The Role of Alloy Microstructure During Oxidation in the Nb-Ti-Al System. Ph.D. Thesis

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1993

OSTI ID:236779

Brady, M P

The role of the alpha(Nb2Al + Ti), gamma(TiAl + Nb), and beta(Nb-Ti-Al solid solution) phases during oxidation of Nb-Ti-Al alloys at 1200 C and 1400 C in air was investigated. Oxidation kinetics were monitored continuously by thermogravimetry for exposure times up to 72 hours. Scanning electron microscopy, electron microprobe analysis, x-ray diffraction, in-situ high temperature x-ray diffraction, optical microscopy, and quantitative metallography were used to characterize oxide scales and alloy microstructures. At 1200 C in air, a systematic study of a series of alpha + gamma alloys selected from the same tie line revealed that the rate of oxidation was inversely proportional to the alpha/gamma interfacial area and proportional to the alpha/gamma volume fraction ratio. Further, a transition from paralinear oxidation kinetics to parabolic oxidation kinetics was observed when the gamma phase was continuous in the alloy microstructure. However, a protective alumina scale was not formed by any of the alloys and the rate of oxidation was too high to be considered protective. A phenomenological model which defined the roles of the alpha and gamma phases during the oxidation of two-phase alpha + gamma alloys at 1200 C in air was presented. When the temperature of oxidation was raised from 1200 C to 1400 C, a transition to protective alumina formation was observed for alloys which fall within the gamma phase field. The rate of oxidation was approximately equal to that of NiAl extrapolated to 1400 C. Alloys which fall within the beta phase field at 1400 C were borderline alumina formers and exhibited nonprotective paralinear oxidation kinetics. The temperature dependence of the kinetics of alumina formation was identified as the key factor in the transition to protective alumina formation observed at high temperature. A phenomenological model which defined the roles of the gamma and beta phases during oxidation at 1400 C in air was presented.

• Research Organization: Florida Univ., Gainesville, FL (United States)
• OSTI ID: 236779
• Report Number(s): N--96-21539; NIPS--96-34116
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM ALLOYS
METALLOGRAPHY
MICROSTRUCTURE
NIOBIUM ALLOYS
OXIDATION
PHASE TRANSFORMATIONS
REACTION KINETICS
SOLID SOLUTIONS
TEMPERATURE RANGE 1000-4000 K
TITANIUM ALLOYS