Development and characterization of a high-Cr-content Co–Ni–Al–V–Ta–Cr superalloy: Microstructure, mechanical properties and oxidation resistance

Abstract

Enhancing the oxidation resistance of Co-based superalloys by adding a high content of Cr, while simultaneously ensuring the stability of the γ/γ′ phases, presents a significant challenge. This study evaluated the alloying potential of Co–30Ni–10Al–5V–4Ta using the CALPHAD method, revealing promising characteristics. The developed Co–30Ni–10Al–5V–4Ta–12Cr alloy characterized by high Cr content and γ/γ′ two-phase structure, demonstrating high γ′ solvus temperature of 1139°C, low density of 8.48 g/cm3, minimal γ/γ′ lattice misfit of +0.28%, high compressive yield strength of 651 MPa at 800°C, and excellent oxidation resistance with a weight gain of 6.5 mg/cm3 after 200 h at 1000°C. Examination of the oxidation behavior at 1000°C revealed an oxide layer consisting of a porous outer CoO, NiO, and V3O4 (CNV) oxide and a denser inner mixed oxide layer comprising CoO, NiO, and V3O4 (CNV) oxide, Al2O3, Cr2O3, CoO, and NiO (CNAC) oxide, and TaO2, CoO, and NiO (CNT) oxide.

1. Introduction

In the preceding decade, there has been a growing interest in the advancement of Co-based superalloys, largely fueled by the need for improved materials in aviation and power generation [1–2]. In 2006, a significant advancement was achieved by Professor Ishida of Japan, who introduced a Co–Al–W-based superalloy exhibiting a dual-phase γ and γ′ microstructural configuration [3]. This breakthrough established γ′ phase-strengthened Co-based superalloys as strong contenders for future applications requiring high-temperature structural performance. Recent years have witnessed significant progress in the exploration of novel Co-based superalloys, extending beyond Co–Al–W/Mo/V-based [3–9], notable systems including Co–Ti-based [10–13], Co–V–Nb/Ta-based [14–16], Co–W–Ga/Ge-based [17–18], and so on. These developments have substantially enriched the research landscape and the potential for innovation within the Co-based superalloy field.

The incorporation of Cr into Co-based superalloys is well-acknowledged for its role in enhancing oxidation resistance. Research conducted by Li et al. [19] has demonstrated that while the presence of Cr can destabilize the γ/γ′ microstructure, it concurrently provides oxidation resistance levels comparable to those found in commercial Ni-based superalloys. Numerous investigations have focused on elucidating the mechanisms behind Cr oxide formation and its influence on oxidation resistance in Co-based systems, aiming to identify the optimal Cr concentration necessary for enhanced performance [20–23]. Furthermore, the incorporation of Cr affects various material properties, including precipitate morphology and lattice parameter misfit. Yan et al. [20] noted that Cr alloying can destabilize the γ/γ′ structure by promoting the formation of topologically close-packed (TCP) phases, whereas the addition of Ni serves to expand the γ/γ′ phase region, thereby aiding in the restoration of the γ/γ′ microstructure. In a systematic study, Povstugar et al. [24] analyzed the effects of increasing Cr levels in Co–Al–W alloys, concluding that Cr reduces the lattice misfit between the γ and γ′ phases. Collectively, these studies indicate that while the incorporation of Cr significantly improves oxidation resistance and overall service performance, it poses substantial challenges to maintaining the stability of the γ/γ′ two-phase microstructure. Consequently, optimizing Co-based superalloys with high Cr content is critically important.

In 2019, Chen et al. [5] reported a novel category of W-free Co-based superalloys, namely, Co–Al–V-based superalloys. Among these, the Co–30Ni–10Al–5V–4Ta alloy was notable for its extensive γ/γ′ two-phase region, a high γ′ solvus temperature of 1172°C, and a relatively low density of 8.70 g/cm3. These findings underscore the considerable potential of such alloys to serve as the next generation of advanced superalloy. This study aims to alloy the Co–30Ni–10Al–5V–4Ta base alloy with Cr to minimize the γ/γ′ lattice misfit and improve oxidation resistance. However, the addition of Cr could promote the formation of TCP phases, necessitating a thorough assessment of the alloying potential and phase stability of the base alloy.

Recent research has highlighted the effectiveness of the CALPHAD method for thermodynamic calculations, aiding in the efficient composition design and performance optimization.