HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of δNb₁₁Si₄ usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss ⟶ δNb₁₁Si₄ + Nbss₁. In some cases, the δNb₁₁Si₄ precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr₂Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/γNb₅Si₃ interfaces. The coupling precipitation behaviors and precipitation reactions of δNb₁₁Si₄, Cr₂Nb and γNb₅Si₃ in the Nb-Ti-Si based alloys have been discussed.