Influence of twins found in adiabatic shear bands on dynamic recrystallization of a near β Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy

Much efforts have been made on the studies of the microstructure evolution near adiabatic shear bands (ASBs) in titanium alloys. However, the deeper evolution mechanism is not very clear. Therefore, in the present, the dynamic compression test utilizing split Hopkinson pressure bar at room temperature and a strain rate of ~3000/s was carried out on a hot-rolled near beta-typed Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy, and the microstructures of the central region and the transition region of the adiabatic shear bands were carefully investigated by highangle annular dark field-scanning transmission electron microscope (HAADF-STEM). It was found that the central region of ASBs was composed of the softening regions (SRs) and the hardening regions (HRs). Moreover, detailed observation indicated that in the SRs, the microstructure consisted of dynamic recrystallization (DRX) grains. While in the HRs, the {11-22}< 11-2-3 > twin in alpha grains and the {332}< 113 > twin in beta grains were observed, and a great number of dislocations were accumulated at the interface of twins and parent phases (alpha and beta phases) to form dislocation walls, thus promoting the DRX process in the ASB central region. It illustrated a new DRX mechanism in ASBs. Meanwhile, no twins were found in the ASB transition region. Further research on the average compositions of alpha and beta phases in the HRs of ASBs by energy dispersion spectrum method combined with d-electron theory revealed that the twin in alpha grains was mainly caused by the severe deformation under dynamic loading, while that in beta phase was the result of the instability of beta phase.