Microstructural Effects on the Mechanical Response of a γ-TiAl Based Alloy Studied with In Situ High-Energy X-Ray Diffraction and Finite Element Simulations

Intermetallic γ-titanium aluminide (TiAl) based alloys are of great interest to the aviation industry, i.e. for use in turbine engines, as this material class has excellent creep properties, high oxidation resistance at elevated temperatures and outstanding specific mechanical properties due to its low density. In service, temperatures up to 750°C are reached and especially tensile properties are of utmost concern. Therefore, a novel setup combining high-temperature tensile testing with in-situ high-energy X-ray diffraction (HEXRD) is utilized in this work. It allows to investigate the evolution of lattice strains up to macroscopic plastic deformation and subsequent failure of the specimen. Furthermore, the experimental observations are compared with the outcome of finite element simulations (FEM) to verify the ongoing deformation mechanisms.<br/>The alloy of interest is the so-called TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B, at.%). Tailored heat treatments allow to adjust different microstructures comprising the ordered phases γ (L1₀ structure), α₂ (D0₁₉ structure) and β_o (B2 structure). The different crystal lattices, combined with the phase-specific morphologies within the prevailing microstructure, result in varying mechanical responses, which are also naturally dependent on the testing temperature. Therefore, a nearly lamellar β microstructure, comprising γ/α₂ lamellar colonies and a β_o seam at the colony boundaries, as well as a microstructure with additional globular γ grains are investigated at temperatures ranging from room temperature to 800°C to highlight differences in the macroscopic as well as microscopic mechanical behavior.<br/>The experiments are conducted at the beamline P07B at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. The setup is based on a tensile testing rig with a 20 kN load cell. The measurements at elevated temperatures are achieved by induction heating of the sample. By analyzing the positional change of the individual peaks in the diffractogram during the in-situ tensile HEXRD experiments it is possible to determine the lattice strain of individual sets of lattice planes and furthermore the phase strain of the microstructural phases. Comparing the plastic onset of the lattice strain - macroscopic stress curves, one can gain insights into the apparent load partitioning between the phases and the different deformation mechanisms depending on the testing temperature.<br/>Additional finite element analysis of the samples investigated by HEXRD is conducted using ABAQUS. The necessary microstructures are simulated using a software called Neper, as well as an in-house routine to derive random 3D periodic lamellar structures, conforming to the experimentally observed microstructural characteristics, e.g. α₂/γ colonies with varying lamellar spacings obeying the Blackburn orientation relationship as well as the subdivision of single lamellae into domains. Since the HEXRD experiments at the synchrotron only provide part of the strain tensor, as the information gained from one diffractogram is just two-dimensional, the missing information can be filled with these FEM investigations. Ultimately, the connection between experiment and simulation can be drawn via the present phase strain, which can be calculated from both methods.