Thermal analysis of W-free Co–(Ni)–Al–Mo–Nb superalloys

Abstract

In this investigation, the thermal analysis of W-free cobalt-based superalloys based on Co–Al–Mo–Nb and Co–Ni–Al–Mo–Nb systems was performed. The analysis was performed at different stages of heat treatment process. The differential thermal analysis (DTA) was utilized for the determination of characteristic temperatures related to microstructural changes. First of all, the DTA analysis was carried out for discussing as-cast alloys in the temperature range of 40–1500 °C. The results showed thermal effects connected with melting and important order–disorder transition. The temperature range of 1200–1250 °C was chosen for performance of a first heat treatment operation for the investigated alloys. Specimens were annealed at selected temperature for 5 h. The microstructure of alloys after solution heat treatment was analyzed as well. Afterward, the solutionized specimens were subjected to the further thermal analysis in order to select the aging temperature according to the order–disorder transformation related to formation of γ′ phase with overall formula Co3(Al,X). Five aging variants were performed in the temperature range of 800–1000 with a step of 50 °C. After each stage of heat treatment, SEM/EDS analysis and hardness measurements were performed.

Phase Transitions in the Co–Al–Nb–Mo System

“…Makineni et al [15] reported on the formation of intermetallic compounds Co 3 (Mo,Nb) with the D019 structure and CoAl with the B2 structure in the Co-10 at.% Al-5 at.% Mo-2 at.% Nb alloy after annealing for 35 h at 800 • C. These phases, as suggested, were formed due to the decomposition of the ordered γ -phase (Co 3 (Al,Nb,Mo)). On the other hand, in the same alloy Co-10Al-2Nb-5Mo, the presence of the Co 2 Nb Laves phase was reported by Damian Migas et al [16] (Table 2). In order to accurately establish the crystal structure of intermetallic phases in the studied quaternary alloys of the Co-Al-Nb-Mo system, first, we conducted a study of binary (Co-10Al) and ternary alloys (Co-10Al-X%Nb and Co-10Al-Y%Mo).…”

Quantitative and Qualitative Assessment of As-Cast Microstructure and Microporosity of γ-γ′ Co-based Superalloys

“…[20][21][22][23][24][25][26] Typically, for this type of alloys, treatment consisting in homogenization and supersaturation or only supersaturation at a temperature range of 1250-1300 °C for up to 24 hours with cooling in water, followed by late supersaturation in the range of the solvus temperature of the L1 2 phase, i.e., 900-1000 °C. [27][28][29] In the study, we analyze qualitatively and quantitatively the c-c 0 type Co-based alloys. For this purpose, different ternary, quaternary and quinary alloys were prepared, including two c 0 -Co 3 (Al, W) forming alloys and two W-free ones.…”

Isothermal oxidation behavior of W‐free Co–Ni–Al‐based superalloy at high temperature

“…[ 14 ] An appropriate content (about 2–5 at%) of Mo to stabilize γ' precipitates in Co‐based superalloys is also a favorable alloying method. [ 15 ] The addition of Ti can promote the formation of γ'‐(Co, Ni) 3 (Al, Ti) precipitates and significantly increase the volume fraction of γʹ. Furthermore, Cr can improve the oxidation resistance of Co‐based superalloy and help to improve the toughness of materials.…”

“…However, it will inversely decrease the oxidation resistance due to the evaporation of Mo‐containing oxide at high temperatures. [ 15,23 ]…”

“…However, it will inversely decrease the oxidation resistance due to the evaporation of Mo-containing oxide at high temperatures. [15,23] Generally, oxides formed on the surface of Co-based superalloys at high temperatures include a Co-containing outer layer, an inner layer with complex spinel oxides containing Co, Al, and an Al 2 O 3 -rich internal oxidation zone (IOZ). [23][24][25][26][27][28] Although Al-rich spinel oxides can play a significant role in inhibiting element diffusion, unfortunately, the production of spinel oxide is accompanied by the invalidation of Al 2 O 3 and the Cr 2 O 3 protective layer.…”