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Metals 2018, 8(9), 706;

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

Institut für Werkstoffe, TU Braunschweig, 38106 Braunschweig, Germany
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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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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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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