Mesoscopic Structural States at the Nanoscale in Surface Layers of Titanium and Its Alloy Ti-6Al-4V in Ultrasonic and Electron Beam Treatment

Abstract

Ultrasonic and electron beam treatment of commercial titanium VT1-0 and its alloy VT6 (Ti-6Al-4V) produces a nonequilibrium grain-subgrain hierarchical substructure in the surface layer, which causes a multiscale fragmentation of the material and reveals a damping effect. When cooled in the gradient temperature field (during electron beam treatment) and when the β phase of the initial alloy is destroyed by ultrasound, the high-temperature bcc structure of the surface layer undergoes a nonequilibrium phase transition into an hcp α-phase structure. The excess specific volume of the β phase is hierarchically distributed in the a phase through the growth of nonequilibrium α′ and α″ martensite, and in the form of local ω-phase precipitation along the grain boundaries of the α phase. The specific volume of the nonequilibrium phases exceeds the specific volume of the α phase. This eliminates the formation of micropores and causes material fragmentation at the micro- and nanoscale structural levels during the nonequilibrium β → α phase transition. The growing α′ laths cause the fragmentation of the α phase at the microscale level. The α″ laths grow within the nonequilibrium α′ laths; they have a thickness of ∼1.5 nm and fragment the material at the nanoscale level. This process is controlled by the electronic subsystem that creates nanoscale mesoscopic structural states for the formation of nonequilibrium martensite phases. The reversible elastoplastic deformation of the nonequilibrium martensite phases at the nanoscale level governs the damping effect of the surface layer subjected to ultrasonic or electron beam treatment. The generation of nanoscale mesoscopic structural states and the related new mechanism of reversible deformation in the conditions of broken translational invariance of the lattice in a deformable solid has been confirmed experimentally.