Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

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• 作者：S. K. Makineni ; B. Nithin ; D. Palanisamy ; K. Chattopadhyay
• 刊名：Journal of Materials Science
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：51
• 期：17
• 页码：7843-7860
• 全文大小：8,747 KB
• 刊物类别：Chemistry and Materials Science
• 刊物主题：Chemistry<br>Materials Science<br>Characterization and Evaluation Materials<br>Polymer Sciences<br>Continuum Mechanics and Mechanics of Materials<br>Crystallography<br>Mechanics<br>
• 出版者：Springer Netherlands
• ISSN：1573-4803
• 卷排序：51

文摘

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 γ–γ′ alloys of base composition Co–10Al–5Mo–2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 °C of the Co–10Al–5Mo–2Nb alloy, needle-shaped DOb>19b>-ordered precipitates with stoichiometry of Cob>3b>(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 γ-Co with composition different from the original matrix, CoAl with B2 ordering and Cob>3b>(Mo, Nb) with DOb>19b> ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the γ′ phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 °C without any significant effect on γ′ volume fraction (76 %). However, at 950 °C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Crb>3b>Si-type cubic phase, a hexagonal Laves phase, rhombohedral μ phase, and solid solution of Co phase. The γ–γ′ microstructure in the Co–10Al–5Mo–2Nb and Co–30Ni–10Al–5Mo–2Ta alloys is not stable at 800 and 950 °C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 °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 γ′ precipitates, even after aging at 950 °C for 100 h.Electronic supplementary materialThe online version of this article (doi:10.1007/s10853-016-0026-1) contains supplementary material, which is available to authorized users.