Morphology and competitive growth during the development of the parallel lamellar structure by self-seeding in directionally solidified Ti-50Al-4Nb alloy

Abstract

Bridgman-type directional solidification was conducted for the parallel lamellar structure of the TiAl-Nb alloys by self-seeding technique. The growth morphology and the competitive growth were investigated during the growth of the non-preferred crystals. The primary α phase and the final α2/γ lamellar structure are obtained at the selected growth rates. The non-preferred crystals with the parallel lamellar always have the asymmetry morphology with the tilting primary arms and the developed second arms at the front-lateral direction, which can affect the non-preferred growth advantage greatly. For the converging growth process the competitive growth takes on the struggle of the primary arms, while the second arms for the diverging growth, in which the parallel crystals always have the advantage to growth out and eliminate the misorientated grain formation at the crucible sidewall. Generally the parallel columnar crystals with the parallel lamellar structure can be achieved during the cellular-dendritic growth at the proper growth rates by self-seeding technique.