The precipitation reaction in a Ag-modified TiAl based intermetallic, as studied by TEM

The precipitation reaction of L12-(Al, Ag)3Ti in a L10-TiAl(Ag) intermetallic compound with a nominal composition of Ti-54 at.% Al-2 at.% Ag has been studied by transmission electron microscopy (TEM), high resolution transmission electron micrograph (HRTEM), EDX and supporting microhardness test. TEM observations revealed that the alloy quenched from 1273 K is of L10 structure without any precipitates. After aging at 1073 K for 1 h, spherical L12-(Al, Ag)3Ti particles were formed via homogenous nucleation. The compositions of the precipitates and the matrix have been determined. After further aging at 1073 K up to 65 h, growth and coarsening of L12-(Al, Ag)3Ti precipitates and a change in the shape of the precipitates from spherical to elliptical were observed. The particle size distribution, number density and aspect ratio as functions of aging time have been determined. Microhardness tests showed that maximum hardness appeared in the sample aged at 1073 K for 17 h with an average precipitate diameter of about 40 nm. The results were discussed in terms of an anisotropy of the energy of interphase boundaries and the theories of coarsening and precipitation hardening.