MICROSTRUCTURE AND MICROSEGREGATION IN A Ni-BASED SINGLE CRYSTAL SUPERALLOY DIRECTIONALLY SOLIDIFIED UNDER HIGH THERMAL GRADIENT

In order to understand the effect of high thermal gradient on the microsegregation of refractory elements in Ni-based superalloys, a Ni-based single crystal superalloy containing 4% Re (mass fraction) was prepared by dual heating zone melting and liquid-metal cooling (LMC) directional solidification technique. Comparing with the traditional high rate solidification (HRS) method with thermal gradient G=20-40 K/cm, withdrawal rate V=50-100 $\mu$m/s and primary dendritic arm spacing λ1=200-400 μm, this technique can significantly increase the thermal gradient (up to 238 K/cm) and withdrawal rates (up to 500 $\mu$m/s). Planar-like and cellular-like solid-liquid interfaces, coarse dendrite and fine dendrite were sequentially obtained with increasing withdrawal rates. Under the condition of $G$=238 K/cm and $V$=500 $\mu$m/s, the primary dendritic arm spacing λ1 and the mean size of γ' precipitates (in dendrite core) obviously decreased to 61.3 and 0.04 μm, respectively. In addition, the microsegregation increased initially and then decreased with increasing withdrawal rate, especially for the microsegregations of W and Re. EPMA line scan indicated that solid-back diffusion has an obvious influence on the microsegregation for the fine dendrite structure under high thermal gradient directional solidification.