X Gu, r.. Hand 20Al+2Li2CO3+1502=4LiAl3O8+2CO2.(4) LiAIO2+ 2Al,O3(film)=LiAls Og. (10) As LiyO is never observed it is thought that these Thus LiAls Os could start to react with aluminium reactions occur preferentially to the high-temperature liquid, resulting in a cyclic directed melt oxidation decomposition of the carbonate to the oxide ction sequence similar to that outlined above Lithium aluminates are therefore formed on the surface of the parent metal and aluminium liquid continues to penetrate this lithium aluminate layer by Conclusions capillary action and react with lithium aluminates Byker et al. claimed that LiAlsOs spinel is stable Composite Al, o, /Al ceramics have been obtained over a substantial range of stoichiometry and thus by y directed melt oxidation of pure aluminium exter- ny LiAlO, could be transformed to LiAl,OR on nally doped with a Li source(Li,CO,).Products contact with excess Al have been produced by directed melt oxidation into LiAlO,+4Al+30,+LiA1 Og. (5) both free space and particulate preforms comprising Al,,. As no other dopants were present, LiAls O can also react with liquid Al to produce Li can initiate directed oxidation reactions and is therefore an effective dopant for the production of LiAl OS+ Al= 3Al2O3+ li (6) AlO, from Al by directed melt oxidation With Li the directed melt oxidation process was The resulting Li vaporizes easily to the reaction initiated by the formation of LiAls Og, which aids the front and would reform LiAl Og again producing breakdown of the stable oxide film that would the Li-rich layer seen on the outermost surface of normally form on aluminium. Subsequently the pro- the products cess involves motion of Li from within the growth Li+ 5Al+402FLiAl-O (7) to the reaction front; this can occur because of the This cycle of oxidation re the directed high vapour pressure of Li at the reaction tempera- oxidation Al2O Al body growth although some Li ture. Thus, a Li-containing non-protective lithium may be lost to the environment aluminate layer was formed on the outward surface As the lithium aluminate layer is constantl of product growth. This layer was instrumental veloping the subsequent cyclic reaction sequence being broken down and reformed, the orientation in a similar fashion to the Mg-doped directed of lithium aluminate grains formed at a later time is not related to those formed earlier. High-angle grain oxidation system boundaries are therefore observed in the lithium aluminate layer(Fig. 10). By comparison, after the Acknowledgement initial development of an AlO3 layer there is always Al,O, present in the system. Hence the orientation This work was undertaken whilst one of us(XG) of subsequently grown Al,O: is related to the pre- was in receipt of a Sheffield University Scholarship existing Al,O3 grains, and low-angle grain bound aries are seen between Al,O, grains(Fig. 12) n content may be exhausted so that the remaining References Al liquid may react with nitrogen(present in air 1. Newkirk, M.S., Urquhart, A. w.& Zwicker, H. R, For- which was used as the oxidizing atmosphere)to Ition of lanxide ceran form aIN l(1986)81-9 In the Al2O,Al growth into a preform body. Rescelberg A.S. Observations on the role of Me and Si 2.N fine Al2O, particles were used as filler. Before in the directed oxidation of Al-Mg-Si alloys. J. Mater. Res.,7(l992)265-8 directed melt oxidation reactions started, the fol 3. Aghajanian, M.K., Macmillan, N. H, Kennedy, C. lowing reactions between Al,O3 filler and Li, CO 3 Luxzcz, s.J.& Roy, R, Properties and microstructures would occur. Li,CO3+ Al2O3(filler)=2LiAIO2+ CO2( 8) 4. Xiao, P, Derby, B, Alumina/aluminum composites of alumi and magnesia as a surface dopant. J, Am. Ceram. Soc., 77 (1994)1961-70 Li,CO3+ 5Al2O3(filler)2LiAl5 O8 CO..(9) 5. Gu, x.& Hand, R. J, The production of reinforced luminium/alumina bodies by directed melt oxidation J When aluminium liquid infiltrates into the mixture Eur. Ceram. Soc., 15 (1995)823-31 of dopant and reinforcement, LiAlO, leads to break- 6. Lee, J. D, in Concise Inorganic Chemistry. D. va down of the Al,, protective layer on the alu- Nostrand Company Ltd, London, 1965, pp 69-76 7. Breval, E, Aghajanian, M. K.& Luszcz, S. J minium surface by the following reaction Microstructure and composition of alumina/alumi934 X. Gu, R. J. H&d 20A1 + 2Li,CO, + 150, + 4LiA1508 + 2C02. (4) As Li,O is never observed it is thought that these reactions occur preferentially to the high-temperature decomposition of the carbonate to the oxide. Lithium aluminates are therefore formed on the surface of the parent metal and aluminium liquid continues to penetrate this lithium aluminate layer by capillary action and react with lithium aluminates. Byker et ~1.‘~ claimed that LiAl,08 spine1 is stable over a substantial range of stoichiometry and thus any LiAIOz could be transformed to LiAl,OB on contact with excess Al: LiAlO, + 4Al + 30, +LiAl,O,. (5) LiAl,08 can also react with liquid Al to produce A&O, LiAl,O, + Al + 3Al,O, + Li (6) The resulting Li vaporizes easily to the reaction front and would reform LiAl,O, again producing the Li-rich layer seen on the outermost surface of the products: Li + 5Al + 40, +LiAl,O,. (7) This cycle of oxidation reactions leads to the directed oxidation A120,/Al body growth although some Li may be lost to the environment. As the lithium aluminate layer is constantly being broken down and reformed, the orientation of lithium aluminate grains formed at a later time is not related to those formed earlier. High-angle grain boundaries are therefore observed in the lithium aluminate layer (Fig. 10). By comparison, after the initial development of an Al,O, layer there is always Al,O, present in the system. Hence the orientation of subsequently grown A&O, is related to the pre￾existing Al,O, grains, and low-angle grain bound￾aries are seen between Al,O, grains (Fig. 12). Within the base of growth product the oxygen content may be exhausted so that the remaining Al liquid may react with nitrogen (present in air which was used as the oxidizing atmosphere) to form AIN.S In the Al,O,/Al growth into a preform body, fine A&O, particles were used as filler. Before the directed melt oxidation reactions started, the fol￾lowing reactions between A&O3 filler and Li,CO, would occur: L&CO, + A&O, (filler) + 2LiA102 + CO* (8) and Li,C03 + 5Al,O, (filler) +2LiA150, + COZ. (9) When aluminium liquid infiltrates into the mixture of dopant and reinforcement, LiAIOz leads to break￾down of the A&O, protective layer on the alu￾minium surface by the following reaction: LiA102 + 2Al,O, (film) + LiAl,O,. ’ (10) Thus LiAl,Os could start to react with aluminium liquid, resulting in a cyclic directed melt oxidation reaction sequence similar to that outlined above. Conclusions Composite Al,OJAl ceramics have been obtained by directed melt oxidation of pure aluminium exter￾nally doped with a Li source (L&CO,). Products have been produced by directed melt oxidation into both free space and particulate preforms comprising pure a-A&O,. As no other dopants were present, Li can initiate directed oxidation reactions and is therefore an effective dopant for the production of A120, from Al by directed melt oxidation. With Li the directed melt oxidation process was initiated by the formation of LiAl,O,, which aids the breakdown of the stable oxide film that would normally form on aluminium. Subsequently the pro￾cess involves motion of Li from within the growth to the reaction front; this can occur because of the high vapour pressure of Li at the reaction tempera￾ture. Thus, a Li-containing non-protective lithium aluminate layer was formed on the outward surface of product growth. This layer was instrumental in developing the subsequent cyclic reaction sequence in a similar fashion to the Mg-doped directed oxidation system. Acknowledgement This work was undertaken whilst one of us (X.G.) was in receipt of a Sheffield University Scholarship. References 1. 2. 3. 4. 5. 6. I. Newkirk, M. S., Urquhart, A. W. & Zwicker, H. R., For￾mation of lanxide ceramic composite materials. J. Murer. Rex, 1 (1986) 81-9. Nagelberg, A. S., Observations on the role of Mg and Si in the directed oxidation of Al-Mg-Si alloys. J. Muter. Res., 7 (1992) 265-8. Aghajanian, M. K., Macmillan, N. H., Kennedy, C. R., Luxzcz, S. J. & Roy, R., Properties and microstructures of lanxide Al,O,-AI ceramic composite materials. J. Mater. Sci., 24 (1989) 658-70. Xiao, P. & Derby, B., Alumina/aluminum composites formed by the directed oxidation of aluminum using magnesia as a surface dopant. .I Am. Ceram. Sot., 77 (1994) 1961-70. Gu, X. & Hand, R. J., The production of reinforced aluminiumialumina bodies by directed melt oxidation. J. Eur. Ceram. Sot., 15 (1995) 823-3 1. Lee, J. D., in Concise Inorganic Chemistry. D. Van Nostrand Company Ltd, London, 1965, pp. 69-76. Breval, E., Aghajanian, M. K. & Luszcz, S. J., Microstructure and composition of alumina/aluminum