Papers by Author: Shigenari Hayashi

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Authors: Toshio Narita, Takeshi Izumi, Takumi Nishimoto, Yoshimitsu Shibata, Kemas Zaini Thosin, Shigenari Hayashi
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-(2050)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 1250at%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.
Authors: Shigenari Hayashi, Mikihiro Sakata, Shigeharu Ukai, Toshio Narita
Abstract: High temperature oxidation / creep deformation behavior of a diffusion barrier coated Hastelloy-X alloy, with large grain size ~500μm, was investigated at 970°C in air with external tensile stress of 22.5, 27.5, 32, and 40MPa. The diffusion barrier coating formed on Hastelloy-X consisted of a duplex structure with an inner diffusion barrier layer of Re-Cr-Ni alloy, and an outer oxidation resistant layer of β-NiAl. Un coated bare Hastelloy-X alloy with same grain size was also examined under the same conditions for comparison. The composition of the as-coated diffusion barrier coating was (15~21)Ni, (33~37)Cr, (30~33)Re, (11~15)Mo, and (9~14)Fe. This composition corresponds to σ-phase in the Ni-Cr-Re ternary system, which is known as a topologically close packed, TCP phase. The composition of this diffusion barrier layer did not change during the experiment. The oxide scales formed after creep testing on the coated and un-coated alloy surfaces were needle-like θ-Al2O3, and Cr2O3 with small amount of FeCr2O4, respectively. Grain boundary oxidation was also found in the subsurface region of the un-coated alloy. The Al2O3 scale exhibited severe spallation, and many cracks were formed perpendicular to the stress direction. However, no spallation or cracks were observed in the Cr2O3. The creep rupture times for the diffusion barrier coated alloy were about 1.5 times longer than those for bare alloy at all creep stress conditions. The fracture surface after rupture indicates that fracture occurred along alloy grain boundaries in both the coated and un-coated alloy substrate. Many cavities and cracks were observed within the diffusion barrier coated alloy substrate. These cavities and cracks tended to propagate from the substrate toward the diffusion barrier layer, and then stopped at the Re-Cr-Ni / β-NiAl interface. Cracks formed in the un-coated alloy initiated at the tip of grain boundary oxides, and propagated into alloy substrate. However no major cavities were observed inside the alloy substrate. The stress index, n, for both specimens was about 6, and this indicates that the deformation mechanism of both samples was dislocation creep. These results suggest that the Re-Cr-Ni diffusion barrier layer acts as a barrier against the movement of dislocations at the interface with the alloy surface.
Authors: Kosuke Saito, Shigenari Hayashi, Toshio Narita, Isao Iwanaga, Ryohei Tanaka
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.
Authors: Shigenari Hayashi, Toshio Narita, Brian Gleeson
Abstract: The early-stage oxidation behavior of γ '-Ni3Al-based alloys of composition (in at.%) Ni-22Al and Ni-22Al with 10, 20, and 30Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150°C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the γ '-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al2O3 scale during heating the γ '-based alloys through to about 700°C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in γ '. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L12 crystal structure of γ ', which has the effect of increasing the Al:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ ' surface would kinetically favor the formation of Al2O3 relative to NiO. A further contributing factor is that the Pt-containing γ '-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment would reduce Ni availability and increase the Al supply to the evolving scale, thus kinetically favoring Al2O3 formation.
Authors: Takumi Nishimoto, Shigenari Hayashi, Toshio Narita
Abstract: The effects of coatings on the creep and oxidation behavior of Ti-50Al alloy were investigated at 1173K in air at a constant loading of 30MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixture layer, and a Cr diffusion zone. Creep tests were also carried out with sole Cr or Al coated TiAl and also of uncoated TiAl. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed several exfoliations. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.
Authors: Yoshimitsu Shibata, Shigenari Hayashi, Toshio Narita
Abstract: A coating with duplex structure of a outer β-NiAl and an inner α-Cr layer, was formed on a Ni-40Cr-3Re (in at%) alloy with or without Zr addition, and the coated alloys were oxidized under thermal cycling in air for up to 2300ks. The coated alloys containing Zr showed a two-step parabolic oxidation, the kp1st in the early stage of oxidation was 2.6~5.4×10-11 kg2 m-4 s-1 for four alloys tested, and kp 2nd for the longer oxidation increased with increasing Zr content from 2.6~5.4×10-11 for the alloy with 0.1at%Zr to 9.6×10-11 kg2  m-4 s-1 for the alloy with 1.0at%Zr. The rapid oxidation for the alloy with 1.0at%Zr is due to the formation of ZrO2 as an internal oxide. The oxide scale in the 1st stage consisted of both α- and θ- Al2O3 with whiskers, and with further oxidation the α-Al2O3 became the major product in the 2nd stage. After the oxidation for 2300ks the as-prepared, outer β-NiAl was changed into a mixture of β-NiAl and γ’-Ni3Al for the Ni-40Cr-3Re alloy containing Zr, while in the coated Ni-40Cr-3Re alloy the outer layer became a mixture of γ’-Ni3Al and γ-Ni(Al,Cr). It was concluded that the addition of Zr into the coated Ni-40Cr-3Re alloy helps maintain high Al contents in the outer Ni-aluminide layer by forming a protective Al2O3 layer.
Authors: Xiao Chao Wu, Shigeharu Ukai, Ryota Miyata, Yoshito Sugino, Naoko Oono, Shigenari Hayashi, Satoshi Ohtsuka, Takeji Kaito
Abstract: The hot rolling at temperature range of 1100 °C to 862 °C and subsequent air-cooling induce a formation of the coarse ferrite grains in the 9CrODS steels. This coarse ferrite is produced by transformation from the severely hot rolled γ-grains to ferrite. Formation process and mechanism of the transformed coarse ferrite are interpreted in terms of a nucleation, growth and coalescence of the same variant ferrite grains under a variant restriction rule.
Authors: Toshio Narita, Stewart Ford, Takayuki Yoshioka, Takumi Nishimoto, Takeshi Izumi, Shigenari Hayashi
Abstract: A duplex layer, outer Pt-modified γ’-Ni3Al + γ-Ni and inner multi-barrier σ- Re(Cr,Ni,W), coating system was formed on a Ni-based single crystal 4th generation superalloy. Oxidation behavior of the coated alloy was investigated under thermo-cycling conditions, and analyzed by EPMA and XRD. During cyclic oxidation 1hr at 1100°C and 20 min at room temperature, a slow growing α-Al2O3 formed for up to 400 cycles and its spallation was rare. The parabolic rate constant of mass change was 6.3x10-16 kg2m-4s-1. The Pt-modified γ’-Ni3Al + γ-Ni contained 19Al, 12Pt, 4Cr, and 3Co in at%, and their concentration profiles were almost flat across the outer layer. The multi-barrier, σ-Re(Cr,Ni,W) contained 40Re, 23Cr, 17Ni, 7Al, 4W, 3.5Mo, and 3Co in at%. Furthermore, the γ’-Ni3Al containing Pt was newly formed between the multibarrier and bulk alloy substrate. It was concluded that the σ-Re(Cr,Ni,W) is compatible with the Ptmodified γ’-Ni3Al in the multi-diffusion barrier coating on Ni-based single crystal, 4th generation superalloy at high temperatures.
Authors: Shinya Mikuni, Shigenari Hayashi, Toshio Narita
Abstract: The effects of the third element on the high temperature oxidation of γ'-Ni3Al with 5at%X (X=Ti, Ta, Nb, Cu, Co and Fe) alloys were investigated at 1173K in air, and oxidation behavior could be classified into three groups. The first group, comprised of alloys with Cu and Co, showed good oxidation performance with Al2O3 formation. A second group contains Ti, Ta, and Nb as alloying elements, and showed poor oxidation performance. With Fe or Mn addition the alloy oxidation performance was intermediate between the first and second group. The effects of these elements are discussed associate with partitioning factors for each element in the γ'-phase.
Authors: Hiroyuki Matsumaru, Shigenari Hayashi, Toshio Narita
Abstract: A diffusion couple study was carried out with Ni-based superalloy / γ’-Ni3Al with or without a Re-based alloy layer. The Re-based alloy containing Re, W, Cr, and Ni was formed on a second generation, single crystal Ni-based superalloy by using electroplating processes to form films of 70at%Re-Ni and Ni-20at%W, followed by Cr-pack cementation at 1573K for 36ks. The superalloy with or without the Re-based alloy layer was bonded to a γ’−Ni3Al alloy in vacuum. Diffusion couples were annealed in an evacuated quartz ampoule at 1423 K for 90 and 360ks. After the 360ks diffusion treatment the Re-based alloy remained sound and suppressed inward Al diffusion from the γ’-Ni3Al, it was further found that outward diffusion of alloying elements from the alloy substrate to the γ’-Ni3Al was also significantly reduced by the Re-based alloy layer.
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