Single Crystal Superalloy DD6 Research Articles

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.

Effect of cooling rate after solution on microstructure and creep properties of single crystal superalloy DD3

The effect of different cooling rate on microstructure and creep properties of DD3 single crystal superalloy after solution treatment was investigated. 10 °C/min cooling rate is adverse to the creep properties performed at all the conditions of 760 °C/786 MPa, 982 °C/210 MPa and 1038 °C/161 MPa. The best creep lives can be obtained with 149 °C/min cooling rate specimens at 760 °C/786 MPa. At 982 °C/210 MPa and 1038 °C/161 MPa, a cooling rate of 38 °C/min shows the best creep properties in the four kinds of bars. The experimental results show that the size of γ′ precipitates decreases with the increasing of cooling rate from 10 °C/min to 149 °C/min. During the creep test, the directional coarsening of γ′ precipitates occurred and γ′ rafting is formed.

Effect of Hf Content on the Microstructures and Stress Rupture Properties of DD6 Single Crystal Superalloy

Abstract The effect of Hf content on the microstructures after heat treatment and stress rupture properties of DD6 single crystal superalloy was investigated. The results show that with increasing of Hf content in the range of current research, the residual γ/γ' eutectic increases apparently, the size of γ' particles in dendritic cores has no obvious change and that in interdendritic decreases after heat treatment. With the increase of Hf content, the stress rupture life increases at first and decreases afterwards, and the elongation displays fast increasing first, and slow rising afterwards at 1100 °C, 140 MPa. At last the relationship between the microstructures and stress rupture properties of DD6 alloy with different Hf contents was discussed.

Surface Recrystallization in Nickel Base Single Crystal Superalloy DD6

The samples of single crystal superalloy DD6 are grit blasted and then heat treated either with the standard heat treatment procedure or in the temperature range of 1 000-1 250 °C for 4-16 h at vacuum atmosphere, then the recrystallization behavior of DD6 alloy is investigated. The results show that the equiaxed recrystallization grains form in the γ phase region where the as-cast γ′ phases have been dissolved completely, and cellular recrystallization forms in the region where the as-cast γ′ phases have been dissolved partially. The cellular recrystallization area consists of cellular grains, and the cellular grain consists of cubic γ′ phase, lamellar γ′ phase and γ+γ′ The coexistence of the equiaxed recrystallization zones and cellular recrystallization zones is a recrystallized characteristic of the cold worked single crystal samples which are heat treated at a temperature lower than the solution temperature. When the heating temperature is higher than 1 150 °C, with the increase of heat treating temperature, the equiaxed recrystallization zone expands, whereas the cellular recrystallization zone shrinks. All the deformed regions are consumed by equiaxed recrystallization after annealing at solution temperature.

Effects of carburization on recrystallization behavior of a single crystal superalloy

The samples of single crystal superalloy DD6 were grit blasted and heat treated in the solution and aging treatment at vacuum atmosphere, then the effects of carburization on recrystallization of single crystal superalloy DD6 were investigated. The results showed that carbon was introduced into the grit blasted samples during carburization, producing a homogenous distribution of MC-type carbides in the form of submicron dispersed precipitates within the grit blasted region. These MC-type carbides hold back the recrystallized grain boundaries. As temperature increases, MC-type carbides dissolve and secondary M 6C-type carbides precipitate. The interaction between these M 6C-type carbides and recrystallized grain boundaries further hinders recrystallization. Thus the average depth of the recrystallization region and number of recrystallized grains observed in carburized grit blasted samples are less than those in the samples which underwent grit blasting process alone. Carburization can significantly reduce recrystallization in single crystal superalloy DD6 during solution heat treatment.

Application of inverse method to estimation of boundary conditions during investment casting simulation

Inverse method was used in single crystal superalloy DD6 processing simulation during solidification. Numerical modeling coupled with experiments has been used to estimate the interface heat transfer coefficient (IHTC) between the surface of slab casting and inner mold. Calculated temperature dependent values of IHTC were obtained from a numerical solution. The calculated temperatures agreed well with the measurement of cooling profile.

REFINED DENDRITE AND PRECISE ORIENTATION OF NICKEL-BASED MONOCRYSTAL SUPERALLOY WITH MELT SUPERHEATING TREATMENT

Ni -based monocrystal superalloy with fine dendrite, precise orientation and finally elevated service performance is required as turbine blades in advanced aeroengine. In this paper, the melt superheating treatment is carried out for DD3 single crystal superalloy during directional solidification processing. The thermal gradient of solidification interface remains constant for different melt superheating temperatures with the appropriate superheating treatment regulation. It is shown that with the melt superheating temperature increased from 1500°C to 1640°C and then to 1780°C respectively, both the primary and the secondary dendrite arms are obviously refined and the dendrite segregation ratios of Mo , W and Ti are reduced while the deviation angle of (001) crystal orientation can all be controlled within 5°. The dendrite structure refinement and the crystal orientation consistence should be attributed to the melt structure change and heredity which is caused by the superheating treatment.

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.

Development of a nucleation inhibitive coated mold for the rapid solidification of superalloy melt

The development of a glass-lined mold from gels in the ternary system of SiO 2–ZrO 2–B 2O 3 by adopting the shell molding technique and sol–gel processing has been investigated. The crystallization characteristics of ZrO 2 particles and the high temperature structure stability of Si–Zr–B coating were demonstrated. The inhibitive effect of B 2O 3 on crystallization in the Si–Zr–B coating was explained. Finally, the result that DD3 single crystal superalloy can obtain high undercooling and realize rapid solidification in it establishes that the Si–Zr–B coated mold has an ideal non-catalytic nucleation inhibition for superalloy melt.

Solidification of superalloy in a SiO 2–ZrO 2–B 2O 3 coating mould

The preparation of glass-lined coating mould from gels in the ternary system of SiO 2–ZrO 2–B 2O 3 has been investigated. The crystallization characterization and high temperature structure stability of this coating mould are demonstrated. We can find that the crystallization of t-ZrO 2 as well as the tetragonal to monoclinic phase transformation are, respectively, retarded and impeded owing to the encasement of SiO 2 matrix. While the inhibitive effect of B 2O 3 on crystallization of the SiO 2–ZrO 2–B 2O 3 coating mould is explained. Finally, DD3 single crystal superalloy melt can realize highly undercooled rapid solidification by adopting this coating mould, which further evinces that SiO 2–ZrO 2–B 2O 3 coating mould has an ideal nucleation inhibition for superalloy.

Solidification of undercooled molten Ni-based alloys

The effect of undercooling on grain structure is investigated in pure nickel, Ni75Cu25, and DD3 singlecrystal superalloy by employing the method of molten salt denucleating combined with thermal cycling. Meanwhile, a comparison of factors that may be related to structure formation is performed and the difference in the refined structure between Ni75Cu25 alloy and DD3 single-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low and high undercoolings in Ni75Cu25 and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dendrite remelting driven by the chemical superheating produced in recalescence, and the second one at high undercoolings is due to the recrystallization process as a result of the high stress provided in the rapid solidification after high undercooling. Dislocation morphology evolution in as-solidified structure is also provided by the transmission electron microscopy (TEM) technique to further verify the recrystallization mechanism.

Special shell mold for solidification of undercooled nickel-based alloy melts

Shell molds composed of SiO 2, SiO 2–B 2O 3, SiO 2–ZrO 2–B 2O 3, and ZrO 2, Al 2O 3, as well as ZrSiO 4, were prepared. Various undercoolings were obtained with pure nickel, Ni–Cu alloy, and the commercial DD3 single crystal superalloy melts in these molds. It was found that all these molds have a good non-catalytic nucleation effect on pure nickel and Ni–Cu alloy melts, whereas this effect is not obvious with respect to DD3 single crystal superalloy. Only in shell mold composed of ZrSiO 4, SiO 2–ZrO 2–B 2O 3, and ZrO 2 can the DD3 single crystal superalloy be undercooled substantially. Based on the investigation of the structure of shell molds and the calculations using classical nucleation theory, it is demonstrated that ZrSiO 4, SiO 2–ZrO 2–B 2O 3, and ZrO 2 shell molds have an ideal non-catalytic nucleation effect on the DD3 single crystal superalloy melt. Large undercooling and rapid solidification of DD3 single crystal superalloy can be achieved in these three shell molds.

γ′ Formation and morphology evolution in the undercooled structure of DD3 Single crystal superalloy

An investigation of the γ′ formation and its morphology evolution in the bulk undercooled DD3 single crystal superalloy is performed. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been respectively discussed with respect to undercooling, nucleation and solute segregation during solidification and post-solidification, by employing the classical nucleation theory. Furthermore, TEM and SEM technique are adopted to reveal the γ′ microscopy and to illustrate other factors correlated to the γ′ precipitation. It is found that the size, fraction, and distribution of precipitated γ′ in as-solidified DD3 single crystal superalloy are all influenced by melt undercooling.

Crystallization of gels in the SiO2-ZrO2-B2O3 system

The preparation of glasses from gels in the ternary system of SiO2-ZrO2-B2O3 has been investigated. The crystallization character of ZrO2 particles and high-temperature structure stability in the SiO2-ZrO2-B2O3 gels were analyzed. The effect of B2O3 as the inhibitor of crystallization was illustrated. Finally, the result that the purified DD3 single crystal superalloy melt can retain high undercooling in the SiO2-ZrO2-B2O3 coating mold indicates that the coating has an ideal non-catalytic nucleation property.

Dendrite growth in undercooled DD3 single crystal superalloy

Crystal growth in the undercooled DD3 single crystal super-alloy is investigated in this paper. With the increase of undercooling, two kinds of dendrites are obtained. When ΔT < 30 K, dendrite growth is dominantly controlled by solutal diffusion, and the as-solidified morphologies are similar to the conventional as-cast highly branched dendrites. In the undercooling range of 78–150 K, the severe solute trapping that originates from high dendrite growth velocity weakens the effect of solutal diffusion on the dendrite growth. Thermal diffusion, controlled by heat gradient, instead of solutal diffusion, is found to become the key factor in determining the formation of largely developed fine dendritic morphologies. Furthermore, the measured dendrite arm spacing are explained by employing the current dendrite growth theories.

Structure evolution in undercooled DD3 single crystal superalloy

A substantial undercooling up to 210 K was produced in the DD3 single crystal superalloy melt by employing the method of molten salt denucleating combined with thermal cycle, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–210 K, the solidification structure could be classified into four categories. At the lower range of undercooling, 0–25 K, the dendrite growth is dominantly controlled by solute diffusion, so the formed dendritic morphologies are similar to those of the conventional as-cast structure. With the increase of undercooling, the alloy undergoes two grain refinements. The first grain refinement occurs in a certain range of undercooling, 30–70 K, because of the dendrite break-up or ripening owing to remelting. At a higher range of undercooling, 78–150 K, however, the severe solute trapping that results from high dendrite growth velocity weakens the effect of solutal diffusion on the dendrite growth. In this case, highly developed fine dendrite is formed as a result of the restrained ripening process. The decrease of the grain size, i.e. the formation of fine equiaxed structure above the critical undercooling (Δ T*=180 K), can be attributed to the stress that originates from the extremely rapid solidification process, which results in the dendrite distortion, disintegration and recrystallization finally.