Optimization of heat treatment process for Ti551 titanium alloy based on phase transformation regulation

The effects of heat treatment parameters including solution temperature, aging temperature and cooling rate on the microstructure and mechanical properties of a new medium-strength and high-toughness Ti551 alloy were investigated by means of optical microscopy, scanning electron microscopy and universal tensile testing machine. With the same solid solution temperature at 900 ℃, no obvious changes were observed in the content and size of primary α phase of alloy, while the lamellar thickness of secondary α phase increased from 0.26 μm to 0.42 μm with the aging temperature increasing from 550 ℃ to 650 ℃. With the same aging temperature of 550 ℃, the content of the primary α phase decreased from 45% to 15% as the solution temperature increased from 900 ℃ to 950 ℃, along with continuous refinement of grains and morphological transformation from short rod-like to equiaxed. The lamellar thickness of the secondary α phase increased accordingly, reaching up to 0.72 μm after alloy subject to solid solution treatment at 950 ℃. The morphological feature and size of the microstructure are decisively influenced by the cooling rate: an almost single fully equiaxed primary α phase microstructure is obtained by furnace cooling, whereas the primary α phase is refined and the precipitation of coarse lamellar secondary α phase is promoted by moderate cooling. The results show that optimal strength-toughness combination of the Ti551 alloy is achieved under the composite heat treatment of 900 ℃×2 h+ air cooling, then followed by 550 ℃×6 h aging, which is closely associated with the regulatory role of cooling rate. Acicular secondary α phase can be induced in the transformed β microstructure by higher cooling rate, and the alloy’s strength-plasticity matching effect is further enhanced by the subsequent aging treatment.