Advances in Superalloys

Abstract

This topic concerns advances in superalloys, and is sponsored by the TMS High Temperature Alloys Committee. Near-term developments on nickel-base superalloys are presented, as well as longer-term considerations for new superalloys. The High Temperature Alloys Committee sponsored a very successful International Symposium on Superalloy 718 and Derivatives on October 10–13, 2010. A wide range of processing, properties, and applications of these alloys was considered in depth. The entire published proceedings are available at http://www.wiley.com/WileyCDA/WileyTitle/ productCd-0470943165.html. The High Temperature Alloys Committee also recently sponsored a symposium entitled ‘‘Advances in Processing’’ at the TMS 2011 Annual Meeting that was very well attended. Several interesting papers are presented from these two symposia. Superalloy 718 has a very versatile microstructure which can include c, c¢, c¢¢, and d phases. Rick Kearsey and coworkers describe how varied grain size, precipitate size, morphology, and phase fraction influence mechanical properties at high temperatures in one improved derivative alloy, ATI 718Plus . This alloy is shown to offer promising combinations of monotonic and cyclic mechanical properties in comparison with a superalloy employed in many turbine disk applications, Waspaloy. Some aerospace applications require small, complex-shaped components. A near-net shape metal injection molding process is investigated for use with Superalloy 718 by Eric Ott and coworkers. This process requires injection of a powder–binder blend into complex tooling, removal of tooling, debinding, densification, heat treatment, and final machining. With sufficient control of process and materials through appropriate industry specifications, this process is shown to have promising potential for aerospace components. Processing of superalloys for aerospace applications often includes shot peening after machining, to introduce compressive residual stresses near the surface that can prevent surface cracking. Verification of the stress magnitude is often required. R. Chandrasekar and co-authors describe the use of an advanced eddy current inspection technique to measure these stresses in Superalloy 718, and the effects of different microstructures on response. As the oil and gas industry has advanced to drill deeper wells with higher pressure, temperature, and corrosive media, conventional steel components have been selectively replaced with superalloys. John ‘‘Jack’’ deBarbadillo describes how Superalloy 718 and derivatives have been adapted to these demanding requirements. More advanced cast and wrought superalloys are being optimized to withstand extreme conditions in deep-water sour gas and oil drilling and production applications. Advanced ultrasupercritical power generation plants have the potential to attain significantly higher efficiency, reducing fuel consumption and gas emissions. Here also, many steel components will need to be replaced with superalloys for the associated increases in temperature. Paul Jablonski and coworkers describe the potential for cast versions of selected wrought superalloys to attain the temperature–stress requirements, in large cast components. Several candidates show promising results in screening tests at relevant conditions. Dispersion strengthening can be used to provide exceptional strength in superalloys at high temperatures. However, this can make the superalloys very difficult to fabricate, due to poor hot working and joining capability. Mike Fahrmann and co-author describe an innovative new Co-base superalloy that uses fine-scale stable nitrides for dispersion strengthening, introduced by a gas nitriding process after fabrication processes are completed. Thermodynamic and kinetic properties are shown to strongly influence the nitridation process. The mission of the TMS High Temperature Alloys Committee is to provide a means of communication among those interested in superalloys and other high-temperature alloys. Emphasis is placed on JOM, Vol. 64, No. 2, 2012