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• Investigated the influence of sliver defects on the creep rupture life of nickel-based single crystal superalloys. • Revealed the damage mechanism of sliver defects on the creep life of single crystal superalloys through SEM and EBSD observations. • Developed creep constitutive equations for specimens containing sliver based on crystal plasticity theory, explaining the reasons for the decrease in sample life caused by sliver. • Predicted the damage tolerance of sliver defects with different orientations. Sliver defects are common casting imperfections in the preparation of single crystal blades, significantly impacting the high-temperature mechanical performance of blades and potentially leading to blade failure. A detailed study was conducted on the microstructure of sliver in the second-generation nickel-based single crystal superalloy CMSX-4, focusing on its influence on creep performance at 980 °C/250 MPa. Results indicate a substantial impact of sliver on the creep performance of single crystal superalloys, with a 26.37 % reduction in creep rupture time for samples with sliver compared to those without. Fracture analysis reveals that sliver primarily contributes to performance deterioration by promoting the extension of grain boundary microcracks through grain boundary sliding during the creep process, accelerating sample fracture. Utilizing crystal plasticity theory, a successful fit was achieved for the creep rupture life of samples with sliver, allowing the prediction of fracture life for samples with different orientations of slivers. Predictive results suggest that, for CMSX-4 alloy, the orientation of the sliver should be restricted to within 8°. [ABSTRACT FROM AUTHOR]

The effect of hot isostatic pressing on the tensile property of single crystal superalloys at elevated temperature is studied experimentally. The results show that the high temperature elongation of nickel based single crystal superalloys is obviously enhanced by hot isostatic pressing, while the yield strength is unchanged. Besides, it is found that the hot isostatic pressing causes the change of fracture surface. The enhancement of elongation and the change of fracture behavior are discussed. • HIP on the tensile property of CMSX-4 at elevated temperature is studied. • High temperature elongation is obviously enhanced by HIP. • The fracture mechanism is changed based on surface fracture analysis. • Micropores healed by HIP affect the formation and propagation of the initial crack. [ABSTRACT FROM AUTHOR]

The effect of a high magnetic field on the γ′ phase of Ni-based single crystal superalloy during directional solidification is investigated experimentally. The results clearly indicate that the magnetic field significantly reduces the γ′ phase size. Further, the quenching experiment is carried out, and the results found that the length of mushy zone is obviously decreased under a high magnetic field. Based on both experimental results and nucleation mechanism, it is found that the decrease of γ′ phase size should be attributed to the fact that a high magnetic field causes the increase of temperature gradient in front of solid/liquid interface and leads to the increase of undercooling of γ′ phase. [ABSTRACT FROM AUTHOR]

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/ γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant ( p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field. [ABSTRACT FROM AUTHOR]

The suppression of stray grains during seed melt-back of single-crystal superalloy through thermal resistance technique has been investigated based on both experimental observations and numerical simulation. The results indicate that the introduction of thermal resistance layer significantly suppresses the stray grain formation of single-crystal superalloy. Based on both theoretical analysis and numerical simulation, above results should be attributed to the decrease of radial heat transfer of sample in the thermal resistance layer. [ABSTRACT FROM AUTHOR]

The effects of a high magnetic field on microstructures during seed melt-back of superalloy were investigated. Experimental results indicated that the high magnetic field significantly modified the melt-back interface shape and the melt-back zone length. In addition, stray grain on the edge of sample was effectively suppressed in the high magnetic field. Based on experimental results and quantitative analysis, the above results should be attributed to the increasing temperature gradient in a high magnetic field. [ABSTRACT FROM AUTHOR]

The creep properties of nickel-base single crystal superalloy CMSX-4 with and without high magnetic field at 980 °C/250 MPa are investigated. Superalloy CMSX-4 is shown with an excellent rupture life under high magnetic field. Fracture surface results indicate that small square-shaped facets are shown and microporosities are formed at the bottom of dimple without high magnetic field, while the dimple surface shows large square-shaped facets and microporosities disappear from the bottom of dimple with high magnetic field. The mechanism of the enhancement of rupture life and the change of fracture behavior are explored in detail. [ABSTRACT FROM AUTHOR]

• The interface reaction of superalloy CMSX-4 and ceramic core is carried out during directional solidification. • A continuously distributed Al 2 O 3 layer is formed at superalloy/silicon oxide ceramic core interface. • A discontinuously distributed Ta/Ti enrichment carbide ((Ta/Ti)C) layer is formed at Al 2 O 3 layer/superalloy interface. • The formation mechanism of the interface reaction is analyzed. To clarify and comprehend the interfacial compatibility of single crystal superalloy and ceramic core, the interface reaction of nickel-based single crystal superalloy CMSX-4 and silicon oxide ceramic core during directional solidification is investigated. The results revealed that a continuously distributed Al 2 O 3 layer is formed at the interface of superalloy / silicon oxide ceramic core. Moreover, it is also found that a discontinuously distributed Ta/Ti enrichment carbide layer, (Ta/Ti)C, is formed at the interface of Al 2 O 3 layer/superalloy. The formation mechanism of interface reaction between the nickel-based single crystal superalloy and silicon oxide ceramic core is analyzed. [ABSTRACT FROM AUTHOR]

In order to improve the microstructure and enhance the mechanical properties of nickel based single crystal superalloys, a heat treatment under an alternating magnetic field is investigated. Compared with traditional heat treatment without alternating magnetic field, the microstructure at an alternating magnetic field exhibits the uniformly distributed and high volume fraction of γ′ precipitates, the decreases of residual eutectic, segregations and microproposity. The rupture life is enhanced from 62 h to 106 h at 980/250MPa. Present work provides a new method to optimize the microstructure and mechanical properties of nickel based single crystal superalloy blades. [ABSTRACT FROM AUTHOR]

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k , or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity. • Super-high purity of nickel-based single crystal superalloys was explored. • The PDAS, micro-segregation, eutectic and pores all greatly decreased with the increase of purity. • Clyne-Kurz model was used to predict the volume fraction of eutectics. • The elemental segregation at γ/γ′ interface changed remarkably with the variation of the purity. [ABSTRACT FROM AUTHOR]

Laser-directed energy deposition (L-DED) is considered as an effective repair method for damaged single-crystal (SX) superalloy turbine blades. However, controlling stray grains (SGs) and epitaxial growth is still the major problem. This study investigated the influences of pre-solution treatment and intrinsic heat treatment (IHT) on the formation of SGs and epitaxial growth of L-DED René N5 SX superalloy. The γ/γ' eutectics and coarse γ' precipitates at the inter-dendrite region can be eliminated after pre-solution treatment procedures before the L-DED experiment. A lower temperature in pre-solution treatment resulted in the retention of residual γ/γ' eutectics and coarse γ' precipitates. Thus, the aggravated element segregation within the local melt pool and interface collapse during the L-DED process can result in stray grains (SGs) formation. Improved pre-solution treatment procedures can also help reduce SGs caused by the carbides in the substrate. Besides, the SG formation in the deposited layers can be attributed to the recrystallization as a result of high thermal stress during the IHT in thermal cycles. The optimized pre-solution treatment procedure reduced the elemental segregation of the substrates , resulting in a larger primary dendrite arm spacing (PDAS) from the bottom of the deposited layers. Due to IHT during thermal cycles in the deposited layers, the equivalent heat treatment process promoted the re-precipitation and growth of γ' phase. Meanwhile, the optimized pre-solution treatment procedure decreased the size of the heat-affected zone (HAZ) caused by thermal cycles. In summary, by optimizing the substrates' pre-solution treatment procedures to eliminate the γ/γ' eutectics ,coarse precipitaes and decrease the carbides in the subsrtates, the formation of SGs resulting from substrate segregation and heterogeneous microstructure can be controlled. This work provides the potential guidance of substrate pre-treatment for L-DED repair of SX superalloys. [Display omitted] • Pre-solution heat treatment procedure is benefit to the control of stray grains (SGs) in the bottom of the molten pool. • SGs found in the deposited zone were strongly associated with recrystallization. • Improper pre-solution treatment procedure increased the length of heat-affect zone (HAZ). • Intrinsic heat-treatment (IHT) caused by thermal cycles led to different microstructure evolution of γ' precipitates in the sample. [ABSTRACT FROM AUTHOR]

Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture. • Creep behaviors of a new 4th generation Ni-SX are compared at 1150 °C and 1100 °C. • Temperature-and-time dependent TCP precipitation and creep inconsistency affect the creep. • Depending on temperature, the two factors display competitive behaviors. • Higher temperature of 1150 °C enlarges the influence of TCP precipitation. • Creep inconsistency on dendritic structures dominants the creep at 1100 °C. [ABSTRACT FROM AUTHOR]

Abstract: The effect of magnetic field on precipitation phases in directional solidified single-crystal nickel-base superalloy was investigated. The magnetic field significantly decreases the size of γ′ and the content of carbides and γ/γ′ eutectic. The decreased magnitude is firstly sharp and then gentle with the increasing magnetic field intensity. The γ′ phase refinement could be attributed to a decrease in nucleation activation energy during solid phase transformation. The segregation reduction of alloying elements is responsible for the content decrease of carbides and γ/γ′ eutectic. The magnetic- field-assisted directional solidification provides an effective way to control the size and content of precipitation in superalloys. [Copyright &y& Elsevier]