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 (View the MathML source10)LaB6//(1View the MathML source.0)ZrB2. Crack extensions along and perpendicular to the direction of fibers exhibit toughness of 8.7 and 17.8 MPa m1/2, 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 1.5×104–7.2×104 Pa. The rate of weight change is described by kp=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, La2O3·B2O3 and La2O3·3B2O3 being the primary scale constituent with ZrO2 deficiency in the outer scale.