A metallography coupled with the microprobe method was used to study the solidification process of superalloys. It was found that the final solidification temperatures of the conventional superalloys are generally very low, about lOtSO-112OC, the solidification ranges are large enough, reaching 180-24OC, that makes the solidification segregation serious. Strictly controlling the contents of the minor elements P,Zr,B and Si in superalloys can raise the final solidification temperature to 1180-1280°C and contract the solidification temperature range to 50-12O"C, that results in alleviating the solidification segregation obviously. After controlling the contents of the minor elements, a series of new superalloys with low segregation and excellent properties have been developed.

Phosphorus is generally considered as a most common impurity and detrimental element in nickel-base superalloys. In contrary, the beneficial effect of P on 650 "C stress rupture life and creep behavior has been shown in this paper. For understanding the P behavior in Ni-Cr-FeMo-Nb-Ti-Al system, the role of P has been studied in Ni-Cr-Fe and Ni-Cr-Fe-MO system alloys and the beneficial effect of P on 650 "C stress rupture life and ductility has been also confirmed in INCONEL 7 18. Auger analysis has demonstrated the grain boundary segregation of P and its severe interaction with MO. Experimental results suggest grain boundary strengthening effect of P. The detail mechanism and theoretical explanation is still undertaken in research.

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Phosphorus and sulfur are generally regarded as the most common impurities and detrimental elements in nickelbase superalloys. For further understanding the role of P and S in Inconel 718 nine experimental heats were melted on the base of conventional Inconel 718 chemical composition with variation of P (from 10 to 130 ppm) and S (from 15 to 175 ppm) respectively. Phosphorus and sulfur both have almost no influence on strengths and ductilities at room temperature tensile test, Phosphorus also has no effect on the yield and ultimate strengths and elongation at 650°C tensile test, However, sulfur has an obviously decreasing effect on 650°C tensile elongation but no effect on yield and ultimate strengths.

Segregationdue to solidification of highly alloyed superalloy is a very complex process.