The influence of Mo on Suzuki-segregation-related microstructure evolution and mechanical properties of Co−Ni-based superalloy

Highlights

• Influence of Mo on microstructure of Ni-Co-based super alloy was investigated.

• Influence of Mo on mechanical property was investigated.

• Mo segregated at stacking faults in all alloys after pre-cold-rolling and aging.

• Higher Mo content enhanced the precipitation of μ phase.

• The quasi-μ phase precipitated first via Mo-segregation in lower Mo alloy.

Abstract

Mo, as one of the most important elements in superalloy, dramatically affected the microstructure and mechanical properties via solution strengthening, forming Mo-enriched phase, and atomic segregation (Suzuki effect). To verify the influence of Mo on the microstructure and mechanical properties of Ni–Co-based superalloy, two types of alloys containing 4 wt% (A4) and 8 wt% (A8) Mo were prepared. The microstructure was examined using X-ray diffraction, scanning electron microscope, and transmission electron microscope, while the mechanical properties were evaluated using tensile tests at both room temperature and elevated temperature (700 °C). The segregation of Mo at stacking faults was observed in both alloys after aging with pre-cold-rolling. At the same time, in alloy A8, direct precipitation of μ phase both along grain boundary and in the interior of matrix was observed after aging. In alloy A4, segregation of Mo along stacking fault ribbons resulted in formation of quasi-μ phase. The quasi-μ phase developed into μ phase during prolonged aging. There was no precipitate along grain boundary in A4 after aging resulting in poor ductility at both room and elevated temperature. The strength was enhanced by net effect of Suzuki segregation, γ′ phase strengthening, and fine μ phase.

Introduction

Ni-based and Co‒Ni-based superalloys are widely used as turbine disk materials and other applications, owing to their superior strength and creep properties at elevated temperatures [[1], [2], [3], [4]]. There is a great advance in a recent research, where a novel Co−Ni-based superalloy, exhibiting exceptionally superior mechanical properties at temperatures ranging from room temperature to 800 °C, was proposed. The excellent high-temperature mechanical properties of this alloy were at least partially ascribable to its high Mo (8 wt%) and Nb (3 wt%) contents [5], which were attributable to solid-solution strengthening and the Suzuki segregation [6,7].

Suzuki effect has been extensively used to enhance the strength of alloys via chemical interaction between solute atoms and extended Shockley partial dislocations at elevated temperatures [8]. Many experiments regarding the Suzuki effect have been conducted in Co-based alloy [[9], [10], [11]]. For example, in 1999 and 2001 by Chiba et al. [6,7] firstly used the Suzuki effect model in Co−Ni-based superalloy to explain the mechanism of high work-hardening rate at elevated temperatures. The dynamic strain aging (DSA), which leads to the high work-hardening rate, in the stress relaxation test at elevated temperatures is considered to be related to the dislocation locking effect from Suzuki segregation [6,7]. The attempt to give the direct evidence for Suzuki segregation was conducted by Han et al. in MP159 superalloy during the study on plastic flow behavior at high temperature [12]. The extension of dissociated dislocation was observed and Mo and Al were suggested to segregate at stacking faults ribbons. The Suzuki segregation at 700 °C in Co−Ni-based superalloy with Nb addition was investigated using both experimental and phase field simulation methods by Koizumi et al. [13]. The stacking fault is considered to be a local HCP structure embedded in the FCC matrix. The simulation results suggest Cr and Mo atoms segregated at stacking fault ribbons as a result of the decreasing stacking fault energy during the aging. Based on these experimental and simulation results, a novel design concept of alloy, which is the combination of γ′ precipitate and Suzuki segregation in a Co−Ni-based superalloy, was proposed and realized successfully by Bian et al. [5]. The segregation of solute atoms at stacking faults is performed via cold-deformation followed by an aging at 800 °C. Mo, Co and Cr atoms are confirmed to segregate at stacking fault ribbons by STEM-HAADF and EDS mapping. This Co−Ni-based alloy can demonstrate the yield stress as high as approximately 1600 MPa at room temperature and 1200 MPa at 700 °C, respectively, due to the simultaneously strengthening effect by γ′ precipitate and Suzuki effect.

The formation of Suzuki segregation is depended on the type and content of solute atoms such as Mo in Co−Ni-based superalloy. It is necessary to emphasize the influence of these solute atoms on the mechanical properties because the Suzuki segregation behavior and the microstructure could be greatly changed by these elements. However, by now there is no study focusing on the effect of the different content of Mo on the Suzuki segregation in Co−Ni-based superalloy. Therefore, in the present study, a systematic investigation of Co‒Ni alloys containing two Mo concentrations was carried out for the first time.