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

In situ composite of LaB 6 -ZrB 2 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 ZrB 2 fibers was ∼0.2-1.2 μm. with fiber lengths up to 100 μm. LaB 6 was the first phase to nucleate when eutectic growth occurred, and ZrB 2 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 (110) LaB6 //(12.0) 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 LaB 6 -ZrB 2 eutectic in situ composites was studied in the temperature range of 912-1223°C and in the pressure range 1.5× 10 4 -7.2× 10 4 Pa. The rate of weight change is described by k p = 9.71 × 10 -3 exp(-31000/RT) mg 2 cm -4 min -1 in the range 912-1094°C. Above 1094°C, rapid oxidation kinetics were observed, La 2 O 3 .B 2 O 3 and La 2 O 3 .3B 2 O 3 being the primary scale constituent with ZrO 2 deficiency in the outer scale.