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Phase Transformation of a Superalloy


The NSF-funded IGERT Trainee, Michael Benson, from UT, has investigated the strain-induced face-centered cubic [FCC] to hexagonal-close-packed [HCP] phase transformation in a cobalt-based superalloy. Monotonic tension, monotonic compression, stress-controlled high-cycle fatigue, and strain-controlled low-cycle fatigue experiments have been performed with in-situ neutron diffraction in light of close interactions with Prof. Mark Daymond of Queen’s University, Canada. Co-based superalloys may undergo a strain-induced phase transformation from the metastable FCC structure to the stable HCP structure at room temperature. This phase transformation has been previously documented to occur at room temperature during cyclic loading. Although the previous work did an excellent job of characterizing the fatigue behavior with thermographic and fractographic analyses, only ex-situ laboratory x-ray studies were used to investigate the strain-induced transition. The previous work did not elucidate the onset of the phase transformation under cyclic-loading conditions.

Conventional x-ray diffraction patterns on specimens failed during high-cycle fatigue [Figure 1a] and low-cycle fatigue [Figure 1b] at different maximum-stress and total-strain levels are shown, respectively. Jiang’s work showed that the strain-induced phase transformation occurs during high-cycle fatigue above a maximum stress level of 586 MPa and during low-cycle fatigue above a total strain level of 0.6 % [1,2]. However, Jiang’s work only exhibited that the phase transformation occurred, but did not show the onset of the transformation or the rate of the accumulation of the new phase [1,2]. In order to calculate these new aspects, in-situ neutron-diffraction experiments were conducted.

In-situ neutron-diffraction experiments showed that the HCP phase demonstrated logarithmic dependence upon fatigue cycles. From this information, the onset of the transformation and the accumulation rate can be calculated, with the results shown in Table 1. The accumulation rate was much greater in tension than in compression. Therefore, the phase transformation is much more favorable deformation mechanism under the tensile deformation as opposed to the compression deformation. During the stress-controlled high-cycle fatigue, the HCP phase formed during the first fatigue cycle with no further phase transformation as the deformation cycle continued. This observation is in an extreme contrast to the case of the strain-controlled low-cycle fatigue, where the HCP phase first formed at fatigue cycle three and, then, gradually accumulated. The phase transformation did not saturate until around 100 fatigue cycles. The results suggest that the tensile plastic work is required for the phase transformation to occur.

1 Jiang, L., Brooks, C. R., Liaw, P. K., Dunlap, J., Rawn, C. J., Peascoe, R. A., and Klarstrom, D. L., Metallurgical and Materials Transactions A, 35, 785 2004.

2 Jiang, L., Brooks, C. R., Liaw, P. K., Wang, H., Rawn, C. J., Peascoe, R. A., and Klarstrom, D. L., Metallurgical and Materials Transactions A, 14, 162 2001.

Address Goals

This research was noteworthy because, although some work had been done on understanding the allotropic FCC to HCP phase transformation, no systematic approach had been applied to gain a detailed understanding of the strain-induced phase transformation under different loading modes. There was a need for the application of an in-situ nondestructive experimental technique to characterize the onset of the phase transformation and the rate of the accumulation of the HCP phase.

Neutron diffraction offers a nondestructive method to characterize materials behavior in-situ while a load is applied. Specifically, this advantage of neutron diffraction has led to studies of deformation-induced phase transformations for a wide range of materials in the current literature. However, no comprehensive set of experiments has been applied to investigate the strain-induced FCC to HCP phase transformation in cobalt. In the current investigation, the transformation is studied under four loading modes, including monotonic tension, monotonic compression, stress-controlled high-cycle fatigue, and strain-controlled low-cycle fatigue, with in-situ neutron diffraction. The experiments revealed the transformation onsets and the accumulation rates of the HCP phase for the four loading conditions. The knowledge gained from this work will provide the valuable insight, not only for understanding the deformation mechanisms of Co-based superalloys, but also advanced materials as a whole. This work will build the nation’s research capability through critical investments in advanced instrumentation, facilities, and experimental techniques and tools.