Coupling effect of second phase and phase interface on deformation behaviours in microscale Ti-55531 pillars

The metastable-β Ti-55531 alloys with different second phases were chosen to investigate coupling effect of second phase and interface as well as the dislocation interaction details under uniaxial compression. In Ti-55531 alloy, nanoscale ω phase was acquired by solution treatment, which was coherent with the β matrix. The orientation relationship between hexagonal ω and bcc β lattice was (0001)_ω//(111)_β and [112‾0]_ω//[1‾10]_β. Sub-microscale α phase was acquired by ageing process, following the Burgers orientation relationship (BOR), {110}_β//{0001}_α and <11‾1>_β//<112‾0>_α. The characters of mechanical behaviors and deformation microstructures in this alloy have been analyzed by combining the results from uniaxial compression, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM. Ti-55531 alloy micropillars with deformable α phase show great plastic stability and high strength, which are attributed to the deformable α phase and the interaction between the dislocations and the high-density α/β interface. However, Ti-55531 alloy micropillars with nanoscale ω phase exhibit size effect and poor plastic stability. The deformation mechanism of the Ti-55531 pillars with ω phase is dislocation shearing the ω phase, which will cause the ω→β phase transformation. The ω-free channels result in the collective movement of dislocations, performed as the strain bursts in the stress-strain curves. But the dislocation slip between the α phase and the β matrix follows a harmony slip mode, which is similar to “slip relay” behaviour. The results reveal that tuning the morphology of phase interface and the density of second phase to govern the activation, multiplication and movement of dislocation is an effective method to tailor the strength and plastic deformation of materials. The deformation model between the dislocations and the phase interface has great significance to the design of the high performance materials by optimizing the microstructures.