The need for corrosion-resistant alloys applied to support increasingly harsh environments at high temperatures resulted in the development of high chromium-containing nickel-base superalloys. It is well established in several complex multicomponent alloy systems, the concurrence of several modes of precipitation phenomena. Perhaps the most intriguing is the occurrence of concomitant general precipitation (GP, homogenous and heterogeneous) and discontinuous mode of precipitation (DP). While the former is controlled by lattice diffusion the latter is driven by grain boundary (GB) diffusion and migration. It is well established that the mechanical and corrosion properties depends critically on the phase stability under service conditions. This study is focused on identification of the different phases precipitated at a specific grain boundary. Here, the precipitation takes place in an high performance superalloy 33 with austenitic matrix and chemical composition (wt.%) of 32,75Cr-32,53Fe-31,35Ni-1,49Mo-0,54Cu-0,4N-0,012C-0,63Mn-0,30Si. For this purpose, a JEOL JEM-ARM200CF aberration-corrected scanning transmission electron microscope (STEM) has been used at Lehigh.
Figure 1A presents a complex precipitation product at a specific GB in this alloy upon direct-aging at 700 °C for 100 h. Different phases have nucleated and grown, driven by the diffusion of a moving GB. The two diffraction patterns of Figs 1B and 1D correspondto both grains across the GB and the dark field (DF) TEM image of Fig. 1C shows the adjacent grain being consumed by the advancement of the GB reaction front of the DP. In a previous study , X-ray elemental maps show the degree of partition of different substitutional elements (Fe, Ni, Cr, Mo) and interstitial N. In fact, one can identify more than two phases in this precipitate colony. In general, such complex GB precipitation products depend on the structure and thermodynamic properties of individual GBs .
Figure 2 shows a high angle annular dark field (HAADF) image of the region boxed in red in Fig. 1A. This TEM session was conducted in a ChemiSTEM FEI Titan at UFRJ with the foil upside down respect to Fig 1A. This study reveals that the discontinuous precipitation at grain boundary results in three different phases. The precipitate indicated by n° 1 at the GB reaction front itself has been identified as η phase, Cr3Ni2SiN, with diamond cubic crystal structure. At the original position of the GB, however, two different phases have precipitated: precipitate n° 2 is a M23C6 carbide with FCC structure and precipitate n° 3 is an intermetallic σ phase with tetragonal structure. The precipitation of the latter is associated with embrittlement . XEDS spectra and elemental mapping have corroborated such phase identification .
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Acknowledgements: This research was sponsored by CNPq (Brazil) and NSF-MWN (US: DMR-0303429) joint CIAM Program. One co-author acknowledges the financial support from FAPERJ. The authors are grateful to the Nucleo de Microscopia Electronica (UFRJ) for access to their TEM.
To cite this abstract:Julio Cesar Spadotto, Masashi Watanabe, Jean Dille, Ivan Guillermo Solórzano; Phase identification of complex grain boundary precipitation in a high Cr and Ni superalloy upon direct-aging. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/phase-identification-of-complex-grain-boundary-precipitation-in-a-high-cr-and-ni-superalloy-upon-direct-aging/. Accessed: April 3, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/phase-identification-of-complex-grain-boundary-precipitation-in-a-high-cr-and-ni-superalloy-upon-direct-aging/