Generation Single Crystal Superalloy DD6 Research Articles

Microstructure Evolution and Stress Rupture Properties of DD6 Single Crystal Superalloy after Overheating

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.

Stress Rupture Properties and Fracture Behavior of DD6 Single Crystal Superalloy

The stress rupture properties, fracture behavior and microstructure evolution of the second generation single crystal superalloy DD6 at different conditions were investigated. The results show that the alloy has excellent stress rupture properties. The fracture mechanism of stress rupture of the alloy at 760°C/785MPa, 760°C/750MPa and 760°C/720MPa shows quasi-cleavage mode and the fracture mechanism at 980°C/250MPa, 1070°C/160MPa and 1100°C/140MPa shows dimple mode, while the fracture mechanism at 850°C/550MPa shows quasi-cleavage and dimple mixture mode. Only a little change in the morphology of γ′ phase occurred at 760°C/750MPa. The γ′ rafts form at temperature above 760°C and the thickness of rafts increases with increasing temperature. The dislocation shear mechanism including stacking fault formation is operative at lower temperature and high stress. The dislocation by-passing mechanism occurs to form networks at γ/γ′ interface under the condition of high temperature and lower stress.

Effects of Long-Term Aging on the Microstructures and Tensile Properties of Ni-Based Single Crystal Superalloy

Abstract The microstructure evolution and the tensile properties of the second generation single crystal superalloy DD6 were investigated after full heat treatment and aging process at 980–1200 °C. The results indicate that the morphological instability occurs at relatively high temperature, and the γ ' phase gradually connects and rafts after aged for 800 h at 1050 °C. After aged for 100 h at 1200 °C, the γ / γ ' microstructure cubicity gradually becomes weaker and transforms into spherical boundary, accompanied with a few finer matrix phase embedded into strengthening phases. Whereas the dislocation lines are homogeneously distributed in the γ / γ ' interface area, and dislocation motions are also strengthened with increasing of aging time and temperature. After aged at 1050 °C for 1000 h, the precipitation phase in the form of blocky appears in the enrichment areas of solution strengthening elements and its brittle characteristics will pile up dislocation motions at lower temperature. The ultimate tensile strength, yield strength and elongation percentage decrease with increasing of aging time, while a fluctuation of contraction area will occur at 760 °C

Tensile Behavior of the Second Generation Single Crystal Superalloy DD6

Tensile properties of the second generation single crystal superalloy DD6 were investigated from 20 °C to 1 100 °C. Microstructure evolution and fracture mechanism were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the tensile strength decreases slightly with increasing temperature from 20 °C to 400 °C. The tensile strength of the alloy increases with the increase of temperature from 400 °C to 800 °C. Above 800 °C, the yield strength of the alloy decreases greatly with increasing temperature. The elongation and contraction of area almost present opposite tendency in contrast to changes of the tensile strength. At lower and intermediate temperature (from 20 °C to 850 °C), the tensile fracture mechanism shows quasi-cleavage mode, while at high temperature (980 °C and 1 100 °C), it is dimple mode. The γ′ precipitate morphology still maintains cubic after tensile fracture at lower and intermediate temperature. The γ′ phase changes into rectangular solid at high temperature. The γ′ phase is sheared by anti-phase boundary (APB) or stacking faults at lower and intermediate temperature. At high temperature, dislocations overcome γ′ through by-passing mechanism.

Kinetics of cellular discontinuous precipitation in a Re-containing single crystal superalloy

Cellular recrystallization behavior of a second generation single crystal superalloy DD6 was studied experimentally by annealing at sub-solvus temperature of 1100°C in this work. During cellular growth, the microstructure in the direct vicinity of recrystallization reaction front was investigated by combined electron microscopy techniques. The results exhibited strong recovery and subgrain formation in the grit-blasted original matrix. Compositional mapping measured across the advancing recrystallization reaction front shows that a thin layer of γ-forming elements always exists in the recrystallization reaction front. By diffusion ahead and inside the reaction front, the chemical deviation from local equilibrium at the interface results in a force pulling the grain boundary forward.

Effect of Hf Content on Oxidation Resistance of Single Crystal Superalloy at 1000°C

The specimens of single crystal superalloy DD6 with 0.10% Hf and 0.47% Hf were prepared in the directionally solidified furnace. The effect of Hf content on the isothermal oxidation resistance of the second generation single crystal superalloy DD6 was studied at 1000°Cin ambient atmosphere. Morphology of oxides was examined by SEM, and their composition was analyzed by XRD and EDS. The experimental results show that the oxidation resistance of DD6 alloy with 0.47% Hf is better than that of the alloy with 0.10% Hf. The alloy with different Hf content all obeys parabolic rate law during oxidation for 100h at 1000°C. The increase of Hf content can promote the Al2O3 formation and decreases the proportion of NiO. The oxide grain size and the thickness of the oxide layer all reduce with increasing of Hf content. The oxide scale of the alloy with different Hf content is made up of an outer NiO layer with a small amount of Co3O4, inner Al2O3 and Cr2O3 layer with a small amount of TaO2.

Effect of Hf on Stress Rupture Properties of DD6 Single Crystal Superalloy After Long Term Aging

The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1040 °C for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980 °C/250 MPa of the alloy after long term aging was investigated. The results show that the γ' coarsening and rafting and no topologically close packed phase (TCP) are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1040 °C for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980 °C/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed.

Effect of LAB on the Stress Rupture Properties and Fracture Characteristic of DD6 Single Crystal Superalloy

The slabs of the second generation single crystal superalloy DD6 were cast using bi-seeded crystals with symmetric twist low angle grain boundaries. The influence of low angle boundary (LAB) on the stress rupture properties of the alloy was investigated at 760 °C/758 MPa, 850 °C/550 MPa and 980 °C/250 MPa. SEM was used to study the fracture surface and the failure mechanism. The results show that when the misorientation angle of LAB is relatively small it has a less effect on the stress rupture properties of the alloy. The stress rupture properties decrease with the misorientation increase on the same test conditions. The fracture characteristic of specimen shows intergranular rupture trend with the misorientation increases on the same conditions or with increasing of temperature and decreasing of stress for the same misorientation of LAB.

Isothermal oxidation behavior of single crystal superalloy DD6

The isothermal oxidation behavior of the second generation single crystal superalloy DD6 was studied at 1050 °C and 1100 °C in ambient atmosphere. Morphology of oxides was examined by SEM and their composition was analyzed by XRD and EDS. The experimental results show that DD6 alloy obeys subparabolic rate law during oxidation of 100 h at 1050 °C and 1100 °C. The oxide scale exposed at 1050 °C is made up of an outer NiO layer with a small amount of Al2O3 and an inner Al2O3 layer. The oxide scale exposed at 1100 °C is made up of an outer Al2O3 layer with a small amount of NiO, an intermediate layer, mainly composed of Cr2O3 and TaO2, and an inner Al2O3 layer. The γ′-free layer was formed under the oxide scale at two temperatures.

Microstructures of Low Angle Boundaries of the Second Generation Single Crystal Superalloy DD6

The specimens of low angle boundaries were machined from the second generation single crystal superalloy DD6 blades. The microstructures of low angle boundaries (LAB) were investigated from three scales of dendrite, γ′ phase and atom with optical microscopy (OM), scanning electron microscope (SEM), transition electron microscope (TEM) and high resolution transmission electrion microscopy (HREM). The results showed that on the dendrite scale LAB is interdendrite district formed by three dimensional curved face between the adjacent dendrites. On the γ′ phase scale LAB is composed by a thin layer γ phase and its bilateral imperfect cube γ′ phase. On the atom scale LAB is made up of dislocations within several atom thickness.

High cycle fatigue behavior of the second generation single crystal superalloy DD6

The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf (in mass fraction) was subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C in ambient atmosphere. SEM was used to determine the initiation site and the failure mechanism. Evolution of the microstructure was investigated by TEM observation. The results show that fatigue limit of DD6 alloy with 0.34%Hf is a little smaller than that of the alloy with 0.10%Hf. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along {111} octahedral slip planes, rather than perpendicular to the loading axis of specimen. Typical fatigue striation formed in steady propagation of fatigue crack. The fracture mechanisms of the high cycle fatigue of DD6 alloys with 0.10%Hf and 0.34%Hf are quasi-cleavage fracture. Different types of dislocation structures were developed during high cycle fatigue deformation.

Stress Rupture Properties of the Second Generation Single Crystal Superalloy DD6 after High Temperature Exposure

The effects of high temperature exposure simulating service conditions on stress rupture properties were studied for the second generation single crystal superalloy DD6. The specimens with [001] orientation were exposed in air at temperatures of 980°C and 1070°C for 100h to 1000h. They were then tested using conventional mechanical tests at 1070°C/140MPa to determine the effects of exposure on stress rupture properties. The analysis indicated that stress rupture life decreased with increasing exposure time. At the temperature of 980°C, the stress rupture life is more than 180h after exposure for 1000h. When the test temperature increased to 1070°C, the stress rupture life exceeds 100h after 800h exposure. The morphology of γ prime phase after exposure was observed by using scanning electron microscopy (SEM). Morphologies evaluations have shown that alloy DD6 exhibits excellent microstructure stability after exposure. TCP (Topologically Closed Packed) phases have not been observed. It has been also found that the morphology and size of γ prime affected stress rupture life of the alloy. The decrement in stress rupture life with increasing exposure is a result of γ prime rafting.

Creep Behavior of Single Crystal Superalloy DD6 at 760°C and 980°C

The tensile creep properties of the second generation single crystal superalloy DD6 with [001], [011] and [111] orientations were investigated under different stress levels at 760°C and 980°C, and the creep mechanism of DD6 alloy was discussed. The results show that DD6 alloy possesses good creep properties, and obviously exhibits anisotropic behavior. The deviation degree between stress axis and [001] crystallizing orientation gradually deceases, while deviation degree between stress axis and [011], [111] crystallizing orientations increases due to lattice rotation during creep. The alloy with [001] orientation provides geometry hardening tendency, but the alloy with [011] and [111] orientations supplies geometry softening tendency. The creep deformation of DD6 alloy with three orientations mainly concentrates on the matrix channels and y' phase is stable at 760°C. The alloy demonstrates complicated creep behavior and the creep deformation with three orientations is situated at both matrix channels and y' phases at 980°C, and the directional coarsening of γ' phase occurs.