β-Grain Refinement of α+β-Type Ti–4.5Al–6Nb–2Fe–2Mo Alloy by Using Rare-Earth-Oxide Precipitates

抄録

The effect of the addition of small amounts of rare-earth elements such as La (0.01 mass%), Y (0.1 mass%), Er (0.1 mass%) and Ce (0.1 mass%) on the refinement of β-grains in an α+β-type Ti–4.5Al–6Nb–2Fe–2Mo alloy was investigated in the temperature range 1173–1573 K. The β-grain size and the rare-earth-oxide precipitates obtained after heat treatment were evaluated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. Upon heating, alloys exhibited rapid β-grain growth above a threshold temperature, and this temperature depended on the added rare-earth elements. The fine precipitates of rare-earth oxides formed in the alloy suppressed the β-grain growth through pinning. Dissolution of the precipitates in the β-matrix caused rapid β-grain growth. Yttrium was found to be the most effective element for the suppression of β-grain growth at high temperatures such as 1573 K.