Research On The Directionally Solidified Microstructures Evolution Of Ni-10.6%NbC Eutectic Alloy At Constant And Changing Growth Rates

In this paper, directionally solidified microstructures of Ni-NbC alloy were investigated both at constant and changing growth rates. The growth mechanism of NbC phase and solidification behavior were presented and discussed. At the same time, the adjustment mechanism in eutectic spacing was researched at an abruptly changing growth rate in directional solidification.At a given temperature gradient of 300K/cm and the growth rates less than 20μm/s in directionally solidification, coupled growth regular eutectics with plane front were obtained. The growth rate in experiment was higher than the critical velocity calculated by the CS (Constitutional Supercooling) theory.The growth of NbC is involved in three stages which are the initial growth, competitive growth and steady-state growth stages with the branch of NbC phase in its preferred growth direction.As the growth rate was less than 20μm/s, Ni matrix and NbC fiber coupled growth together and the phase morphologies of NbC mainly consisted of trilobar and rods. When the growth rate was increased to 50μm/s, cellular eutectic structure occurred in directionally solidified microstructures. Some of trilobar and rod NbC phases transited to plate NbC phases. Simultaneously, the regular eutectic spacing decreased with the growth rate increasing and the volume fraction of NbC phase increased in directional solidification.When the growth rate was increased suddenly from the same initial growth rate of 2μm/s in directional solidification, the solidified structures were still regular eutectics at the growth rate changing ratio smaller than 10. However, at the larger ratio, cellular eutectics and coarse primary Ni dendrites were firstly precipitated from the liquid. If the growth rate was changed to the same growth rate of 50μm/s from different initial growth rates, the solidified structure varied from regular eutectics and the formation of primary Ni dendrites were obtained.The directionally solidified microstructures can be refined by changing growth rate abruptly in directional solidification. The regular eutectic spacing of Ni-NbC decreased in the experiment was originated from the nucleation of NbC phase and the branch process of NbC phase ahead of the liquid/solid interface.