Microstructural effects on the tensile properties and deformation behavior of a Ti-48Al gamma titanium aluminide |
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Authors: | G Babu Viswanathan Michael J Mills Vijay K Vasudevan |
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Affiliation: | (1) the Department of Materials Science and Engineering, Ohio State University, 43210 Columbus, OH;(2) the Department of Chemical and Materials Engineering, University of Cincinnati, 45221-0012 Cincinnati, OH |
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Abstract: | The effects of microstructure on the tensile properties and deformation behavior of a binary Ti-48Al gamma titanium aluminide
were studied. Tensile-mechanical properties of samples with microstructures ranging from near γ to duplex to fine grained, near- and fully-lamellar were determined at a range of temperatures, and the deformation structures
in these characterized by transmission electron microscopy (TEM). Microstructure was observed to exert a strong influence
on the tensile properties, with the grain size and lamellar volume fraction playing connected, but complex, roles. Acoustic
emission response monitored during the tensile test revealed spikes whose amplitude and frequency increased with an increase
in the volume fraction of lamellar grains in the microstructure. Analysis of failed samples suggested that microcracking was
the main factor responsible for the spikes, with twinning providing a minor contribution in the near-lamellar materials. The
most important factor that controls ductility of these alloys is grain size. The ductility, yield stress, and work-hardening
rate of the binary Ti-48Al alloy exhibit maximum values between 0.50 and 0.60 volume fraction of the lamellar constituent.
The high work-hardening rate, which is associated with the low mobility of dislocations, is the likely cause of low ductility
of these alloys. In the near-γ and duplex structures, slip by motion of 1/2<110] unit dislocations and twinning are the prevalent deformation modes at room
temperature (RT), whereas twinning is more common in the near- and fully-lamellar structures. The occurrence of twinning is
largely dictated by the Schmid factor. The 1/2<110] unit dislocations are prevalent even for grain orientations for which
the Schmid factor is higher for <101] superdislocations, though the latter are observed in favorably oriented grains. The
activity of both of these systems is responsible for the higher ductility at ambient temperatures compared with Al-rich single-phase
γ alloys. A higher twin density is observed in lamellar grains, but their propagation depends on the orientation and geometry
of the individual γ lamellae. The increase in ductility at high temperatures correlates with increased activity of 1/2<110] dislocations (including
their climb motion) and twin thickening. The role of microstructural variables on strength, ductility, and fracture are discussed.
This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented
at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint
Committee on Mechanical Behavior of Materials. |
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