Microstructural Evaluation During Mechanical Milling and Consolidation of ODS LI 2 -(Al, Cr) 3 Ti
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nanometers and several micrometers. However, due to the nature of the dispersoids used different par-
ticle diameters were found after consolidation and annealing. Although known to be the thermodynamically most stable oxide the initial Y20 3 powder has formed a mixed yttrium-aluminum oxide during manufacturing and, hence, significant particle growth has been observed. While CeO2 shows a similar behavior, the line compound A120 3 remains stable, but the average particle size of this oxide also increases during the heat treatment. First preliminary mechanical tests supported by TEM observations show the significant influence of the grain size on the creep resistance at high temperatures. INTRODUCTION Recently, it has been shown [1-3] that the class of ternary trialuminides of type AI67Ti25X8 (with X = Cr, Fe, Mn, Cu, Ni, Ag) shows excellent compressive room temperature ductility due to its higher crystal symmetry (ordered L1 2-structure) as compared to the binary A13Ti intermetallic compound (tetragonal D022 structure). Besides their low density and very good oxidation resistance a potential application of such intermetallics as high temperature structural materials particularily depends on their resistance against creep deformation. The addition of a small amount of incoherent oxide particles via mechanical alloying or milling techniques can retain extraordinary creep resistance up to about 90% of the melting temperature. The choice of a suitable oxide type is defined by several factors: on one hand the oxide under consideration should have a negligible solubility within the metallic matrix, low diffusivity of the constituent atoms in the matrix, a high negative heat of formation and a high melting point. With respect to these prerequisites, Y20 3 seems to be most promising and has been used for dispersion strengthening of nickel- and iron-base superalloys, e.g. [4]. On the other hand, formation of mixed oxides (especially with Al) has been observed [5,6]. This can lead to significant dispersoid growth during consolidation and annealing of the material and, hence, deteriorates the creep resistance at high temperatures. Consequently, in this paper the effect of different oxide particle types, namely Y20 3, A120 3 and CeO 2 on the microstructure and the high temperature creep behavior of ODS (AI,Cr) 3Ti L12-intermetallic compounds was analyzed. Particular attention was paid to the development of the dispersoid size and distribution after the milling process, the consolidation and the heat treatment.
KK8.34.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
EXPERIMENTAL ODS A167Ti25Cr8 alloys were manufactured by mechanical milling of small crushed pieces of ingots of the respective nominal composition prepared by arc melting under vacuum. The prealloyed powder was mixed before milling with 3 vol.% A120 3, Y20 3 or CeO2. In addition, a pure A167Ti25Cr, alloy and an ODS alloy with 1 vol.% Y20 3 were manufactured in the same way. The designation of the alloys used in the following paragraphs is gi
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