THE MICROSTRUCTURE OF ELECTRODEPOSITED TITANIUM-ALUMINUM ALLOYS

The microstructure of electrodeposited titanium-aluminide alloys containing 3.6 to 24.1 at. pct Ti was studied by transmission electron microscopy. The surface morphology of the deposits showed that they contained nodular and faceted grains which tended to be less faceted at higher Ti contents. Extensive {111} twinning was observed in all deposits. and growth striations parallel to {111} were observed in the low Ti deposits. The growth of nodules was linked to the presence of these twins; it was hypothesized that the twin boundaries act as easy atomic attachment points and, therefore, enhance the growth rate. The presence of twins and striations was used to propose a growth mechanism. The 5.3, 15.8, and 24.1 at. pct Ti deposits were single-phase grains of the L1(2) crystal structure, as opposed to the expected equilibrium two-phase mixture of face-centered cubic (fcc) Al (saturated with Ti) and D0(22) Al3Ti. Calculated electron diffraction intensity data were used to demonstrate that the decrease in intensity of the superlattice reflections in the substoichiometric deposits is due to a reduction in the difference in atomic scattering factors between the two lattice site types.