Microstructure and compressive strength of as-cast and aged Nb-rich Nb-Al-Ti intermetallics

Author

Dah-Liang Guan

Date of Award

5-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Charlie R. Brooks

Committee Members

Peter K. Liaw, John D. Landes, Easo P. George

Abstract

As-cast Nb-Al-Ti alloys containing 15 at.% A1 and 10, 25 and 40 at.% Ti were tested in compression at 298, 673, 873 and 1173 K (25, 400, 600 and 900°C) at strain rates from 10^-5 to 5x10^-2 s^-1 to a strain of 5%. The yield strength decreased with increasing temperature for each alloy at all strain rates, and increased with increasing strain rates. The stress-strain curves showed serrated yielding at test temperatures of 673 and 873 K (400 and 600°C) and the lower strain rates. The cause of this was not determined. All alloys showed a yield point drop when tested at 1173 K (900°C), and transmission electron microscopy indicated this was due to recovery occurring during testing. The Ti content (and hence the Nb content) had a marked effect of strength. At a given strain rate and test temperature, the strength increased considerably with increasing Nb content (decreased with increasing Ti content). This was attributed to solid solution strengthening of the ordered B2 structure, and in the 10% Ti (75% Nb) alloy, to the presence of some δ phase. Aging treatments were carried out at 873, 1173 and 1373 K (600, 900 and 1100°C) after which compression testing was conducted at 1173 K (900°C) at a strain rate of 10^-4 s^-1 Aging the 40% Ti alloy at 873 and 1173 K (600 and 900°C) for 10 and 100 hours, and 10 hours at 1373 K (1100°C), did not change the strength significantly. Aging the 10 and 25% Ti alloy for 10 hours at 1173 K (900°C) did not significantly change the strength, but aging for 100 hours increased it about 20%. Aging for 10 hours at 1373 K (1100°C) increased the strength 30 to 40% above that of the as-cast level. For a given alloy, the increase in strength correlates with increasing amounts of δ formed during aging. As was found for the as-cast condition, there is a marked increase in strength with less Ti (more Nb) content and this is believed due to solid strengthening of the B2 matrix.

Recommended Citation

Guan, Dah-Liang, "Microstructure and compressive strength of as-cast and aged Nb-rich Nb-Al-Ti intermetallics. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/9263