Evolution behavior of superlattice phase with Pt 2 Mo-type structure in Ni-Cr-Mo alloy with low atomic Mo/Cr ratio
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ution behavior of Ni2(Cr, Mo) phase with Pt2Mo-type structure in the Ni–Cr–Mo alloy with a low atomic Mo/Cr ratio subjected to a long-term thermal exposure of 100–340 h at 600 °C was investigated using transmission electron microscopy and microhardness. Results demonstrate that there is a linear relationship between major axis cube (L3) of ordered domain and thermal exposure time (t) followed by a coarsening regime described by the Lifshitz–Slyozov–Wagner model, as well as between the aspect ratio (D) of ordered domain and thermal exposure time. The volume fraction of ordered domain increases with increasing thermal exposure time, whereas the hardening of samples decreases due to growth-coarsening of ordered domain. Prolonged thermal exposure time led to the coarsening of ordered domain by rate of (3.39 6 0.02) 1030 m3/s without changing their crystallography and ordering characteristics during thermal exposure. Plastic deformation before thermal exposure do not lead to decomposition of initial Ni2(Cr, Mo) phase, but both plastic deformation and thermal exposure affect their morphology and distribution.
I. INTRODUCTION
Ni–Cr–Mo-based alloys have been widely applied in chemical, aerospace, and nuclear industry for their good oxidation resistance and corrosion resistance.1–3 It has been reported that Hastelloy C2000 alloy, with a nominal composition of Ni–26.62Cr–10.03Mo (at.%), is a promising candidate as heat exchanger in molten salt reactor at temperatures varying in the range of 300–700 °C.1,4–7 Among the precipitates8 formed in the alloy during service temperature, particles of ordered phase are mainly responsible for strengthening of the alloy.9,10 However, the precipitation of ordered phase deteriorates corrosion resistance of the alloy in application due to partitioning of Mo.11 The evolution behavior of ordered phase has been an attractive subject of experimental and theoretical investigation in Ni–Cr–Mo-based alloys.12–14 The evolution behaviors of ordered phase, such as thermal stability and solvus temperature, are dependent on composition of the Ni–Cr-based alloys, especially an atomic X/Cr ratio (X 5 Mo and W).13,15 Hu et al.16 investigated the stability of Ni2Cr1xMx (M 5 Nb, Mo, Ta, and W) superlattice by first principle calculation based on the density functional theory, and found that all Ni2Cr1xMx superlattice phases were energetically more stable relative to Ni2Cr when value of x up to 0.25, indicating the alloying elements can improve the thermal Contributing Editor: Eric A. Stach a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.34 J. Mater. Res., Vol. 31, No. 4, Feb 29, 2016
stability of the Ni2Cr superlattice. Pai et al.17 studied the solvus temperature (Tf) of Ni2(Cr, Mo) phase in Ni0.66Cr1xMox alloy with x varying from 0 to 0.5, i.e., Tf increases with increasing an atomic Mo/Cr ratio (R) when 0 , R , 1. Similarly, Tf of Ni2(Cr, Mo) also increases with R even though R . 1 according to the results of Dymek et al.18 and Tawancy and Aboelfotoh.9,10
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