•Inserting graphite block into the platform can improve local cooling condition.•Improved local cooling condition flattened the concave solidification interface.•Improved local cooling condition promoted the dendritic to cellular transition.•Improved local cooling condition refined the dendrite microstructures.•Freckle formation can be suppressed by improving local cooling condition.In order to suppress the freckle formation in specimens with abruptly varying cross-sections, superalloy CMSX-4 was directionally solidified, and the local cooling method was investigated, in which a graphite block was inserted into the bottom of platform of specimen. The experimental and simulation results were coupled to evaluate this method in terms of freckling number, primary dendrite arm spacing (PDAS), dendrite morphology, and solidification interface shape. The results show that, with the application of local cooling method, freckles appearing in the specimens were absolutely removed. Dendrite microstructure was refined with reduced PDAS, and a dendritic to cellular transition was promoted. In addition, the concave solidification interface was flattened. Therefore, the local cooling method with the application of graphite block can be employed to reduce the freckling tendency in directionally solidified CMSX-4 superalloy specimens with abruptly varying cross-sections.Download high-res image (263KB)Download full-size image

The microstructure and elevated temperature tensile property of Fe-doped NiAl–Cr(Mo)–(Hf,Dy) eutectic alloy before and after heat treatment were investigated by SEM, TEM and elevated temperature tensile test. The result shows that the as-cast alloy has higher elevated temperature strength. The heat treatment leads to the coarsening and local dissolution of lamellar structure, and thus results in the weak decrease of elevated temperature strength and the increase of the ductility. Moreover, the observation of side surface reveals that the micropores in the heat-treated alloys, hindering the facture of tensile specimen, are less than those in the as-cast alloy. Meanwhile, more dimples and lamellar neckings are observed in the fracture surface of heat-treated alloys, even the dislocation substructure occurs in Cr(Mo) phase. This demonstrates that the heat treatment results in the decrease of elevated temperature strength and the increase of ductility.

•Fully eutectic microstructure was obtained in NiAl–Cr(Mo) hypereutectic alloy.•The volume fraction of Cr(Mo) strengthening phase increased with increasing content of Cr + Mo.•The highest fracture toughness of 26.15 MPa m1/2 was obtained in NiAl–Cr–Mo alloy system.The microstructures and room temperature fracture toughness of directionally solidified NiAl-xCr-6Mo (x = 28, 32 and 36 at%) alloys were investigated. Fully eutectic microstructure could be obtained in the alloys over a wide composition range. High temperature gradient could increase the planar/cellular transition rate and expand the eutectic coupled growth zone. The volume fraction of Cr(Mo) strengthening phase increased with the increasing content of Cr, accordingly, the fracture toughness of NiAl–Cr(Mo) alloys also gradually increased. The fracture toughness of 26.15 MPa m1/2 was obtained in the NiAl-36Cr-6Mo hypereutectic alloy solidified at withdrawal rate of 10 μm/s and temperature gradient of 600 K/cm, which is the highest value in the NiAl–Cr–Mo alloy system until now. Well-aligned microstructure was beneficial to the enhancement of the fracture toughness, while the existence of primary phase seriously deteriorated the toughness. All the directionally solidified NiAl–Cr(Mo) alloy failed as brittle quasi-cleavage fracture. Some toughening mechanisms, such as crack bridging, crack nucleation, crack blunting, crack deflection, interface debonding and shear ligament toughening as well as linkage of microcracks were observed. In addition, mobile dislocation generated from the interface also had significant influence on the toughness.

•Microstructure stability of Ti–48Al–2Cr–2Nb alloy during heat treatment is studied.•Lamellae orientation of Ti–48Al–2Cr–2Nb alloy is retained well upon rapid heating.•A quasi-seed is used to align the lamellar microstructure of Ti–48Al–2Cr–2Nb alloy.The stability of lamellar microstructures in the Ti–48Al–2Nb–2Cr alloy during heat treatment depends more on heating rate and less on holding time and cooling rate. When the heating rate is higher than 61 °C/min, the original lamellae orientation at room temperature can still maintain unchanged even though the lamellae are transformed to single α grains at a temperature close to melting point. Therefore, the high-temperature α grains, which are transformed from the α2/γ lamellae within a quasi-seed, are successfully employed to align the lamellar microstructure by directional solidification.

As a promising electromagnetic process to obtain TiAl samples without contamination, electromagnetic confinement and directional solidification was successfully applied for lamellar microstructure control in Ti-47Al alloy. Seeded by a Ti-43Al-3Si seed, columnar α grains grew stably and the lamellae within these grains were aligned parallel to the growth direction. However, stray α or β grains, in which the lamellae were complex, nucleated and grew in the sample edge. The possible reasons for the formation of stray grains were discussed by analyzing the temperature gradient at the solid/liquid interface and the macrosegregation of Ti5Si3 particles and Al solute along the sample radius. Moreover, the fluid flow induced by electromagnetic force, which led to the macrosegregation, was also discussed by using a simple model.

The microstructure evolution and room temperature fracture toughness of NiAl–Cr(Mo)–(Hf,Dy)–4Fe alloy during heat treatment have been investigated. After heat treatment (1250 °C), the NiAl and Cr(Mo) phases become coarse, and a local dissolution occurs in Cr(Mo) lamellae. Simultaneously, fine Hf solid solution appears at the NiAl/Cr(Mo) phase interface. Moreover, the addition of Fe can improve the fracture toughness of NiAl–Cr(Mo)–(Hf,Dy) alloy, and heat treatment can enhance fracture toughness further. The relevant toughness mechanism is also discussed.

•Microstructure evolution are controlled by traveling magnetic field.•New microstructure are obtained.•The solute distributions induced by TMF has been studied numerically.Based on the simulation results, directional solidification experiments under traveling magnetic field were performed to obtain thin samples with 7 mm diameter. The experimental results demonstrate that banding is overgrown by peritectic coupled growth or island banding structures as solidification proceeds. Numerical results indicate that this phenomenon should be attributed to the different solute distribution at the S/L interface with different TMF. The effects show that it is possible to control solidification microstructure evolution by using a traveling magnetic field during directional solidification of peritectic alloys.

A phase and microstructure selection map used for peritectic alloy directionally solidified under convection condition was presented, which is based on the nucleation, constitutional undercooling criterion (NCU criterion), and the highest interface temperature criterion. This selection map shows the relationships between the phase/microstructure, the G/V ratio (G is the temperature gradient, V is the growth velocity), and the alloy composition under different convection intensities and nucleation undercoolings. Comparing with the results from directional solidification experiments of Sn–Cd peritectic alloys, this selection map was generally in agreement with the experimental results.

•The modest addition of Dy improves the efficiency of obtaining the full eutectic.•Compared to 6 and 90 μm/s, the refined effect of Dy is more apparent at 30 μm/s.•The addition of 0.1 wt.% Dy refines the intercellular zone and the lamellas.•The Dy-containing phase can form when the Dy content is no less than 0.1 wt.%.The effect of various Dy content on the microstructure of Ni–31Al–32Cr–6Mo hypereutectic alloy was studied at the withdrawal rates of 6, 30 and 90 μm/s. The results show that the solid–liquid interface morphology has an evolutionary process of planar → cellular → dendritic interface with the increasing withdrawal rate. The primary Cr(Mo) dendrites are gradually weeded out through competitive growth between the primary phase and the eutectic phase. The volume fraction of primary Cr(Mo) dendrites decreases with the modest addition of Dy (0.05 wt.%) at 6 μm/s. When the withdrawal rate increases to 30 μm/s, the appropriate addition of Dy (0.1 wt.%) refines the microstructure, such as the width of intercellular zone and the lamellar thickness in the intercellular zone. With the increase of withdrawal rate to 90 μm/s, the addition of Dy has no significant effect on the microstructure. In addition, the white Dy-containing phase can occur in the boundary of eutectic cells when the Dy content is no less than 0.1 wt.%.

Directional solidification experiments have been carried out in Fe-Ni peritectic alloys to study microstructure evolution in diffusive regime. A numerical modeling of melt convection was developed and discussed to investigate the convective velocity in samples of different diameters. The simulation results show that convection effects can be reduced by decreasing the sample diameter, and diffusion-controlled growth can be achieved if the sample diameter is smaller than about 1 mm. Based on the simulation results, experiments were performed in thin samples of 1 mm diameter to obtain microstructures in diffusive regime. Different kinds of microstructure evolutions were observed in the directionally solidified Fe-Ni alloys. The time-dependent microstructure evolution implies that the solidification process seemed to be in non-steady state rather than in steady state. Based on the transient model, solute distributions in the liquid ahead of the primary and peritectic phases were discussed to reveal the microstructure evolution. Since the solute partition coefficients of the primary and peritectic phases are different, the magnitudes of solute element rejected by the two solid phases are different in two-phase growth conditions. And within the boundary layer, the solute fields ahead of the primary and peritectic phases are different owing to the weak effect of solute mixing in diffusion-controlled regime. Furthermore, microstructure evolution in peritectic alloys was discussed based on the analysis of solute redistribution.

A new initiating phenomenon of peritectic coupled growth was observed in directionally solidified Fe–Ni alloys under diffusion-limited growth condition. Although it is generally accepted that peritectic coupled growth is initiated by island banding, the present experimental results show that two phases coupled growth could also be initiated by planar primary phase in samples of different compositions. Two models were proposed to explain the new initiating mechanism of peritectic coupled growth. It was found that due to the solute pile-up at the planar primary phase interface, the liquid at the planar primary phase interface will be suitable for nucleation of the peritectic phase or constitutionally undercooled with respect to the growing primary phase. The simultaneous growth of the nucleated peritectic phase with the primary phase or the cellular primary phases with the intercellular peritectic phase can induce the formation of coupled growth.Highlights► New initiating phenomenon of peritectic coupled growth was observed. ► Two models were proposed based on the diffusive solute transport. ► Simultaneous growth of nucleated γ phase with δ phase can induce coupled growth. ► δ phase transition from planar to cellular front can trigger coupled growth.

The microstructure and room temperature fracture toughness of pseudo-binary NiAl–ⅹMo (ⅹ = 7.8, 9, 13 and 16 at. %) in situ composites were investigated. For all four alloys examined, uniform and well-aligned Mo fibrous structures were directionally solidified by the liquid metal cooling process at the growth rate of V = 6 μm/s, respectively. With the increase of the Mo content, the spacing of the Mo fibers decreased and the volume fraction of Mo fibers increased. Significant influence of Mo addition on the toughness was observed for the directional solidification alloys, and the fracture toughness was found to increase with volume fraction of Mo rods. Scanning electron microscopy was used to characterize the fracture behavior. Based on the results of theoretical analysis, the composite structures of these alloys provided improvement in fracture toughness over binary NiAl primarily by crack trapping and crack bridging. Interface debonding, crack deflection and microcrack linkage provided further resistance to crack growth.Highlights► Regular fibrous structure of the NiAl–Mo eutectics were produced. ► The relative volume fractions of the constituent phases of eutectic can be changed. ► The fracture toughness of NiAl–Mo composites were significantly improved. ► Crack trapping and crack bridging are the main toughen mechanisms.

Solidification microstructure and growth interface morphology of directionally solidified Ni–31Al–32Cr–6Mo(at.%) hypereutectic alloy were studied. The experiments were carried out at higher temperature gradient of about 250 K cm−1 with different withdrawal rates of 4–500 μm s−1. When the withdrawal rate was less than 50 μm s−1, the primary Cr(Mo) dendrites were gradually eliminated through competitive growth between the primary phase and the eutectic phase. When the withdrawal rate exceeded 100 μm s−1, no primary phase formed, eutectic phase grew directly. Fully eutectic microstructures with lamellar morphology were observed at all withdrawal rates. With increasing withdrawal rate V, the growth interface changed from planar to cellular and then dendritic, the solidification microstructure also transformed from planar eutectic to two-phase cellular eutectic and dendritic eutectic. The microstructure was refined and the eutectic interlamellar spacing λ decreased according to the relationship of λ = 4.82V−0.42. Compared to the alloy at eutectic composition, the volume fraction of Cr(Mo) strengthening phase was increased obviously.Highlights► Fully eutectic structure was obtained in NiAl–Cr(Mo) hypereutectic alloy. ► Two-phase cellular and dendritic eutectics were observed. ► λ2V = constant was also applicable for cellular and dendritic eutectic growth. ► The volume fraction of Cr(Mo) strengthening phase was significantly increased.

•The effects of axial convection on freckle formation were studied.•The evolution of convection patterns as cross-section enlarges are presented.•The intensity of axial flow can be enhanced through increasing melt temperature.•Axial convection ahead of interface increases notably as cross-section enlarges.The effects of convection patterns on freckle formation of directionally solidified Nickel-based superalloy sample with abruptly varying cross-sections were investigated experimentally and numerically. The experimental results demonstrate that freckles were only observed at the bottom of larger cross-section. Numerical results indicate that this phenomenon should be attributed to the different convection patterns at front of solidification interface. As the withdrawal rate increased, the primary dendrites spacing has an obvious influence on freckle formation. A more in-depth investigation of the convection patterns can provide a better understanding of freckle formation and perhaps offer methods to minimize freckles in turbine blades.

The influences of melt flow generated by traveling magnetic fields (TMFs) on dendrite growth during the upward-directional solidification of Pb–33 wt% Sn binary alloy were investigated under the condition of 1×g gravity. When the direction of the TMF was changed from upward to downward, the primary dendrite spacing gradually increased, and the peak of distribution of the primary dendrite spacing shifted to the narrower values. This was caused by the different intensities of melt flow, which was controlled by TMFs. The effects of TMFs on melt flow were similar to those of adjustments in gravity levels; thus, the primary dendrite spacing varied. The effective gravity acceleration, which was used to modulate melt flow, decreased for downward-TMFs and increased for upward-TMFs. As the drawing speed increased, the modulated flow showed more significant influences on primary dendrites spacing.