Microstructure evolution of 1 mol % calcia-doped yttria

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The microstructure of 1 mol % calcia-doped yttria under different sintering condition was investigated, and the thermal shock resistance of yttria with and without additive was compared in present work. The results showed that by adding 1 mol % calcia, the sinterability of yttria is greatly improved and the sample can be sintered as low as 1300°C. Increasing heating temperature and prolonging soaking time are beneficial to form coarse grains with lots of closed pores in calcia-added sample at temperatures higher than 1500°C. By theoretical analysis, it is known that because of the solid solution of Ca2+, defects are generated in the yttria lattice, and the activation energy for diffusion is decreased. Therefore, the grain boundary mobility of yttria is accelerated and closed pores are left inside coarse grains when pores cannot move with yttria grain boundaries anymore. The water quench test reveals that the non-additive sample heated at 1500°C has the highest thermal shock resistance and it cannot be utilized under high temperature in industry. On the contrary, the calcia-doped sample has superior thermal shock resistance by the effect of closed pores, and it might be utilized to melt and cast high purity titanium and its alloys through vacuum induction melting (VIM) process.