Microstructural Evolution and Refinement Mechanism of a Beta-Gamma TiAl-Based Alloy during Multidirectional...

Area of Science:

• Materials Science
• Metallurgy
• Mechanical Engineering

Background:

• Titanium alloys, particularly those based on TiAl, are crucial for high-temperature applications due to their low density and high specific strength.
• Achieving a desirable microstructure in these alloys often requires advanced processing techniques to overcome challenges like brittleness and limited ductility.

Purpose of the Study:

• To investigate the microstructural evolution and mechanical property changes in a Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B alloy subjected to multidirectional isothermal forging (MDIF).
• To elucidate the dynamic recovery and recrystallization mechanisms governing the refinement of the alloy's microstructure during MDIF.

Main Methods:

• Multidirectional isothermal forging (MDIF) was applied to the alloy to produce a crack-free pancake.
• Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were employed to analyze microstructural evolution.
• Tensile testing at ambient and elevated temperatures (800 °C) was conducted to evaluate mechanical properties.

Main Results:

• MDIF successfully transformed the initial near-lamellar microstructure into a refined and homogeneous duplex microstructure.
• Gamma (γ) grain size was significantly reduced from 3.6 μm to 1.6 μm after the second forging step.
• Ultimate tensile strength at room temperature and elongation at 800 °C showed substantial increases post-forging.
• Phase transitions, including β/B2→α2 and α2+γ→β/B2, were observed during intermediate annealing and forging, respectively.
• The refinement mechanism involved lamellar conversion (continuous dynamic recrystallization and γ lath DDRX) and γ phase bulging in the first step, with DDRX of γ grains dominating the second step.

Conclusions:

• MDIF is an effective processing route for refining the microstructure of the studied titanium alloy.
• The refined duplex microstructure achieved through MDIF leads to significant improvements in both ambient and high-temperature mechanical performance.
• Understanding the dynamic recrystallization mechanisms is key to optimizing the forging process for advanced titanium alloys.