Microstructure and mechanical properties of additively manufactured γ-TiAl with dual microstructure

Abstract

The latest research on electron beam powder bed fusion (PBF-EB), additive manufacturing (AM) process, reveals the possibility of establishing a dual microstructure and differing mechanical properties inside a complex-shaped component made of titanium aluminides (TiAl). The functionally graded material (FGM) parts are processed with two different, well-adjusted AM process parameter sets leading to a significant change in localized aluminum (Al) loss via evaporation during PBF-EB. By heat treatment, the as-built microstructure, containing two different levels of Al, is transformed into customized microstructures: fully lamellar (FL) and nearly lamellar (NL + γ). This study aims to determine the microstructural and mechanical characteristics of PBF-EB processed 4th generation TiAl alloy TNM. Tensile and creep tests are performed for single (FL or NL + γ) and dual microstructure specimens with two orientations. The element distribution of aluminum is determined, the microstructures are characterized, and the fracture-inducing defects are identified. The presented new FGM processing route is a groundbreaking benefit from the PBF-EB process and is necessary for superior mechanical performance. The stress-strain and the creep properties of the dual microstructure specimens are situated between the single microstructure specimens. The mechanical characterization shows that the interface between FL and NL + γ does not cause any weakening of the specimens. The failure always takes place in the weaker microstructure (tensile: FL; creep: NL + γ). For the first time, AM components such as TiAl turbine blades can consist of creep-resistant airfoil sections and high-strength root sections taking advantage of new microstructural FGM design possibilities.