The effects of alloying elements (Ta, Hf) on the thermodynamic stability of γ′-Co3(Al,W) phase
Graphical abstract
Highlights
► We report alloying effects on the stability of phase in Co–Al–W based alloys. ► 2.2 at. % Ta addition does not stabilize the phase at 900 °C. ► A small addition of Hf stabilizes the phase at 900 °C. ► Thermodynamically stable phases can be formed in Co–Al–W–Hf alloys.
Introduction
The γ′-Co3(Al, W) phase with an L12 structure was discovered in the Co–Al–W ternary system by Ishida's group [1]. γ/γ′ cuboidal microstructures aligned along <100> directions, as seen in Ni-base superalloys, are formed in the alloys due to a small misfit between the γ and γ′ phases. The addition of Ta or Ni to the system was also reported to stabilize the γ′ phase toward higher temperatures [1], [2]. These findings open possibilities for developing a new-type of Co base superalloys strengthened with coherent γ′ precipitates [1], [3]. Quaternary alloying effects on the γ′ solvus temperature [4], [5] and mechanical properties of Co-base γ/γ′ microstructures [3], [6], [7], [8] were recently investigated. Physical properties of γ′ single crystals were also studied [9].
According to Refs [1], [6], the γ′ phase is in equilibrium with the γ-Co(A1), CoAl (B2) and Co3W (D019) phases at 900 °C, and exists as a metastable phase at 1000 °C. However, our previous study using a diffusion-couple technique revealed that the γ′ phase is metastable and three phases of γ, CoAl and Co3W are in equilibrium with each other even at 900 °C in the system [10]. Such low stability of the γ′ phase was confirmed by supplementary experiments conducted using heat treated bulk alloys [11], [12].
The low thermodynamic stability of the γ′ phase may cause the decomposition of the γ/γ′ microstructure, and thereby degrading the mechanical properties during long-term exposures at high temperatures. The aim of the present study is to investigate the effect of quaternary alloying elements (X: Ta, Hf) on the phase equilibria in the Co–Al–W ternary system with a close attention to the thermodynamic stability of the γ′ phase. Ta and Hf were chosen as a fourth alloying element because of the following reasons: (1) Ta was reported to increase the γ′ solvus temperature and to partition preferentially to the γ′ phase [1], [13]; and (2) Hf is expected to have a high partition coefficient between the γ′ and γ phase, according to Fig. 1 in the literature [13].
Section snippets
Experimental procedures
The nominal compositions of the alloys studied with their designations are listed in Table 1. The composition of the alloy 2.2Ta was selected to have a 60%/40% Vf of γ′/γ phases, respectively, based on our preliminary chemical analysis on the γ/γ′ two phases (Co-9.9Al-3.7W-0.6Ta/Co-9.4Al-10.7W-3.3Ta) formed after a short annealing in a diffusion-couple sample. In the alloy 1.1Hf, Hf was added in a way that Al and W would be replaced by Hf in the same Al/W ratio as in the Ta added alloy since
Co–Al–W–Ta quaternary system
Microstructures of the Ta containing alloy after heat treatments were similar to those of Co–Al–W ternary alloys reported in [11], [12] and of Ti containing quaternary alloys reported in Ref. [5]. Fig. 1 shows BSE images of the alloy after homogenization and heat treatments at 900 °C. The homogenized sample exhibited a fine γ/γ′ cuboidal microstructure with a homogeneous γ′ particle distribution (Fig. 1(a)), which indicates that the γ′ phase precipitated during cooling from the γ single-phase
Summary
This paper investigated the effects of alloying elements (Ta, Hf) on the thermodynamic stability of the γ′-Co3(Al,W) phase with an L12 structure at 900 °C, based on microstructure observation and electron microprobe analysis on bulk alloy samples heat-treated for periods up to 2000 h. The main results are:
- (1)
An addition of up to 2.2 at. % Ta does not change the phase equilibria in the Co–Al–W ternary system: the γ′ phase is metastable and the three phases of γ, CoAl and Co3W are in equilibrium.
- (2)
A
Acknowledgment
This study is partially supported by the Grant-in-aid the Ministry of Education, Culture, Sports, Science and Technology-JAPAN.
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Currently at National Institute for Materials Science in Japan.