Mechanical properties of Si- and C-doped directionally solidified TiAl–Nb alloys

The microstructure and mechanical properties of directionally solidified Ti–47Al–3Nb, Ti–47Al–3Nb–0.2Si–0.1C, Ti–47Al–3Nb–0.3Si–0.2C alloys were evaluated. The lamellar thickness of Si- and C-doped alloys was fine and uniform, and little or no precipitates were found within the lamellae or at inter-lamellar interfaces. Room temperature yield stress increased rapidly with increasing Si and C contents, likely due to solid solution hardening as well as due to a reduction in lamellar spacing distribution. However, fracture toughness of “type-I” specimens of the alloys decreased marginally as Si and C were introduced and as their content increased. It is argued that the α2 volume fraction decreases with increasing Si and C contents so that the frequency of delamination-type separation decreases, and this feature is thought to principally contribute to the high crack growth resistance of “type-I” specimens. Furthermore, fracture toughness of Ti–47Al–3Nb–0.2Si–0.1C alloy specimens with “type-III” orientation, polycrystalline form, and with “type-I” orientation increased in order of “type-III” → polycrystal → “type-I”. These results are discussed and the ability to obtain a respectable combination of strength and toughness through Si and C addition to a TiAlNb alloy produced by directional solidification is demonstrated.