Vacuum Melt Infiltration of 2D Tyranno SA3® Ceramic Matrix Composites with Cr-25(wt.%)Si Intermetallic Alloy

The potential and limitations of vacuum melt infiltrating woven preforms of 2D Tyranno SA3 fibers
with Cr-25(wt.%)Si alloy were investigated. Microstructural observations revealed that the volume
fractions of open voids infiltrated by the molten metal varied between 3.7 and 10.0% irrespective of the magnitudes of the melt infiltration temperatures and hold times, which was attributed in part to substantial volatilization of the silicide charge based on computational thermodynamics. In the absence of experimental data, poor wettability of the melt could not be ruled out unambiguously. Other possible reasons are carefully examined and discarded. Ring microstructures were observed around the fibers mainly at the outer surfaces of the infiltrated preform, where the BN and chemical vapor infiltrated SiC coatings had vaporized in the high vacuum environment. In contrast, they were absent in the interior of the preform, where the coatings were still present with no evidence of molten metal attack of the coatings. Details of thermodynamic calculations are presented to confirm the volatilization of the BN and CVI SiC
coatings thereby resulting in the ring microstructures due to the filling of voids by the molten silicide. It is concluded that successful melt infiltration of Tyranno SA3® with Cr-25%Si can be only achieved under sufficient positive pressure to prevent volatilization of the coatings and the molten silicide.