Dislocations in plastically deformed L1 2 compounds based on Al 3 Ti
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The fine structure of dislocations in lightly deformed samples of several cubic ordered alloys with composition based on Al3Ti has been examined by weak beam electron microscopy. For all the materials examined the dislocations tend to dissociate into two 1/2(110) partials separated by APB. Dislocation dissociation is not complete at very small strains and the strain required to dissociate, as well as the dissociation distance, varies from one alloy to another. Improvements in ductility achieved by alloying are directly related to the ease and extent of this dissociation.
I. INTRODUCTION Trialuminide alloys of titanium are receiving much attention as potential materials for high temperature applications where low density and good oxidation resistance are of critical importance. These materials offer reasonable strength, even up to high temperatures, but are generally extremely brittle. Recent efforts try to understand the underlying causes of this low ductility or toughness and then to improve the overall material properties. The binary Al3Ti alloy has the tetragonal D0 22 structure, is extremely brittle, and deforms at room temperature mostly by twinning.1 At higher temperatures slip processes start to become important. Two approaches have been attempted to alloy this material and improve ductility: by alloying the D022 material attempting to enhance slip or twin mechanisms,2 or by alloying to change the crystal structure to the higher symmetry L l 2 structure. It is apparently this second approach that is producing the more promising results. Additions of transition metal elements such as Fe, Cr, and Mn have been shown to produce L l 2 ordered materials with reasonable strength and good ductility in compression, both at room temperature as well as at higher temperatures. 35 The alloys with Cr and Mn additions have been shown to have a slight ductility when tested in bending,5 rather than only in compression as for all previous tests. Systematic studies on cubic Al3Ti based alloys containing such transition metals6'7 suggest that alloys modified by additions of Cr or Mn are the most ductile, and are the softest of the alloy series examined. Transmission electron microscope examinations show that dislocations may be clearly dissociated after room temperature deformation, in contrast to earlier studies, for example, on Ni modified alloys8 where no dissociation was apparent, and more distinctly dissociated than on alloys modified with Fe.9 In recent studies10-11 of Cr and Mn modified alloys, it is shown J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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that both bend and tensile ductility can be achieved, albeit limited, and that the materials become stronger when tested at low temperatures (down to liquid nitrogen temperature). Similar low temperature strengthening has also been reported for an Fe modified alloy.12'13 Observations of strengthening at high temperatures have been interpreted in terms of a cube cross slip model, as for Ni 3 Al, 101314 a model based on dislo
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