Effect of Solidification Condition and Alloy Composition on Formation and Shape of Pores in Directionally Solidified Ni-Al Alloys

The effect of the solidification condition and alloy composition on the formation of cylindrical pores oriented along the solidification direction was investigated in Ni(100-x)Al (x) (x = 20, 25, 30, and 50 at. pct) alloys that were solidified unidirectionally in hydrogen atmosphere. It was revealed that the uniformity of the pores strongly correlates with the width of the mushy zone (i.e., the region of solid-liquid coexistence) in the solidification front. In alloys with x = 25 and 50 (i.e., NiAl and Ni3Al intermetallic compounds, respectively), uniform cylindrical pores were formed, reflecting small freezing intervals, which lead to narrow mushy zones. On the other hand, irregular pores were formed in x = 20 and 30 two-phase alloys comprising Ni solid-solution and Ni3Al phases and Ni3Al and NiAl phases, respectively, that had large freezing intervals leading to wide mushy zones. This is because the large amount of primary crystals with dendritic structures prevents the growth of directional pores in the mushy zone. For the x = 20 and 30 alloys, the increase in the temperature gradient of the solidification front, which decreases the mushy zone width, clearly enhances the uniformity of the pores. Consequently, decreasing the mushy zone width results in the growth of uniform cylindrical pores.