Analysis of directional solidification of molybdenum(5)silicon(3)-molybdenum silicide aligned lamellar eutectics and the effect of residual stress induced cracking

The Mo-Si phase diagram exhibits a Mo₅Si₃-MoSi ₂ eutectic at the 54% Si composition. Since the terminal phases have comparable melting points and are equidistant from the eutectic composition, there is the possibility of obtaining lamellar microstructures in this system. In addition, if the alloys are directionally solidified, there is the further possibility of obtaining aligned lamellae. In this study, a high temperature (xenon-arc-lamp) optical floating zone furnace is utilized to directionally solidify Mo-Si alloys of the eutectic composition. Growth conditions are systematically varied to investigate their effects on the solidification microstructure. Growth rates and rotation speeds are identified that result in lamellar microstructures.; Eutectic microstructures obtained in previous research revealed simply the so-called script lamellar in which the two phases form a random checkerboard pattern rather than well aligned lamellae. Script Lamellar microstructures are known for their inability to relieve residual thermal stresses from solidification because of the anisotropy of both phases present, therefore, micro-cracked microstructures occur after processing. Well-aligned lamellar microstructures can reduce the residual thermal stresses in the system so as to eliminate micro cracking due to directional solidification processing. The thermal residual stresses on the well aligned lamellar specimens can be estimated by mechanical modeling which shows a reduction in Von Mises stresses in comparison to published stresses on Script lamellar microstructures.