How Rapid Heating and Quenching Cycles Affect Phase Evolution in Advanced γ-TiAl Alloys—An In Situ Synchrotron Radiation Study

Additive manufacturing (AM) processes are increasingly considered as an alternative manufacturing route to investment casting and conventional forging to produce complex aircraft components out of γ-TiAl based alloys. Due to their extremely fast heating and cooling rates, AM processes are very dynamic and often take place under thermodynamic and chemical disequilibrium conditions. While with conventional research methods, AM parts can only be studied after processing and all interpretations are based on the final microstructures and properties, <i>in situ</i> high-energy X-ray diffraction (HEXRD) experiments in a synchrotron radiation source enable a highly time-resolved and direct observation of the evolution of phases, strains, and texture during the dynamic process.<br/>We simulated and studied the intrinsic heating cycles, which occur below the top layer during an AM process using a quenching dilatometer DIL 805 A/D that has been slightly modified to operate in the Hereon run HEMS beamline of the Petra III synchrotron radiation source at DESY. An advanced γ-TiAl based alloy was exposed to several cycles with cooling rates up to 500 K/s and the diffraction patterns were continuously recorded with a frame rate of 10 Hz. These in situ experiments allow to determine the influence of the cooling rate and a chosen powder bed temperature on lattice parameters and phase fractions, and to observe changes in the crystal structure and atom site occupancy for all phases present, namely α/α₂, β/β_o and γ/γ_m. Based on the results, e.g. a powder bed temperature above 700 °C can be recommended for AM processes in order to allow chemical equilibration in the produced γ-TiAl parts.