Tailoring the Properties of a Ni-Based Superalloy via Modification of the Forging Process: an ICME Approach to Fatigue Performance

Abstract

Traditionally, material design and property modifications are usually associated with compositional changes. Yet, subtle changes in the manufacturing process parameters can also have a dramatic effect on the resulting material properties. In this work, an integrated computational materials engineering (ICME) framework is adopted to tailor the fatigue performance of a Ni-based superalloy, RR1000. An existing fatigue model is used to identify microstructural features that promote enhanced fatigue life, namely a uniform, fine grain size distribution, random orientation, a distinct grain boundary distribution (specifically high twin boundary density and limited low-angle grain boundaries). A deformation mechanism map and process models for grain boundary engineering of RR1000 are used to identify the optimal thermo-mechanical processing parameters to realize these desirable microstructural features. For validation, small-scale forgings of RR1000 were produced and heat-treated to attain fine grain and coarse grain microstructures that represent the conventionally processed and grain boundary engineered (GBE) conditions, respectively. For each of the four microstructural variants of RR1000, the twin density and grain size were characterized and were in agreement with the desired microstructural attributes. In order to validate the deformation mechanisms and fatigue behavior of the material, high-resolution digital image correlation was performed to generate strain maps relative to the microstructural features. The high density of twin boundaries was confirmed to inhibit the length of slip bands, which is directly attributed to extended fatigue life. Thus, this study demonstrated the successful role of models, both process and performance, in the design and manufacture of Ni-based superalloy disk forgings.

Application of ICME to Engineer Fatigue-Resistant Ni-Base Superalloys Microstructures

“…This result is in agreement with past measurements of slip band length in coarse grain RR1000 materials. 22 Moreover, these observations are consistent with the fatigue modeling results, whereas the formation of longer PSBs represented a higher degree of stored energy and were attributed to a shorter fatigue life. 22 Lastly, fatigue crack initiation has been observed extensively throughout the years at or near twin boundaries, stemming back to the work of Boettner et al 28 and Thompson 29 and more recently in Nibased superalloys.…”

“…22 Moreover, these observations are consistent with the fatigue modeling results, whereas the formation of longer PSBs represented a higher degree of stored energy and were attributed to a shorter fatigue life. 22 Lastly, fatigue crack initiation has been observed extensively throughout the years at or near twin boundaries, stemming back to the work of Boettner et al 28 and Thompson 29 and more recently in Nibased superalloys. 24,30,31 Moreover, there exists a high prevalence of annealing twin boundaries inherent to Ni-based superalloys, such that the majority of cracks initiate due to twin boundaries.…”

“…Additionally, the limits associated with recrystallization and the effect of grain growth were investigated to gather a significant amount of experimental results and derive constitutive models to describe the formation of twin boundaries in Ni-base superalloy RR1000. 21,22 The models were used to predict the length fraction and density of R3 boundaries as a function of the grain size following annealing and amount of deformation (or stored strain energy) following isothermal forging. Using the deformation mechanism map for RR1000 as a guide (Fig.…”

“…For more information about the coarse grain specimens, the reader is invited to refer to Ref. 22. The grain boundary maps for the sub-solvus annealed forgings are shown in Fig.…”

Microstructural‐based strain accumulation during cyclic loading of Ni‐based superalloys: The role of neighboring grains on interconnected slip bands

“…The process parameters were selected based on a process deformation map to identify pertinent microstructural features, as discussed further in Detrois et al 33 The γ 0 precipitate structure of the RR1000 variants was revealed by etching with a modified Kallings reagent, by EOS Laboratories, which etched the γ phase. The underlying γ 0 structures were imaged on a FEI Quanta 3D field emission gun (FEG) dual-beam scanning electron microscopy (SEM), as shown in Figure 1A,B.…”