| Today, superalloys are widely used for manufacturing the large size blades of utility gas turbines (UGT) and forging dies operated at elevated temperature. The important aims of developing new alloys are to increase the operating temperature and decrease the cost of materials. Alloy containing low Cr and high additive refractory elements such as W, Mo, Nb, Ta and Re can meet high temperature strength demand. Among them, elements Ta and Re are extremely expensive metals. The alloy cost is boosted to over one magnitude due to the Ta and Re addition, and the cost factor restricts the application of alloys seriously. To overcome this shortcoming, some attempts have been made recently to partially substitute Ru for Re. The effects of Ta and Ru on the microstructures and properties of cast Ni-base superalloys with low Cr and high W content are described in this paper. The purpose of this study is to explore the possibility and the way of decreasing the amount of Ta and replacing Re by Ru by understanding the effect mechanism of Ta and Ru.The systematical research on the microstructure, oxidation behavior and stress-ruptureproperties of Ta-containing or Ta-free alloys and the microstructure, solid-solution heat treatment and solidification behavior of Ru-bearing or Ru-free superalloys were carried out, based on investigating the microstructural stability of a base compositional cast Ni-base superalloy. The specimens of as-cast and after different treatments were analyzed by optical metallography, quantatitive metallography, SEM, EDS and XRD.The microstructures of as-cast and 850 /3000h, 950 /1000h and 1100 /50h~500h thermal exposure specimens for the base compositional (Ta, Ru)-free alloy were investigated. The results show that the as-cast microstructure consists of MC carbides, eutectic ' in dendritic region and the secondary y' phase distributed in the y matrix. The section size of specimen directly affects the volume fraction and sizes of phases. The eutectic y' in heavy section of specimen appears to be lower volume fraction and larger size. Section size has no significant effects on the volume fraction of MC carbides, but the increase of MC size will accompany with the enlargement of the section size. This superalloy exhibits a surprisingly good microstructural stability, due to no harmful TCP phase precipitates after long term thermal exposure in the 850 to 1100 range. Furthermore, in this temperature range, the carbide reactions are the slow decomposition of MC and precipitation of M6C carbides along the grain boundary, interdendritic zone and periphery of eutectic y', and the volume fraction of MeC can reach 1.4% after 1100 /500h exposure. In addition, after 1100 /500h thermal exposure there are still about 50% (volume fraction) MC carbides to be reserved from the original as-cast microstructure.Microstructures of as-cast and thermal exposure at 1100 /500h specimens of three heats of the low Cr and high W content superalloys with 0, 4, 6Wt.% Ta were investigated. Results indicate that the sorts of precipitates are not changed by Ta addition. Ta is a strong MC carbide former, its tendency of forming MC carbide isstronger than that of element Ti, but weaker than Nb. Ta is strong y' former and promotes the formation of eutectic y'. Substitution of Al for Ta on the basis of equivalent atomic percent does not change the quantity of eutectic y'. Excessive content of Ta leads to form excessive quantity of eutectic y'. The W and Mo above the limit of solubility can form a (W, Mo) solid solution due to the low solubility of element W and Mo in eutectic '. M6C is more stable carbide than MC in cast nickel base superalloys containing high level of W, and the addition of Ta does not inhibits the reaction, in which MC carbides transformed to granular or plate M6C carbides after 1100 /500h thermal exposure. In addition, Carbon from the decomposed MC carbides promotes the reaction a (W, Mo)+C-M6C.The static oxidation tests of three heats of Ta-free and Ta-containing superalloys were carrie...
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