Deformation Behavior of High Temperature Intermetallic TiAl Alloyed with Gallium
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DEFORMATION BEHAVIOR OF HIGH TEMPERATURE INTERMETALLIC TiAl ALLOYED WITH GALLIUM B. KAD*, and B. F. OLIVER* *Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 ABSTRACT TiAl (y) intermetallic has been investigated with ternary additions of up to 10 at% Ga. Optical microscopy reveals that gallium can be retained in solution to at least 7 at%. Microhardness data reveals that Ga substituted for Al such that Ti/(Al+Ga)=l causes solution softening. Deformation behavior of the single phase alloy in compression is reported. Perfect dislocations of the a/21110) type and superdislocations of the a[011] type and a/2[112] type have been observed. TEM results indicate that primary deformation is contributed by a/2[1101 type dislocations and twinning on (111) planes with wavy slip optically observed. INTRODUCTION TiAl is brittle at ambient temperatures. Considerable work has been devoted to understanding the mechanism of this brittle behavior [1-71. Researchers have attempted to improve the ambient temperature ductility by ternary additions to TiAl. Most notably Ag, V, Nb, Cr and Mn additions have been reported to improve room temperature ductility [5,8-111. Micro additions of Ni and B also enhance the ductility and castability of the TiAl base alloys 112]. Current research reports on the modified deformation behavior of TiAl when alloyed with Ga. Efforts were limited to improve the room temperature properties while still maintaining a single phase, L10 compound. Additions of Nb, V, Mo, Mn and Ga were chosen to substitute for either Ti or Al. Ga proved to be most promising based on preliminary hardness measurements and compression tests.
EXPERIMENTAL PROCEDURE Alloying Guides a) b)
c)
The choice for gallium as an alloy addition was based on the following: Ga exhibits a good solubility in both Al and Ti. Hence, Ga additions might still retain a single phase microstructure. Ga forms an L10 compound with Ti as does Al. Intuitively, Ga may substitute preferentially for Al in the matrix while still retaining the Li0 structure since Ga belongs to the same periodic group as Al and the electron/atom ratio is conserved. TiAl has similar lattice parameters (a=0.3997 nm, c=0.4075 nm, c/a=1.0195) to TiGa (a=0.3970, c=0.3970, c/a= 1.00) [131. It was thought that Ga additions may decrease the tetragonality of the TiAl lattice and enhance the plastic deformation behavior.
Experimental Techniques The alloys were prepared from the individual elements of 99.99% or better purity. Alloy buttons were made in an enclosed chamber on a water cooled copper hearth using a non-consumable tungsten electrode under a constant 30 cubic feet/hour argon flow. The buttons were remelted at least Mat. Res. Soc. Symp. Proc. Vol. 133. -1989 Materials Research Society
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six times, turning the button after each melt pass for better mixing. The weight loss was less than 1%for all melts accepted for further study. The alloy buttons were wrapped in Ta and encapsulated in silica tubing and evacuated to 10-6 tor
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