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The eutectie system Ni-

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Directionally solidified (DS) Ni-33(at. pct)Al-33Cr-1Mo and Ni-33(at. pct)Al-31Cr-3Mo eutectic alloys were grown at different rates varying from 7.6 to 508 mm h Ϫ1 . The microstructures consisted of eutectic colonies with parallel lamellar NiAl/(Cr,Mo) plates for solidification rates at and below 12.7 mm h Ϫ1 . Cellular eutectic microstructures were observed at higher solidification rates, where the plates exhibited a radial pattern. Room-temperature fracture-toughness tests were conducted using a modified ASTM E-399 technique. The average fracture-toughness values for specimens with planar eutectic and cellular microstructures were about 12 to 15 and 17 MPaΊm, respectively, for both alloys. However, the Ni-33(at. pct)Al-33Cr-1Mo specimens grown at and above 254 mm h Ϫ1 exhibited fracture toughness values of about 8 MPaΊm due to the presence of short (Cr,Mo) plates. The fracture toughness values for the Ni-33(at. pct)Al-31Cr-3Mo alloy were also correlated with quantitative microstructural data in an attempt to identify the relevant elements of the microstructure determining resistance to fracture. A phenomenological fracture model is presented in an attempt to rationalize the present observations.