Effect of Post-ECAP Annealing on the Microstructure and Mechanical Properties of Commercially Pure Titanium

An ultrafine-grained (UFG) microstructure with a mean grain size of 197 nm was produced in commercially pure titanium by equal-channel angular pressing (ECAP) to explore the effect of annealing treatment (temperature: 100-600 degrees C, holding time: 60 min) on the mechanical properties of severely deformed Ti. The evolution of microstructure after annealing treatments was studied in different length scales using light and transmission electron microscope, and mechanical properties were evaluated by room temperature tensile tests and microhardness measurements. The results show that UFG Ti exhibits high thermal stability up to 300 degrees C and thereafter recrystallization and grain growth phenomena occur with increasing annealing temperature. The microhardness of ECAP-ed material shows a sudden drop after annealing at 400 degrees C corresponding to the occurrence of significant grain growth in the microstructure (from 197 to 1.12 mu m), whereas it decreases slightly after annealing temperatures of 100 and 200 degrees C. A different behavior was observed at 300 degrees C. Annealing at 300 degrees C not only preserves the high strength (YS = 735 MPa) of as-ECAP state, but at the same time, it leads to an improvement in tensile ductility (from similar to 16 to similar to 19%). Fractography of tensile specimens by scanning electron microscope reveals a ductile fracture mode with typical dimple morphology in all processing conditions including starting material, as-ECAP state, and ECAP with subsequent annealing.