β Grain Refinement due to Small Amounts of Yttrium Addition in α+β Type Titanium Alloy, SP-700

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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 1223 to 1573 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 precipitates 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.
社団法人日本鉄鋼協会の論文
2006-01-15

β Grain Refinement due to Small Amounts of Yttrium Addition in α+β Type Titanium Alloy, SP-700

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