Microstructure and high-temperature strength in MoSi2/NbSi2 duplex silicides

Pseudo-binary (Mo0.90Nb0.10)Si-2 and (Mo0.85Nb0.15)Si-2 containing duplex C11(b) and C40 phases were prepared in a slow solidification process using the floating zone method to obtain basic knowledge regarding control of their microstructure. In (Mo0.90Nb0.10)Si-2 a thick band-like C40 phase was formed along grain boundaries in the C11(b) matrix, while peculiar fine lamellar colonies composed of the C11(b) and C40 phases appeared in (Mo0.85Nb0.15)Si-2 accompanied by constituent large C11(b) and C40 grains. The orientation relationship between the two phases at the lamellar boundary was determined to be (0001)(C40)\\(110)(C11b) and [<11(2)over bar 0>](C40)\\[1(1) over bar 1](C11b). Although the atomic stacking sequence changed from threefold periodical layers on (0001) planes in the C40 phase to two-fold layers on (110) planes in the C11(b) phase at the lamellar boundary, the lattice mismatch at the interface was very small (within 3%), resulting in good thermal stability of this microstructure and high strength at high temperatures. The phase transformation must proceed by the motion of all four 1/6[1(1) over bar 2]-type partial dislocations on two layers of every three (0001) layers in the C40 phase to avoid large lattice distortion. (C) 1998 Elsevier Science Limited. All rights reserved.