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ELECTRON MICROSCOPY STUDIES OF MoSSi3-MoSi2 EUTECTIC COMPOSITES MODIFIED BY ERBIUM ADDITIONS D. P. Mason and D. C. Van Aken, Department of Materials Science and Engineering, The University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109-2125 J. F. Mansfield, Electron Microbeam Analysis Laboratory, The University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109-2143 ABSTRACT Erbium additions to Mo5Si3-MoSi2 eutectic composites have been investigated in an effort to reduce oxygen impurity levels found in the MoSi2 starting powders. Additions of 0.35 atomic percent Er were found to refine the script lamellar microstructure and increase the hardness of the composite as compared with the unmodified material. This increase in hardness was maintained over the temperature range 25°C to 1300'C. Oxygen rich Er particles have been observed in the Er-modified material suggesting that Er has acted as a gettering agent for oxygen. An intermetallic phase Er2Mo3Si4 was also observed in the Er-treated material. The character of this phase was determined by convergent beam electron diffraction (CBED), wavelength dispersive spectroscopy (WDS) and X-ray dispersive spectroscopy (XEDS). The particles of Er2Mo3Si4 were found to be heavily faulted which caused a reduction in the experimental diffraction symmetries. Thus, ambiguities arose during CBED analysis. Therefore, a standard sample of Er2Mo3Si4 was prepared and used to aid in the identification of the faulted particles. INTRODUCTION High oxygen contents have been reported in commercial MoSi2 powders and is present in the form of Si0 2 [l]. This has been suggested as one of the reasons for the poor mechanical properties in MoSi2-based materials[1,2]. Thus, SiO 2 may be the cause of the low strength values reported for Mo5 Si 3 -MoSi 2 eutectic composites[2]. Therefore, alloying with the addition of Er was investigated in an effort to reduce the oxygen concentration in these materials. It was believed that the Er would act as a gettering agent and remove the oxygen from the melt by forming Er203. Furthermore, the presence of rare-earth oxides may contribute to the creep strength of the eutectic microstructure. Thus, arc-melted samples of the eutectic composition were modified by the addition of 0.35 at% Er. Two microstructural changes resulted from the addition of Er. First, the scale of the script microstructure was reduced; and second, the occurrence of pro-eutectic MoSi2 was greatly diminished. (Fig. la,b) The results of the solidification study are reported elsewhere[3]. The addition of Er also had a beneficial effect on the mechanical properties. Hot hardness tests performed on Er-treated and untreated samples showed that the treated material had a higher hardness at all test temperatures (Fig. 2a). This increase in hardness may be due to an increasing constraint placed upon the MoSi2 by the Mo5Si3 as the microstructure becomes more refined. However, in monolithic MoSi2, Maloy et. al. have attributed the increase in hardness to the elimination of Si02 by adding 2 wt% C [1].