Ni3Al-based Single Crystal Superalloy Research Articles

Study on Ni3Al-Based Single Crystal Superalloy Joints Brazed by Vacuum Brazing with Zr-Containing Filler

Melting point depressants (MPDs) are required to lower the melting point of filler for brazing. In this study, Zr was used as the MPD, and powder filler was prepared by adjusting the Zr and Mo content referring to Thermo-Calc calculations. The prepared filler was used to braze a high-Mo Ni3Al-based single crystal superalloy, IC21, for 1200 °C/30 min. The effects of adjusting the Zr and Mo content on the microstructure and tensile properties of the joint were investigated. The increase in Zr content promotes the formation of Ni7Zr2 in the joint, leading to a decrease in the tensile strength of the joint. The increase in Mo content forms diffusion barriers between the BM and filler, resulting in an enhancement in the tensile strength of the joint. However, continued increases in Mo content leads to an increase in the P-topologically close packed phase, causing a decline in the tensile strength of the joint. When the Zr content was (11.8–12.2) wt.% and the Mo content was (7.3–7.7) wt.%, the tensile strength of the joint at 980 °C reached a maximum of 550 MPa. This study provides a potential direction for the design of brazing filler composition for high-Mo Ni3Al-based superalloys.

Study on microstructure evolution and failure behavior of Ni3Al-based single crystal superalloy joints brazed with Ni-based filler

The microstructural characteristics of brazing seams significantly influence the mechanical properties of brazed joints. In this study, a novel Ni3Al-based single crystal superalloy(Alloy 1) with high Mo content was brazed with a Ni–B–Si–Ti filler. The evolution of microstructure and mechanical properties of brazed joints were investigated after thermal exposure at 1000 °C up to 300h. The brazed joint was mainly composed of the athermally solidified zone (ASZ), isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and base metal (BM). As the increase of thermal exposure time, the tensile strength of the joint at 980 °C decreases from 625 MPa at 0h of thermal exposure to 443 MPa after 300h of thermal exposure. It is suggested that as the duration of thermal exposure increases, the skeleton phases gradually aggregate at the ASZ/ISZ interface, reducing the relative barrier effect of the skeleton in ASZ. Simultaneously, the ASZ/ISZ interface becomes smoother over time, which in turn leads to rapid crack propagation.

Tensile crack inhibition of a PtReAl coated Ni3Al-based single crystal superalloy after long-term oxidation

Tensile behavior of Pt-aluminide (PtAl) coating with and without a middle NiRe layer on a Ni3Al-based single crystal superalloy after different oxidation times were investigated at room temperature (RT) and 1100 °C. The results revealed that the existence of the NiRe layer not only improved the oxidation resistance of the coating, but also performed a significant inhibition of crack propagation, contributing to a superior ductility of the oxidized Re modified PtAl (PtReAl) coated specimen. The tensile and cracking behaviors of the coated specimens at RT and 1100 °C were discussed in detail.

Investigation on the tensile properties of PtAl and PtReAl coated Ni3Al-based single crystal superalloy

Tensile tests of uncoated, diffusion Pt-aluminide (PtAl) coated and Re modified PtAl (PtReAl) coated Ni3Al-based single crystal superalloy were conducted at room temperature (RT) and 1100 °C to investigate the effect of coating on the tensile behaviors and the cracking mechanism. The results revealed that the PtAl and PtReAl coatings had significantly impaired the ultimate tensile strength of the superalloy by about 10% at room temperature, ascribed to the decreasing true bearing section of the coated specimens caused by the cracking of the brittle coatings. At 1100 °C, the PtAl and PtReAl coating improved the oxidation resistance and ductility of the superalloy. However, due to the low high-temperature strength of the coatings, the yield strength and the ultimate tensile strength of the PtAl coated superalloy showed a reduction of about 19% and 17% when compared to the uncoated superalloy. Moreover, the PtReAl coated specimen presented relatively lower strength than PtAl coated specimen, which is attributed to the internal oxidation of the Re-rich layer during the tensile test process at 1100 °C.

Thickness Debit Effect in Creep Performance of a Ni3Al-Based Single-Crystal Superalloy with [001] Orientation

With the complexity of the structure of aero-engine turbine blades, the blade wall thickness continues to decrease. It is found that when the blade wall thickness decreases to a certain extent, its mechanical properties will decline significantly. It is extremely important to study this phenomenon of a significant decline in mechanical properties caused by wall thickness. In this paper, the creep behavior of a second-generation Ni3Al-based single crystal superalloy with different wall thicknesses and [001] orientation at 980 °C/220 MPa has been studied and compared with the creep life of Φ4 round bar. The experimental results show that the second orientation and the surface affected zone are not the main reasons for the reduction of the life of thin-walled samples under this experimental condition. By analyzing the fracture morphology and deformed microstructure of thin-walled samples with different thicknesses, it is found that the thickness debit effect of the single crystal alloy occurs since the effective stress area of the alloy changes due to internal defects and surface affected zone during the creep process. For thicker samples, the creep life of the alloy can be extended by making the samples undergo certain plastic deformation through better plastic deformation coordination ability, while for thinner samples, the plastic coordination ability is poor, and the ability to extend the creep life through plastic deformation is also weaker when the effective stress area of the alloy changes, which leads to the thinner samples being more prone to fracture.

Interdiffusion and oxidation behaviour of Pt/Ru modified Ni–30Al–0.3Dy coating on Ni3Al-based superalloy

ABSTRACT A high purity of Ru or Pt was deliberately deposited as a middle layer between an alumina-forming Ni–30Al–0.3Dy coating and a Ni3Al-based single-crystal superalloy. It was found that the Ru modified Ni–30Al–0.3Dy coating effectively suppressed the interdiffusion between the coating and the substrate and inhibited the precipitation of topologically close-packed (TCP) phases which impaired the mechanical properties of the superalloy. The 1100°C annealing and oxidation tests suggested that, different from Pt, Ru restricted the inward diffusion of Al from the coating to the substrate, thereby hindering the interdiffusion. And the existence of Ru layer can simultaneously improve the oxidation resistance of the coating.

Effect of temperature and stress on high temperature creep behavior of Ni3Al-based single crystal superalloy

The creep behavior and dislocations mechanism of the Ni3Al-based single crystal alloy IC6SX with [001] orientation were investigated under the testing conditions of 1100 ​°C/137 ​MPa, 1100 ​°C/120 ​MPa and 1070 ​°C/137 ​MPa. It was observed that the temperature and stress had a significant effect on the high temperature creep life of the single crystal alloy. As the temperature was reduced from 1100 ​°C to 1070 ​°C, the creep life increased from 65.07 ​h to 313.8 ​h. As the stress was reduced to 120 ​MPa, the creep life increased to 243.3 ​h. Under the high temperature and low stress condition the dislocations entered the γ′ phase by climbing caused by the atomic diffusion, instead of slipping.

Mechanisms of femtosecond laser ablation of Ni3Al: Molecular dynamics study

Detailed knowledge of the physical essence of femtosecond laser and Ni3Al interactions is of great significance for the effective processing of film-cooling holes in Ni3Al-based single crystal superalloy turbine blades. Although attempts have been made to understand the femtosecond laser processing of Ni3Al-based single crystal superalloys via experiments or mesoscopic simulation, several phenomena and material removal mechanisms at the atomic level remain obscure. The information obtained in previous studies cannot be directly related to the femtosecond laser processing of Ni3Al-based single crystal superalloys due to the differing intrinsic properties of materials. In this paper, using TTM-MD, three regimes of material response to femtosecond laser irradiation are identified in simulations with a single pulsed 500 fs laser: melting, photomechanical spallation and phase explosion. Several key physical parameters related to the thermodynamic and dynamic behaviour of Ni3Al are obtained at the atomic level, including the fluence threshold, crystal lattice thermal stability and thermodynamic critical temperature. At low fluence, the material behaviour is characterized by heterogeneous surface melting and homogeneous subsurface melting. Photomechanical spallation occurs at higher fluence, which can be interpreted as void nucleation and growth, microcrack formation and eventual large liquid layer ejection. Under extremely high laser fluence, the material behaviour follows the phase explosion mechanism, forming an ablation plume with a layered structure. The dominant driving force change under different fluences causes the mechanism change. Additionally, an integral visual picture is successfully created based on the key parameters obtained, through which the mechanisms throughout the laser spot are revealed.

The role of Re in improving the oxidation-resistance of a Re modified PtAl coating on Mo-rich single crystal superalloy

A novel Re modified β-(Ni,Pt)Al coating was prepared on a Mo-rich Ni3Al-based single crystal (SC) superalloy by electro-deposition of Re/Pt dual films and low-activity aluminizing. The isothermal oxidation behavior of the Re modified β-(Ni,Pt)Al coating and traditional β-(Ni,Pt)Al coating was comparatively studied at 1100 °C. Apparent spallation of oxide scale was found on the surface of traditional β-(Ni,Pt)Al coating after 300 h isothermal exposure, which mainly resulted from the evaporation of Mo-containing oxides. It is further found that the outward diffusion of Mo from the SC substrate was effectively inhibited in the Re modified β-(Ni,Pt)Al coating by forming the σ-MoRe phase both at the coating/superalloy interface and grain boundaries. It is also revealed that the addition of Re can stabilise Mo to form the σ-MoRe phase, allowing the improved oxidation resistance of the Re modified β-(Ni,Pt)Al coating.

Improving cyclic oxidation resistance of Ni3Al-based single crystal superalloy with low-diffusion platinum-modified aluminide coating

A low-diffusion NiRePtAl coating ((Ni,Pt)Al outer layer in addition to a Re-rich diffusion barrier layer) was prepared on a Ni3Al-base single crystal (SC) superalloy via electroplating and gaseous aluminizing treatments, wherein the electroplating procedures consisted of the composite deposition of Ni-Re followed by electroplating of Pt. In order to perform a comparison with conventional NiAl and (Ni,Pt)Al coatings, the cyclic oxidation performance of the NiRePtAl coating was evaluated at 1100 and 1150 °C. We observed that the oxidation resistance of the NiRePtAl coating was significantly improved by the greater presence of the residual β-NiAl phase in the outer layer and the lesser outward-diffusion of Mo from the substrate. In addition, the coating with the Re-rich diffusion barrier demonstrated a lower extent of interdiffusion into the substrate, where the thickness of the second reaction zone (SRZ) in the substrate alloy decreased by 25 %. The mechanisms responsible for improving the oxidation resistance and decreasing the extent of SRZ formation are discussed, in which a particular attention is paid to the inhibition of the outward diffusion of Mo by the Re-based diffusion barrier.

Rotating bending fatigue property of the Ni3Al-based single crystal superalloy IC6SX at 900°C

The high cycle fatigue behavior of a Ni3Al base single crystal superalloy IC6SX has been investigated at 900°C in this work. The specimens used for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests were carried out at 900°Cin air, the stress ratio of R(σmax/σmin) was -1, and the rotating speed of the fatigue tests was 6500r/min(108Hz). The stress-fatigue cycle life (S-Nf) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). It has been found that the median fatigue strength is 457.5MPa and the safety fatigue strength is 413.93MPa. And the fatigue fracture was composed of three different characteristic regions.

Research on the creep mechanism of a Ni3Al-based single crystal superalloy IC6SX under 980°C/205MPa

The Ni3Al-base single crystal superalloy IC6SX used in this work was prepared by spiral grain selection method. And the creep behaviors of the IC6SX alloys was studied systematically under 980°C/205MPa. The microstructure evolution, movement of dislocations, formation of the dislocation networks and dislocation configuration for the alloy during the creep process were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that the creep performance of the single crystal alloy IC6SX is excellent under the condition of 980°C/205MPa. The results showed the creep mechanism included dislocation slipping and diffusion for Ni3Al-based Single Crystal Superalloy IC6SX under 980°C/205MPa.

Effect of heat treatment and long-term age on microstructure of a Ni3Al-based single crystal superalloy

A new Re-containing Ni3Al-based single crystal superalloy was recently developed by our group. In order to obtain uniform microstructure and further improve mechanical properties of superalloy, the heat treatment regime (i.e. solution treatment and age treatment) was investigated by experimental analysis and dynamics calculation (by JMatPro), and the optimal heat treatment regime was 1335–1355°C/16 h, air cooling (AC)+1140°C/2 h, AC+870°C/32 h, AC. Meanwhile, completely heat-treated experimental alloy was long-term aged at 1150°C for 25–1000 h, which showed very good microstructural stability and the coarsening behaviour of γ′ phases agreed well with the typical Lifshitz–Slyozov–Wagner (LSW) theory.

Optimization of Ta Content of a Third Generation Ni<sub>3</sub>Al Base Single Crystal Superalloy

In order to optimize the Ta content of a newly developed 3rd generation Ni3Al-based single crystal superalloy, a series of alloys with the varying of Ta content of 3.0, 4.2, 4.5, 6.0wt. % were prepared. The experimental results showed that with the addition of Ta, the size and volume fraction of bulky γ' phase significantly increased while the primary dendrite arm spacing decreased. Meanwhile, the difficulty of solution treatment significantly increased with increasing Ta content. And the thermodynamic calculation results showed that the volume fractions of γ' phase in this series of alloys were significantly higher than those of typical Ni based single crystal superalloys, indicating that the 3rd generation Ni3Al based single crystal superalloys can serve in the temperature of 1150oC to 1200oC.

Effect of Al Content on Microstructure and Creep Behavior of Low Density Ni<sub>3</sub>Al-Base Single Crystal Superalloy

The influence of Al content on microstructure and creep behavior of Ni3Al-Base single crystal superalloy was investigated. Two Ni3Al-Base single crystal alloys with different Al contents were prepared in a vacuum induction furnace by screw selection crystal method.Optical microscope (OM), differential scanning calorimetry (DSC),scanning electron microscope (SEM),energy dispersive spectrometer (EDS),and X-ray diffraction (XRD) were used to observe the microstructure of the alloys.The experimental results showed thatthe volume fraction, mean size of the eutectic phase γ-γ′, γ′before and after heat treatment display differences with the Al content. It has been found that the stress-rupture lives increased with theAl content.The reasons may be as follows.Firstly, as content of Al increases, the reinforcing phaseγ′increases, resulting in the improvement of the precipitation strengthening.And secondly, the increasing of Al content leadsthe decreasing lattice mismatch of γ/γ′ phase, which enhances the coherent strain field around theγ′ phase and the hence increase the strengthening effect to the alloy.

The influence of thermal exposure on the microstructure of a Ni3Al-based single crystal superalloy

The effect of thermal exposure on the microstructure of an Ni3Al-based single crystal superalloy was investigated. The Ni3Al-based single crystal superalloy used in this experiment contained a high content of molybdenum (Mo) and a low content of chromium (Cr), which had good high-temperature strength and oxidation resistance. The alloy was exposed at 1020–1200°C for different time periods. The microstructure after thermal exposure was examined by scanning electron microscope (SEM) and field emission microscope (FEM). The results indicated that precipitation of needle-like topologically close-packed (TCP) phases was only observed at certain intermediate temperature intervals (1020–1080°C). Moreover, once the TCP phases precipitated, they would not dissolve back even after thermal exposed for 1000 h. No precipitation phases were observed after high-temperature thermal exposure at 1100–1200°C for 25 h.

Investigation on Creep Mechanism of a Ni<sub>3</sub>Al-Based Single Crystal Superalloy IC6SX under 760°C/540MPa

The creep behaviors of a Ni3Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically under the testing condition of 760/540MPa. The microstructure evolution, movement of dislocations, formation of the dislocation networks and dislocation configuration during the creep process were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the creep performance of the single crystal alloy IC6SX is excellent under 760/540MPa. The experimental results showed that the creep curve of the Ni3Al-base single crystal superalloy IC6SX was divided into three stages, including decelerating creep stage, steady-state creep stage and accelerated creep stage. The microstructure and the dislocation configuration were different at different stage during the creep and the raft microstructure has not been formed. The creep mechanism was main slipping characterized by dislocation glide mechanism.

Grain Competition Mechanism of a Ni<sub>3</sub>Al-Based Single Crystal Superalloy IC6SX

The grain competitive growth and elimination during the directional solidification of a Ni3Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically. The experimental results revealed that there were 5 kinds of mechanism during the grain competitive growth and elimination. The grains with preferred growth direction and smaller deviation angle to growth direction have stronger competitiveness, and the mutual thwarting of dendrites played an important role in the processing of grains competitive growth. The results can explain the competitive growth mechanism during the directional solidification and can be used to optimize processing parameters to lay an important foundation for improving preparation processes of single crystal superalloys.

Microstructure and Oxidation Behavior of Modified Aluminide Coating on Ni3Al-based Single Crystal Superalloy

Al, Al-Si and Cr-Al-Si coatings are prepared onto IC20 alloy by powder pack cementation to improve the oxidation resistance of the alloy. The isothermal oxidation behaviors of the coatings and the uncoated IC20 alloy at 1 100 °C are comparatively investigated. For the coatings, less weight gains are obtained compared to the uncoated alloy after 100 h thermal exposure. The Al-Si coating exhibits the best oxidation resistance, while the addition of Cr accelerates the scale spallation. Phase transformation from β-NiAl to γ and γ′ occurs in the coating after oxidation for extended periods. The oxidation protection and degradation mechanisms for these coatings are discussed.

Kinetics and microstructural evolution during recrystallization of a Ni3Al-based single crystal superalloy

The recrystallization kinetics and microstructural evolution of a Ni3Al-based single crystal superalloy were presented, especially the different recrystallization behaviors between the dendrite arm and the interdendritic region. The single crystal alloy was deformed by grit blasting. A succeeding annealing under inert atmosphere at 1280°C for different time led to the formation of recrystallized grains close to the grit blasting surface. It was found that the recrystallization depth and velocity in the dendrite arm were respectively deeper and faster than those in the interdendritic region where the Y-NiMo phase existed. The recrystallization process in the interdendritic region was significantly inhibited by the Y-NiMo precipitates. However, the pinning effect gradually weakened with the annealing time due to the dissolution of the Y-NiMo phase, and the recrystallization depth in the dendrite arm was deeper than that in the interdendritic region.