In order to promote the strengthening of the γ "phase, niobium-based superalloys have been modified by replacing niobium atom by atom with tantalum and then heat treating the resulting alloys at temperatures higher than those normally used for niobium-based alloys. It is found that the strength of molybdenum alloy is increased and the phase stability is higher than that of niobium alloy.
Existing nickel-based superalloys continue to be improved, but they still have many weaknesses in strength or ductility. This is especially true at high temperatures, that is, temperatures above 1200°F. These alloys are generally nickel-based with one or more chromium, iron, and cobalt. In addition, these alloys can contain a variety of elements present in most combined ways in order to produce the desired effect. Some elements used in nickel-based superalloys to provide or improve one or more of the following properties are: molybdenum, tantalum, tungsten, rhenium (strength); Chromium, aluminum (oxidation resistance); Nickel (phase stability); Or cobalt (to increase the number of beneficial secondary precipitation objects). Other elements are added to form hardened precipitates, such as γ '(Al, Ti) and γ "(Nb) phases. Minor elements (C, B) are added to form carbides and borides, and other elements (Ce, Mg) are added to control inclusions. Some elements (B, Zr, Hf) are also added to promote favorable particle boundary effects. Although the addition of many elements such as Co, Mo, W, And Cr is beneficial to the properties of the alloy, in some cases they can precipitate out to form undesirable phases such as σ, μ, and Lavers phases.
Tantalum plays a different role than niobium in nickel-based superalloys. Moreover, tantalum has been found to produce an alloy with better phase stability and different phase relationships than the corresponding niobium alloy. This difference in phase stability allows tantalum-containing alloys to have much higher strength at higher temperatures than their niobium-containing counterparts. In addition, the γ "phase in the alloy of the invention is not easily transformed into δ phase, which often occurs in the corresponding niobium containing alloy.
