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

Abstract

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 Lit inter-lamellar interfaces. Room temperature yield stress increased rapidly with increasing Si and C contents. likely due to solid solution hardening as well as clue 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 alpha(2) 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. polvcrystalline form, and with "type-I" orientation increased in order of "type-III" --> polycrystal --> "type-I". These results Lire 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. (C) 2004 Elsevier Ltd. All rights reserved.