The impact of directional annealing on the microstructure and 900 °C tensile properties of γ-TiAl alloy

Abstract

This study investigates the impact of columnar and equiaxed microstructures of γ-TiAl alloy on tensile properties at 900 °C, as well as the corresponding high-temperature deformation microstructure. The characterization of twin and dislocation morphologies is conducted using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The dislocation and twinning behavior of γ_M and γ_L following high-temperature stretching is the primary focus of this investigation. The findings reveal that the columnar structure, characterized by reduced transverse grain boundaries, demonstrates enhanced high-temperature performance during the tensile process at 900 °C. This results in improved high-temperature strength and greater capacity for permanent deformation after necking. During high-temperature deformation, γ_M forms dislocation walls comprising $⟨0\bar{1}1]$ superdislocations and triggers numerous <110] true twins, potentially originating from the dissociation of $⟨0\bar{1}1]$ stacking faults. Concurrently, a broad spectrum of dislocation cross-slip activities is observed in γ_L, involving $1/2⟨1\bar{1}0]$ ordinary dislocations, as well as $⟨0\bar{1}1]$ and $1/2⟨11\bar{2}]$ superdislocations, along with some <011] true twins in γ_L facilitating additional strain adjustment. Furthermore, the study identifies that micro-holes and micro-cracks tend to develop at the interface of γ_M and the interface of (α₂+γ) lamellar colonies. These results offer valuable insights for enhancing the comprehension of the benefits associated with the high-temperature mechanical properties of α columnar crystal structures characterized by a high aspect ratio, as well as the alterations in micro/nano structures that occur during high-temperature deformation.