Improvement of Strength and Ductility of α+β Titanium Alloy by Stress Induced Transformation and Fining Microstructure

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The effect of short-time duplex heat treatment on the microstructure, mechanical properties and fatigue strength of Ti-6Al-4V alloy was investigated. This heat treatment consisted of solution treatment at 1203 K for 60 s and subsequent water-quenching plus aging at 753〜953 K for 40 s. For the heat-treated materials, we carried out detailed metallographic examinations including the observations of microstructures by an optical microscope and TEM, X-ray diffraction and selected area electron diffraction measurements as well as the hardness measurements, tensile test and fatigue test. The static strengths and fatigue strength of the alloy were significantly increased by the above heat treatments. Especially, when the alloy was heat-treated with the conditions of the solution treatment at 1203 K for 60 s and aging at 803 K for 40 s, the improvement rates of the tensile strength and fatigue strength reached about 29% and 22% respectively. It was thought that this strengthening was caused by the formation of α'martensite phase with quenching after the short-time solution treatment and the precipitation of fine αphase in the retained βphase during the short-time aging. In spite of the remarkable improvement in the strengths, the ductility of the heat-treated materials remained above the level of the untreated material, most likely because strain-induced martensite transformation occurred in β phase which was retained even after the short-time aging.