High temperature precipitation hardening in a rapidly quenched AlTiNi alloy II. Precipitate characterisation

Changes in the morphology and structure of second phase particles, formed during isothermal ageing of rapidly quenched Al6Ti1.5Ni (wt.%) alloy at temperatures in the range 300–500°C, have been examined using a combination of conventional amplitude contrast imaging and electron microdiffraction. The precipitation-hardening response is initially attributable to a fine-scale distribution of coherent particles of metastable L12 phase and the orientation relationship between the metastable precipitate and α-aluminium matrix is such that (001)L12//(001)α, [100]L12//[100]α. During isothermal ageing, the metastable precipitates evolve with a transitional, three-dimensional cross-like morphology defined by three orthogonal sets of ellipsoidal elements forming in pairs. The precipitates comprise variants of a series of one-dimensional tetragonal superlattices that vary in structure from L12 to body-centred tetragonal D023; the c axis of the tetragonal cells is parallel to the major axis of each ellipsoidal segment and the orientation relationship is such that the principal axes of the ordered product superlattices are parallel to those of the matrix phase. The structural transformation from metastable L12 to equilibrium D022 phase, via an intermediate D023 structure can be modelled assuming aperiodic shear displacements of 1/2[110](001) within one-dimensional clusters of L12 unit cells, to create a range of one-dimensional tetragonal superlattices with differing c parameters.