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Notes: Conclusions 1. The effect of α' and α" phases on the mechanical properties of commercial titanium alloys of the martensite type (VT6, VT14, VT3-1, VT16) was investigated. A substantial strengthening effect (as much as 30%) in alloys VT6, VT14, and VT3-1 is due to the formation of α' phase in quenching. The best combination of strength and ductility was found for alloys of the martensite type quenched from temperatures in the α+β region near the end of the α+β→β transformation. 2. The effect of Mo, V, Fe, Cr, and Mn concentrations on the properties of titanium martensite (α') was investigated. It was found that increasing the amount of β stabilizer in the α' phase increases its strength, lowering the ductility. The largest increase in strength was observed in alloys with the highest supersaturation of α' phase with the β stabilizing element. 3. With increasing transformation of α' to α" phase in quenched Ti−Mo and Ti−V alloys the strength decreases gradually, although α" phase impoverished in β stabilizing elements has the same mechanical properties as α' phase. With increasing amounts of β stabilizing element the strength decreases, the ductility increasing. 4. The addition of aluminum to titanium alloys with β stabilizing elements leads to an increase in the strength of α' phase and a larger increase of the strength after quenching. 5. Transmission electron microscopy revealed changes in the fine structure of metastable α' martensite with increasing amounts of β stabilizers within the limits of the primary phase transformation (β→α') and in α" phase with the β→α" transformation, which determine the mechanical properties of the alloys. The net results are basically as follows. With increasing amounts of β stabilizer within the limits of the β→α' transformation (1–4% Mo) the number of regions with twins decreases; the number of α' needles with a high dislocation density increases, and also the number of orientations. With larger amounts of β stabilizers (5–9% Mo) the number of α' needles decreases and the number of α" plates increases, the characteristic feature of which is internal twins, more even orientation of the plates, and a lower dislocation density than α' phase.