Mechanical properties of nanocrystalline silicide dispersion strengthened (SDS) intermetallic TiAl alloys

With respect to high-temperature structural applications finely dispersed Ti5Si3 Silicides were incorporated into intermetallic TiAl alloys by an in-situ reaction process. After mechanical alloying TiAlSi alloys amorphous powders were obtained. The powders were compacted via hot-isostatic pressing (HIP) between 800 degrees C and 1000 degrees C. Depending on KIP temperature the material exhibits a globular microstructure of the equilibrium phases, Ti5Si3+alpha(2)-Ti3Al or Ti5Si3+gamma-TiAl, with mean grain sizes in the nanometer or submicron range, respectively. Microhardness measurements show a Hall-Fetch type dependence on grain size. In compression tests the submicron-grained SDS-gamma-TiAl (d=154 nm) tested at room temperature showed very limited plastic deformation before fracturing at high stress of 2940 MPa. At 800 degrees C a yield strength of sigma(1.25) = 140 MPa was obtained on this material. Coarsening the microstructure (d=280 nm) allowed the SDS-gamma-TiAl to be deformed to more than 5% plastic strain at room temperature in compression (sigma(1.25)=1920 MPa) and resulted in an improvement of the high-temperature strength of more than a factor 2 (sigma(1.25)=337 MPa). Further improvement is expected by optimizing the heat treatment and alloy compositions.