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Title: Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C

Abstract

The environmental behavior of an orthorhombic titanium-aluminide alloy, Ti-22Al-25Nb, was studied in dry and humid air between 650 and 1,000 C by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti{sub 3}Al-based titanium aluminides at and below 750 C. Between 800 and 1,000 C, the oxidation resistance of Ti-22Al-25Nb was as good as {gamma}-TiAl base aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1,000 C. The mixed oxide scale consisted of TiO{sub 2}, AlNbO{sub 4}, and Al{sub 2}O{sub 3} with TiO{sub 2} being the dominant oxide phase. Underneath the oxide scale, a nitride layer formed in the temperature range investigated and, at 1,000 C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 {micro}m below the oxide scale when exposed to air at 900 C for 500 hr. The peak hardness dependedmore » on exposure time and reached five times the average hardness of the bulk material under the above conditions.« less

Authors:
 [1]
  1. DLR-German Aerospace Center, Cologne (Germany). Inst. of Materials Research
Publication Date:
OSTI Identifier:
684337
Resource Type:
Journal Article
Journal Name:
Oxidation of Metals
Additional Journal Information:
Journal Volume: 52; Journal Issue: 5-6; Other Information: PBD: Dec 1999
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; HYDROGEN EMBRITTLEMENT; MICROHARDNESS; TITANIUM ALLOYS; ALUMINIUM ALLOYS; NIOBIUM ALLOYS; AIR; TEMPERATURE DEPENDENCE; KINETICS; SCALING

Citation Formats

Leyens, C, and Oak Ridge National Lab., TN. Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C. United States: N. p., 1999. Web. doi:10.1023/A:1018816300624.
Leyens, C, & Oak Ridge National Lab., TN. Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C. United States. doi:10.1023/A:1018816300624.
Leyens, C, and Oak Ridge National Lab., TN. Wed . "Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C". United States. doi:10.1023/A:1018816300624.
@article{osti_684337,
title = {Environmental effects on orthorhombic alloy Ti-22Al-25Nb in air between 650 and 1,000 C},
author = {Leyens, C and Oak Ridge National Lab., TN},
abstractNote = {The environmental behavior of an orthorhombic titanium-aluminide alloy, Ti-22Al-25Nb, was studied in dry and humid air between 650 and 1,000 C by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti{sub 3}Al-based titanium aluminides at and below 750 C. Between 800 and 1,000 C, the oxidation resistance of Ti-22Al-25Nb was as good as {gamma}-TiAl base aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1,000 C. The mixed oxide scale consisted of TiO{sub 2}, AlNbO{sub 4}, and Al{sub 2}O{sub 3} with TiO{sub 2} being the dominant oxide phase. Underneath the oxide scale, a nitride layer formed in the temperature range investigated and, at 1,000 C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 {micro}m below the oxide scale when exposed to air at 900 C for 500 hr. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.},
doi = {10.1023/A:1018816300624},
journal = {Oxidation of Metals},
number = 5-6,
volume = 52,
place = {United States},
year = {1999},
month = {12}
}