Effect of Zr substitution by Ti on growth direction and interface structure of LaB_6-Ti_xZr_(1-x)B_2 directionally solidified eutectics

摘要

Boride ceramics were directionally solidified around LaB_6-(Ti_xZr_(1-x))B_2 eutectic compositions. The directionally solidified rods were composed of LaB_6 rich concentric belts surrounding large eutectic regions with a dense and uniform distribution of (Ti_xZr_(1-x))B_2 fibers of 0.6 μm in diameter inside a LaB_6 matrix. The di- and hexa-boride mole fractions at eutectic points have been more accurately defined for x=0-0.76. Fiber growth directions were modified by the addition of titanium. Diboride (subscript 2) fibers grew along [0001]_2 for x = 0 and along (1 1 0 0)_2 for x ≠ 0. The orientation of the hexaboride (subscript 6) matrix was unchanged with or without titanium addition. The favored crystallographic orientation relationship was: [001]_6//[0 0 01]_2 and (110)_6//(l 1 2 0)_2 and (110)_6//(l 1 0 0)_2 for any x value. The dominant interfaces were {1 1 0}_6// {1 1 2 0}_2 for x = 0 and (001)_6//(0001)_2 for x ≠ 0, the latter being expected to be energetically more favorable, as predicted by a 2D geometrical model.