Research on the coarsening mechanism of precipitations and its effect on toughness for nickel-based weld metal during th
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Research on the coarsening mechanism of precipitations and its effect on toughness for nickel-based weld metal during thermal aging Tongjiao Chu1, Huali Xu1, Haichao Cui1, Fenggui Lu1,a) 1
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China a) Address all correspondence to this author. e-mail: [email protected] Received: 14 December 2018; accepted: 22 January 2019
Coarsening mechanism of precipitations was investigated in a weld metal of Alloy 617 during long-term aging at 750 °C, and its effect on impact toughness was clarified distinctly. The needle-like M6C phases at the grain boundary nucleated and coarsened at 2000 h and then presented a stable size with aging to 8000 h. Spherical c9 phase grew rapidly with the rate of 0.0121 nm/h when aged at 1000 h; then, its ripening rate (RR) reduced to 0.0033 nm/h at 8000 h and stabilized around it. The coarsening of M6C and c9 was, respectively, controlled by interface diffusion and volume diffusion with the coarsening rate constant of 7.865 × 10−20 m2/s and 1.519 × 10−27 m3/s. Interaction of M6C and c9 could facilitate their coarsening and cause dramatic decrease in toughness at the early stage. At aging to 8000 h and more, the lower RR of needle-like M6C phases and c9 phases helped to form stable toughness at a later stage.
Introduction Nickel-based alloy has been preferentially selected to use in aerospace, power industry, etc., for its high strength and better resistance to corrosion and oxide at elevated temperatures [1, 2]. As one of nickel-based alloys, Alloy 617 is mainly composed by Ni–Cr–Co–Mo, which is expected to serve at elevated temperatures [2]. Thus, the mechanical behavior of Alloy 617 at high temperatures is significantly important, which is mainly influenced by phase precipitations. As a kind of alloy with solid-solution strengthening and aging strengthening, the nickel-based alloy contains various types of precipitations. Cozar et al. [3] reported the nanoscaled c9-Ni3Al (cubic or spherical shape) and c0-Ni3Nb precipitations (lens-like disk shape) in Inconel 718. M23C6 could lose Cr atoms and form M6C as follows: M23C6 1 Mo ! M6C [4, 5]. Yang et al. [6] reported that M (metal atoms) directly reacted with C (carbon) as follows: 6M 1 C ! M6C. These precipitations would coarsen during long-term thermal exposure, which was controlled by coarsening kinetics [7, 8]. Li et al. [9] described the coarsening process of Ni3Al and Ni3V in Ni75AlxV25x alloys through Langer and Schwartz model obtained from Ostwald ripening theory. The coarsening
ª Materials Research Society 2019
behavior of c9 phases was in accordance with matrix diffusion and had very low interfacial energies [10]. Wu et al. [11] suggested that the coarsening of c9 phases followed the cube rate law at the early stage and the square rate law at the later stage. However, only limited information is available in general describing and qualitatively determining
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