THE NATURE OF COMPLEX PRECIPITATES ASSOCIATED WITH THE ADDITION OF BORON TO A GAMMA-BASED TITANIUM ALUMINIDE

Ingots of Ti-47 Al-2 Cr-2 Nb (in at%) have been prepared with and with out additions of between 0.1 and 1.0 at% of boron. The boron was added either as TiB2 or AlB12 particles. Samples were examined in the as-ca st condition and after heat treatment. Optical microscopy and scanning and transmission electron microscopy have been used to assess the inf luence of these boron additions on the microstructure. Optical microsc opy showed that long ribbon-like precipitates were evident for all lev els of boron addition but that the grain size was significantly reduce d only by addition of boron over about 0.5 at%B. The lamellar structur e observed in this alloy was found to be stabilised against coarsening at all levels of boron addition. The structure of the ribbons was stu died in detail using TEM and SEM and they were found to consist of a v ery fine two-phase lamellar structure with plate thicknesses down to a few nm. The lamellar plates were identified as TiB2 and B2 with an or ientation relationship of [0001](TiB2)//[001](beta) and [I<$(1)over ba r$>00](TIB2)// [100](beta). The B2 was found to be the continuous phas e.