Development of Cast Superalloys for Gas Turbines in China

As in other countries,significant achievements in the research of cast superalloys for many years have also been obtained in China.These results are important contribu- tion to the development of aero and land-based gas tur- bine engines.

R & D OF CAST SUPERALLOYS AND PROCESSING FOR GAS TURBINE BLADES IN BIAM

Research and development of cast superalloys and processing for turbine blades in BIAM during the last 35 years have been reviewed briefly in this paper.

Effects of cerium addition on the microstructure and stress rupture properties of a new nickel-based cast superalloy

The microstructure and stress rupture properties of a new nickel-based cast superalloy were investigated with the cerium(Ce)additions of 0,19,50,96,150,and 300 ppm,respectively.The results indicated that Ce was mainly found in M C and M 23 C 6 carbides,and it was also found to exist in the form of cerium-rich phases or inclusions.According to the microstructure evidence,Ce promoted the formation of M C carbides and aggravated the inhomogeneity of M 23 C 6 carbides along grain boundaries.It was also identi-fied that the average sizes of primary and secondaryγ’phases all decreased with the rising Ce content.The acceleration of Ti,Nb,and C segregations during solidification was attributed to the influence of cerium on the variation of carbides along grain boundaries.The stress rupture life experienced a signifi-cant drop as the Ce content increased from 19 to 300 ppm.Explorations showed that the degradation was mainly attributed to the severe degradation of M C carbides and the easily forming micro-voids around them caused by the Ce addition.In addition to that,the increments in the inhomogeneous distribution of M 23 C 6 carbides at grain boundaries and the accelerated coarsening rate ofγ’phases both induced the fracture under complex stress conditions.

Effect of Phosphorus on Microstructure and High Temperature Properties of a Cast Ni-base Superalloy

Effect of phosphorus on the microstructure and high temperature properties of a cast Ni-base superalloy M963 has been investigated. SEM observation and EDS analysis showed that P was mostly enriched in the interdendritic region, and the P-rich phase was formed in the front position of finally solidified eutectics in high P doped alloys. It was found that the P-rich phase, as preferred initiation and propagation site of cracks, could aggravate the fracture process at high temperature in high P doped alloys. Consequently, high P addition would reduce remarkably the ductility and creep life of M963 superalloy at high temperature.

Characteristics of High Temperature Rupture of a Cast Ni-Based Superalloy M963

The rupture behavior of a cast Ni-base superalloy M963 at high temperature has been investi- gated. The microstructure examination shows that there exists a large amount of the carbide and γ-γ' eutectic, which is very harmful to the mechanical properties of M963 superalloy. The tensile strength of M963 superalloy both at room temperature and at high temperatures is higher than that of K17G alloy, but the tensile ductility of the former is much lower than that of the latter. In tensile fracture process with the high strain rate, the open carbides are the initiation site and the carbide/matrix interface is the propagation path of cracks. But in fracture process with the low strain rate, the carbide/matrix interface and cast microvoids are the initiation sites, and the carbide/matrix interface is the propagation path of cracks. The effective ways to improve ductility of M963 superalloy are also suggested.

Effect of Melt Superheating on the Morphology of MC Carbide in a Cast Ni-base Superalloy M963

The effect of the melt superheating temperature on the as cast microstructure of a cast nickel base superalloy M963 has been investigated. The results show that the as cast microstructure of the alloy consists of γ solid solution matrix,γ′ precipitate in cubic shape, (γ+γ′) eutectic and MC carbide, and the morphology of MC carbide in the microstructure can be varied from coarse scriptlike, fine scriptlike to fine cubelike or discontinuous particles by increasing the melt superheating temperature. The mechanism of melt superheating is discussed by means of differential thermal analysis (DTA) technique.

SEGREGATION OF ALLOYING ELEMENTS IN CAST Ni-BASE SUPERALLOYS

An investigation was made on the possibility of using Ni-x phase diagrams to judge the sym- bol and degree of segregation of alloying element x in cast nickel-base superalloys.The pre- diction coincides with the experimental data.

Effect of Magnesium and Cerium Microelement on Microstructures and Mechanical Properties of K465 Nickel-Base Superalloy

0.02%(mass fraction) Mg and 0.02% Ce were added into nickel-base superalloy, K465, in order to study their effect on microstructures and mechanical properties. The results showed that without addition of Mg and Ce, the grain boundaries was flat, the MC carbide precipitate was noncontinuous with particle shape. With 0.02% Mg addition the MC carbide tended to have tiny granular and blocky morphology, while it had a cursive script-like morphology with 0.02% Ce addition. It was also shown that without any addition of Mg and Ce, the room temperature properties, average tensile strength (σb) and yield strength (σ0.2) were 960 and 790 MPa, the creep rupture life under the condition of 1248 K, 230 MPa was 28.1 h. With the addition of 0.02% Mg, the room temperature properties, average tensile strength and yield strength had a certain enhancement, the creep rupture life (1248 K, 230 MPa) was up to 66.7 h. When the Ce addition was 0.02%, average tensile strength and yield strength were up to 1020～1030 MPa and 855～885 MPa, respectively, but the average elongation percentage fell down to 3.0%, only reaching the required value. The most expressive thing was that the creep rupture life (1248 K, 230 MPa) was only 17.25～18.77 h, far below the required value.

Thermal Fatigue Behavior of Ni_3Al Based Superalloy IC6E

The thermal fatigue behavior of Ni3Al based superalloy IC6E during the cycles between 900 ℃/1 000 ℃ and the room tempera- ture was investigated. The experimental results indicate that the primary and secondary thermal fatigue cracks initiate inside or round the borides and then spread away along grain boundaries and/or in interdendritic areas. The fracture of borides and their separation from the matrix at interfaces are mainly responsible for the crack initiation and its spreading. At temperatures higher than 1 000 ℃, the grain boundary oxidation combined with cyclic stresses accelerates the crack growth.

DirectionaHy Solidified High-strength Superalloy DZ22

Alloy DZ22 possesses rather high intermediate and elevated temperature mechanical properties comparable with the advanced commercial DS al- loy PWA1422.The alloy composition is based on PWA1422 alloy,only the content ranges of Hf,C and Zr are changed for systematic study.The pres- ent melting technology for master alloy ingot and DS process parameters for components are suitable for this alloy.The alloy revert which has been specially processed can be reused.The mould with- drawal rate during directional solidification affects the crystal orientation,alloy structure and the me- chanical properties of DS castings.The increment of solution-treatment temperature increases obvi- ously the longitudinal stress-rupture life at high temperature but decreases transversal one at inter- mediate temperature.The production practice in batches has demonstrated that alloy DZ22 has not only high mechanical properties but also excellent processing performance,including castability, weldability and machinability.It is indeed a favorable material for advanced aero and land gas turbine engine blades with complex inner cooling passage.

Preparation and tensile properties of DD5 single crystal castings

The preparation procedure of DD5 single crystal castings was optimized. The microstructure characteristics of DD5 single crystal superalloy were investigated by microstructure observation and segregation behavior examination. The results show that the grain orientation is optimized by constraining the spiral crystallizer in [001] orientation and spatial scale. Also, the γ＇ phase of inter-dendrites is larger and more irregular than that in dendrite arms. High temperature tensile tests of DD5 single crystal castings exhibit that the peak stress increases with increasing temperature, while the area reduction shows an opposite trend, when the temperature is below 800℃; meanwhile, when the temperature is between 800℃ and 1000℃, the fracture stress of the alloy is the same as the peak stress. The fracture mode changes from shear to ductile with increasing temperature from 600℃ and 1000℃.

Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900℃ and stress rupture test was performed at 950℃ with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ＇phase and the size of γ/γ＇eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ＇ eutectic decreased.In the cooling rate range of 1.42℃s^-1–0.849 s^-1,the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ＇ eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84℃ s^-1 showed the best comprehensive mechanical properties.

Effect of Cooling Rate on Microstructure and Mechanical Properties of K465 Superalloy

K465 superalloy, as a material for production of turbine nozzle, shows high mechanical properties as well as microstructure stability under critical and severe service conditions. The appropriate microstructure and strength of this alloy can be obtained by solid solution strengthening mechanism. Heat treatment parameters such as time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from solid solution temperature affect the microstructure of the alloy. Among these parameters, cooling rate from solid solution temperature is the most effective. Therefore, the effect of cooling rate on microstructure and mechanical properties （tensile and stress properties） was investigated. For this purpose, three different cooling rates were applied to the cast K465 specimens subjected to solution treatment at 1 483 K for 4 h. Microstructures of the specimens then were studied using optical and electron microscopy. Also, tensile tests were performed at room temperature and stress rupture tests were performed under the condition of 1 248 K and 230 MPa. It was found that with increasing the cooling rate, the size of the γ＇ precipitates decreases and the mechanical properties of specimens increases. Also, it was found that the shape and volume fraction of primary γ＇ particles are largely influenced by the cooling rate after solution treatment at 1 483 K for 4 h.