Microstructural stability and mechanisms of fatigue and creep crack growth in Ti–24Al–11Nb |
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Authors: | P B ASWATH R D GOOLSBY L W GRAHAM |
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Affiliation: | (1) Department of Mechanical and Aerospace Engineering, Texas, 76019, USA;(2) Materials Science and Engineering Program, University of Texas at Arlington, Arlington Texas, 76019, USA |
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Abstract: | Titanium intermetallics are being developed for long term applications at elevated temperatures, particularly those alloys
based on the alloys Ti3Al and TiAl. Typical approaches include the design of appropriate microstructures for room and elevated temperature fatigue
and creep resistance. However, a little explored area is the stability of these microstructures at elevated temperature and
its effect on fatigue crack growth. The present investigation documented the microstructural stability, fatigue crack behaviour,
and stress rupture of Ti-24Al-11Nb, a Nb modified Ti3Al alloy. A coarse two phase α2+β Widmanstatten microstructure was found to exhibit the best resistance to fatigue crack growth. Microstructural stability
and elemental segregation were studied as a function of exposure time for up to 500 h at 800°C using transmission electron
microscopy (TEM). Results indicate that the Widmanstatten microstructure is metastable and the β phase breaks up into particles.
The absence of a continuous β phase surrounding the α2 phase reduces the resistance of the microstructure to fatigue crack growth at room temperature. At elevated temperature the
microstructure stability does not play a role in determining the fatigue resistance. A fine Widmanstatten microstructure has
the best resistance to creep deformation. Stress rupture tests were conducted in vacuum and air at 649°C and 760°C. Two types
of failure mechanisms were seen in stress rupture; these include transgranular and intergranular failure within prior β grains.
When tested in air at 760°C a combination of transgranular and intergranular failure occurred. Specimens that exhibited a
higher proportion of transgranular failure had longer lives. When tested in vacuum at 760°C the predominant failure mode was
intergranular. At 760°C extensive microstructural changes like breakup and spherodization of the β phase occurred under stress
while the rate of coarsening without any stress was much slower. At 649°C the specimens tested in vacuum consistently exhibited
longer lives. The creep crack growth when tested in air at 649°C was always a brittle transgranular mode while the specimens
tested in vacuum always failed by an intergranular mode.
This revised version was published online in November 2006 with corrections to the Cover Date. |
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