New Cobalt Based Superalloys Strengthened by L12 Phase: Designing and Manufacturing, Heat Treatment, Microstructural Analysis, Properties Characterization and Application

Dear Colleagues,

We would like to invite you to submit your recent works to this Special Issue on “New Cobalt-Based Supersalloys Strengthened by L1₂ Phase: Designing and Manufacturing, Heat Treatment, Microstructural Analysis, Properties Characterization and Application.

The main course of turbine engine development is the improvement of service temperature concomitant higher efficiency and lower CO₂ emission. Elements of turbine engines are exposed to an aggressive service environment, which requires the utilization of alloys with specific properties. Creep, oxidation, sulphide and hot corrosion are strong contributors to engine element degradation. The service life of elements working under these conditions may be decreased due to the use of low-quality fuels, containing sulphur, sodium and halides impurities. An atmosphere of hot, contaminated gas causes the formation of liquid flux, which can dissolve protective coatings and accelerate the degradation of the substrate alloy. Another danger is connected to the diffusion of sulphur, resulting in deleterious sulphide corrosion. Although commercial Ni-based superalloys possess excellent creep properties due to γ′ strengthening, their behaviour in hot, corrosive environments is insufficient.

Cobalt seems to be an appropriate replacement for Ni-based alloys due to its higher melting point and better corrosion resistance compared to Ni-based alloys. Furthermore, these alloys possess appropriate oxidation resistance and excellent thermal fatigue resistance. In the past, there has been much research concerning conventional Co-based superalloys. These alloys are strengthened by carbides and a solid solution of refractory metals in fcc cobalt matrix. Unfortunately, their strength at high temperature is considerably lower in comparison to γ′- strengthened alloys from an Ni–Al system, and, therefore, conventional Co-based superalloys have not achieved wide recognition. Attempts to create Co-based superalloys with γ-γ′ microstructure have been performed successfully. Although the Co₃Al phase, analogous to Ni₃Al, is thermodynamically unstable in a binary Co-Al system, the presence of stable γ′-Co₃Ti phase in Co-Ti system has been reported. Nonetheless, these alloys are characterized by many disadvantages, i.e., inappropriate lattice misfit, low volume fraction of γ′, low temperature of dissolution in cobalt matrix and precipitation in an unfavourable form, and, hence, have not become an alternative for Ni-based analogues.

A breakthrough for superalloys development came in 2006, due to the work of Sato et. al. The γ′ intermetallic phase was found in Co-Al-W ternary system (as well as in the systems of Co-Al-Mo, Co-Al-Nb and Co-Al-Ta type). In this case, the lattice misfit between the γ-matrix and γ′-Co₃(Al,X) ternary compound is no more than 0.53%. The intermetallic compound with L1₂ structure is stable to 990 °C, and, hence, Co-based superalloys strengthened by this phase are an attractive alternative for Ni-based analogues. The high-temperature strength of new superalloys has been studied by many authors. Furthermore, it was important to understand, how creep properties may be tailored through alloying. The improvement of creep resistance may be obtained by addition of B, Ta, Nb, Mo, Hf and Ti, which also strengthen grain boundaries. Although some papers have been published concerning the properties of this new group of superalloys of Co-Al-W and Co-Al-Mo-Nb type, these materials are in an early phase of development and are still not suitable to replace Ni-based superalloys.

To address the information gap concerning these materials, this Special Issue of Materials will describe the current state of the art in the area of new Co-based superalloys.

In particular, topics of interest include, but are not limited to the following:

• Theoretical description of issues facing the design processes of new Co-based superalloys
• Technological aspects of the casting processes used for the manufacture of new Co-based superalloys and their plastic deformation
• Designing heat treatment and verification of its effects
• Characterization of microstructure of new Co-based superalloys
• Characterization of properties of new Co-based superalloys, especially high-temperature phenomena, etc.

Prof. Grzegorz Moskal
Dr. Agnieszka Tomaszewska
Guest Editors

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• Co-based superalloys
• Co-Al-W-X alloys
• W-free Co-based superalloys
• single crystals
• microstructure
• mechanical properties
• oxidation resistance
• corrosion resistance
• heat treatment
• high-temperature properties