The effect of ultrafine grained microstructures on the hot-workability of intermetallic/ceramic composites based on γ-TiAl

Intermetallic/ceramic composites are prepared by mechanical alloying and subsequent powder consolidation. The bulk material of the composition Ti–45Al–2.4Si (at.%) exhibits a homogeneous microstructure of equiaxed γ-TiAl grains (dγ=196 nm) with a volume fraction of about 10% ξ-Ti5(Si,Al)3-silicides finely dispersed along the grain boundaries. Large fully dense specimens (m=500 g) allow for the demonstration of the easy hot-workability of ultrafine grained materials by industrial processing techniques like isothermal forging and extrusion. The temperatures for isothermal forging can be kept between 800 and 900°C for maximum strain rates in the range of 10−4 s−1. Deformations up to 75% at stresses below 160 MPa are achieved. Due to the stabilizing effect of the Ti5Si3-silicides, only slight grain coarsening is observed. Taking into account the high strain rate sensitivity of the material, extrusion tests (ε̇=1.5 s−1) are performed at temperatures between 1025 and 1250°C. While significant coarsening of both the γ-TiAl matrix and the silicide dispersion is observed at high deformation temperatures, extrusion at 1025°C retains the fine grained and homogeneous microstructure.