Super Alloy Parts in Aviation

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Super Alloy Parts in Aviation

Cross-section of an aircraft wheel and brake assembly

Super Alloy Parts in Aviation

Super Alloy Parts in Aviation

By John Routledge

The first successful flights of jet engine-powered airplanes (in World War II, by the German and British military) were made with materials-limited engines of relatively modest performance. As they advanced, jet engines continued to be materials oriented. Nonetheless, examination of materials progress since 1942 shows a spectacular series of developments that permitted uninterrupted increases in temperature and operating stress. The developments were both process- and alloy-oriented, and often a combination of the two. As a result the net 800-lb thrust of the 1942 Whittle engine has risen to the level of 65,000 lb-a factor of 80 in a little over 40 years.

Initially, cobalt-base alloys emerged as the leaders for blade manufacture, while iron-base alloys served for lower temperature requirements, disks, for example. From more or less improved conventional practice, wrought alloys, such as S-816, gave way to the coarse-grained precision-cast cobalt-base alloy parts. Then, industry learned how to control the grain size and structure, designers learned how to live with less-than-desired ductilities, and operating temperatures climbed to 815 °C (1500 °F). Precision casting of super alloy parts, then and now, continue to play a commanding role in the super alloy world.

There were parallel developments in Ni-base systems, the valuable, flexible, and now dominant -y/,y'-strengthened alloys. Here, it took the process development of vacuum metallurgy to make possible the production of strong "high alloy" compositions by controlling the impurity levels. Then still higher alloy contents, leading to greater strength and temperature potential, were realized through the development of remelting technologies, of which vacuum arc remelting is the most outstanding. These developments required unparalleled efforts by research and development groups to demonstrate and evaluate the roles of alloy composition and structure, to use the benefit of purity levels previously considered unattainable, and to develop advanced techniques to further modify the structures and the chemistries to solve special problems. Ultimately, this led to the exciting developments of directionally solidified and single-crystal blades, the latter reaching engine application only very recently.

Austenitic super alloy parts.

Throughout this period, the concern among metallurgists, designers, and manufacturers was always that the nickel-base and cobalt-base alloys ultimately would have to be replaced with higher-melting alloy systems, the refractory metals. This is hardly surprising when one realizes that increased alloying tends to produce lower-melting alloys; here were alloys being used at higher and higher fractions of their melting temperatures!

At first, major efforts were made with alloys of molybdenum and columbium (niobium). These were without success for the then-planned operating temperatures and anticipated lifetimes, but they may still hold promise for temperatures above about 1100 °C (2000 °F) if suitable coatings can be found. Excellent strength levels were realized and some promising coatings were developed, but expected lifetimes were not realized. Later, chromium-base alloys looked to be a natural, but ultimately were not successful because of brittleness problems.

We must also mention the early trials with cermets, and the first of a series of ceramic-age developments from 1950 onwards, both of which produced interesting solid structures, but still no acceptable applications in the super alloy competition. The austenitic super alloys remained dominant.

With the advent of rapid solidification processing, alloys of still more complexity are being developed and studied, now with the advantage of even closer control over impurity segregation and structure of desired phases. Further, production of superfine grain sizes and structures in the powder metallurgy area makes superplasticity easy to achieve and use. Nominally, cast alloys such as IN-100 and Mar-M 509 are made very strong at low and intermediate temperatures and are easily formable into complex shapes, including near-net-shapes. In the 1960s, who would ever have predicted that IN-100, a casting alloy, could be made to be superplastic and a candidate for disk applications at about 650-700 °C (1200-1300 °F)? Superplastic structures can be expected to have a major impact on super alloy technology.

ODS super alloy parts.

Finally, we are beginning to see significant applications of ODS (oxide-dispersion-strengthened) alloys, again using a blend of processes and alloying techniques developed over the intervening years. Mechanical alloying, and now the use of RS (rapid solidification; fine, fully alloyed powders), will permit use of ODS nickelbase and cobalt-base alloys to temperatures in excess of 1100 °C (2000 °F).

Use at 1100 °C (2000 °F) and above for alloys melting under 1400 °C (2550 °F)? Use in excess of 80% of the absolute melting temperature? Yes, that time has arrived. Even higher fractions of the melting point may be achieved with metalmatrix composites.

In summary, the extremely effective interplay of alloying processes with alloy compositions and structures, coupled with excellent supporting scientific studies of structures, properties, and stability have given the super alloys an engineering position never dreamed of by their early proponents!

Alternative alloys and materials are being sought but have not yet emerged. These new materials are being studied to replace or supercede super alloy parts.

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John Routledge

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