By James M. Dahl
Product Application Manager
Aerospace & Power Generation
Richard B. Frank
Carpenter Specialty Alloys
EDITED BY STEPHEN J. MRAZ
Aircraft engines must perform in extremely hot and cold environments. A plane taking off from Antarctica, for example, can have its internal engine temperatures go from a bone-chilling -50° to 1,000°F in a matter of minutes. To keep the aircraft operating efficiently, gas-turbine-engine components such as compressors, exhaust casings, rings, and seals must be made of superalloys with low, controlled thermal-expansion characteristics. These materials must provide strength, ductility, and dimensional stability over a wide temperature range.
The materials' expansion properties must allow for small clearances between rotating and stationary components. Smaller and more constant clearances means better fuel efficiency through takeoff, cruise, and landing when operating temperatures and stresses can vary greatly. Thermal stability is also essential because engine components made of these alloys must withstand prolonged exposure at or near maximum service temperatures without substantial degradation of mechanical properties or microstructure.
Two relatively new alloys are proving effective in both aerospace and land-based gas-turbine engines, Thermo-Span and Pyromet CTX-909.
THERMO-SPAN AND PYROMET
Both alloys are precipitationhardenable and have low and relatively constant coefficients of thermal expansion over a broad temperature range. They offer good thermal fatigue resistance and similar tensile strengths at both room and elevated temperatures. The mechanical properties of both are similar to the widely used nickel-based, high-temperature 718 superalloy.
Like other controlled-thermal-expansion alloys, they combine the expansion characteristics of Kovar-base alloys with the thermal stability and high-temperature strength of superalloys. In general, controlled-expansion alloys have coefficients of expansion about 40% lower than those of the standard alloys.
Thermo-Span is a Ni-Fe-Co-Cr-based austenitic alloy. The chromium in Thermo-Span is of special significance. Before Thermo-Span, chromium could not be used in alloys without sacrificing low-expansion properties.
With 5.5% chromium added to an optimized Ni-Fe-Co-Cr base, composition, uncoated Thermo-Span resists oxidation at temperatures up to 677°C (1,250°F). Parts made of CTX-909 and earlier alloys, however, must be coated to withstand temperatures above 540°C (1,000°F).
Coating adds time and expense to the manufacturing process, but it is the only way chromium-free Fe-Ni-Co-based alloys can survive temperatures above 540°C (1,000°F).
CTX-909, like Thermo-Span, is a high-strength Fe-Ni-Co superalloy with a low and relatively constant coefficient of thermal expansion over a broad temperature range. It also has good thermal-fatigue resistance.
In oxidation tests of Thermo-Span and CTX-909, cylinders of both materials were held in a resistance-heated, static-air environment at temperatures up to 677°C (1,250°F) for 20 hr, then held at room temperature for 4 hr. Specimens were weighed every four cycles (80 hr at heat) for a total of 130 cycles (1,040 hr at heat). Both alloys exhibited similar oxidation weight gain after the first few exposure cycles. In subsequent cycles, however, CTX-909 gained weight rapidly, indicating a nonprotective oxide scale. After the same initial series, the rate of gain for Thermo-Span was distinctly reduced, indicating formation of an adherent oxide scale. The difference is primarily related to the 5.5% added chromium.
Unlike other controlled thermal expansion alloys, Thermo-Span provides a level of oxidation resistance previously unattainable with low thermal expansivity.
In salt-spray corrosion tests, polished alloy specimens were exposed to a fog of 3.5% NaCl solution for 200 hr at 35°C (95°F). The chromium again made a difference. An average of only 5% of the surface rusted on Thermo-Span specimens, while the CTX-909 specimens averaged about 50%.
THERMAL EXPANSION AND STABILITY
The Invar effect found in these Kovarbased alloys determines the thermal expansion behavior of Thermo-Span and Pyromet CTX-909. Both have low expansivity below their Curie temperature (the temperature below which they are ferromagnetic). This low thermal expansivity is related to spontaneous volume magnetostriction where lattice distortion counteracts normal lattice thermal expansivity. At temperatures above the Curie temperature, nickel-iron alloys and 36% nickel alloys expand at higher rates because they are no longer ferromagnetic.
Both superalloys have almost identical thermal expansion profiles up to Thermo-Span's Curie temperature of about 320°C (610°F). At this point, Thermo-Span goes from ferromagnetic to paramagnetic. The expansion rate for CTX-909 alloy continues at a low rate until reaching its Curie temperature of 415°C (780°F).
TENSILE AND NOTCH STRENGTH
The development of low-thermal-expansion alloys has been driven by the quest for the best combination of short-term tensile strength and high-temperature notch rupture strength. But good tensile properties have been easier to attain than good notch strength. This was especially true for chromium-free controlled-expansion alloys.
Thermo-Span and CTX-909 have similar tensile strengths at both room and elevated temperatures, despite differences in both their matrix and hardener-system compositions. There are some variations, however, in their notch and combination smooth/notch stress-rupture properties.
Notch-rupture tests are important in assessing the tendencies of alloy parts to crack at machined features subject to relatively high stress concentrations. These tests are particularly significant in evaluating controlled-expansion alloys because residual chromium in these alloys contributes to a phenomenon called stress-accelerated grain boundary oxidation (SAGBO). In stress rupture testing, this leads to notch-brittle behavior.
Alloys generally undergo SAGBO embrittlement when they are exposed to a combination of high temperatures (especially around 540°C (1,005°F)), stress, and oxygen. All three conditions are found in gas turbines, so an alloy's resistance to SAGBO is important.
While both low-expansion alloys meet minimum levels of rupture life and elongation required by industry specifications, Thermo-Span provides consistently superior rupture life. For example, the stress-rupture performance of Thermo-Span at 675°C (1,250°F) is similar to that of CTX-909 alloy at 650°C (1,200°F).
Fatigue resistance is also important in controlled thermal expansion alloys because aircraft engines often produce vibrational and cyclic thermal stresses. Rotating beam fatigue tests show that Thermo-Span has a higher fatigue strength than CTX-909.
Thermal stability, of course, is critical to the performance of controlled expansion alloys in gas-turbine engines. They must retain their essential mechanical characteristics during long-term exposure to high temperatures in all flight conditions.
Tests comparing room-temperature strength and ductility of the two alloys after 500, 1,000 and 5,000 hr of static exposure at 650°C (1,200°F) reveal some interesting characteristics. For example, despite the lower yield and tensile strengths of Thermo-Span versus CTX-909 alloy before thermal exposure, its ductility remains the same after 500 or 1,500 hr at temperature exposure. At the same time, CTX-909's ductility shows a significant drop after 1,000 hr at temperature. Thermo-Span also showed only a 2 to 7% decrease in strength after testing, while CTX-909 had a 12 to 23% drop in strength. This shows the superior thermal stability of Thermo-Span and indicates that the overaging reaction is more extensive in CTX-909 than in Thermo-Span.
|Composition of two low-expansion alloys (wt%)|