Materials Science Forum Vols. 539-543

Abstract: Specimens of single crystal superalloy SC16 have been pre-deformed at 1223 K to strain of 15 % under tensile load of 150 MPa. The profiles of the 001 superlattice reflection of the γ’ precipitate phase as well as the profiles of the 002 reflections of the fcc matrix phase and of γ’-precipitate phase have been measured between RT and 1073 K by means of X-ray diffraction (XRD). From these measurements the lattice distortion of the γ’ precipitates and the lattice misfit between γ and γ’ phase have been determined as a function of temperature. The width of the 001 reflection as well as the lattice misfit decreased as a function of temperature. The results are ascribed to the anisotropic arrangement of dislocations at the γ/γ’ interfaces and to the different thermal expansion coefficients of the γ- and γ’-phase. A comparison of lattice misfit with previous measurements on moderately strained specimens suggests that the measured changes in lattice structure are predominantly created in stage I of creep-deformation.

Abstract: The effect of phosphorus in some typical wrought superalloys including IN718, GH761 and GH4133 has been studied. Phosphorus was not dissolved in the γ matrix of superalloys and was segregated in the residual liquids seriously during solidification, and the solidification of the alloys added with phosphorus was always finished by the precipitation of a phosphorus bearing phase. Two factors controlling the segregation tendency of a minor element were determined, one is the solubility of the element in the matrix, another is the precipitation temperature of a phase in which the element is absorbed. The detrimental effect of phosphorus on the solidification could be eliminated by a proper homogenization treatment. Phosphorus improved the stress rupture properties of the alloys with a fine grain structure. By combining the grain refinement with phosphorus addition, the properties of GH761 alloy was improved efficiently.

Abstract: The profiles of 001 and 002 reflections have been measured at 1173 K as a function of time by means of X-ray diffraction (XRD) on tensile-creep deformed specimens of single crystal superalloy SC16. Decrease in line width (full width at half maximum: FWHM) by about 7 % and increase in peak position by about 3x10-4 degrees was detected after 8.5x104 s. Broadening of the 002 peak profile indicated a more negative value of the lattice misfit after the same time period. The results are discussed in the context of the anisotropic arrangement of dislocations at the γ/γ’ interfaces during creep and their rearrangement during the thermal treatment at 1173 K.

Abstract: Forged parts, specifically designed for the validation of microstructural models, have been manufactured in Inconel alloy 718 using a wide range of thermo-mechanical histories. The microstructural evolution observed in the forged samples has been compared with predictions from two models for dynamic recrystallisation and grain growth.

Abstract: It is here demonstrated that combinations of various modeling techniques can be used to predict the service behaviour of heat resistant alloys, and sometimes to design “made-to-measure” alloys for specific high temperature applications. A first example is given, where an affordable creep-resistant nickel-base superalloy for operation around 750°C has been designed without any experiment. Based on the analysis of huge databases on existing alloys, Gaussian processes were used to predict its thermomechanical properties. Thermo-Calc allowed to design its fabrication process and to assess its weldability and its thermodynamical stability at service temperature. The alloy has then been tested and the validity of the modeling approach verified a posteriori, in particular a creep rupture life of 100 000 h at 750°C under 100 MPa. A second example is given, in the case of high-carbon Fe-Ni-Cr based alloys for reformer tube applications (HP steels). Their mechanical properties are predicted through the analysis of existing data with artificial neural networks. Parallelly, their thermodynamical stability in operating conditions is assessed using Thermo-Calc in combination with Dictra, to simulate the precipitation of carbides in the austenite matrix during service. It is therefore tried to understand microstructural evolution in service, damage mechanisms, and durability.

Abstract: The carbon and nitrogen free new alloys which were composed of the supersaturated martensitic microstructure with high dislocation density before the creep test have been investigated systematically. These alloys were produced from the new approach which raised creep strength by the utilization of the reverse transformed austenite phase as a matrix and intermetallic compounds such as Laves and μ-phases as precipitates during creep test. It is important that these alloys are independent of any carbides and carbo-nitrides as strengthening factors. Creep behavior of the alloys is found to be different from that of the conventional high-Cr ferritic heat resistant steels. The minimum creep rates of the Fe-Ni alloys at 700°C are found to be much lower than that of the conventional steel, which is due to fine dispersion strengthening useful even at 700°C in these alloys. As a result carbon and nitrogen free alloys exhibited superior creep properties at temperatures more over 700°C, and steam oxidation resistance.

Abstract: Selection of alloys for components in industrial environments at high temperature is often based on long-term tests in environments that simulate as closely as possible the actual service conditions. However, particularly for alloys that rely on formation and retention of oxide scales for protection, such tests may not be sufficient for this purpose. Several nickel-base alloys have been exposed in an 80% CO-20% H2 gas mixture at 650oC for up to 5,000h, in order to give conditions similar to those that can occur in petrochemical plant where high carbon and low oxygen activities can lead to scale breakdown and the onset of metal dusting damage. The study has shown that small changes in test conditions, such as the introduction of contaminants, the addition of CO2, the proximity to alloy surfaces on which carbon deposition and metal dusting have already been initiated and the proximity to other surfaces that can initiate such deposition, can lead to very significant decreases in the induction period, prior to breakdown of the scale and the onset of dusting.

Abstract: In Alloy 718 ingot cogging process, dynamic and metadynamic recrystallizations, and static grain growth occur, and also the presence of δ phase plays a key role in controlling the grain size. In this study, the evolution of grain structure in VIM/VAR-processed Alloy 718 ingots during post-cogging heat treatments is dealt with. Compression tests were made on VIM/VAR-processed Alloy 718 ingot at temperatures between 900oC ~ 1150oC. Heat treatments were made on the compression-tested specimens, and the variation of grain size was evaluated. Constitutive equations for the grain growth are established to represent the evolution of microstructures. Special attention is paid to the evolution of grain structure under the condition of dynamic and metadynamic recrystallizations, and grain growth. The grain growth rate depends mainly on the presence of δ-phase below the δ-solvus temperature, and on the difference in the grain boundary characteristics above it.