Influence of solidification conditions and alloying elements Re and Ti on micropores formed during homogenization of Ni base single crystal superalloy

The influence of withdraw rate during directional solidification and elements Re and Ti on the formation of micropores during homogenization in Ni base single crystal superalloy CMSX-4 is investigated by X-ray tomography method. The results show that the number of micropores below 5 μm in sample solidified at 10 mm/min after homogenization at 1300 °C/3 h is greater than that in the case of 6 mm/min. The number of micropores below 10 μm in Re free and Ti free alloys is rather smaller than that of normal CMSX-4 alloy, while the number of micropores between 15 μm and 25 μm in Ti free alloy is relatively greater than that in Re free alloy. The variation of number of micropores is analyzed based on measurement of primary dendrite arm spacing (PDAS) and segregation ratios of alloying elements, the causes are mainly attributed to reverse segregation tendencies of Re and Ti and their remarkable diffusion coefficient difference, the increasing of PDAS also enhances the propensity of micropores formation during homogenization.