Nobufumi Ueshima

Impact of interplay between magnetic field, transformation strain, and coarsening on variant selection in L10-type FePd

Variant selection of L10-type ferromagnetic alloys has been numerically investigated using the phase-field modeling, to clarify the phenomena at greater temporal and spatial resolution and to reveal the underlying mechanism. The duration for which the external magnetic field is effective is found to be very short, and variant selection is significantly affected by not only direct response to the external magnetic field but also their interplay between the field, intrinsic transformation strain, and various thermodynamic energy components involved in the course of microstructure evolution. The detailed mechanism of the interplay was quantitatively analyzed in terms of the driving force for the variant selection, by partitioning it into the various energy components. Careful examination of the variant selection at the very early stage revealed that the slight difference in size and configuration of variants during disorder-to-order transition realized by the interplay between transformation strain and exter...

Impacts of Interface Energies and Transformation Strain from BCC to FCC on Massive-like δ-γ Transformation in Steel

Interface energies of δ/γ, γ/γ, δ/δ, L/δ, and L/γ interfaces, at first, as a function of misorientation were evaluated with an aid of atomistic simulations with embedded atom method. Then, under geometric constraints where grains or interfaces compete each other to minimize overall free energy, effective interface energies for those interfaces were quantified. It is found that neither the minimum nor effective δ/γ interface energies, 0.41 or 0.56 J/m2, respectively, is significantly higher than those of other interfaces including liquid/solid interfaces, but the δ/γ interface energy is significantly high for the small entropy change upon δ-γ massive-like transformation, resulting in significantly higher undercooling required for γ nucleation in the δ phase matrix than in solidification. Detachment of δ-phase dendrite tips away from γ-phase dendrite trunks can be explained only from a viewpoint of interface energy if small misorientationis introduced at the δ/γ interface from the perfect lattice matching between BCC and FCC crystal structures. Examining the BCC-to-FCC transformation strain on the γ nucleation in the massive-like transformation, the γ nucleation is prohibited 170 K or more undercooling is achieved unless any relaxation mechanism for the transformation strain is taken into account.

Numerically-quantified two dimensionality of microstructure evolution accompanying variant selection of FePd

Through three-dimensional (3D) simulations of microstructure evolution by phase-field modeling (PFM), microstructures have been quantified during their time evolution by an image processing technique with particular attention to the shape of variants in the course of variant selection. It is found that the emerging variants exhibit planar shapes rather than 3D shapes due to the elastic field around the variants arising upon disorder-to-order transition to the L10 phase. The two-dimensionality is more pronounced as variant selection proceeds. Although three equivalent variants compete for dominance under an external field, one of the three variants vanishes before final competition occurs between the remaining variants, which can be explained by the elastic strain energy. These numerical analyses provide better understanding of the microstructure evolution in a more quantitative manner, including the small influence of the third variant, and the results obtained confirm that the understanding of variant selection obtained from two-dimensional (2D) simulations by PFM is valid.

Microstructure Prediction for TMW-4M3 During Heat Treatment

The alloy TMW-4M3 has been developed as a novel cast and wrought alloy based on a concept of combining Ni-base and Co-base superalloys. This alloy contains higher amounts of Co and Ti than Alloy 720Li. For practical applications, it is very important to control the size and distribution of γ’ phase as an intended microstructure. However, precipitation behavior of this type of alloy greatly depends on heat treatment conditions. In this study, we made a modification to the thermodynamic database in order to obtain reasonable γ/γ’ phase boundary in the range of high Co composition. By using it, both the nucleation rate calculation based on classical nucleation theory and the microstructure evolution prediction based on the phase field method were applied to the precipitation of intragranular γ’ particles during the heat treatment process. The simulated microstructures under different temperature history conditions agree well with experiments in both the size and the morphology of γ’ precipitates.

Microstructure Controlling of U720-Typed Superalloys to Improve a Hot and Cold Workability by Using Incoherent Gamma Prime

For high performance air craft engine disks, high-performance Ni-based wrought superalloys (e.g. U720LiTM, AD730TM) with over 40vol% of the γ′ phase at 600–700 °C have been developed. Applying them to gas turbines is expected to improve the efficiency. However, due to their low workability, it is difficult to make large size components. We have developed a new innovative process (the MH process) to improve the workability of high-performance Ni-based wrought superalloys. The strengthening mechanism of these γ′ precipitation-typed superalloys is widely known to be enhanced by the coherent interface between the γ phase and γ′ phase. We focused on the incoherent γ′ phase that precipitated on the γ phase grain boundary during forging and it was found that the incoherent boundary shows no strengthening effect. Formation of γ/incoherent γ′ two-phase microstructure would dramatically improve their workability. Cold working was achieved by applying the MH process to the U720-type alloy AD730. The MH process made it possible to fabricate cold rolled sheets, cold drawn wires and the forged turbine blades. It also exhibited excellent ductility at high temperature; we obtained 99% reduction of area at over 920 °C and 500% elongation at 950 °C by applying the MH process. It was demonstrated that U720Li can be processed by die forging at low temperature and high strain rate without using superplastic forming. In this study, we investigated how the heat treatment condition affects the formation of unique microstructures with good hot and cold workability.

Influence of Cu and Mg addition on age-related deterioration in strength and creep behavior of Zn-12Al die casting alloys

Abstract The influence of the chemical composition of Zn-12Al-yCu-xMg (y = 1, 5; x = 0.05–1.4 mass%) die casting alloys on age-related deterioration in mechanical strength and creep behavior was investigated. Tensile tests were performed after aging at room temperature for 1 day and at 100 °C for 1 week. Creep tests were carried out under tensile stresses of 25, 50 and 100 MPa at 100, 130 and 160 °C. Cu addition suppressed age-related deterioration of proof stress by suppressing the coarsening of the eutectic structure during the aging. Creep resistance was improved by the Mg addition, whereas the Cu addition has a deleterious effect on creep resistance. The results show that the age-related deterioration of Zn-12Al alloys can be improved without sacrificing creep resistance by the complex addition of Cu and Mg.