Creep Deformation Studies in Directionally Solidified MoSi 2 -Mo 5 Si 3 Eutectics
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Creep Deformation Studies in Directionally Solidified MoSi2-Mo5Si3 Eutectics D. P. Mason and D. C. Van Aken, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2125 ABSTRACT The high temperature deformation behavior of directionally solidified (DS) MoSi2-Mo5Si3 eutectics was studied and compared to powder processed MoSi2-Mo5Si3 composites having the same volume fraction of Mo5Si3. Decremental step strain rate tests were performed in the temperature range of 1100-13000 C and at strain rates between 10 -4 to 10-6/s. A considerable increase in the flow stress was observed for the directionally solidified material. At 12000 C and a strain rate of I0-6/s the flow stress of the DS eutectic was 255 MPa as compared to 20 MPa for the powder processed composite. The high temperature strength of the DS eutectic was unaffected by changes in the scale of the lamellar microstructure and these results were modeled using a constitutive relation for power law creep. A stress exponent of 4.5 and an activation energy of 300 kJ/mol was determined for the DS eutectic. Evidence of dislocation glide and climb was observed in the MoSi2 lamellae whereas the dislocation density was small in the Mo5Si3 phase. The improved creep strength of the eutectic is believed to be a result of both the fibrous morphology and a stronger interface structure as compared to the powder processed composite. INTRODUCTION Molybdenum disilicide (MoSi2) has recently received considerable attention as a possible matrix for high temperature structural materials [1,2]. The two main factors limiting its use are poor high temperature creep strength and low temperature ductility. Many researchers are applying composite engineering to address these issues and the viability of many different reinforcements, reinforcement morphologies, and processing paths are being assessed yet, only recently have structure-property relationships been addressed [2,3,4,5]. The authors have shown that the room temperature hardness of MoSi2 and MoSi2-Mo5Si3 eutectics is dependent upon the microcrystalline scale of the MoSi2 and that the hardness follows a Hall-Petch relationship [6]. This scale effect was also observed at elevated temperatures, but the difference in hardness decreased continuously with increasing temperature. In the present study, the relationship between microstructural scale and mechanical behavior is further examined for directionally solidified MoSi2-Mo5Si3 eutectics. EXPERIMENTAL PROCEDURE The materials used in this study were all of eutectic composition, i.e. 44.5 volume percent Mo5Si3 dispersed in a MoSi2 matrix. Directional solidification (DS) was accomplished by a Czochralski method using a tri-arc furnace and a pull rate of 39 mm/hr. Two DS eutectic rods were produced by this method and one alloy contained 0.35 atomic percent erbium. A composite was also produced by hot pressing (HP) elemental molybdenum and Johnson Matthey MoSi2 powders at 1625'C for 2 hours under a pressure of 23 MPa and two billets were produced in
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