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The influence of yttrium on the sintering of Al2O3
Journal of the Less-Common Metals, 70 (1980) 0 Elsevier Sequoia S.A., Lausanne -Printed
115 - 117 in the Netherlands
115
Letter
The influence of yttrium on the sintering of AlsO,
D. DELAUNAY,
A. M. HUNT2
and P. LACOMBE
Loboratoire de ~~t~il~rgie ~~y~iq~e, associ& au CNRS no 177, Uniue~it~ Pnr~-~~d, 91405 Orsay Ckdex (France) (Received
July 30,1979)
The influence of minor additions of active elements such as yttrium on the adherence of oxide scales developed on heat-resisting alloys has been extensively studied [l - ‘7J . Many assumptions have been proposed to explain the mechanisms of their beneficial influence, and it has been suggested that yttrium should have an effect on oxide defects and enhance the plasticity of the oxide scale [6 - 81. Previous studies of the oxidation of AlaOs former alloys such as Fe-Ni-Cr-Al-Y, where yttrium was either added at low concentration in cast alloys or implanted, have shown that YsOs is incorporated in the inner AlzOs layer [9, lo]. Thus in order to investigate the mechanisms, the influence of minor additions of YzOs on the sintering kinetics of AlzOs was studied. Sintering mech~isms are related to diffusion phenomena and we expected to obtain in this way information about the effect of YzOs on oxide defects. A powder mixture of 1 at,% Y,Os in AlaOs was prepared (the characteristics of the initial powders are summarized in Table 1) and O.l%, 0.01% and 0.002% mixtures were obtained by successive dilutions. Each mixture was dried for 4 h at 150 ?Z and was mixed for 2 h. Samples of 25 mm X 5 mm X 5 mm were uniaxially compacted under 160 kgf cmm2. This low pressure is used to avoid cont~ination of the mixtures by binder. Dilatome~ic measurements were carried out for 4 h at 1300 “c in room air. Compacts reached this temperature in 4 h. The good reproducibility of the densification curves indicated that the powder mixtures were homogeneous. Figure 1 shows the shrinkage curves obtained on mixtures with increasing contents of Y,Os particles of constant size (1000 a) that was greater than the Al,Os particle size (50 A), It is clear that the sintering kinetics of Al,Os are increased by small additions of Y,Os. For an addition of 1% YzOs the kinetics are slower than for pure Al,Os. The influence of the Y,Os powder size on the sintering of Al,Os was investigated with an addition of 1% Y,Os (Fig. 2). The sintering kinetics increase with decreasing size of the YsOs particles and the optimum rate is reached for approximately equal sizes of the YsOs and Al,Os particles.
116 TABLE 1 Characteristics of the powdersa
Powder
Average grain size (4
99.995b
50
A12o3
1000
y2°3
Purity (wt.%)
99.5
aProvided by Mme Lejus (Laboratoire de Materiaux de Haute Temperature et Desordres Structuraux. ENSC. Paris) and MM. Revcolevschi and Berton (Laboratohe de Chimie Appliqu&, UPS, O&y). ’ bPurified by DJEVA.
‘C
I300 lOO0
500
0 123456
7
0 tlwrl
Fig. 1. Influence of Y2O3 additions (0.002 - 1 at.%, powder size 1000 A) on the densification rate of Al203 (powder size, approximately 50 A) at 1300 “C. The compacting pressure was approximately 160 kgf cmd2. Fig. 2. Influence of the powder size of Y3O3 on the densification rate of Ai203-lat.% Y3O3 compacts at 1300 “C in comparison with the pure AI303 densification curve (alumina powder size, approximately 50 A; compacting pressure, 160 kgf cme2).
A comparison of Figs. 1 and 2 leads to the conclusion that Y3O3 additions in Al,03 play a beneficial role on the sintering rate, at least for Y3O3 contents up to 1% and for a comparable size of the two powders. These results are only valid for the first stages of sintering at 1300 “c and with a low initial pressure of the mixtures. It is known that sintering is controlled by diffusion processes. Consequently these results show that minor additions of Y3O3 increase the diffusion rate in Al303 and hence the mobility of the vacancies. This increase in mobility should improve the plasticity of the AlzO3. These proposals agree with recent investigations which show that an addition of 90 ppm Y in an Fe-Ni43-Cr,-Al, alloy decreases the stresses in the oxide scale developed by oxidation at 1000 “c [ll] .
117 1 J. K. Tien and F. S. Pettit, Metall. Trans., 3 (1972) 1587. 2 C. S. Giggins, B. H. Kesr, F. S. Pettit and J. K. Tien, Metall. Trans., 5 (1974) 1685. C. S. Giggins and F. S. Pettit, Pratt and Whitney Aircraft Rep. ARL751 6234, 1975. 3 J. S. Stringer, E. M. Allam and D. P. Whittle, Thin Solid Films, 45 (1977) 377. 4 E. J. Felten, Trans. Metall. Sot. AZME, 245 (1969) 1349. 5 M. S. Seltzer, B. A. Wilcox and J. Stringer, Metall. Trans., 3 (1972) 2391. 6 H. T. Michels, Metall. Trans. A, 7 (1976) 379. 7 H. T. Micheis, Metali. Trans. A, 9 (1978) 873. 8 C. S. Pettit, AGARD Conf. Proc., 120 (1973) 155. 9 J. C. Pivin, D. Delaunay, C. Roques-Carmes, A. M. Hunts and P. Lacombe, Corros. Sci., to be published. 10 J. C. Pivin, C. RoquesCarmes, J. Chaumont and H. Bemas, submitted to Corros. Sci. 11 D. Delaunay, A. M. Huntz and P. Lacombe, submitted to Corros. Sci.
115 - 117 in the Netherlands
115
Letter
The influence of yttrium on the sintering of AlsO,
D. DELAUNAY,
A. M. HUNT2
and P. LACOMBE
Loboratoire de ~~t~il~rgie ~~y~iq~e, associ& au CNRS no 177, Uniue~it~ Pnr~-~~d, 91405 Orsay Ckdex (France) (Received
July 30,1979)
The influence of minor additions of active elements such as yttrium on the adherence of oxide scales developed on heat-resisting alloys has been extensively studied [l - ‘7J . Many assumptions have been proposed to explain the mechanisms of their beneficial influence, and it has been suggested that yttrium should have an effect on oxide defects and enhance the plasticity of the oxide scale [6 - 81. Previous studies of the oxidation of AlaOs former alloys such as Fe-Ni-Cr-Al-Y, where yttrium was either added at low concentration in cast alloys or implanted, have shown that YsOs is incorporated in the inner AlzOs layer [9, lo]. Thus in order to investigate the mechanisms, the influence of minor additions of YzOs on the sintering kinetics of AlzOs was studied. Sintering mech~isms are related to diffusion phenomena and we expected to obtain in this way information about the effect of YzOs on oxide defects. A powder mixture of 1 at,% Y,Os in AlaOs was prepared (the characteristics of the initial powders are summarized in Table 1) and O.l%, 0.01% and 0.002% mixtures were obtained by successive dilutions. Each mixture was dried for 4 h at 150 ?Z and was mixed for 2 h. Samples of 25 mm X 5 mm X 5 mm were uniaxially compacted under 160 kgf cmm2. This low pressure is used to avoid cont~ination of the mixtures by binder. Dilatome~ic measurements were carried out for 4 h at 1300 “c in room air. Compacts reached this temperature in 4 h. The good reproducibility of the densification curves indicated that the powder mixtures were homogeneous. Figure 1 shows the shrinkage curves obtained on mixtures with increasing contents of Y,Os particles of constant size (1000 a) that was greater than the Al,Os particle size (50 A), It is clear that the sintering kinetics of Al,Os are increased by small additions of Y,Os. For an addition of 1% YzOs the kinetics are slower than for pure Al,Os. The influence of the Y,Os powder size on the sintering of Al,Os was investigated with an addition of 1% Y,Os (Fig. 2). The sintering kinetics increase with decreasing size of the YsOs particles and the optimum rate is reached for approximately equal sizes of the YsOs and Al,Os particles.
116 TABLE 1 Characteristics of the powdersa
Powder
Average grain size (4
99.995b
50
A12o3
1000
y2°3
Purity (wt.%)
99.5
aProvided by Mme Lejus (Laboratoire de Materiaux de Haute Temperature et Desordres Structuraux. ENSC. Paris) and MM. Revcolevschi and Berton (Laboratohe de Chimie Appliqu&, UPS, O&y). ’ bPurified by DJEVA.
‘C
I300 lOO0
500
0 123456
7
0 tlwrl
Fig. 1. Influence of Y2O3 additions (0.002 - 1 at.%, powder size 1000 A) on the densification rate of Al203 (powder size, approximately 50 A) at 1300 “C. The compacting pressure was approximately 160 kgf cmd2. Fig. 2. Influence of the powder size of Y3O3 on the densification rate of Ai203-lat.% Y3O3 compacts at 1300 “C in comparison with the pure AI303 densification curve (alumina powder size, approximately 50 A; compacting pressure, 160 kgf cme2).
A comparison of Figs. 1 and 2 leads to the conclusion that Y3O3 additions in Al,03 play a beneficial role on the sintering rate, at least for Y3O3 contents up to 1% and for a comparable size of the two powders. These results are only valid for the first stages of sintering at 1300 “c and with a low initial pressure of the mixtures. It is known that sintering is controlled by diffusion processes. Consequently these results show that minor additions of Y3O3 increase the diffusion rate in Al303 and hence the mobility of the vacancies. This increase in mobility should improve the plasticity of the AlzO3. These proposals agree with recent investigations which show that an addition of 90 ppm Y in an Fe-Ni43-Cr,-Al, alloy decreases the stresses in the oxide scale developed by oxidation at 1000 “c [ll] .
117 1 J. K. Tien and F. S. Pettit, Metall. Trans., 3 (1972) 1587. 2 C. S. Giggins, B. H. Kesr, F. S. Pettit and J. K. Tien, Metall. Trans., 5 (1974) 1685. C. S. Giggins and F. S. Pettit, Pratt and Whitney Aircraft Rep. ARL751 6234, 1975. 3 J. S. Stringer, E. M. Allam and D. P. Whittle, Thin Solid Films, 45 (1977) 377. 4 E. J. Felten, Trans. Metall. Sot. AZME, 245 (1969) 1349. 5 M. S. Seltzer, B. A. Wilcox and J. Stringer, Metall. Trans., 3 (1972) 2391. 6 H. T. Michels, Metall. Trans. A, 7 (1976) 379. 7 H. T. Micheis, Metali. Trans. A, 9 (1978) 873. 8 C. S. Pettit, AGARD Conf. Proc., 120 (1973) 155. 9 J. C. Pivin, D. Delaunay, C. Roques-Carmes, A. M. Hunts and P. Lacombe, Corros. Sci., to be published. 10 J. C. Pivin, C. RoquesCarmes, J. Chaumont and H. Bemas, submitted to Corros. Sci. 11 D. Delaunay, A. M. Huntz and P. Lacombe, submitted to Corros. Sci.