MICROSTRUCTURE AND HIGH-TEMPERATURE STRENGTH IN MOSI2 NBSI2 DUPLEX SILICIDES/

Pseudo-binary (Mo0.90Nb0.10)Si-2 and (Mo0.85Nb0.15)Si-2 containing dup lex C11(b) and C40 phases were prepared in a slow solidification proce ss using the floating zone method to obtain basic knowledge regarding control of their microstructure. In (Mo0.90Nb0.10)Si-2 a thick band-li ke C40 phase was formed along grain boundaries in the C11(b) matrix, w hile peculiar fine lamellar colonies composed of the C11(b) and C40 ph ases appeared in (Mo0.85Nb0.15)Si-2 accompanied by constituent large C 11(b) and C40 grains. The orientation relationship between the two pha ses at the lamellar boundary was determined to be (0001)(C40)\\(110)(C 11b) and [<11(2)over bar 0>](C40)\\[1(1) over bar 1](C11b). Although t he atomic stacking sequence changed from threefold periodical layers o n (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 th e interface was very small (within 3%), resulting in good thermal stab ility of this microstructure and high strength at high temperatures. T he 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) 1 998 Elsevier Science Limited. All rights reserved.