Stoichiometry effects: on the deformation of binary TiAl alloys
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I. 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 Ll 0 crystal structure, which is based on an ordered face-centered tetragonal cell in which the Ti and Al atoms occupy alternating (002) planes (Fig. 1). The formation of this compound involves a peritectic reaction, with the TiAl phase field existing primarily on the Alrich side of the stoichiometric composition, from —5065 at. % Al.3 A number of researchers have studied the deformation behavior of single-phase TiAl alloys which had compositions on the Al-rich side of stoichiometry.4"8 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 Ll 0 crystal structure were found, which are, in order of increasing energy, a/2(110) unit dislocations ("easy" slip type4) and a/2(112> and a/(101) superdislocations (Fig. 1). (b) The superdislocations were found to undergo complex dissociation reactions. The a[101] dislocations can first dissociate into the other two dislocations, namely a[101] -> a/2[110] + a/2[lT2].
(1)
The a/2[112] superdislocation can then further dissociate according to a/2[lT2] -> a/6[lT2] + a/3[U2]
t SF
(2)
J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
http://journals.cambridge.org
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O Al
FIG. 1. The Ll 0 unit cell of TiAl, with the three lowest energy slip vectors in the {111} plane marked.
which creates a superlattice extrinsic stacking fault (SF). Additional dissociations of the a/2[110] or a/3[lT2] dislocations would lead to the creation of an antiphase domain. It has been postulated4 that the antiphase domain boundary energy of TiAl is too high to allow these further dissociations, but this has recently been called into question by Hug et al.,9 who report observing additional complex dissociations of the superlattice dislocations into partials separated by stacking faults and antiphase domains. (c) A large number of sessile faulted dipoles are observed after the room temperature deformation of these alloys. These dipoles are formed by local dissociations of the superdislocations by reactions (1) and/or (2), followed by pinning of the a/6(112) partials. The pinning has been suggested by Hug et al.6'8 to be caused by a complicated cross-slip/redissociation mechanism. © 1989 Materials Research Society
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E. L. Hall and S-C. Huang: Stoichiometry effects
(d) Twinning on {111} planes was also shown to be an important deformation mode.4 In the present work, these deformation studies have been extended to include two-phase TiAl (y) + Ti3Al (a2) allo
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