HIGH-TEMPERATURE PRECIPITATION HARDENING IN A RAPIDLY QUENCHED AL-TI-NI ALLOY .2. PRECIPITATE CHARACTERIZATION

Changes in the morphology and structure of second phase particles, for med during isothermal ageing of rapidly quenched Al-6Ti-1.5Ni (wt.%) a lloy at temperatures in the range 300-500 degrees C, have been examine d using a combination of conventional amplitude contrast imaging and e lectron microdiffraction. The precipitation-hardening response is init ially attributable to a fine-scale distribution of coherent particles of metastable L1(2) phase and the orientation relationship between the metastable precipitate and alpha-aluminium matrix is such that (001)( L12)//(001)(alpha), [100](L12)//[100](alpha). During isothermal ageing , the metastable precipitates evolve with a transitional, three-dimens ional cross-like morphology defined by three orthogonal sets of ellips oidal elements forming in pairs. The precipitates comprise variants of a series of one-dimensional tetragonal superlattices that vary in str ucture from L1(2) to body-centred tetragonal D0(23); the c axis of the tetragonal cells is parallel to the major axis of each ellipsoidal se gment and the orientation relationship is such that the principal axes of the ordered product superlattices are parallel to those of the mat rix phase. The structural transformation from metastable L1(2) to equi librium D0(22) phase, via an intermediate D0(23) structure can be mode lled assuming aperiodic shear displacements of 1/2[110](001) within on e-dimensional clusters of L1(2) unit cells, to create a range of one-d imensional tetragonal superlattices with differing c parameters.