Materials Science Forum Vols. 638-642

Abstract: The process of new fine grain formation was studied in compression of a Ni-20%Cr alloy at temperatures ranging from 500°C (0.46Tm) to 950°C (0.73 Tm) at a strain rate of 710-4 s-1. Two types of deformation behaviors with different features of microstructure evolution were observed, depending on processing conditions. The deformation behavior under flow stresses below about 500 MPa (relatively high temperatures) was typical for hot working associated with discontinuous dynamic recrystallization (DRX). The extensive local migration (bulging) of both initial and deformation induced high angle boundaries (HAGBs) resulted in the development of nuclei, which grew out leading to the formation of recrystallized structure with grain size of D > 1 m. Numerous annealing twins were observed within these DRX grains. On the other hand, continuous DRX gave a major contribution to the formation of new grains at applied stresses above 500 MPa (relatively low temperatures). This fact was attributed to a low mobility of grain boundaries. The new grains with size of D < 1 m were evolved due to gradual transformation of deformation induced low angle boundaries (LAGBs) into HAGBs.

Abstract: New experimental methods developed by the authors for characterisation of the structure of single-crystal nickel-base superalloys are presented: X-ray diffraction at defined areas of the dendritic structure, none destructive SEM investigation of ’-morphology changes for different creep stresses and times with one specimen, and high resolution SEM investigations of dislocations in the ’-interfaces. Application of these techniques allowed to obtain new results, e.g. about the mosaicity of dendritic subgrains, distribution of the ’-misfit within a single dendrite and kinetics of rafting during creep in the superalloy CMSX-4 in wide temperature and stress ranges.

Abstract: In 1999, we proposed the concept of a virtual gas turbine system which is a combination of turbine design and material design programs. Using this system, it has become possible to design a gas turbine engine and a combined cycle automatically, by inputting some basic information such as power output, turbine inlet temperature and material specifications. The derived outputs are turbine gas path dimensions, gas and cooling air flow rates, thermal efficiency, CO2 emissions, etc. We use the system to evaluate the potential improvement if a newly developed material is to be used in building the engine. Based on the virtual gas turbine system we have begun developing the virtual jet engine system, which can simulate the operation of a jet engine or a gas turbine engine to predict the degradation of materials used in the high temperature parts of the engine. The system consists of a thermal and aerodynamic analysis of the engine, a thermal and stress analysis of hot parts, and a material degradation analysis. Actual engine dimensions, operation data and material specifications are used to perform the analyses. In this paper, we will show some of the results of the use of the virtual gas turbine system, and then describe the development plan and the preliminary output of the virtual jet engine system.

Abstract: The formation of serrated grain boundaries and its subsequent effect on creep resistance have been investigated in a wrought nickel based superalloy Nimonic 263. The grain boundaries were considerably serrated without the presence of γ' phases or M23C6 when a specimen was slow-cooled from the solution treatment temperature. The high resolution observation on the lattice image of the serrated grain boundary suggested that the grain boundaries tended to serrate to have specific segments approaching to one {111} low-index plane at a boundary in order to have lower interfacial free energy of grain boundary. The grain boundary serration led to a change in M23C6 carbide characteristics: the carbide morphology from granular to planar, a lowered density and their coherency pattern to two neighboring grains from consistent to zigzag. The improvement of creep resistance was noticeably observed by the introduction of GB serration without deterioration of basic mechanical properties. This improvement in creep resistance by the serration was associated with a lower rate of cavitation and crack propagation through the modification of carbide characteristics as well as grain boundary configuration.

Abstract: The effects of thermal physical parameters and boundary conditions on investment solidification parameters were obtained using a computer simulation system. Directional solidification parameters of single crystal superalloy include the temperature distribution, the position and the shape of the solid/liquid interface in the mushy zone of the solidifying blade casting. Commercial finite-element analysis software, ProCAST, was used to simulate the solidification processes of the castings of single crystal DD6. The simulation results indicate that the predictions of the temperature show little sensitivity to the thermal physical parameters and boundary conditions. Further, it has also been shown that the location and the shape of solid/liquid interface is related to the boundary conditions of simulation. Increasing the value of interface heat transfer coefficient decreases the width of mushy zone.

Abstract: The effect of rhenium on the microstructure and mechanical properties of single crystal superalloys with a nominal composition of Ni-3Cr-12Co-1Mo-6W-6Al-8Ta-0.1Hf-(0, 2, 4) Re has been studied. With the rise of rhenium content, the size of as-cast  particles becomes smaller. Rhenium addition elevates the incipient melting temperature and slows down the solid solution process. Even after 2000h prolonged aging both at 950°C and 1050°C, no topologically close-packed phase precipitation is found in the three alloys with Re content up to 4%. Re retards the  coarsening and is beneficial to improving the stress rupture life. The deformation mechanisms together with the dislocation configuration have been studied and discussed.

Abstract: This study deals with the fabrication of high strength ferritic/martensitic steels by a control of both the carbon concentration and the fabrication process parameters. The 9Cr-2W steels containing a carbon concentration of 0.05, 0.07 and 0.11 wt% were normalized at 1050oC for 1 h, followed by a tempering at 550 and 750oC for 2 h, respectively. The results of the tensile tests at room temperature indicated that the tensile strengths were increased with an increase of the carbon concentration from 0.05 wt% to 0.07 wt%, but no more increase was observed when the carbon concentration was increased further up to 0.11%. After a cold rolling from a 4 mm to a 1 mm thickness without/with an intermediate heat treatment and a final heat treatment, the results of the tensile tests exhibited that superior tensile properties were obtained when the fabrication processes were composed of a tempering at 550oC, and a cold rolling with several intermediate heat treatments. These results could be attributed to the finely distributed precipitates in the partially recrystallized matrix. The optimized carbon concentration and the controlled fabrication process parameters are thus suggested to fabricate a high strength 9Cr-2W steel sheet.

Abstract: The rate controlling mechanism for creep deformation of the single crystal Ni-based superalloy, CMSX-4, at 1073-1273K and 250-700MPa was investigated. Constant load tensile creep tests and creep interrupted tests up to the minimum creep stage, were conducted in air. And TEM observations carried out on creep interrupted specimens. Stacking faults in the ’ were observed on creep interrupted specimens at the temperature lower than 1223K and the stress higher than 500MPa. The number of stacking faults increases monotonously with a decrease in temperature, and remarkably with an increase in stress. The stacking fault formation depends on creep temperature and stress conditions. The stress exponent of minimum creep rate, n value, and the activation energy for creep, Qc, were constant for all creep test condition range. From these results, the stacking fault formation has no influence on creep resistance and the rate controlling mechanism for creep deformation at the low temperature and high stress condition was not thought to be shearing the ’, but movement of the mobile dislocations in the  channel as well as at the high temperature and low stress condition.

Abstract: When Fe-10.3mol%Ni-14.3mol%Al alloy is heated at 1173 K for 8.64104 s, a number of B2 precipitates are dispersed in the A2 matrix. When the two-phase microstructure of A2+B2 is aged at 973 K, the phase-separation of B2 precipitate particles takes place to form a new A2 phase in each B2 particle. In the course of further ageing at 973 K, the new A2 phase grows but decreases in number, and finally only one A2 particle is left in the individual B2 particles. The appearance of new A2 phase in each B2 precipitate is due to the difference in the volume fraction of A2 phase that should exist in A2+B2 two-phase system depending on the heating temperature: i.e., the phase-separation of B2 precipitates starts with the aid of chemical free energy.

Abstract: This work assesses the effects of the surface recrystallization of the processing of water grit blasting, grit blasting and mechanically polishing on the microstructures and creep rupture properties of DD6 alloy, a low-cost second generation single crystal superalloy. The results demonstrate that the possibility and the depth of the surface recrystallization of DD6 show an increase as the annealing temperature increases from 1050°C to 1250°C. No surface recrystallization happens when the specimens of the alloy have been undergone at 1100°C for 4 hours after water grit blasting, but the surface recrystallization occurs at the annealing temperature above 1200°C for 4 hours after water grit blasting. The test indicates that the increasing pre-deformation decreases the surface recrystallization temperature. The creep rupture life of DD6 alloy without processing is 274.4 hours, 341.1 hours at the conditions of 980°C/250MPa, 1070°C/140MPa respectively. After annealing at 1100°C for 4 hours, the creep rupture life of the alloy with the processing of water grit blasting is equivalent to that of the alloy without processing. The surface recrystallization of the alloy happens with the processing of grit blasting after the annealing at 1100°C for 4 hours, and there is a reduction of the creep rupture life at the conditions as mentioned above. The creep properties of DD6 alloy meet the needs of blades and vanes of single crystal for advanced aeroengines when the surface recrystallization of the alloy occurs during manufacturing and processing.