IN-SITU STRENGTHENING OF TITANIUM WITH YTTRIUM - TEXTURE ANALYSIS

In situ processing consists of heavily deforming a two-phase alloy of mutually immiscible elements to produce composite sheet or wire. In the well-studied Cu(fcc)-Nb(bcc) system, severe deformation by swaging and drawing reduces as-cast Nb filament phase thicknesses several hundred-fold after deformation. Cu-20 vol % Nb ultimate tensile strengths exceed 2000 MPa for material deformed to a true strain of eta = 12, where eta = In (area(original)/area(final)). In an earlier study of in situ strengthening in immiscible hexagonal close-packed metals, Ti-50 vol % Y and Ti-20 vol % Y alloys were deformed by hot extrusion, hot and cold swaging. The composites were deformation processed to true strains as high as eta = 7.6 to form a filamentary microstructure with filament thicknesses on the order of 0.1 mu m. The deformation processing of these composites increased their ultimate tensile strengths from 318 to 945 MPa, but the specimens' original diameters were too small to allow deformation processing to the very high true strains achieved with the Cu-Nb composites. In this study, a larger casting of Ti-20 vol % Y was deformation processed to eta = 12.8 in an attempt to achieve further refinement of the filament thickness. This composite formed the same filamentary microstructure up to eta = 7.27 observed in the earlier study of Ti-Y composites; however, at higher eta values the filaments recrystallized into equiaxed grains, decreasing the ultimate tensile strength. X-ray texture analysis of the composite specimens showed a strong [<10 (1)over bar 0>] fibre texture in both the Ti and Y phases in the deformation processing range 2.25 less than or equal to eta less than or equal to 7.27. This texturing is thought to constrain both the Ti and Y phases to deform by plane strain, which produces severe geometric restrictions on the ability of the plane straining filaments to achieve high eta values without fracturing or recrystallizing.