Phase evolution and crystallography of precipitates during decomposition of new ``tungsten-free'' Co(Ni)-Mo-Al-Nb gamma-gamma ` superalloys at elevated temperatures

Makineni, SK and Nithin, B and Palanisamy, D and Chattopadhyay, K (2016) Phase evolution and crystallography of precipitates during decomposition of new ``tungsten-free'' Co(Ni)-Mo-Al-Nb gamma-gamma ` superalloys at elevated temperatures. In: JOURNAL OF MATERIALS SCIENCE, 51 (17). pp. 7843-7860.

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Abstract

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free gamma-gamma' alloys of base composition Co-10Al-5Mo-2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 degrees C of the Co-10Al-5Mo-2Nb alloy, needle-shaped DO19-ordered precipitates with stoichiometry of Co-3(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc gamma-Co with composition different from the original matrix, CoAl with B2 ordering and Co-3(Mo, Nb) with DO19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the gamma' phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 degrees C without any significant effect on gamma' volume fraction (76 %). However, at 950 degrees C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr3Si-type cubic phase, a hexagonal Laves phase, rhombohedral mu phase, and solid solution of Co phase. The gamma-gamma' microstructure in the Co-10Al-5Mo-2Nb and Co-30Ni-10Al-5Mo-2Ta alloys is not stable at 800 and 950 degrees C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 degrees C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of gamma' precipitates, even after aging at 950 degrees C for 100 h.

Item Type:	Journal Article
Publication:	JOURNAL OF MATERIALS SCIENCE
Publisher:	SPRINGER
Additional Information:	Copy right for this article belongs to the SPRINGER, 233 SPRING ST, NEW YORK, NY 10013 USA
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	25 Aug 2016 10:21
Last Modified:	25 Aug 2016 10:21
URI:	http://eprints.iisc.ac.in/id/eprint/54317

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