Abstract: The relationship of microstructure and tersile ductility (ψ) with fracture toughness (KIc) and crack propagation (da/dN) of TCI 1 Ti alloy is researched into. The results show that the relation of KIC and da/dNwith ψ is dependent on structure type of alloy. For plate-like strucrture KIC increases with increase of ψ, and then invariable. But for equiaxed structure with increase of ψ KIC firstly increases, and then decreases, da / dNof plate-like structure changes along a curve with minimum, but for equiaxde structure da/dNis invariabe with increase of ψ in the researched range up to 30% ψ KIC of equiaxed structure increases with incerase of thickness of a plates preaent in the transformed ft, cooling slowly through temperature range of 950~850°C during heat-treatment, many particles of Ti3Al will precipifate along the periphery of widmanstatten a platelets, resulting in serious decrease of ψ and KIC Optimum synthetic mechanical properties can be obtained by using the heat- treatment of 990°C, 80min ,A.C. +950°C, 2h A.C. +530°C , 5h A.C. Maximum KIC and minimum da/dNmay be obtained by 1030°C , 80min, A.C. +850°C, 2h A.C. +530°C, 5h A.C., notwithstanding the ψ of alloy is very low (6.9%).

Key words: titanium-alloy microstructure, tensile ductility, fracture toughness, crack propaga- tion