Microstructural Studies of the Deformation of TiAl Alloys
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MICROSTRUCTURAL STUDIES OF THE DEFORMATION OF TiAl ALLOYS ERNEST L. HALL AND SHYH-CHIN HUANG GE Corporate Research and Development Center, PO Box 8, Schenectady, NY 12301
ABSTRACT The mechanical behavior and microstructures of TiAl alloys after tensile and bend tests at room temperature and elevated temperatures were studied. The results for two-phase Ti52 A148 alloys are compared with those of single phase Ti 48A152 , and the effect of adding 3 at. pct. vanadium as a substitute for Ti in these two alloys is considered. It is shown that Ti52A148 has greater strength and ductility than Ti48A152 at room temperature and elevated temperatures up to 871PC (1600'F). Adding vanadium increases the ductility of both binary alloys. The microstructure of the Ti 52 A14 8 alloy deformed at room temperature contains primarily twins and 1/2 easy slip dislocations, and associated pinned faulted whereas the similar Ti48 A152 sample exhibits superdislocations dipoles. If these samples are deformed at 540 0 C (1000'F) or above, the Ti52 A148 exhibits extensive twinning, and the pinned faulted dipoles in the Ti 48 A152 sample disappear. The vanadium additions do not noticeably change the deformation microstructure at room temperature. It is suggested that the strength and ductility of these alloys may be controlled by tetragonality, bonding, interstitial element, and grain size effects, which in turn are affected by the presence of second phases and by the alloy composition. INTRODUCTION There is at present a great deal of interest in the intermetallic compound TiAl, since this compound possesses attractive properties for high temperature structural applications. These properties include high melting temperature, low density, high modulus, and good oxidation resistance. The major problem limiting the use of this material is its poor low temperature ductility [1,2]. The deformation behavior of TiAl is related to its L10 crystal structure, which is based on an ordered face centered tetragonal cell in which the Ti and Al atoms occupy alternating (002) planes. The formation of this compound involves a peritectic reaction, with the TiAl phase field existing primarily on the Al-rich side of the stoichiometric composition, from - 50 - 65 at. pct. Al [3,4]. A number of researchers have studied the deformation behavior of single-phase TiAl alloys which had compositions on the Al-rich side of stoichiometry [5-9]. The results of these investigations can be summarized as follows: a) The slip plane for deformation is exclusively the {111 } plane. Dislocations with the three lowest energy Burgers vectors on this plane in the Li 0 crystal structure were found to be, in order of increasing energy, 1/2[110] unit dislocations ("easy" slip type [51) and 1/2 and superdislocations. b) The superdislocations were found to undergo complex dissociation reactions. The [101] dislocations can first dissociate into the other two dislocations (1/2[1 10] + 1/2[112]), and then the 1/2[112] superdislocation can then further dissociate according to 1/2[112] --+ 1/6[1
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