EFFECTS OF GROWTH RATE ON PRIMARY PHASE AND MICROSTRUCTURES OF DIRECTIONALLY SOLIDIFIED Ti-46Al-2Cr-2Nb ALLOY

Abstract

TiAl alloys with fully lamellar structure has been intensively studied for excellent fracture toughness and creep properties. Directional solidification is an effective way to control the lamellar structure. Thus it is important to investigate the effect of solidification parameters on the structure of TiAl alloys during directional solidification. In this work, microstructures of directionally solidified Ti-46Al-2Cr-2Nb (atomic fraction, %) alloy has been investigated through Bridgman-type directional solidification experiment at constant temperature gradient and wide range of growth rates (v=10—120 µm/s). The type of primary phase, the primary dendritic arm spacing (�1), the secondary dendritic arm spacing (�2) and the lamellar spacing (�e) are investigated. It is found that: the primary phase of the specimens transformed fromphase tophase with the increase of growth rates; the transformation trend of primary phases with growth rates can be predicted by the phase selection model based on the highest interface temperature criterion. �1, �2, ande all decrease with the increasing of v. The relationship between �1 and v is not affected by the transformation of primary phases, which follows �1=700.6v −0.24 relationship. However the relationship between �2 and v is associated with the