Effect of Zr substitution by Ti on growth direction and interface structure of LaB6-TixZr1−xB2 directionally solidified eutectics

Abstract : Boride ceramics were directionally solidified around LaB6-(TixZr1−x)B2 eutectic compositions. The directionally solidified rods were composed of LaB6 rich concentric belts surrounding large eutectic regions with a dense and uniform distribution of (TixZr1−x)B2 fibers of 0.6 μm in diameter inside a LaB6 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 [0 0 0 1]2 for x = 0 and along View the MathML source〈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: [0 0 1]6//[0 0 0 1]2 and View the MathML source(1 1 0)6//(1 1 2¯ 0)2 and View the MathML source(1¯ 1 0)6//(1¯ 1 0 0)2 for any x value. The dominant interfaces were View the MathML source{1 1 0}6//{1 1 2¯ 0}2 for x = 0 and (0 0 1)6//(0 0 0 1)2 for x ≠ 0, the latter being expected to be energetically more favorable, as predicted by a 2D geometrical model.