Improvement of Surface Cracking Risks by Controlling Dilution of Cobalt-Based Hardfacings

Cobalt-based hardfacings have been widely applied to the main valve seats of steam turbines for the purpose of improving durability. With the aim of decreasing the surface cracking risks of the hardfacings, the dilution of the hardfacing on their surface was controlled and the properties of the dilution-controlled materials were investigated in this study. Mixtures of cobalt-based hardfacings and base materials were cast in order to simulate the dilution of base material to hardfacing, which occurs in the process such as PTA welding. It was confirmed that the grain size matched that produced by PTA welding by controlling the cooling rate. To investigate the effect of aging on the microstructures and mechanical properties of the cast samples, the samples were aged in electric furnaces where the temperature was controlled to the steam temperature of steam turbines and above. The test results show that the phase transitioned from face-centered cubic to hexagonal closed-packed in low-dilution samples, while carbides precipitated along the grain boundaries in high-dilution samples after aging. Both samples showed an increase in hardness and reduction in ductility and fracture toughness. In addition, the variation in microstructures and strength properties was suppressed for a 20 % dilution sample. To validate the influence of dilution as investigated with the cast samples, test specimens were machined from the surface and bottom layers of multi-layered hardfacing that had different dilutions by PTA welding. It was confirmed that the influence of dilution on the strength properties of hardfacing layers had a similar tendency to the cast samples. The results above lead to the possibility of reducing the surface cracking risks by controlling the dilution of hardfacings and suppressing embrittlement after aging. We have applied dilution-controlled hardfacings to steam valves and successfully reduced surface cracking.

Novel Fe hydrides and magnetism in the Fe-H system at high pressures

The structure and properties of metal-hydrides at extreme conditions is key to the understanding of hydrogen-storage technologies, high-temperature superconductivity, and the dynamics of planetary cores. Here we investigate the phase relations and magnetic properties of iron hydrides, including two novel FeHx compounds, up to 63 GPa and 1800 K by Synchrotron Mössbauer Source spectroscopy and single-crystal X-ray diffraction. We observe the formation of a novel monoclinic iron hydride phase Fe2H∼3 at 63 GPa and 1000 K, which breaks down to a stoichiometric hexagonal closed packed FeH phase upon decompression below 52 GPa at 300 K. The long-range magnetic order in the two newly-synthetized phases persists to higher pressures compared to the well-known double hexagonal closed packed and face-centered cubic phases of FeH. The formation of magnetic Fe-H phases with high hydrogen concentration may influence the magnetic behavior of planetary metallic cores.

Intrinsic asymmetric ferroelectricity induced giant electroresistance in ZnO/BaTiO3 superlattice

We combine the piezoelectric wurtzite ZnO and the ferroelectric (111) BaTiO3 as a hexagonal closed-packed structure and report a systematic study on the ferroelectric behavior induced by the interface and the transport properties between electrodes.

Zigzag Dissociation Mode of 〈c + a〉 Dislocations on the 101¯1 Plane in Magnesium Alloys

Fundamental understanding of the dissociation mode of 〈c + a〉 dislocations on the 101¯1 plane is required before the goal of improving the ductility of Mg alloys attained. In this study, our density-functional theory calculations reveal that the atoms in the 101¯1 plane slip along a zigzag trace through a low-energy pathway. We thus propose a novel zigzag dissociation mode based on this slip trace. In particular, the shuffling motion of atoms is observed at the position of stable stacking fault, which is closely related to the c/a ratio of the hexagonal closed-packed lattices.

The First Principle Approach in Estimating Bulk Modulus Based on Explicit Expression of Canonical Partition Function

The bulk modulus is one of the most important characteristic features of solids. Accordingly, we have developed a statistical-mechanical treatment based on an equation which enables us to calculate the bulk modulus for solids with the minimum manifold of input data. In our model, a conjunction between Gruneisen parameter and canonical partition function has been established. We have found out that the volume dependency of Gruneisen parameter is critical in estimating bulk modulus. The result for hexagonal closed- packed (hcp) iron is very good and commensurate with the best measurements. This framework can be extended to the other elemental solids or a variety of compounds.