MORPHOLOGY AND INTERFACE STRUCTURE OF MO5SI3 PRECIPITATES IN MOSI2

Two distinctly different morphologies of Mo5Si3 precipitates have been observed in a single crystal MoSi2. Both precipitates are laths elong ated along the [1 (1) over bar 0] direction common to the matrix and t he precipitate but differ in their cross-sectional shape and lattice o rientation. Type I precipitates exhibit a rectangular cross-section, w ith interfaces parallel to low-index planes (110)(m) parallel to(002)( p) and (00 (2) over bar)(m) parallel to(220)(p), while type II precipi tates are parallelograms, with their major interface at 13 degrees to the low-index planes (00 (2) over bar)(m) parallel to(220)(p) (the sub scripts m and p denote the MoSi2 matrix and Mo5Si3 precipitate, respec tively). The orientation relationships corresponding to the two charac teristic morphologies differ by a 1.8 degrees rotation around the lath axis. A periodic array of dislocation loops and associated ledges env eloping the precipitates were revealed by high-resolution electron mic roscopy and selected area electron diffraction. The Burgers vector of these dislocations was determined unambiguously from high-resolution i mages in orthogonal viewing directions. The differences between the tw o characteristic morphologies and their orientation relationships are due to a difference in the stacking sequence of ledges.