Shigenari Hayashi

Corrosion behavior of Fe-40Al sheet in N2–11.2O2–7.5CO2 atmospheres with various SO2 contents at 1273 K

A 200 mum thick Fe-40 at.%Al sheet, produced by powder metallurgy and hot extruding, was corroded at 1273 K for up to 360 ks in N-2-11.2 vol.%O-2-7.4 voI-%CO2 atmospheres containing 100, 500, or 2000 ppmSO(2). The alloy oxidized rapidly at the very initial stage of corrosion, forming sulfides in addition to both theta- and alpha-Al2O3 scales, it was suggested that sutfides may be formed by direct reaction Of SO2 gas molecules with the alloy. Sulfide or dissociated sulfur at the alloy-scale interface may promote exfoliation of the external scale. With further oxidation, the oxide scale consisted solely of alpha-Al2O3, and grows obeying a parabolic rate law. The parabolic rate constants are between 5.5 x 10(-12) and 7.2 x 10(-12) kg(2) m(-4) s(-1), independent Of SO2 concentration. After 360 ks of corrosion the Al content in the alloy sheet remained at about 38 at. %Al, with an almost flat profile across the entire sheet

Advanced Coatings on High Temperature Applications

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.

Two-step Cr and Al diffusion coating on TiAl at high temperatures

Abstract The formation of an oxidation resistive coating layer on a TiAl alloy was investigated with a two- step Cr (at 1573 K for up to 72 ks ) and then Al (at temperatures between 1273 and 1573 K for 36 ks) pack diffusion process at high temperatures. The coated TiAl was oxidized in air at 1173 K for up to 1252.8 ks under a thermal cycling condition. The Cr diffusion coating layer consisted of γ, β, and Laves phases, which were transformed during cooling from the β-phase formed at 1573 K. The coating layer formed by the Al diffusion at 1473 and 1573 K consisted of an outermost TiAl 2 , outer Al-rich γ-TiAl, intermediate γ, β, and Laves phases, and a diffusion zone; there is little Al-diffusion at 1273 and 1373 K. The TiAl coated by the two-step Cr and Al diffusion process at 1573 K showed very good oxidation resistance in air at 1173 K due to the formation of a protective α-Al 2 O 3 scale. After oxidation for up to 1,252.8 ks the coating layer maintained a structure with three phases γ, Laves, and β, which acts as a diffusion barrier to Al and Ti.

Effect of Water Vapor on the Oxidation Behavior of Fe–1.5Si in Air at 1073 and 1273 K

The oxidation behavior of Fe–1.5Si was investigated at 1073 and 1273 K in air, air–H2O, Ar–H2O, O2–H2O, and O2 atmospheres. The extent of corrosion in atmospheres containing H2O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O2–H2O, air–H2O, and Ar–H2O. With increasing H2O contents in air–H2O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker was located between the external Fe2O3 (Fe3O4 included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO+Fe2SiO4 layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO+Fe2SiO4 layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO+Fe2SiO4 mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner FeO+Fe2SiO4 layer.

Grain characteristic and texture evolution in friction stir welds of nanostructured oxide dispersion strengthened ferritic steel

Abstract The present paper investigated the grain and texture characteristics in a nanostructured oxide dispersion strengthened ferritic steel subjected to friction stir welding. The ‘onion rings’ structure obviously exhibited in the macrostructure overview of the welds. The electron backscatter diffraction (EBSD) work revealed that the ‘onion rings’ comprised alternate layers made by coarse and fine grains, while no strong texture was exhibited in the alternate layers of the ‘onion rings’. Image quality maps of EBSD indicated that layers of fine grains were deformed under high strain conditions. Textures within the stir zone and thermomechanically affected zone were weak and exhibited some characteristics of bcc simple shear textures. Results of grain boundary revealed that the mechanical action in welding process promoted the transformation of low angle to high angle boundaries and contributed to the grain refinement.

Effect of coating layer structures and surface treatments on the oxidation behavior of a Ti–50at.%Al alloy

Abstract A Ti–50at.%Al alloy, coated by a two-step Cr (at 1573 K for 7.2 and 18 ks) and Al-diffusion (at 1573 K for 36 ks) process, was oxidized in air under thermal cycling between 1173 K and room temperature for up to 3600 ks. The coated layer consisted of a three layer structure: an outermost TiAl 2 (τ phase), outer TiAl (Al-rich γ-phase), and intermediate γ, β, and Laves phase layers. The coated TiAl with an intermediate layer of a fine, three phase structure oxidized slowly due to a protective α-Al 2 O 3 layer, while a coarse structured coating was oxidized catastrophically due to the formation of cracks. Vickers hardnesses of a β and Laves mixture as well as a γ and Laves one are around 700–800 Hv, whereas the τ phase and a γ and τ mixture showed 300–400 Hv. The Al content and its profile in the outermost and outer layers were almost unchanged before and after the 3600 ks oxidation, and it seems to arise from a slow diffusion of Al, Ti, and Cr through the three-phase intermediate layer. It was suggested that the fine, three-phase structured coating layer was very effective to suppress degradation due to both mutual diffusion and cracking.

Competitive effect of water vapor and oxygen on the oxidation of fe-5 wt.% Al alloy at 1073 K

The effect of oxygen on the oxidation of Fe–5wt.% Al alloy was investigated at 1073 K in N2–12.2 vol.% H2O, O2–12.2 vol.% H2O, and N2–O2–12.2 vol.% H2O with various amounts of oxygen. The results showed S-shaped oxidation curves that consisted of three stages: slow-incubation, rapid transition, and relatively slow oxidation. The amount of oxidation increased with increasing oxygen contents up to 0.9 vol.% O2 and then rapidly decreased. On the oxygen-rich side, a slow incubation oxidation stage was observed and its duration increased with increasing oxygen content. The extent of oxidation decreased gradually with decreasing oxygen content from the critical value and the incubation period disappeared. In the transient period, Fe2O3 was formed on the lean oxygen-content side and elongated voids were formed in the outer Fe3O4 and FeO layer. It was suggested that the differences in the morphology of Fe2O3 formed on the surface affected by the dissociation and gas-transport process due to differences in oxygen partial pressure at the gas–scale interface.

Sulfidation properties of TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys at 1173 K and 1.3 Pa sulfur pressure in an H2S–H2 gas mixture

Abstract TiAl–2 at. % X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys were sulfidized at 1173 K for 86.4 ks at a 1.3 Pa sulfur pressure in an H2–H2S gas mixture. The structure, phases, and compositions of the external sulfide scale and alloy surface layer were measured using EPMA and X-RD. The TiAl–2Ag and –2Cu alloys sulfidized faster than TiAl, and the alloy surface layer was thicker than that of TiAl. Sulfidation amounts of the TiAl–2X (X=V, Co, Fe, Mo, W and Nb) alloys were almost the same as that of TiAl, while the thickness of the alloy surface layer decreased in the order: V>Co>Fe>Mo>[Cr (by Narita T, Izumu T, Yatagai M, Yoshioka T. Intermetallics, 2000;8:371)]>W>Nb. The sulfide scale was composed of multi-layer structures: an outermost (rich in Ti-sulfides), an outer (rich in Al2S3), an inner (a mixture of Ti-sulfides and Al2S3), and an innermost (rich in Ti-sulfides) layer. The alloy surface layer also had a multi-layer structure, and was classified into four groups: group 1 for TiAl–2V and –2Co alloys as well as TiAl binary alloy where the surface layer consists of alloy substrate/TiAl2/TiAl3/sulfide scale, group 2 for TiAl–2Nb, –2Mo, and –2W (and also– 2Cr) alloys with alloy substrate/TiAl2/TiAl3/(Nb, Mo, W or Cr)–Al alloy/sulfide scale, group 3 for TiAl–2Cu and –2Ag alloys with alloy substrate/TiAl2/Ti (Al, Ag or Cu)3 with an L12 structure/TiAl3/sulfide scale, and group 4 for TiAl–2Fe alloy with alloy substrate/TiAl2/Ti(Al,Fe)3 with an L12 structure/TiAl3/FeAl3/sulfide scale. Diffusion paths for these four groups were shown in a tentative Ti–Al–X ternary phase diagram.

The Role of Bond Coat in Advanced Thermal Barrier Coating

A novel diffusion barrier bond coat with a duplex layer structure, a sigma phase Re-Cr-Ni barrier and Ni aluminides as an aluminum reservoir was formed on a Ni based superalloy (TMS 82+) and Hastelloy X. The oxidation behavior of both alloys with and without the sigma- Re-Cr-Ni -phase as a diffusion barrier was investigated at temperatures of 1373K (Hastelloy X) and 1423K (TMS-82+) for up to 360ks. It was found that the Re-Cr-Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.

Oxidation behavior of sulfidation processed TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys at 1173 K in air

Abstract The oxidation behavior of sulfidation processed TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag, and W) alloys was investigated at 1173 K in air for up to 630 ks under a heat-cycle condition between 1173 K and room temperature. Oxidation behavior was classified into two groups: group A with alloys containing Fe, Nb, Mo and W, and group B containing V, Co, Cu and Ag. The TiAl binary alloy belonged to group B. During the sulfidation processing, the alloying elements in group A formed aluminides on the TiAl 3 layer, while the group B alloys formed a TiAl 3 (TiAl 2 included) layer including a small amount of the third element. The cross-sectional microstructures for group A show the sequence; oxide scale/TiAlX/TiAl 2 /alloy substrate, and for group B oxide scale/Ti 3 Al/alloy substrate. The alloys in group A (except Nb containing alloy) showed some scale exfoliation at the initial stage of oxidation, but only very little exfoliation after long oxidation times, whereas alloys in group B showed little exfoliation at the first several cycles, and then tended to exfoliate significantly, resulting in very rapid oxidation. The TiAlX/TiAl 2 layers formed by the reaction between the X-aluminide and TiAl 3 improve the oxidation properties of the group A alloys.