The gradient microstructure and its thermal stability of TC17 titanium alloy induced by ultra high strain rate deformation of LSP were systemically examined by transmission electron microscope (TEM). The microstructure near substrate consisted of high density deformation twins and dislocations; The microstructures featured of high density dislocation tangles

dislocation cells and subgrains closer to the surface and deformation twins became less prevalent due to the higher strain and strain rate which led to insufficient time for atoms to reposition to the twinned orientation to accomplish the twinning deformation; The original coarse grains with size of tens of micrometers (average grain-size 43μm) were refined instantly to hundreds of nanometers (average grain-size 396nm) in the top surface of TC17 titanium alloy after LSP. The quantitative calculation of recrystallization kinetics proved that the instant grain refinement of TC17 titanium alloy after LSP was the result of rotation dynamic recrystallization (RDR) mechanism. The LSPed TC17 titanium alloy samples were annealed at 573K

623K

673K

and 723K for 1h

respectively. Comparison of the gradient microstructures studied at different depths of the LSPed and LSP+573K/1h and LSP+673K/1h annealed specimens indicated that the dislocation density clearly decreased and the dislocation cells became clearer in the 573K/1h annealed specimen while there were no great changes in deformation twin density and the average grain size at the surface did not change greatly. After annealing at 673K for 1h

the dislocation density and deformation twins density both decreased greatly at different depths and the average grain size at the surface increased compared to those of the LSPed and 573K/1h annealed specimens. The average grain-sizes in the top surface layers of the LSPed and 573K/1h

623K/1h

673K/1h

and 723K/1h annealed samples were 396nm

422nm

493nm

1.04μm

and 2.46μm

respectively. The critical temperature at which the microstructures of the LSPed TC17 titanium alloy changed significantly was 673K; The fine gains at the top surface began to grow abnormally when the annealing temperature exceeds that temperature. There was a gradient distribution of hardness values of the LSPed surface layer. The hardness values decreased significantly when the annealing temperature reached 673K. It can be concluded that 673K was the critical temperature below which the microstructures induced by LSP were thermally stable in the TC17 titanium alloy.