Analysis of the Chemistry of Ni-Base Turbine Disk Superalloys Using An Alloys-By-Design Modeling Approach
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NI-BASE superalloys are remarkable for the range and number of alloying elements used in their constitution. Typical grades call for as many as ten alloying element additions. For example, (1) chromium and cobalt are added to promote resistance to oxidation and corrosion, (2) aluminum, titanium, and tantalum to impart precipitation hardening, (3) tungsten and rhenium to impart resistance to creep deformation, and (4) zirconium and boron for grain boundary strengthening. At least in part, this situation arises because nickel displays a capacity to dissolve significant quantities of its neighbors from the d-block of transition metals. In this way, one can argue that superalloys—as a class of structural alloys—possess a degree of chemical complexity which is rare and arguably unique. It has been pointed out recently[1] that this situation presents a challenge since the many possible alloying elements mean that the existing grades of Ni-base superalloys are unlikely to be optimized for their intended applications. Thus, alloy compositions superior to those currently available are likely to exist, waiting to be discovered or possibly be designed. Traditionally, alloy design has invoked considerable use of trial and error-based approaches involving costly and exhaustive processing backed up by empirical property testing. Consider, for example, the development DAVID J. CRUDDEN, Postgraduate Student, NILS WARNKEN, Lecturer, and ROGER C. REED, Director of Research, are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Contact e-mail: djc000@bham. ac.uk BABAK RAEISINIA, Research Scientist, formerly with School of Metallurgy and Materials, University of Birmingham, and is now with Novelis Inc., Kennesaw, GA. Manuscript submitted February 16, 2012. Article published online December 14, 2012 2418—VOLUME 44A, MAY 2013
of alloy N18.[2] In this instance, alloys Rene´ 95, with its high strength, and Astroloy, with its superior resistance to crack propagation, were blended in various proportions with the aim of developing a more balanced alloy. In total, over 50 different alloy chemistries were evaluated to reach the chemistry of alloy N18, which was subsequently selected for more comprehensive, scaledup evaluation.[3] The prevalent use of empirical methods in the design of Ni-base superalloys is also evident from Figure 1, in which the evolution of turbine disk alloy compositions patented over the past 50 years, binned into 5-year classes, is depicted. In this figure, each point represents the mean within each 5-year class and the whiskers point to the minimum and maximum in that class. It is clear that as time has progressed, no particular trend toward higher or lower levels of any of the alloying elements has occurred. This level of arbitrariness in the composition of the alloys, albeit in part, can be attributed to the reliance of alloy design methods on experience and empiricism. Given the breadth of available alloy design space, one cannot hope to find new grades of su
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