Phase equilibria and microstructural evolution in ternary Co-Al-W between 750 °C and 1100 °C.

Area of Science:

• Materials Science
• Metallurgy
• Physical Chemistry

Background:

• Understanding phase equilibria in ternary alloys like Cobalt-Aluminum-Tungsten (Co-Al-W) is critical for developing advanced materials.
• The Co-rich portion of the Co-Al-W system is of interest for high-temperature applications, but detailed phase diagrams are incomplete.

Purpose of the Study:

• To investigate the phase equilibria in the Co-rich region of the Co-Al-W ternary system between 750 °C and 1100 °C.
• To determine the stability and transformation kinetics of the gamma prime (γ') phase within this temperature range.
• To provide essential data for developing a new thermodynamic description of the ternary Co-Al-W system.

Main Methods:

• Isothermal annealing of five ternary alloy compositions within the Co-rich Co-Al-W system.
• Microstructural analysis and phase identification at various temperatures (750 °C to 1100 °C).
• Construction of isothermal sections of the phase diagram and a time-temperature-transformation (TTT) diagram.

Main Results:

• Below 900 °C, the phase diagram is characterized by a three-phase tie-triangle involving gamma (γ), D0₁₉, and B2 phases.
• At 1000 °C and 1100 °C, different combinations of γ, D0₁₉, B2, and micro phases dominate the diagram.
• The gamma prime (γ') phase exhibits limited stability, with its persistence likely influenced by kinetic factors rather than thermodynamic stability at higher temperatures.
• A TTT diagram shows a nose-shape, indicating a trade-off between thermodynamics and kinetics for γ' phase transformations.

Conclusions:

• The phase equilibria in the Co-rich Co-Al-W system have been mapped across a significant temperature range.
• The stability of the γ' phase is constrained, and its presence in microstructures is often kinetically controlled.
• The generated phase diagram data and TTT information are vital for refining thermodynamic models of the Co-Al-W system and guiding alloy design.