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The phase relationships among β-Ti, α-Ti and α2-Ti3Al phases in Ti–Al binary system at temperatures around 1473 K have been examined by transmission electron microscopy, particularly paying attention to the size of anti-phase domains (APDs), using Ti–(25–35) at.%Al alloys quenched from various high-temperature phase regions. Regardless of the phases present at high temperatures, all alloys become α2 single phase in the quenched state. The region of α2 phase prior to quench exhibits no APDs in the quenched state. The prior β phase region shows finer APDs with an average size of 40 nm, whereas the prior α phase region exhibits large APDs of 150 nm in size. Based on the difference in APD size as a criterion, together with hardness and transformed morphology, the phase regions at given temperatures can be determined. The results suggest that the α2 phase is congruently in equilibrium with α phase at 1465 K (congruent type), rather than in equilibrium with β phase straight with two peritectoid reactions (peritectoid type). From thermodynamic consideration, oxygen content in alloys is not a factor to change the phase equilibria from congruent type to peritectoid type among these phases. [ABSTRACT FROM AUTHOR]

A stoichiometric tetragonal compound of Ni3V (D022) quenched from disordered fcc (A1) state exhibits a unique morphology of ‘multi-variant structure’ (MVS) where the three variants of D022 are periodically arrayed with coherent {102)D022 and {010)D022 interfaces. The MVS of this compound collapses rapidly accompanied with coherency loss at the variant interfaces during aging at elevated temperatures. The collapse is caused by the rigidity of the variants filling in the {102)D022 and {010)D022 channels between the major variant particles. Introduction of disordered fcc (A1) phase to the channels by alloying addition makes the MVS stable without coherency loss of the interfaces even after prolonged aging. The resulting two-phase microstructure, so called maze and chessboard microstructure, exhibits a different shape of D022 variant with various habit planes, depending on the lattice misfit strains along a- and c-axes of D022, δa(<0) and δc(>0), respectively. The factor to determine the shape of variant is δa, whereas that to determine habit plane is the misfit ratio, δc/δa, The morphology map is constructed as functions of δa and δc, and the shape of the D022 variant changes from plate to prism as the value of &z.sfnc;δa&z.sfnc; becomes smaller than 0.002, results in the chessboard microstructure. [Copyright &y& Elsevier]