Revealing laws of element diffusion and hot deformation characteristics during homogenization process of Alloy 625 Plus

Abstract

Based on microstructure analysis, diffusion theory, and hot deformation experiments, the solidification microstructure and element segregation of the Alloy 625 Plus ingot, the diffusion kinetics of Ti, Nb, and Mo during homogenization and the hot deformation behavior of the homogenized ingot were investigated in this study. The results indicate that: (1) the solidified ingot exhibits a typical dendritic microstructure, and significant element segregation occurs, leading to the presence of Ti, Nb, and Mo-rich precipitates in the interdendritic region; (2) Following homogenization, the degree of element segregation in the ingot is significantly reduced. The diffusion coefficients (D) of Ti, Nb, and Mo under various homogenization conditions were calculated. Subsequently, the diffusion constants (D0) and activation energies (Q) of Ti, Nb, and Mo were obtained to be 0.01432, 0.00397 and 0.00195 cm²/s and 244.851, 230.312, and 222.125 kJ/mol, respectively. Finally, the diffusion kinetics formulas for Ti, Nb, and Mo in Alloy 625 Plus were established. After homogenization at 1220 ℃ for 8 h, the alloy exhibits low deformation resistance, a high degree of recrystallization, and optimal deformation coordination ability. Therefore, this represents a rational single-stage homogenization process.

1. Introduction

Nickel-based superalloys have been extensively utilized in aerospace, nuclear energy, petrochemical industry, and ocean engineering owing to their outstanding high-temperature strength and corrosion resistance [1−4]. However, with the advancement of society, science, and technology, the demand for high-end equipment in various fields is growing increasingly urgent, particularly in energy sectors such as nuclear power and the petrochemical industry. Consequently, the requirements for high-performance materials are becoming more stringent. Alloy 625 Plus is a precipitate-hardening nickel-based alloy that combines the exceptional corrosion resistance of Alloy 625 with the benefits of Alloy 718, which can be strengthened through aging processes. Consequently, it achieves high strength through aging without requiring pre-hot or pre-cold working, while possessing corrosion resistance comparable to that of Alloy 625 [5]. This addresses the challenges posed by the limited strength of Alloy 625 and the insufficient corrosion resistance of Alloy 718 in severe corrosive environments [6−8]. In comparison with conventional Alloy 625 and Alloy 718, the elemental adjustments in the Alloy 625 Plus are intended to expedite the precipitation of the γ'' phase during the short-term aging process, leading to age hardening and the acquisition of exceptional mechanical properties. The most significant modifications include an increase in titanium (Ti) content and a decrease in carbon (C) content. The elevated Ti content is crucial for promoting the precipitation of the γ'' phase during aging, whereas the reduced C content is essential to prevent the formation of primary carbonitrides (Nb, Ti) (C, N), thereby mitigating the risk of cracking due to stress concentration during hot working. The properties of Alloy 625 Plus render it suitable for high-strength components in severe corrosive environments, including deep wells containing sulfur gas, light water reactors, chemical industry, and deep-sea pipelines.

However, due to the redistribution of solute elements at the solidification front during the solidification process of highly alloyed nickel-based alloys, significant element segregation occurs between dendrite arms and interdendritic regions, particularly for elements such as Nb, Mo, and Ti, which readily segregate. This leads to an uneven composition of the alloy ingot and the formation of unexpected harmful phases [9−11]. Furthermore, the transition zone between dendrite arms and interdendritic regions, as well as the interface between the segregated phase and matrix, is susceptible to cracking during deformation, thereby deteriorating the hot workability of materials [12]. Fortunately, high-temperature homogenization is a highly effective method for resolving the issue of element segregation and harmful precipitate formation.