Effect of yttrium additions on microstructure and corrosion resistance of Incoloy 825 alloy

The work is devoted to the study of the effect of microalloying with yttrium (Y) additives to improve the corrosion resistance of Incoloy 825 superalloy. The influence of Y on microstructure was evaluated by metallographic methods using optical and scanning electron microscopes, resistance to pitting and intergranular corrosion was evaluated by electrochemical and chemical methods of analysis. The paper describes changes in the structure, phase composition and hardness of cast samples with yttrium content of 0, 0.01, 0.05 and 0.1 wt. %. The obtained data correlate with the results of thermodynamic calculations of phase formation during crystallization. The influence of additions on the structure after strain hardening was investigated. Small addition (up to 0.01 wt. %) promotes increase of mobility of recrystallized grain boundaries. With increasing Y amount, the grain size decreases and hardness increases. It is shown that the greatest deoxidizing ability is observed at small additions of Y in the amount up to 0.01 wt. %, while the total amount of dissolved [O] decreased five times. Increasing the Y content reduces the ability to remove heavy inclusions from the melt, resulting in an increase in the proportion of oxide inclusions. The effect of additives on nitrogen [N] was not observed, and the volume fraction of nitride inclusions did not change, but the size of nitride inclusions decreased and the character of their distribution changed to uniform than in the alloy without Y. The results of pitting and intergranular fracture resistance tests showed that Y is an element that can be used to improve the corrosion properties of Incoloy 825 alloy. The best combination of resistance to the two types of corrosion was observed for the 0.01 wt. % Y sample.