Abstract: The microstructures of as-cast and specimens heattreated at 1 100 ℃ for 500 h or 1 280 ℃ for 2 min-3 h and stress ruptured properties at 1 100 ℃, 118 MPa for four heats of low Cr and high W content cast Nibase superalloys were investigated. The results indicate that the addition of element Ta increases the amount of eutectic γ′. (α+γ′) eutectic forms in the later stage of solidification as the amount of Nb and Ti maintains at 2.1%, Ta and Al reaches or exceeds 14.4%(mole fraction). Small amount of α phase does not decrease the stress rupture life obviously. However, α phase is unstable during the period of solid solution heat treatment at 1 260 ℃ or thermal exposure at 1 100 ℃ because of its solid solution or transformation to blocky M₆C carbides which result in a deterioration of the stress rupture properties of alloys at elevated temperature. Although tantalum is a beneficial element for increasing the high temperature strength of superalloys, its content should be matched with the suitable Al content. Therefore, the substitution of Ta for Al should follow the principle of equal atomic fraction. The alloy with the composition of Ni-10Co-1.5Cr-16W-2Mo-1Nb-5Al-4Ta possesses the optimum properties.

Key words: Ni-base superalloys; α phase; tantalum; M₆C carbides; microstructure; phase transformations