Effects of Nb content on the solidification characteristics and hot deformation behavior of Alloy 625 Plus

Abstract

Through thermodynamic calculations and microstructural characterization, the effect of niobium (Nb) content on the solidification characteristics of Alloy 625 Plus was systematically investigated. Subsequently, the effect of Nb content on hot deformation behavior was examined through hot compression experiments. The results indicated that increasing the Nb content lowers the liquidus temperature of the alloy by 51°C, producing a denser solidification microstructure. The secondary dendrite arm spacing (SDAS) of the alloy decreases from 39.09 to 22.61 µm. Increasing the Nb content alleviates element segregation but increases interdendritic precipitates, increasing their area fraction from 0.15% to 5.82%. These precipitates are primarily composed of large Laves, δ, η, and γ″ phases, and trace amounts of NbC. The shapes of these precipitates change from small chunks to large elongated forms. No significant change in the type or amount of inclusions within the alloy is detected. The inclusions are predominantly individual Al2O3 and TiN, as well as Al2O3/TiN composite inclusions. Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69, a strain rate of 0.5 s−1, and a deformation temperature of 1150°C. Increasing the Nb content also elevates the peak stress observed in the flow curves. However, alloys with higher Nb content exhibit more pronounced recrystallization softening effects. The Laves phase precipitates do not completely redissolve during hot deformation and are stretched to elongated shapes. The high-strain energy storage increases the recrystallization fraction from 32.4% to 95.5%, significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure. This effect is primarily attributed to the addition of Nb, which refines the initial grains of the alloy, enhances the solid solution strengthening of the matrix, and improves the induction of particle-stimulated nucleation.

1. Introduction

Custom Age 625 Plus alloy (UNS N07716) is a precipitation-hardened nickel (Ni)-based alloy commonly utilized in petrochemical and nuclear power engineering [1–2]. Alloy 625 Plus can achieve a high strength level comparable to Alloy 718 through aging treatment while maintaining corrosion resistance similar to Alloy 625 [3–5]. However, technological advancements and increasing energy demands have led to more stringent performance requirements for alloy materials [6]. As shallow oil and natural gas resources gradually deplete, deep-earth and deep-sea explorations have become essential directions for future oil and gas extraction [7]. Severely corrosive environments, characterized by higher temperature and pressure, present significant challenges to improving the performance of Alloy 625 Plus.

Niobium (Nb) serves as a key alloying element commonly employed to enhance the microstructural stability of superalloys and promote the precipitation of phases through aging, thereby achieving desirable mechanical properties [8–10]. Nb can not only dissolve into the matrix for solid solution strengthening but also promote the precipitation of the γ″ [11] and δ [12–13] phases and Nb-containing carbides [14], which are beneficial for controlling grain size and regulating high-temperature performance [15–16]. However, the excessive addition of Nb leads to significant Nb segregation at interdendritic regions, resulting in extensive precipitation of the γ/Laves eutectic phase. These unstable phase boundaries create pathways for crack propagation, thereby deteriorating the hot workability of the alloys [17–19]. Nevertheless, Nb alloying remains an effective strengthening strategy.

Researchers have extensively investigated the role of Nb in Ni-based alloys. However, the majority of these studies have focused on the precipitation phases that occur after heat treatment and their effects on performance [20–22], with relatively fewer investigations into the solidification characteristics and hot deformation behavior of cast alloys. Specifically, the precipitation of the Laves, δ, and γ″ phases during the solidification of Alloy 625 Plus is strongly influenced by the Nb content [23]. Moreover, the hot workability of the alloy is largely determined by its as-cast solidification microstructure. Although dispersed fine Laves phases have been reported to strengthen alloys.