FORMATION OF NANOCRYSTALLINE STRUCTURE IN TWO-PHASE TITANIUM ALLOYS BY WARM SEVERE PLASTIC DEFORMATION

摘要

Warm severe plastic deformation realized via multiple forging can be used for formation of fine-grained microstructures with grain size of several hundred nanometers or less in titanium alloys. The smaller the grain size, the better the superplastic properties are resulted from lower temperature deformation. It has been established that the smallest grain size not only depends on the deformation temperature, but also the phase volume fraction, phase particle size and interparticle distance. The application of thermohydrogen treatment improves hot workability of titanium alloys at lower temperatures and permits to combine the deformation with metastable phases decomposition. Additional grain refinement is observed after hydrogen removal during vacuum annealing due toβ→αpolymorphic transformation accompanied by recrystallization. The microstructure with a grain size of 25 nm was obtained in two-phase titanium alloy Ti-6.3Al-3.5Mo-l.7Zr (wt.%). This alloy is superplastic at temperature 550℃ and strain rate 2X10~(-4) s~(-1): m=0.52, relative elongation achieves 550%.