Ductile TiAl alloy with a Widmanstätten lamellar structure formed by rapid heating

Abstract

Instead of conventional grain-refinement treatments for improving the ductility of fully lamellar TiAl alloys, multiorientational, lamellar, subcolony refinement with good ductility has been achieved simply by using an electric-current pulse treatment. The microstructural refinement mechanism is attributed to the transformation on heating of γ laths in the prior large-grain lamellar structure to Widmanstätten α in several orientations, which on subsequent cooling forms lamellar structure colonies in multiple orientations. This kind of refined multiple-colony lamellar structure was found to enhance the ductility of the TiAl alloy.

Characterization of Microstructures Formed during Nonequilibrium Thermal Cycles in a TiAl Alloy Fabricated by Direct Metal Deposition

“…According to the BOR, the newly formed (α 2 + γ) lamellae may exhibit different orientations as the original (α 2 + γ) lamellae. In this way, the L W microstructure is formed during the heating/cooling cycles through reprecipitation of α and γ lamellae, and L W microstructure follows the BOR with the host colony . In the second mechanism, it is proposed that twins are generated in α grains when cooling from the α phase field.…”

“…[13] Previous research indicates that L W microstructure generally has two main formation mechanisms. [15][16][17][18] In the first mechanism, when (α 2 þ γ) lamellae is heated to the α phase field, new α lamellae will nucleate and grow on the {111} γ plane of the existing γ lamellae. During subsequent cooling, new γ lamellae will nucleate and grow from (0001) α plane of the newly formed α lamellae, thus forming new (α 2 þ γ) lamellae.…”

“…In this way, the L W microstructure is formed during the heating/cooling cycles through reprecipitation of α and γ lamellae, and L W microstructure follows the BOR with the host colony. [16][17][18] In the second mechanism, it is proposed that twins are generated in α grains when cooling from the α phase field. During subsequent cooling, γ lamellae precipitate from α twins and α matrix simultaneously, which will then transform into the L W microstructure and host colony, respectively.…”

“…[13,14] In addition, when cooling at moderate rates (e.g., air cooling, sand cooling, and oil cooling), [13,15] the L W microstructure can precipitate from (α 2 þ γ) lamellae or from α twins inside α grains. [15][16][17][18] TiAl alloys are characterized as poor ductility; hence, they are suitable to be produced by near-netshape manufacturing techniques such as investment casting and powder metallurgy. [6,19] Compared with traditional manufacturing methods, direct metal deposition (DMD) is an ideal choice to prepare TiAl alloy components as it can build nearnet-shaped components directly without using molds.…”

“…When decreasing the cooling rate, the L F microstructure is generated from (α 2 + γ) lamellae through sympathetic nucleation . In addition, when cooling at moderate rates (e.g., air cooling, sand cooling, and oil cooling), the L W microstructure can precipitate from (α 2 + γ) lamellae or from α twins inside α grains …”