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Metals 2018, 8(9), 706; https://doi.org/10.3390/met8090706

Influence of σ Phase on the Allotropic Transformation of the Matrix in Co-Re-Cr-Based Alloys with Ni Addition

1
Institut für Werkstoffe, TU Braunschweig, 38106 Braunschweig, Germany
2
Institut für Werkstofftechnik, Universität Siegen, 57068 Siegen, Germany
*
Author to whom correspondence should be addressed.
Received: 3 August 2018 / Revised: 3 September 2018 / Accepted: 6 September 2018 / Published: 8 September 2018
(This article belongs to the Special Issue High Temperature Materials Development beyond Ni-Base Superalloys)
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Abstract

Co-Re-Cr alloys are being developed for high-temperature application in gas turbines. In these alloys, the Cr2Re3-based σ phase is stable when the Cr content is higher than 20 atomic %. The addition of Ni is being studied to partially substitute Cr, which aims to suppress σ formation without sacrificing the benefit of Cr in the oxidation resistance of the alloy. The microstructure of the alloys with varying Cr (18–23%) and Ni (8–25%) was investigated by electron microscopy in the present study, primarily to look into the stability of the σ phase and its influence on the Co matrix phase transformation. The σ phase is mainly found in two morphologies in these alloys, where at high temperature only blocky σ phase is present at grain boundaries but cellular σ is formed through a discontinuous precipitation within the grains at lower heat treatment temperatures. The presence of fine cellular σ phase influences the alloy hardness. Moreover, the σ precipitation, which depletes the matrix in Re, also influences the allotropic transformation of the Co matrix. View Full-Text
Keywords: Co-Re-Cr-based alloys; nickel; σ phase; phase transformation Co-Re-Cr-based alloys; nickel; σ phase; phase transformation
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Dörries, K.; Mukherji, D.; Rösler, J.; Esleben, K.; Gorr, B.; Christ, H.-J. Influence of σ Phase on the Allotropic Transformation of the Matrix in Co-Re-Cr-Based Alloys with Ni Addition. Metals 2018, 8, 706.

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