ISSN:
1662-9752
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxidegrowth 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 barrierand outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-basedalloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAlformed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activitypack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves threephases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the twostep Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as adiffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(20[removed info]50)at% Cr alloy changed from the δ-Ni2Al3coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated attemperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). Inthe diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (orHastelloy X) and Ni aluminides containing 12[removed info]50at%Al when oxidized at 1423K for up to 1800ks. It wasfound that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outwarddiffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermostand Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems thatthe inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and theouter/outermost layers
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/02/13/transtech_doi~10.4028%252Fwww.scientific.net%252FMSF.522-523.1.pdf
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