Microstructure, mechanical performance and oxidation mechanism of boride in situ composites

In situ composite of LaB₆-ZrB₂ eutectic prepared by floating zone melting is an interesting candidate for the manufacture of turbine blades because of its excellent mechanical properties. Studies of a composite prepared by floating zone melting showed that the average diameter of the ZrB₂ fibers was ∼0.2-1.2 μm, with fiber lengths up to 100 μm. LaB₆ was the first phase to nucleate when eutectic growth occurred, and ZrB₂ shows non-faceted growth. In an ingot solidified with planar growth, the orientation relationship of the phases was: Growth direction [001]_LaB6//[00.1]_ZrB2, interfacial plane (1̄10)_LaB6//(12̄.0)_ZrB2. Crack extensions along and perpendicular to the direction of fibers exhibit toughness of 8.7 and 17.8 MPa m^1/2, respectively. The oxidation behavior of LaB₆-ZrB₂ eutectic in situ composites was studied in the temperature range of 912-1223°C and in the pressure range 1.5 ×10⁴-7.2 ×10⁴Pa. The rate of weight change is described by k_p = 9.71 ×10^-3 exp(-31000/RT) mg² cm^-4 min^-1 in the range 912-1094°C. Above 1094°C, rapid oxidation kinetics were observed, La₂O₃B₂O₃ and La₂O₃B₂O₃ being the primary scale constituent with ZrO₂ deficiency in the outer scale.