Tadaharu Yokokawa

Partitioning behavior of platinum group metals on the γ and γ' phases of Ni-base superalloys at high temperatures

Abstract The partitioning behavior of platinum group metals (PGMs) on the γ and γ′ phases of Ni-base superalloys has been investigated with alloys containing 5 mass% Re and 1 at.% PGMs. It was shown that Ru partitions preferably into the γ, while Rh, Pd, and Pt partition into the γ′, and Ir partitions almost equally. The partitioning behavior is not affected by the 1 at.% PGM addition.

Rafting mechanism for Ni-base superalloy under external stress: elastic or elastic–plastic phenomena?

Abstract Rafting mechanism in Ni-base single-crystal superalloys has been discussed with the total mechanical energy calculated for typical microstructures. We found that the actual rafting phenomena cannot be explained within the coherent elastic regime. The present calculations reveal that (i) only the transverse rafted structure with laminates normal to the stress direction can be realized, regardless of tensile or compressive stresses, and (ii) the lattice misfit is not relevant to the choice of the rafted structures. However, when the eigenstrain of the spherical (dilatational) symmetry changes into that of the tetragonal symmetry with misfit dislocations on the γ/γ′ interfaces, the signs of lattice misfit and external stress govern the choice of the transverse or longitudinal rafts. It is concluded that the rafting belongs to an elastic–plastic phenomenon.

Elastic anisotropy of rafted Ni-base superalloy at high temperatures

Abstract The elastic constants of the rafted Ni-base single-crystal superalloy, TMS-26, at high temperatures (~1000 °C) have been studied with acoustic resonance methods. The rafted superalloy can be regarded as an elastic body of tetragonal symmetry. The elastic anisotropy factors, c 11 / c 33 and E 1 0 0 /E 0 0 1 , increase with temperature up to 1.010–1.025. Although a micromechanics model can reproduce this trend, the calculated anisotropy (~1.001) is much smaller than the measurements, which can be related with the internal strain due to the lattice misfit. The elastic anisotropy of E 0 0 1 1 0 0 =E 0 1 0 contributes to the transverse rafting under external stress.

Creep mechanisms of a new Ni‐Co‐base disc superalloy at an intermediate temperature

The microstructures of a new Ni‐Co‐base disc superalloy, TMW‐4M3, before and after the creep test at 725 °C/630 MPa have been systematically investigated by transmission electron microscopy (TEM). The crept microstructures were marked as three different deformation stages (I, II and III) corresponding to the gradually increased strain. At stage I, stacking fault (SF) shearing was the main deformation mechanism. The SF was extrinsic and lay on {111} plane. However, deformation microtwinning became the dominant mode at stage II and III. The average spacing of deformation twins decreased from 109 ± 15 nm at stage II to 76 ± 12 nm at stage III, whereas the twin thickness did not change significantly. The influence of stacking fault energy (SFE) of γ matrix on the deformation mechanism is discussed. It is suggested that lower SFE in TMW‐4M3 is partly responsible for the enhanced creep resistance.

Elastic constant measurement of Ni-base superalloy with the RUS and mode selective EMAR methods

This paper reports the elastic constants of the Ni-base single crystal superalloy (TMS-26) with a rafted (lamellar) structure having tetragonal symmetry. The elastic constants have been measured at room temperature with the resonance ultrasound spectroscopy method and the mode-selective electromagnetic acoustic resonance method. The value of the elastic constant C33 (250.4 GPa) is almost equal to that of c11 (252.5 GPa), which indicates that the rafted structure virtually has the elastic anisotropy of cubic system.

Effect of Solution Temperature on the Microstructure and Mechanical Properties of a Newly Developed Superalloy TMW-4M3

The influence of solution temperature on the microstructure and mechanical properties of TMW-4M3 superalloy has been investigated. Comparisons of mechanical properties have also been made between the heat-treated TMW-4M3 variants and the commercial U720Li. The key microstructural variables examined were grain size and the volume fraction and size of the strengthening γ′ precipitates that control the mechanical properties of these alloys. By increasing the solution temperature from 1373 K to 1393 K (1100 °C to 1120 °C), the volume fraction of primary gamma prime dropped from 16.9 pct to 14.5 pct, whereas the average grain size increased from 8.7 μm to 10.6 μm. Compared with an alloy with a smaller grain size, the alloy with a larger grain size exhibited superior resistances to creep and fatigue crack growth without the expense of reduced tensile strength and low-cycle fatigue resistance. This suggested that a higher solution temperature may benefit TMW-4M3 in terms of superior overall properties. The greater overall properties of TMW-4M3 variants than that of commercial U720Li were also demonstrated experimentally. The possible explanations for the improvement of mechanical properties were discussed.

Effects of boron addition on microstructure and mechanical properties of an Ir85Hf15 two-phase refractory superalloy

Abstract The aim of this study is to understand the effects of boron addition on the mechanical properties of the Ir 85 Hf 15 two-phase refractory superalloy, especially on the fracture behaviour and the high-temperature strength. Our results indicated that adding boron was an effective way to raise the strength of Ir 85 Hf 15 alloy at both room and high temperatures when the boron content was below a limited value, which was approximately 200 wppm for Ir 85 Hf 15 alloy. Meanwhile, adding boron did not significantly improve the ductility, but it changed the fracture mode from the morphology of grain-boundary fractures generally observed in polycrystalline Ir and binary Ir 85 Hf 15 alloys to mainly transgranular cleavage seen in the boron-doped Ir 85 Hf 15 . The effects of boron addition are discussed in terms of microstructure change and phase constitution.

Virtual Jet Engine System

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.

Interface Energy Calculation of γ-γ′ in Ni-Al System Using the Cluster Variation Method

A calculation of the interface energy for the Ni-Al binary alloy, including the inter-phase boundary (IPB) energy and the anti-phase boundary (APB) energy, has been performed using the Cluster Variation Method (CVM) with the tetrahedron approximation within the temperature range of 600°C~1300°C. The calculated IPB energies range between 8 and 13 mJ/m2, while the APB energies range between 24 and 46 mJ/m2. Additionally, the dependence of the average composition and the order parameter on distance with the compositionally diffuse interfacial regions has been computed. The calculation also shows the width of the diffuse IPB increases with the temperature linearly.

Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfurization

The direct and complete recycling method for Ni-base superalloy is being developed and studied to reduce the material cost for cost-effective operation of gas turbine systems. Understanding the effect of sulfur contamination is important to determine allowable sulfur content after the recycling. However, in the case of single-crystal superalloys, this effect on material properties is not well known except for the detrimental effect on the oxidation resistance. In the present study, creep tests, aging tests, and cyclic oxidation tests have been performed on PWA1484 with varying sulfur content. The increasing sulfur content has been found to correlate with degradation of properties evaluated here. It is observed that the decrease in creep life in PWA1484 due to sulfur doping is primarily due to coarsening of the γ/γ′ interfacial dislocation network, increase in precipitation kinetics of topologically closed-packed phase, and decrease in oxidation resistance. For recycling purposes, a CaO crucible was used in the casting process, which successfully decreased the sulfur level in the alloy, and the resulting material showed comparable or even better properties in comparison to the low sulfur content material.