Oxidation-induced embrittlement of TiAl alloys

The ductility of oxidised TiAl-based alloys is reduced even when the oxygen-rich region is of the order 100 nm thickness; this loss in ductility is smallest in lamellar samples. Removal of this oxidised region restores ductility. Acoustic events are observed during tensile tests at stresses above 300 MPa and cracks at about 250 MPa. In-situ tensile tests on samples, with part of the oxygen-enriched region removed, have shown that cracks are formed only in regions where the oxygen-rich region is present. X-ray diffraction measurements have shown that the oxygen-rich surface generates a tensile stress in the top 1 or 2 μm of the alloy of about 250 MPa corresponding to a compressive stress in the oxygen-rich layer of 2000 MPa. It is concluded that embrittlement is caused by (i) the tensile stress induced by the oxygen-rich region and (ii) the corresponding ease of crack nucleation in this region. Subsequent propagation is controlled by the fracture toughness.