Identification of partial dislocations and faults in cubic Al 3 Ti
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Dislocation dissociations in Al 3 Ti alloys modified with Mn to stabilize the LI2 cubic structure have been studied with transmission electron microscopy and computer simulation of images. Dissociations of a(110) superdislocations into pairs of a/2(110) partials bounding APB's were observed at all temperatures from room temperature to 700 °C. Asymmetrical image contrast, in which one of the a/2(110) partials gives a much more intense image than the other, was observed at small separations of the partials. Although some investigators have taken such asymmetry to suggest SISF-type dissociations in similar alloys, the current work demonstrates that the asymmetry is fully consistent with APB-type dissociation. Further, the degree of image asymmetry decreases as the spacing of the partials increases. It is concluded that identification of the partial dislocations with "near-invisibility criteria" for fractional values of g • b is unreliable, and that computer simulation of images is useful for identification of the partials. However, as expected, the ability to distinguish simulated bright-field images of APB- and SISF-type dissociations also becomes difficult as the separation of the partials becomes very small. Under these conditions, both weak-beam imaging and simulations are necessary to identify the dissociations. Weak-beam simulations have shown that fringe contrast must be present under certain imaging conditions for SISF dissociations, and this contrast has never been observed in this study or in previously published studies of dissociated single superdislocations in cubic Al3Ti alloys. Finally, APB contrast formed with superlattice reflection imaging has been observed between partials on both {111} and {100} after deformation at 700 °C.
I. INTRODUCTION AI3T1, which is tetragonal in binary form, can be alloyed with Ni,1-2 Cu,1'2 Zn,3 Fe,4 Cr,5'6 Mn,5'7 Co, 8 Ag, 9 Pd,10'11 Pt,11 Au,11 and Rh11 to produce a single-phase cubic L l 2 structure. These materials have low densities, high melting points, and excellent high-temperature strength, but have limited tensile ductilities at low temperatures. In addition, (Al, Cr)3Ti alloys have excellent oxidation resistance.12 As is the case for many materials with the LI 2 structure, such as Ni 3 Al, cubic Al3Ti alloys deform by {111} slip of dissociated superdislocations at low temperatures.10'13'19 However, the dissociation mechanism and the degree of dissociation have been the subject of controversy. It is well known that an a[101] superdislocation lying on (111) can dissociate by two mechanisms, one resulting in an antiphase boundary (APB) and the other a superlatticeintrinsic stacking fault (SISF):
a[ 101] - -£• [ 101] + APB + ^- [ 101] d
(1)
J
a[T01] -» f [211] + SISF + j [TT2]
(2)
J. Mater. Res., Vol. 9, No. 3, Mar 1994
http://journals.cambridge.org
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[For the remainder of this paper, dissociation via Eq. (1) will be referred to as an APB dissociation, while dissociation via Eq. (2) will be referred to as an SISF dissociation.] Three d
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