Physical and Mechanical Properties of MoSi 2 -Er 2 Mo 3 Si 4 Composites
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PHYSICAL AND MECHANICAL PROPERTIES OF MoSi2-Er2Mo3Si4 COMPOSITES D. Keith Patrick and David C. Van Aken, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136 ABSTRACT A study has been conducted to determine the feasibility of using Er2Mo3Si4 as a reinforcement phase to improve the high temperature strength of MoSi2. The melting temperature of Er2Mo3Si4 was determined to be 1930 ± 20°C whereas the eutectic of Er2Mo3Si4-39 vol% MoSi2 melted at 1790 ± 10*C. Elevated temperature microhardness tests show that Er2Mo3Si4 has significantly higher hardness than MoSi2 above 1000'C, e.g. approximately 5.8 GPa versus 1.5 GPa at 1300°C, respectively. A MoSi2/Er2Mo3Si4/20p composite was produced by ball milling and hot pressing arc-melted MoSi2-20 vol% Er2Mo3Si4 materials. At 1300°C the MoSi2/Er2Mo3Si4/20p composite and a directionally solidified Er2Mo3Si4-MoSi2 eutectic exhibited hardnesses of 2.4 GPa and 4 GPa, respectively. Preliminary results from compressive decremental step strain rate tests at 1300'C indicate that the creep strength of the MoSi2/Er2Mo3Si4/20p composite is comparable to that of a MoSi2/SiC/20w composite. The creep stress exponent was determined to be 3.3 at 1200'C and 3.7 at 13000 C. INTRODUCTION The high temperature creep strength of MoSi2 is adversely affected by the presence of amorphous Si02 [1]. This oxide may be present in the initial starting material or may develop as part of the processing of the MoSi2 powders, e.g. ball milling. It has been shown that the Si02 may be eliminated by adding carbon prior to hot pressing [2], or by carefully processing elemental powders in an inert environment [3]. It has also been suggested that the improved properties of MoSi2/SiC composites may in part be a result of the reduction of Si02 by free carbon associated with the SiC reinforcement [4]. Addition of rare earth metals during ball milling has not been as successful as the addition of carbon in reducing the SiO2 [5]. The purpose of this study was to investigate some of the physical properties of Er2Mo3Si4, explore processing paths to develop MoSi2/Er2Mo3Si4 particulate composites, and determine the high temperature creep properties of these composites. It has been reported that Er2Mo3Si4 formed naturally in a MoSi2-Mo5Si3 eutectic as a result of adding erbium (Er) to deoxidize the eutectic liquid [6]. However, information available about Er2Mo3Si4 seems to be limited to crystallographic or x-ray diffraction data [6,7]. Many basic physical and mechanical properties of Er2Mo3Si4 have not as yet been reported. EXPERIMENTAL Samples of MoSi2 and Er2Mo3Si4 were prepared by arc-melting high purity elemental Mo bar stock (99.9%), Si single crystal stock (99.999%), and Er chips (99.9%) in an argon atmosphere. These buttons were then broken up, mixed in the desired proportions, and remelted to produce the MoSi2-20 vol% Er2Mo3Si4 alloy and the Er2Mo3Si4-39 vol% MoSi2 eutectic used in this study. The eutectic arc-melted buttons were then directionally solidified by the Czoc
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