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

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 c-c 0 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 DO 19 -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 c-Co with composition different from the original matrix, CoAl with B2 ordering and Co 3 (Mo, Nb) with DO 19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the c 0 phase. The Nicontaining alloy exhibits no phase decomposition until 100 h of aging at 800°C without any significant effect on c 0 volume fraction (76 %). However, at 950°C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr 3 Si-type cubic phase, a hexagonal Laves phase, rhombohedral l phase, and solid solution of Co phase. The c-c 0 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 c 0 precipitates, even after aging at 950°C for 100 h.

Thermal analysis of W-free Co–(Ni)–Al–Mo–Nb superalloys

“…However, the solvus temperature (861 °C) for Co-Al-Mo-Nb alloy is substantially lower compared to that of Co-Al-W alloys (~ 990 °C) [2]. Moreover, the γ′ phase is not stable during long-term annealing [14] and requires stabilization by other elements.…”

“…This article concerns the influence of W-replacement by Mo and Nb on the thermal effects in Co-based superalloys. Some data regarding W-free superalloys are available in the literature [3,13,14]. The similar alloys based on Co-Al-Mo-Nb and Co-Ni-Al-Mo-Nb systems were prepared by induction vacuum melting.…”

Effect of Cr addition on γ–γ′ cobalt-based Co–Mo–Al–Ta class of superalloys: a combined experimental and computational study

“…The c-c 0 microstructure is stabilized in these alloys by a small addition of Nb or Ta. The solvus temperature as well as the stability of c 0 can be further increased by the addition of Ni (up to 30 at.%) and Ti (2at.%) to the base alloy (Co-10Al-5Mo-2Ta/Nb) [37]. Thus, new Co-Nibased superalloys with densities in the range of 8.2-8.6 gm.cm…”

Effect of Ni on Microstructure and Mechanical Property of a Co-Ti-V-Based Superalloy

“…Also, the Ni element was added to the Co-Al-W ternary system, which has been found to stabilize the γ ′ phase [24]. Following the discovery that the γ/γ ′ two-phase structure of the Co-Al-Mo-Nb alloy by Makineni et al [25][26][27][28], some research studies have focused on Wfree Co-based superalloys with γ/γ ′ strengthening. The L12-ordered phases were discovered in other Co-based superalloys, such as Co-V, Co-Ti, and Co-Ta systems [29][30][31][32].…”