Microstructure evolution and phase transformation behavior of Ti17 and γ-TiAl bimetallic structure fabricated by directed energy deposition

Abstract

A bimetallic structure prepared by directed energy deposition (DED) can simultaneously meet multiple property demands in service. However, in existing studies, formation mechanism and controlling method of transition zone (TZ) remain elusive due to narrow TZ in Functionally graded Materials (FGMs). In this article, a bimetallic structure of Ti17 and γ-TiAl alloy with a wide transition zone prepared by DED was obtained with special powder feeding method, and microstructure of TZ was studied in more details. According to change of element content and microstructure, the wide transition zone (TZ) is distinguished as TZ1 and TZ2. With the increase of Al content from TZ1 to TZ2, α phase grows up and (α2+γ)lamellar precipitates according to a certain burgers orientation relation. Meanwhile, massive γ phase nucleates at α grain boundary, and β phase transforms into ordered phase B2. Microhardness of TZ1 is much higher than that of Ti17 because the effect of solid-solution strengthening. The microhardness of TZ2 is reduced by precipitation of softer phases (α2+γ)lamellar and massive γ. In TiAl, the increase of Al content leads to coarsening of (α2+γ)lamellar and massive γ, so microhardness declines. The sequence of phase transitions in different zones is simulated and the results are consistent with the observed microstructure. Due to low strength microstructure with coarsening α phase and fine (α2+γ)lamellar, the average tensile strength of gradient sample at room temperature is only 167 MPa, and fracture shows typical cleavage fracture characteristic.