β Grain refinement due to small amounts of yttrium addition in α+β type titanium alloy, SP-700

The effect of small amounts of yttrium addition on refinement of β grain size and grain growth behavior in SP-700 alloy with Ti-4.5%Al-3%V-2%Fe- 2%Mo was investigated in the temperature region from 1 223 to 1 573 K. Yttrium and oxygen contents in yttrium-added alloys were varied from 0.007 to 0.070% and 0.017 to 0.086%, respectively. Such a small amount of yttrium addition as 0.007% resulted in marked β grain refinement. Variation of β grain size with the heating temperature exhibited rapid grain growth at the particular temperature, which became higher with the increase of yttrium or oxygen content in alloys. It was confirmed that β grain refinement and grain growth behavior with the heating temperature in yttrium-added alloys were brought about by fine precipitates of yttria and dissolution of these precipitates into the β matrix at the higher temperature. TEM and SEM observations of yttrium-added alloys revealed the cubic shape of yttria precipitates with the size of 0.1 to 0.2 μm, and this range of the precipitate's size was well fitted to results analyzed by modified Zener' model for pinning of grain boundary due to a fine particle. Based on assumption that the onset temperature of rapid grain growth corresponds to the re-dissolution temperature of yttria into the β phase, solubility product of Y₂O₃ in reaction of 2Y+3O⇆Y₂O₃ was obtained in use of data of variation of the onset temperature of rapid grain growth with yttrium and oxygen contents in alloys. It was also confirmed that yttria powder could be used as an alloying additive of yttrium in melting of yttrium-added alloys.