Compaction and characterization of mechanically alloyed nanocrystalline titanium aluminides
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NIUM aluminides (a2-Ti3Al and g-TiAl) are attractive candidate materials for aerospace structural and engine applications due to their low density, high specific strength, elevated temperature strength, modulus retention, and excellent creep resistance.[1–5] However, a major obstacle for the use of these aluminides is their ambient temperature brittleness, both making it difficult to fabricate them into structural components by conventional processing methods and raising concerns as to their ‘‘forgiveness’’ in use. There have been many attempts in recent years to improve the ambient temperature ductility of these aluminides by (1) grain refinement via nonequilibrium processing methods, such as rapid solidification, mechanical alloying (MA), or innovative heavy deformation methods; (2) modification of the crystal structure to a more symmetric cubic lattice to achieve the minimum number of slip systems required for plasticity and a short Burger’s vector (for example, formation of the ordered body-centered cubic (bcc) (B2) phase in Ti3Al alloys by addition of niobium and other bcc metals has been shown to increase the ductility); and (3) changing the nature and proportion of constituent phases by either compositional changes or heat treatment (for example, a two-phase TiAl 1 Ti3Al mixture in g-TiAl type alloys increases the ductility).[1–5] These methods have resulted in significant improvements in ductility for the a2Ti3Al and have led to some improvements in the g-TiAl compound.[1,3,4,5] It has been shown previously[6] that nanocrystalline ceramics behave in a ductile fashion at room temperature because of the increased diffusional creep rate resulting from C. SURYANARAYANA, Professor, formerly with the University of Idaho, Moscow, ID, is now with the School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, 99164. F.H. FROES, Director, the Institute for Materials and Advanced Processes, is a Professor, Department of Metallurgical Engineering, University of Idaho, Moscow, ID 83844-3024. G.E. KORTH, Principal Scientist, is with the Metals & Ceramics Division, Lockheed-Martin Idaho Technologies Co., Idaho Falls, ID 83415-2218. Manuscript submitted February 27, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
a reduction in grain size and an increase in the grain boundary diffusivity. Because MA refines the grain sizes of powder particles down to nanometer levels,[7,8] if the titanium aluminides can be produced in a nanocrystalline state, it is likely that the ambient temperature ductility can be significantly increased. For most commercial applications, the mechanically alloyed powders must be consolidated and further processed to create a final shape. Conventional consolidation and forming processes, such as hot isostatic pressing (‘‘hipping’’) or hot extrusion, require elevated temperature exposures for extended times, which can considerably coarsen the nanometer-sized grains and greatly compromise the novel properties of the as-milled powder.[8,9] Thus, the purpose of the pres
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