Abstract
The effect of a two-step heat treatment on the microstructure and high-temperature tensile properties of β-containing Ti-44Al-4Cr (at%) alloys fabricated by electron beam powder bed fusion were examined by focusing on the morphology of α2/γ lamellar grains and β/γ cells precipitated at the lamellar grain boundaries by a cellular precipitation reaction. The alloys subjected to the first heat treatment step at 1573 K in the α + β two-phase region exhibit a non-equilibrium microstructure consisting of the α2/γ lamellar grains with a fine lamellar spacing and a β/γ duplex structure located at the grain boundaries. In the second step of heat treatment, i.e., aging at 1273 K in the β + γ two-phase region, the β/γ cells are discontinuously precipitated from the lamellar grain boundaries due to excess Cr supersaturation in the lamellae. The volume fraction of the cells and lamellar spacing increase with increasing aging time and affect the tensile properties of the alloys. The aged alloys exhibit higher strength and comparable elongation at 1023 K when compared to the as-built alloys. The strength of these alloys is strongly dependent on the volume fraction and lamellar spacing of the α2/γ lamellae. In addition, the morphology of the β/γ cells is also an important factor controlling the fracture mode and ductility of these alloys.
Highlights
Titanium aluminide (TiAl) alloys have attracted significant interest for high-temperature applications such as aircraft and automotive engines because of their lightweight characteristics, excellent strength at high temperatures, and good oxidation resistance [1,2]
We found that it is possible to obtain TiAl alloys with unique microstructures by repeated and rapid fusion and solidification of metal powders, which is a unique phenomenon during the electron beam powder bed fusion (EB-PBF) process [33,34]
We found that the alloys containing the ultrafine lamellar grains exhibit high strength at room temperature and high temperatures [34]
Summary
Titanium aluminide (TiAl) alloys have attracted significant interest for high-temperature applications such as aircraft and automotive engines because of their lightweight characteristics, excellent strength at high temperatures, and good oxidation resistance [1,2]. The lamellar structure is favorable for high-temperature strength and good fracture toughness at room temperature. A new concept for controlling the microstructure of the TiAl alloys, which is focused on an ordered β phase with the B2 structure, has been proposed by Takeyama et al [5]. The new type of TiAl alloys, such as Ti-Al-NbMo-B (TNM) alloys, developed based on this new concept are categorized as β-containing
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