Phase Composition Effects on Dynamic Behavior and Strain Rate Sensitivity in Metastable beta-Ti Alloys

In this study, high strain rate tension tests are conducted to determine and compare the dynamic mechanical behaviors and deformation mechanisms of different phase composition alpha-beta metastable beta-Ti alloys using a split Hopkinson tension bar. Two typical bimodal equiaxed alpha(p) + beta and lamellar alpha(s) + beta Ti-45551 alloy microstructures are formed through different hot working and thermal processing for investigating the effect of phase composition or microstructure on mechanical properties and strain rate sensitivity. It is demonstrated that dislocation nucleation and motion in the alpha/beta phase and dislocation tangle or pile up at the alpha/beta interface are typical deformation modes in both of the typical dual-phase Ti alloys at quasi-static loading conditions. Under dynamic loading, both the strength and ductility show a clearly positive strain rate dependence, which is directly related to dislocation activation in the alpha + beta Ti-45551 alloy. Based on microstructure characterizations, it is shown that deformation twinning starts to become a major deformation mechanism in equiaxed alpha(p) + beta microstructures under dynamic loading conditions. However, deformation twins are not favored in the lamellar alpha(s) + beta Ti-45551 alloy due to its nano phase size. Finally, the mechanical behaviors and strain rate sensitivity are strongly dependent on the phase composition of metastable beta-Ti alloys.