The formation of adjoining twin pairs in TiAl alloys

Abstract: Twinning is an important deformation mechanism in engineering materials, which can improve the ductility and strength of alloys, especially in the case of low-plasticity TiAl alloys. Herein, a large number of γ twins, twin intersections and adjoining twin pairs (ATPs) are formed in globular γ (γ-glob) phase after high-temperature tensile deformation. The ATPs consist of two sets of deformation twins (DTs) that are mirror symmetric about the original twin boundary (OTB). Its formation mechanism is analyzed with electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). The results reveal that the ATPs are related to the strain coordination between the DTs and the adjacent grains. Specifically, as the DT is formed in the γ-glob and impinges on the OTB, it will induce another γ twin (IT) in the adjacent γ-glob. The combination of the two γ twins generates ATPs, which are formed by the transmission of a twin from one grain to the neighboring grain. Moreover, γ twins with a lower Schmidt factor (SF) of 0.2643 are preferentially formed over another γ twin with a higher SF (0.3098) in the same γ-glob grain, because γ IT accommodates the strain caused by the γ DT in adjacent grain. Besides, the long-period stacking ordering (LPSO) structure of periodic contrasts 1 (PC₁) and PC₂ are observed in γ twins, which are considered to be the intermediate phase for the formation of γ twins. The current work elucidates the formation mechanism of ATPs in TiAl alloy and provides insights into the formation of twins in the γ-glob.