Heterostructure manipulation of Ti2AlNb alloy through directional induction heating: investigation of multi-scale deformation mechanisms in the O phase

In this study, a heterostructure of Ti2AlNb alloy (Ti-22Al-26Nb) was fabricated using directional heating treatment (DHT) technology. The microstructure of the alloy was examined before and after DHT treatment using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The primary objective of the study is to investigate the deformation mode and strain hardening mechanism in the multi-scale O phase. The results indicate that DHT treatment leads to a significant refinement in the grain size of the alloy, resulting in a 14.2% increase in ultimate tensile strength (UTS) and a 29.2% increase in elongation. However, the treatment also causes a decrease in yield strength. The soft phase region, characterized by the microstructure with micron-sized O phase precipitates, and the hard phase region, characterized by the microstructure with submicron-sized O phase precipitates, exhibit different modes of strain adjustment. To mitigate the internal stress arising from the deformation mismatch between the soft and hard phases, the micron-sized O phase activates slip on the {110} and {010} planes during the tensile process at room temperature. Additionally, twinning behavior was observed on the {110} plane. On the other hand, the submicron O-phase activates slip on the {041} plane and occurs a crystal rotation of approximately 90 degrees along the (012) and (100) orientations. These high-angle grain boundaries (HAGBs), which form continuously during deformation, effectively store dislocations and contribute to a continuous strain hardening effect in the alloy. Additionally, the alloy exhibits a high strain hardening ability due to the accumulation of geometrically necessary dislocations caused by the strain gradient resulting from the uneven plastic deformation of the soft and hard phase components, as well as the potential hetero deformation-induced (HDI) hardening. Ultimately, a Ti2AlNb alloy with a favorable balance of UTS and elongation was achieved.