The microstructural evolution occurring during friction stir welding of a near-α titanium alloy, Ti-5111, has been examined by backscattered electron imaging and electron backscatter diffraction. The unaffected baseplate (BP) microstructure consists of millimeter-scale prior β grains containing ~100 μm large colonies of aligned α laths, related to each other by a strain-accommodating Burgers orientation relationship. The α laths are separated by fine, 100 to 150-nm-thick, interlath β ribs. A heat-affected zone (HAZ) is observed ~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the β ribs and the formation of secondary α platelets within them closer to the tool. There is a narrow thermomechanically affected zone (TMAZ), comprised of outer and inner regions, observed ~1.0 to 1.5 mm from the tool surface. Deformation is first observed in a ~200-μm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (α laths) α laths and the lattice orientations rotate to align the close-packed á11[`2]0ñlangle{11bar{2}0}rangle directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a ~300-μm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the β transus. Material in the stir zone (SZ) within ~1 mm of the tool surface consists of fine (~10 to 20-μm diameter) equiaxed prior β grains that are delineated by ~500-nm-thick α and contain 150-500-nm thick α laths. The texture exhibits both D1([`1][`1]2)[111]D_1({bar{1}bar{1}{2}})[111] bcc and P1(1[`1]00)[11[`2]0]{P_1({1}bar{1}00)}[11bar{2}0] hcp shear texture components, indicating that this material exceeded the β transus during welding.
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