Papers by Keyword: Nb-Based Alloys

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. It is found that the hydrogen solubility decreases by the addition of ruthenium, tungsten or molybdenum into niobium. The mobility for hydrogen diffusion during hydrogen permeation is estimated from the linear relationship between the normalized hydrogen flux, , and the product of the hydrogen concentration and the difference of hydrogen chemical potential, . It is found that the mobility for hydrogen diffusion during hydrogen permeation is larger for Nb-based alloys than pure niobium, especially at low temperature. The activation energy of the mobility for hydrogen diffusion decreases by the addition of ruthenium, tungsten or molybdenum into niobium.

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion decreases by the addition of ruthenium or tungsten into niobium.

Abstract: The hydrogen diffusion coefficients are investigated during the hydrogen permeation through Nb-based hydrogen permeable membranes at high temperature. It is found that the hydrogen diffusion coefficient for pure niobium under practical conditions is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.

Abstract: Halide-activated pack cementation and Air plasma spraying methods were utilized to deposit Si-modified MCrAlY coatings. Nb-base alloys were coated with MCrAlY by Air plasma spraying and followed by pack siliconizing process at 1100°C for 8h and 10h. The pack powders were consisted of 5 Wt.℅NH4Cl、35 Wt.℅Si、60 Wt.℅Al2O3. Si-modified MCrAlY coatings have three-layer microstructure. The outer layer was composed of NbSi2 and Nb3Ni2.4Cr1.6Si6, while the middle layer comprised the compound of Nb、Ti、Ni、Cr、Si. The inner layer adjacent to the substrate was silicide. Isothermal oxidation resistance of Si-modified MCrAlY coatings was tested at 1250°C in static air. Si-modified MCrAlY coatings had excellent oxidation resistance, because continuous Cr2O3 and SiO2 scales which serve as obstacle of oxygen diffusion were formed after oxidation.

Abstract: A coating with a duplex layer structure, outer β-NiAl and inner σ-Re-Cr-Ni layers, was formed on the third generation Nb-5Mo-15W-16Si- 5Hf-5C alloy by using successively Re-pack cementation, electroplating of Re-Ni film, and Cr/Al pack cementation. The duplex layer coating changed during high temperature oxidation to form a coating with a four layer structure: an outermost Ni2Al3, an outer Ni, an inner σ-Re-Cr-Ni, and an innermost χ-Re-Nb. The Re-pack cementation was carried in an alumina crucible where the specimen was buried in Re metal powder, in vacuum at 1573K to form a Re film, and then a Re-Ni film was electroplated onto the Re-pack treated alloy. The coated alloy formed a protective α-Al2O3 scale when oxidized at 1573K in air for 14.4ks. It was concluded that the σ-Re-Cr-Ni phase can act as a diffusion-barrier against both inward Al diffusion and outward diffusion of alloying elements from the alloy substrate to the β-NiAl.

Abstract: To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures advanced coatings were developed, and they were classified into four groups, (1) the diffusion barrier coating with a duplex layer structure, an inner σ−(Re-Cr-Ni) phase as a diffusion barrier and outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-based alloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAl formed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activity pack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves three phases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the two step Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as a diffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(20
50)at% Cr alloy changed from the δ-Ni2Al3 coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated at temperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). In the diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (or Hastelloy X) and Ni aluminides containing 12
50at%Al when oxidized at 1423K for up to 1800ks. It was found that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermost and Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems that the inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and the outer/outermost layers.