Effect of Ca Addition on Melt Cleanliness, Microstructure and Mechanical Performance of Cast IN738 Superalloy

This study examines the effects of nominal Ca additions of 0.001, 0.01, 0.10, and 0.20 wt% on the oxygen, nitrogen, and sulfur contents, as well as on the microstructure and mechanical properties, of IN738 alloy. As the nominal Ca addition increases from 0.001 to 0.20 wt%, the O, N, and S concentrations decrease from 13, 9, and 12 ppm to 1, 2, and 1 ppm, respectively. Concurrently, the area fraction of MC-type carbides and γ/γ′ eutectics first increases and then decreases, whereas the area fraction of inclusions gradually decreases. EDS analysis indicates that the inclusions are compositionally consistent with MgAl2O4, CaAl2O4, and CaS. With increasing nominal Ca addition, the coarse equiaxed grains in the central region of the solidification microstructure disappear, accompanied by an increase in refined equiaxed grains. At a nominal Ca addition of 0.10 wt%, skeletal carbides form in the microstructure. Mechanical property tests show that the room-temperature tensile strength and yield strength remain nearly unchanged, while the elongation decreases with increasing nominal Ca addition. When the nominal Ca addition increases from 0.001 to 0.01 wt%, both the high-temperature (650 °C) tensile strength and elongation increase; in contrast, at nominal Ca additions of 0.10–0.20 wt%, both properties decrease. The stress-rupture life and elongation at 760 °C under 585 MPa both exhibit a decreasing trend, with the rupture life decreasing from 67 to 11 h and the elongation decreasing from 10.0% to 3.0%. These results indicate that reasonable control of the nominal Ca addition can effectively reduce the O, N, and S levels and the inclusion area fraction while preserving the alloy’s favorable high-temperature performance at 650 °C.