Creep Deformation Studies in Directionally Solidified MoSi₂-Mo₅Si₃ Eutectics

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 1100-1300 °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 1200 °C and a strain rate of 10-6/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