Regulating the coarsening of the γ′ phase in superalloys

Abstract

The properties of superalloys are typically deteriorated by the coarsening of the nano-sized γ′ phase, which is the primary strengthening component at high temperatures. Stabilizing the γ′ phase represents one of the key challenges in developing next-generation superalloys. Herein, we fabricate a cobalt-nickel-based superalloy with a nanoscale coherent γ′ phase, (Ni,Co) 3 (Al,Ti,Nb), which is isolated by stacking-fault ribbons in the alloy matrix as a result of the Suzuki segregation of alloying atoms. Additionally, we demonstrate that this new nanostructure can slow down the coarsening of the γ′ phase at high temperatures. As a result, the cobalt-nickel-based superalloy displays considerably high tensile yield points, exceeding 1650 MPa at room temperature and 1250 MPa at 973 K, which are markedly higher than those of the commonly used nickel-and cobalt-based superalloys. This study thereby paves a new path for developing superalloys with exceptional mechanical performance and thermal stability. INTRODUCTIONGeometrically close-packed and coherent A 3 B-type (A = Ni, Co; B = Al, Ti, and so on) γ′ phases exist in various high-temperature superalloys, and this substantially affects their mechanical performance. 1,2 One of the most critical issues currently restraining the service life of these superalloys lies in the coarsening of the nanoscale γ′ phase upon exposure to high temperatures. To date, much effort has been devoted to probing the morphology of the γ′ phase and to clarifying its coarsening mechanism upon aging in cobalt-and nickel-based superalloys. 3-9 It has been predicted by the Lifshitz-Slyozov-Wagner model 10,11 that coarsening of the γ′ phase in superalloys follows the general relationship r 3 = kt, where r is γ′ phase particle radius, t is the aging time and k is the coarsening-related rate constant.In general, the coarsening of the γ′ phase in superalloys is a diffusion process where large particles grow at the expense of smaller ones in the alloy matrix; this is termed Ostwald ripening. [12][13][14] The factors that predominantly control the coarsening rate of the γ′ phase include the γ/γ′ phase interfacial energy, the solubility of the γ′ phase in the γ phase and the diffusion coefficient of the constituent elements of the γ′ phase through either the γ/γ′ phase interface or the γ phase matrix. 15 Typical values of the interfacial energy of the γ/γ′ phase have been clearly demonstrated to be~20 mJ m − 2 , 15 a value that is very low and essentially remains stable. In addition, for a given material system, the solubility of small γ′ phase particles and the diffusion coefficient of the constituent elements of the γ′ phase in the

Precipitation behavior of a novel cobalt-based superalloy subjected to prior plastic deformations

“…Al, Ti, and Nb are the main constituent elements of γ′-phase while μ-phase is dominantly composed of Co and Mo [3]. Moreover, γ′-phase precipitates even during slow cooling process [3], while μ phase is observed only in the heat treatment of prolonged duration as indicated in Fig. 9.…”

“…Based on our previous research, it has been proved that the chemical composition of γ′-phase in present alloy was (Co, Ni) 3 (Al, Ti, Nb) while the μ-phase was Co 7 Mo 6 type precipitate. Al, Ti, and Nb are the main constituent elements of γ′-phase while μ-phase is dominantly composed of Co and Mo [3]. Moreover, γ′-phase precipitates even during slow cooling process [3], while μ phase is observed only in the heat treatment of prolonged duration as indicated in Fig.…”

“…The formation of stacking faults along the {111} planes with solute atoms (Mo, Cr, and Co) segregated at the stacking fault ribbons has been described previously using the weak-beam dark-field technique and atom probe tomography [3]. In the case of the investigated alloy, the 〈111〉 fiber deformation texture was induced by the prior plastic deformations, as indicated by the results of the XRD analysis, shown in Fig.…”

“…Recently, we reported a novel Co-Nibased superalloy, Co-35Ni-17.5Cr-8Mo-3Nb-2Al (wt%), which Materials and Design 112 (2016) 1-10 exhibited superior mechanical properties at temperatures ranging from room temperature to 1073 K. The excellent high-temperature mechanical properties of this alloy were ascribable to its high Mo (8 wt%) and Nb (3 wt%) contents [3], which, in turn, were attributable to solid-solution strengthening and/or the Suzuki segregation [4][5][6], while the Al (2 wt%) leads to the formation of γ′ phase (Ni 3 Al) [7,8]. In addition, it has been shown that combination of these elements (such as Mo and Nb) has a high tendency to form topologically closed-packed (TCP) phases such as σ-, μ-, and laves-phases during prolonged exposure to at elevated temperatures [9][10][11][12].…”

“…It has been shown that prior plastic deformations and aging result in a significant increase in the strength of the present alloy [3]. However, it is not clear how the microstructure of this novel cobalt-based superalloy evolves in response to plastic deformations and subsequent aging at various temperatures.…”