The influence of withdrawal rate on the porosity in a third-generation Ni-based single crystal superalloy was investigated by a quantitative evaluation method. The results showed that the withdrawal rate obviously effected on the average area fraction, number and diameter of porosities except their radius ratios. In consideration of the microstructure observation for dendrite arms, an optimized withdrawal rate was obtained with a minimum porosity level as about 125 µm s−1. Simultaneously, a threshold value for the acceptance level of porosities might be set as about 0.1% in order to fulfill the requirements for Ni-based single crystal casting in laboratory scale. Finally, the formation reason of porosity was discussed and it was concluded that the feeding for the volume shrinkage of the last solidified eutectic liquids from the residual liquids and the isolating effect of the morphologies of dendrite arms might be two key factors in controlling the porosities level in Ni-based single crystal superalloy.

To increase the peak temperature during solution heat treatment of a single crystal superalloy over the solidus, a remelting solution heat treatment is investigated. It contains two parts: a) homogenizing at mushy zone temperatures, and ignoring partial incipient melting; b) eliminating incipient melting structure by secondary solution heat treatment. After remelting solution heat treatment the refining γ′ particles uniformly distributed and the residual segregations are less than those after traditional solution heat treatment. Therefore, the rupture life at 1100 °C/150 MPa is improved from 105.3 h after traditional solution heat treatment to 141.1 h.Download high-res image (197KB)Download full-size image

The simulation models of the thermal and macrostructural evolutions during directional solidification of Ni-base single crystal (SX) turbine blades under high rate solidification (HRS) and liquid metal cooling (LMC) have been constructed using ProCAST software, coupled with a 3D Cellular Automaton Finite Element (CAFE) model. The models were used to investigate the tendencies of stray grain (SG) formation in the platform region of turbine blades fabricated by HRS and LMC techniques. The results reveal that the LMC technique can prohibit SG formation by smoothing the concaved isotherm and in turn alleviating the undercooling in the platform ends to let the dendrites fill up the undercooled zone before SG nucleation. The simulation results agreed well with the experimental results, indicating that these models could be used to analyze the macrostructural evolution or to optimize process parameters to suppress SG formation. Using these models, the critical withdrawal rate for casting SX turbine blades without SG formation were determined to be around 75 μm·s-1 and 100 μm·s-1 for HRS and LMC respectively, suggesting that LMC can be used as an efficient technique in fabricating SX turbine blades without any SG defect formation.

•Four zones were identified in a melt-back seed of seeding-grain selection technique.•Seeding-grain selection technique can prevent stray grain from growing into casting body by geometry blocking of spiral passage.•The final orientation did not change a lot after high-order branching in spiral passage.A combined technique denoted as seeding-grain selection (SGS) was designed for fabricating single crystal turbine blades. The microstructural evaluation during SGS process was metallographically analyzed in both starter section and spiral section of this technique. Four zones with distinct dendritic morphologies were identified in seed melt back region. After epitaxial propagating from melted seed and branching through the spiral passage, the lined-up dendritic pattern was observed at the outlet of the passage. The effectiveness of the technique on stray grain and orientation control was also assessed by Electron Back-Scattering Diffraction (EDSD) technique. The results indicated that the technique can control stray grain with no detrimental effect on crystal orientation.

•It is determined that stray grain (SG) originates from heterogeneous nucleation.•SGs with random orientations constitute distinct macro, dendritic and γ′ morphologies in comparison to its matrix grain.•Severe segregation could be identified in GB area due to the compositional enrichment and deprivation.Through comparatively characterizing the platforms with and without stray grain in multi-scale levels, it is determined that stray grain (SG) originates from heterogeneous nucleation with randomly distributed orientations with respect to the matrix grain. This independently nucleated deleterious grain constitutes distinct macro, dendritic and γ′ morphologies and forms a high angle boundary (HAB) in the conjunction area of SG and the matrix grain. Compositional distribution reveals that severe segregation could be identified in GB area due to compositional enrichment and deprivation during the last stage of solidification.

Nickel-based single crystal superalloys oriented along the <001> and <011> lattice directions were produced by a bottom seeding technique in an attempt to understand the evolution mechanism of the dendrite grown along different orientations in the present study. The changes in primary dendrite arm spacing for single crystal with different orientations are also discussed. It was found that the dendrite morphologies of single crystal superalloy grown along <011> were different from that of <001>. Firstly, the dendrites showed the irregular cruciforms and array in rows in a transverse section. Secondly, no typical primary dendrites were observed but the dendrite morphologies appeared like the letter “V” or “W” in a longitudinal section. The primary dendrite arms grew along the <001> orientation from the bottom of the sample in the <001> orientation. However, in the <011> orientation, the single crystal developed by continuous side-branching along the [001] and [010] orientations. The primary dendrite arm spacing was as the function of the deviation angle ϕ. It indicates that with the increase in the deviation angle ϕ, the primary dendrite arm spacing first increased, and then decreased.

The influence of elevated withdrawal rate on the microstructure and segregation behavior of Ni-base single-crystal superalloys containing Re and Ru is investigated. The experimental superalloys are processed under a high thermal gradient of approximately 250 K/cm and withdrawal rates between 10 and 500 μm/s. With increasing withdrawal rate, the dendritic structures and γ′ precipitates in as-cast microstructures are apparently refined. Electron-probe microanalyzer (EPMA) results indicate that the degree of segregation for the constituent elements (e.g. Al, Ta, W, Re etc.) increases initially and then decreases with increasing withdrawal rate. In addition, the Re and Ru additions obviously increase the amounts of γ–γ′ eutectic and the tendency of segregation for Al and Ta.Highlights► We characterize the as-cast microstructure of Re- and Ru-containing single-crystal superalloys. ► Dendrite structure is greatly refined with increasing withdrawal rate. ► The segregation of alloying element increases firstly, and then decreases with increasing withdrawal rates. ► Re and Ru additions promote the tendency of dendritic segregation, and increase the volume fraction of eutectic.

The heat transfer during directional solidification by Bridgman-type directional solidification has been analyzed and a relationship has been established that reflects the effect of alloy properties, process parameters and equipment characteristics on thermal gradients. Based on this relationship, some methods for obtaining high thermal gradients have been developed. By using zone-intensified overheating and liquid-metal cooling, high thermal gradients of up to 800 K/cm were achieved. Application of these methods in the processing of single crystal superalloys indicated that high thermal gradient directional solidification produced more uniform microstructures and optimized mechanical properties.

The interface morphologies of single crystal superalloy CMSX-2 were studied over a range of cooling rate with large variations in withdrawal speeds in directional solidification. A superfine cellular structure was obtained under both high thermal gradient up to 1000 K/cm and fast withdrawal rate up to 1 mm/s. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of γ′ phase, reduced microsegregation of alloying elements and smaller size of γ–γ′ eutectics. The rupture life and plasticity of fine structure samples produced in high thermal gradient directional solidification increase significantly than that in conventional directional solidification process at 1323 K.

Grain size and microstructural features of cast superalloy K4169 were investigated under various melting and casting conditions with and without the addition of grain refiners. It is found that lowering pouring temperature and adding refiners to the melt can lead to grain refinement of γ matrix and improve the proportion of equiaxed grains. At a conventional pouring temperature of 1400 °C, the average size of equiaxed grains could be refined to the order of ASTM 3.2, the proportion of equiaxed grains at transverse cross-section could be improved from 56 to 99%. The results also indicate that the average length of primary dendrite axis is shortened with the addition of refiners, but the secondary dendrite arm spacing keeps almost unchanged because local solidification time remained constant. In addition, the microsegregation of main elements such as Fe, Cr, Nb, Mo and Ti is alleviated with the decrease in grain size, and the grain morphology have transformed from dendrite in coarse- to granulate in fine-grained castings. At higher melt pouring temperature, the amount of microporosity in samples with the addition of refiners can be greatly reduced. The mechanisms of grain refinement and increase in equiaxed grain proportion were proposed.