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

In situ composite of LaB6-ZrB2 eutectic prepared by floating zone melting i s an interesting candidate for the manufacture of turbine blades because of its excellent mechanical properties. Studies of a composite prepared by fl oating zone melting showed that the average diameter of the ZrB2 fibers was similar to0.2-1.2 mum, with fiber lengths up to 100 mum LaB6 was the first phase to nucleate when eutectic growth occurred, and ZrB2 shows non-facete d growth. In an ingot solidified with planar growth, the orientation relati onship of the phases was: growth direction [001](LaB6)//[00.1](ZrB2), inter facial plane ((1) over bar 10)(LaB6)//(1 (2) over bar .0 )(ZrB2). Crack ext ensions along and perpendicular to the direction of fibers exhibit toughnes s 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 9 12-1223 degreesC and in the pressure range 1.5 x 10(4)-7.2 x 10(4) Pa. The rate of weight change is described by k(p)=9.71x10(-3) exp(-31000/RT) mg(2) cm(-4) min(-1) in the range 912-1094 degreesC. Above 1094 degreesC, rapid oxidation kinetics were observed, La2O3.B2O3 and La2O3. 3B(2)O(3) being the primary scale constituent with ZrO2 deficiency in the outer scale. (C) 200 1 Elsevier Science Ltd. All rights reserved.