Microstructural development and segregation effects in directionally solidified nickel-based superalloy PWA 1484

These studies were performed to investigate the effects of thermal gradient (G) and growth velocity (V) on the microstructure development and solidification behavior of directionally solidified nickel-based superalloy PWA 1484. Directional solidification (DS) experiments were conducted using a Bridgman crystal growth facility. The solidification velocity ranged from 0.00005 to 0.01 cm/sec and thermal gradients ranged from 12 to 108°C/cm. The as-cast microstructures of DS samples were characterized by using conventional metallography; chemical composition and segregation of directionally solidified samples were analyzed with energy dispersive spectroscopy in SEM.; A range of aligned solidification microstructures is exhibited by the alloy when examined as-cast at room temperature: dendrites, flanged cells, cells. The microstructure transitions from cellular to dendritic as the growth velocity increases.; The experimental data for PWA1484 exhibits excellent agreement with the well-known exponential equation (λ₁ ∝ G −1/2V−1/4). However, the constant of proportionality is different depending upon the solidification microstructure: (1) dendritic growth with secondary arms leads to a marked dependence of λ₁ on G−1/2 V−1/4; (2) flanged cellular growth with no secondary arms leads to much lower dependence of λ ₁ on G−1/2V −1/4.; The primary dendritic arm spacing results were also compared to recent theoretical models. The model of Hunt and Lu and the model of Ma and Sahm provided excellent agreement at medium to high thermal gradients and a wide range of solidification velocities. The anomalous behavior of λ ₁ with high growth velocity V at low G is analyzed based on the samples' microstructures. Off-axis heat flows were shown to cause radial non-uniformity in the dendrite arm spacing data for low thermal gradients and large withdrawal velocities.; Various precipitates including γ′, (γ ′ + γ) eutectic pool or divorced eutectic γ ′, and metal carbides were characterized. Processing conditions (growth velocity V and thermal gradient G) exert significant influence on both morphology and size of precipitates present.; Freckle defects were observed on the surface of nickel-based superalloy MM247 cylindrical samples but not on the surface of cylindrical PWA 1484 samples. The Rayleigh number (Ra) that represents liquid instability at the interface was evaluated for MM247 and PWA 1484 in terms of a recently proposed theoretical equation. The effects of segregation, sloped solid/liquid interface and the morphology of dendritic/cellular trunks on the mushy zone convective flow and freckle formation are also discussed.