Deciphering the dynamic precipitation and annihilation of GBα deviating from the conventional variant selection rule in Ti-652 during hot deformation

Abstract

Differing from the statically precipitated grain boundary α (GBα) that holds Burgers Orientation Relationship (BOR) with adjacent β grains, in this work, it was found that over 97.77 ​% of the dynamically precipitated GBαs do not hold BOR with either side of the adjacent β grains in hot-forged Ti-652 titanium alloy. The evolution of GBα orientation was decoupled as dynamic precipitation and annihilation, which was quantitatively distinguished by defining three misorientation angles: the deviation form BOR ($\omega$), the misorientation between adjacent β grains ($\theta$) and the misorientation between non-BOR GBαs (${\theta }^{\prime }$). During the entire hot forging, an adequate dynamic precipitation produces piecewise continuous and slender GBαs with $\omega$ <10° typically, manifesting as $\theta$ <25° but ${\theta }^{\prime }$ >45°. Oppositely, a sufficient annihilation leads to the dynamic globularization of GBαs and increase of $\omega$ with $\theta$ ≥25° but mainly ${\theta }^{\prime }$ ≤45°. Dynamic precipitation was driven by the intrinsic BOR preference of GBαs, while annihilation attributed to grain growth and rotation caused by accumulated plastic deformation. Deciphering the dynamic precipitation and annihilation of GBα provides a quantitative criterion to tailor microstructures and mechanical properties in the future.