'roll-molds' whose design and technological charac- teristics were given elsewhere [10-12].

Melting was carried out in an electric arc furnace. The G-OO grade (GOST 6912-74) alu- mina and the TsrO grade (GOST 21907-76) zirconium dioxide were used as the raw materials. The melt was poured off from the furnace and was crystallized during the cooling process.

The structure of the alloys was studied in a microscope under reflected light. Polished metallographic sections of different zones of the ingot were the objects of study. The linear metallographic method was used for determining the structural composition (the content of the primary crystals and the eutectic) of the 75% AI=03 + 25% Zr02 alloys. The absolute error in measurements did not exceed 3%.

The results of our analysis showed that the structure of the hypoeutectic AI=O3--ZrO= al- loys is characterized by the presence of three macrostructural zones (see Fig. i) that are similar to those observed in solidified metals and differ from each other by the extent, shape, size and disposition of the structural constituents (Table i).

The effect of the method of cooling the melt becomes apparent only on the formation of the zones of columnar and equiaxial grains. The fine-grained chill zone (shell) is an invar- iably formed structural element whose thickness and microstructure do not depend on the cool- ing conditions of the melt.

The structural composition of the 75% Al=Os + 25% ZrOz alloys depends to a significant extent On the conditions under which the melt is cooled. In all the cases, the structure of the experimental specimens shows disparity between the quantity of the primary a-AlzOs crys- tals calculated from the equilibrium phase diagram [5] (36.2%* primary a-A1203 crystals and 63.8% eutectic) and the experimentally determined quantity (Table 2).

*Here and elsewhere, structural composition is expressed in terms of volumetric content.

269

Microstructural analysis of the AI203--Zr02 alloys showed that under identical cooling conditions of the melt, the structure evolution of the hypoeutectic and the hypereutectic alloys is different. In contrast to the solidification of the hypereutectic alloys contain- ing 62.9% ZrO= whose fine-grained chill zone shows complete absence of the primary Zr02 crys- tals in its microstructure, the separation of the primary a-A1203 crystals is not suppressed during the solidification process of the 75% Al20s + 25% Zr02 alloys and in this case, the formation of a quasieutectic structure is not observed.

In the alloys, Zr02 crystals are rounded and the primary ~-A1203 crystals have faceted growth. The Al20s--Zr02 eutectic solidifies in the form of colonies duplicating the structure and the external shape of the crystals of ~-Al20s that form its main (base) and leading phase. Under identical conditions of solidification, in relation to the Al20s--Zr02 alloys of other compositions, the dimensions of the primary a-A1203 crystals and the eutectic colonies of the 75% AI20~ + 25% Zr02 alloys are comparable with each other and are minimum. This en- sures optimum physical and mechanical properties of the alloys (for example, the least de- struction of the grinding grains).

The eutectic colonies are not mechanical mixtures; they are represented by two-phase formations (bicrystals) whose matrix consists of a harder and more brittle phase (~-A1203). Degeneration of the eutectic structure was observed during the solidification of the melts whose composition significantly differs from the eutectic composition (less than 10% Zr02); in this case, the structure of the alloy in the plane section of the polished metallographic specimen is represented by partially faceted ~-A1203 crystals surrounded by a Zr02 envelope. The microstructure of the alloys of hypoeutectic compositions (>~I0% Zr02) has a similar ap- pearance in the fine-grained chill zone.

One of the specific features of the solidification of the Al20s--Zr02 alloys is that the macro and microstructures of the hypoeutectic alloys and the macro and micromorphology of the eutectic are determined by the conditions of formation of a-Al20s crystals that exhibit strong anisotropy of properties. A radical change in the structure of the hypoeutectic alloys is ob- tained only when the paired and branched growth of the crystals constituting the eutectic is disturbed.

The results of microstructural analysis and the data on the quantitative relationship (ratio) between the structural constituents of the alloys (see Table 2) permit one to consider that the formation of the eutectic and the structure of the alloys as a whole are determined by the growth rate of the ~-A1203 crystals. The obtained variation of the quantity of the primary ~-Al20s crystals and the eutectic as a function of the cooling conditions of the melt can be treated as a shift of the zone of combined growth (the temperature-concentration region of the quasieutectic formation) [7] towards higher concentration of Zr02. In this case, the eutectic solidifies with an increased content of the leading phase right up to the degenera- tion stage of the eutectic structure as compared to the content corresponding to the eutectic equilibrium.

Thus, in the Al20s--Zr02 system having two high-entropy (ASmp.AI203 > ASmp. Zr02 > - - 1 o - - 1 > 17 J.mole �9 K ; ASmp.AI203/ASmp.Zr02 1.5, where ASmp. is the entropy change during melt-

ing) refractory phases, the zone of combined growth is shifted towards the Zr02 side, i.e., towards the leading phase of the eutectic that has higher melting point and crystallizes from the melt with rounded interfaces. This permits one to explain the discrepancy in determining the composition of the eutectic [1-6] which was experimentally established on the basis of the data of microstructural analysis regarding the presence of primary crystals in the structure.

CONCLUSIONS

Structure evolution of the hypoeutectic 75% A1203 + 25% Zr02 alloys under nonequillbrium conditions takes place with excess primary a-A1203 crystals leading to the displacement of the zone of combined growth towards the region of higher concentrations of Zr02.

LITERATURE CITED

i. H. Wartenherg, H. Linde, and R. Jung, Z. Anorg. Allg. Chem., 176, No. 3, 349-362 (1928). 2. H. Suzuki, S. Kimura, H. Yamada, et al., J. Ceram. Assoc. Jpn., 69, No. 782/2, 52-59

(1961). 3. R. F. Geller, R. J. Vavorsky, B. L. Sterman, et al., J. Res. Nat. Bur. Standards, 36,

No. 3, 277-312 (1946).

270

4. F. Schmid and D. Viechniski, J. Mater. Sci., ~, No. 3, 470-473 (1970). 5. G. Cevales, Ber. Dtsch. Keram. Ges., 4_55, No. 5, 216-219 (1968). 6. A. S. Berezhnoi, Multicomponent Oxide Systems [in Russian], Naukova Dumka, Kiev (1970). 7. Yu. N. Taran and V. I. Mazur, Structure of Eutectic Alloys [in Russian], Metallurgiya,

Moscow (1978). 8. A. A. Bochvar, Investigation of the Mechanism and the Kinetics of Crystallization of the

Eutectic-Type Alloys [in Russian], ONTI, Moscow--Leningrad (1935). 9. Inventor's Certificate No. 547387; B. V. Arkhangel'skii, N. B. Zhekhanova, V. F. Sokolov,

et al., Byull. Izobret., No. 7, 57 (1977). i0. A. M. Breisler, B. V. Arkhangel'skii, N. B. Zhekhanova, et al., Abrasives: Nauch.

Tekh. Ref. Sb. NIImash, NIImash, Moscow, No. i0, 3-4 (1974). ii. A. S. Zubov, V. E. Gladkov, A. A. Fotiev, et al., Izv. Akad. Nauk SSSR, Neorg. Mater.,

2_~1, No. 3, 435-438 (1985). 12. A. S. Zubov, "Investigation and development of the production process of a corundum-based

abrasive material for mechanical grinding," Author's Abstract of Ph.D. Thesis, Chelya- binsk (1982).

271