Plastic deformation and fracture behavior of a Fe-modified Al 3 Ti-base L1 2 intermetallic alloy
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The microstructure of the ordered intermetallic alloy with a nominal composition of Al66Fe9Ti24 is nearly single-phase Ll 2 structure, with a few second phase agglomerates at some grain corners. Room temperature compression tests showed that this material exhibits a plastic strain of about 11% at fracture. Final fracture of the compression specimens occurred by a shear-off process along a surface oriented about 45 degrees to the compression axis. Fractographic analysis revealed that the fracture is transcrystalline and the fracture mode is mainly quasicleavage plus tearing. Transmission electron microscopy (TEM) was used to explore its deformation mechanisms. The dislocation density was low after homogenization, but is greatly increased during deformation. The deformation mode was found to be (110) {111} slip instead of twinning as in Al3Ti. The a(110) superdislocations dissociated into two partials of a/3(211)-type, bounding a superlattice intrinsic stacking fault (SISF) on the {111} slip plane.
I. INTRODUCTION
II. EXPERIMENTAL
The ordered intermetallic Al3Ti exhibits some attractive characteristics such as low density (3.4 g/cc) and high oxidation resistance, but is extremely brittle at room temperature. Since Al3Ti crystallizes into the tetragonal DO22 structure,_the major deformation mode was identified as (111) [112] twinning, and slip occurred only at high temperatures.1 It is encouraging that the DO22 Al3Ti can be changed to the cubic Ll 2 ordered structure by replacing some amounts of Al with Cu, Ni, or Fe,2"4 and also Mn and Cr, as has been reported recently.5 Since the Ll 2 structure is highly symmetrical and may have a sufficient number of slip systems for homogeneous deformation, it is expected that the ductility of modified Al3Ti-base alloys will be greatly improved. This has attracted much attention and has led to a considerable amount of research in recent years. However, the results reported did not come up to the expectation. For instance, the Al3Ti alloys having Ll 2 structure with Fe or Ni addition were also brittle and the fracture mode was still cleavage.4'6 Therefore, further investigations aimed toward understanding the deformation behavior and the causes of low ductility of the Ll2-type Al3Ti alloy are necessary. In this paper, an Ll 2 intermetallic alloy with composition near Al66Fe9Ti24 (at. %) was studied. This polycrystalline material shows appreciable compression ductility at room temperature. To understand the deformation behavior, a microstructural characterization of this ordered alloy was carried out and the deformation mode was studied by TEM analysis.
The Al3Ti-based alloy with nominal composition Al66Fe9Ti24 was prepared by arc-melting in argon with a nonconsumable tungsten electrode on a water-cooled copper hearth. The cast buttons were homogenized at 1373 K for 60 h to eliminate nonequilibrium segregation. Compression specimens with dimensions 4 x 4 x 7 mm were cut from the homogenized ingots. Tests were carried out in a Shimadzu DCS-25T machine. A crosshead speed of 0.05 m
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