A New Single Crystal Superalloy for Power Generation Applications

Abstract

The properties of a newly developed single crystal superalloyknown as STAL-15 -is described which is suitable for use in first stage blades of industrial gas turbines (IGTs). With 15 wt.% Cr and 4.55 wt.% Al, the alloy combines good corrosion and oxidation resistance with sufficient creep and fatigue performance. Traditionally, polycrystalline alloys such as IN792 and IN738LC or the single crystal alloy PWA-1483 have been used for this application; unfortunately they display only limited resistance to environmental degradation. The new alloy does not display this weakness and is therefore highly optimised for IGT applications. The new alloy is shown to be an alumina (Al2O3) former; the mechanisms behind the Al2O3-formation process are studied and the effects arising from changes in the chemical composition have been modelled. In addition, the mechanical properties in terms of creep and fatigue resistance are demonstrated together and the alloy stability evaluated during long term (up to 10,000 hours) exposure. For such applications, the new alloy is superior to existing nickel-based single crystal superalloys designed for aeroengine applications and which are optimised for very high creep resistance. The absence of Re contributes to a lower cost of alloy stock, and enhanced castability.

On Thermomechanical Fatigue of Single-Crystal Superalloys

“…% Si increases the TMF by a factor of 2. In other studies, it has been shown that the addition of Si influences the formation of detrimental TCP phases during high temperature exposure, see [117]. However, it seems that the addition of 0.25 wt.…”

“…In this thesis, STAL-15 is the subject of both Papers VII and VIII. Table 3 displays the chemical composition of STAL-15 and for a detailed description of the alloy development history of this alloy the reader is referred to [117]. It should be noted that during its development, the alloy was first called SCA425 and later SCA425+, before being named STAL-15, and there are some small differences in chemical compositions between these forms.…”

“…However, as mentioned previously in this thesis, even though aero engines and industrial gas turbines are very similar, they are run in different ways, which means that slightly different properties are needed for the two types of applications. One aim behind the development of STAL-15 has been to achieve good corrosion and oxidation properties, and thus high Cr and Al contents are needed, and it has also been shown that STAL-15 is an Al 2 O 3 former [117,118]. The other aim was to combine good TMF properties with sufficient creep properties.…”

“…This means that it should have creep properties comparable to IN792, which is poly-crystalline superalloy. According to [117] it can be argued that the excellent creep properties shown for example by CMSX-4, are now unnecessary for industrial gas turbine blades due to the introduction of advanced cooling techniques. However, the introduction of advanced cooling techniques lead to more stress and temperature gradients within the material which increase the demand for excellent TMF properties, and in [117], it was shown that the TMF life of STAL-15 is comparable to that of CMSX-4.…”

Cyclic Oxidation of High Mo, Reduced Density Superalloys

“…These prior studies did not address oxidation directly. Other more recent high Cr, but lower Al, single crystal alloys were developed (SCA425/STAL-15) to promote Cr2O3 scales for corrosion resistance in stationary gas turbines and helium reactors [15][16][17]. In the present program higher Mo contents have been explored while eliminating W. Mo is a powerful solid solution strengthener, but with a density much closer to Ni than other refractory strengthening elements, such as Re and W. According to the fundamental design study, Mo is about 3× the cost of W, but are both relatively inexpensive for refractory metals [14].…”