Alloys-By-Design: Application to nickel-based single crystal superalloys

Abstract

Design rules are proposed by which the compositions of nickel-based single crystal superalloys can be chosen systematically, using models for the most important characteristics: creep resistance, microstructural stability, castability, density and cost. Application of the rules allows the very large compositional space to be reduced to just a few ideal compositions, which are likely to be close to the optimal ones. The procedures have the potential to remove much of the traditional reliance placed upon empiricism and trial-and-error-based testing. It appears that trade-offs must be accepted, however; for example, the most creep-resistant alloys are more dense, more costly and more inherently susceptible to casting-related defects such as freckles during processing. Compositions suitable for both jet propulsion and land-based applications are proposed, for future experimental testing.

Introduction

The nickel-based superalloys are remarkable for their resistance to mechanical and chemical degradation at temperatures up to 1000 °C and beyond [1], [2]. Consider the most popular single crystal superalloys, which find application in a range of jet engines, land-based turbines and compression/pumping systems. Their chemical compositions [3] (see Table 1) confirm that a significant number of elements are added to nickel, e.g. Al, Ti and Ta to impart strengthening via the ${\text{Ni}}_3(\text{Al,Ti,Ta})$ phase which is known as ${\gamma }^{\prime }$, Re, W and Mo to improve creep resistance, and Al, Cr and Co to impart resistance to oxidation, corrosion and sulphidation. There are few, if any, structural alloys which have this degree of compositional complexity.

It follows from these considerations that these alloys are unlikely to have been optimized as yet, and therefore that there are improved compositions – waiting to be discovered or perhaps designed – which are likely to be superior to those currently available. This can be confirmed by the following thought experiment. Table 1 indicates that at least eight alloying elements or more may be added, with concentrations needing to be chosen to an accuracy of at least 1.0 wt.% or probably better. Assuming for one moment that the optimum concentrations lie in the range 0–10 wt.%, empiricism alone would require approximately ${10}^8$ alloys to be prepared, tested and ranked. Clearly, this is beyond the scope of any practical experimental programme of research. Instead, a degree of analysis and modelling is required if optimal alloys are to be isolated. It is probable that this argument applies to other classes of structural alloy, e.g. those based upon iron, aluminium or titanium.

As yet, there have been very few if any attempts to deal with this problem in a systematic way. Indeed, existing single crystal superalloys have been designed largely by empirical methods (see e.g. [4], [5], [6], [7], [8]). In principle, theoretical analysis and computer modelling tools might conceivably alter the prevailing emphasis on trial-and-error testing; obviously improvements in computational power, which are becoming available at modest cost, are likely to help in this regard. The present paper represents a first attempt to do this, for the case of the nickel-based single crystal superalloys. Any systematic design approach of the type proposed here must make use of suitable composition–microstructure–property relationships; for this reason, a new theory for the composition-dependence of creep deformation in these materials is proposed. The methods are used to propose new versions of these alloys by considering the compositional space available; modelling methods are used to isolate compositions within it which are likely to be close to the optimal ones. Typical compositions are presented for future experimental confirmation of their properties.