Effect of deformation temperature on grain refinement in alpha titanium alloy VT5-1

Alpha titanium alloys exhibit good thermal stability, corrosion resistance, satisfactory toughness, but these alloys cannot be strengthened by heat treatment. One of the ways of their strengthening is to produce fine-grained microstructure during deformation. Grain size in materials subjected to hot and warm deformation depends on flow stresses, which significantly vary with conditions of deformation. In the article, the data on flow stresses and microstructural parameters of alpha titanium alloy VT5-1 (Ti-5Al-2.5Sn) subjected to isothermal deformation at temperatures varying from 940 to 650 degrees C are represented. Technique of electron backscattering diffraction was applied for quantitative analysis of the microstructure. Obtained results were used to define the relationships between i) flow stress and the mean grain size in deformed alloy, ii) temperature of deformation and mean sizes of grain and subgrain, iii) temperature of deformation and tensor dislocation density. It was shown that discontinuous dynamic recrystallization prevailed in the VT5-1 alloy at deformation temperatures higher than similar to 840 degrees C and continuous dynamic recrystallization prevailed at 650-840 degrees C. Decrease in the deformation temperature from 940 to 650 degrees C led to tenfold decrease in the size of dynamically recrystallized grains. Microstructure with the mean grain size of 0.7 micrometers and high dislocation density was obtained after deformation at 650 degrees C. Formation of the fine-grained structure resulted in an increase Brinell hardness of the alloy by almost 1.4 times in comparison with the initial coarse-grained condition.