ON THE MICROSTRUCTURE AND CRYSTALLOGRAPHY OF DIRECTIONALLY SOLIDIFIEDMOSI2-MO5SI3 EUTECTICS

Directionally solidified MoSi2-Mo5Si3 eutectic rods were produced by t he Czochralski method. A script lamellar microstructure was produced a nd an inverse square root dependence upon the pull rate (39-210 mm/h) was observed for the lamellar spacing. The eutectic rods were textured with [001] of Mo5Si3 and [110] of MoSi2 parallel to the rod axis and an orientation relationship consisting of [110]MoSi2 parallel to[110]M o5Si3 and (110)MoSi2 parallel to(002)Mo5Si3 was present in all of the samples. The eutectic grows with a lamellar plate morphology inclined at 15 degrees relative to the rod axis and four growth variants are po ssible. A script lamellar microstructure, as viewed in the transverse plane, was produced by periodic branching of the Mo5Si3 lamellae and t his branching was not associated with any change in crystallography. A ddition of 0.35 at.% erbium produced a more fibrous microstructure and an erbium-rich compound, Er2Mo3Si4 with space group 2(1)/c, formed co ngruently with the Mo5Si3-MoSi2 during solidification. A ledge-terrace growth mechanism was proposed to explain the observed microstructures .