Orthorhombic precipitate variant selection in a Ti2AlNb based alloy

Abstract

Variant selections of intragranular and intergranular orthorhombic precipitates in Ti-22Al-25Nb alloy are studied. It was found that strong variant selections occurred when intragranular orthorhombic phases (O phases) precipitated in microstructures with abundant deformation-introduced dislocations. The selected variants grew into parallel O phase lamellae and developed into cluster structures. In the microstructures with low dislocation intensities, all 12 can-be-selected variants could be observed. The interaction energy between the transformation strain and the stress field of prior dislocations were calculated. The results suggested that dislocations could indeed promote variant selections of intragranular O phases. Intergranular O phases (GBO) at high angle boundaries usually select the variants holding the specific orientation relations {001}O//{110}B2 and ⟨11¯0⟩O//⟨1¯11⟩B2 (OBOR here after) with one of the two adjacent B2 grains. In a special case where the adjacent B2 grains share a common {110}B2 plane, the two variants that have their {001}O parallel to the common {110}B2 would be both selected. The two variants keep OBOR with the two B2 grains respectively and form so-called ploy-twins structures. Moreover, O phase colony structures would be formed when a GBO grew inward the adjacent B2 grain that holds OBOR with it.