In situ analysis of the high temperature deformation and fracture mechanisms of a γ-TiAl alloy

摘要

Gamma titanium aluminides are intermetallic alloys with great potential for aerospace applications in low pressure turbines (LPT) because they can provide increased thrust-to-weight ratios and improved efficiency under aggressive environments at temperatures up to 750 °C. Due to that, γ-TiAl alloys are projected to replace the heavier Ni-base superalloys currently used for LPT blades manufacturing.The objective of this research work is to study the deformation and fracture mechanisms of a γ-TiAl alloy, Ti-45Al-2Nb-2Mn(at.%)+0.8(vol.%)TiB2 (Ti4522XD), at service temperatures, and to relate them to specific microstructural features.An array of microstructures was first generated by processing the investigated alloy by centrifugal casting (CC), in the form of LPT blades and rectangular specimens, and by powder metallurgy (PM) techniques, including hot isostatic pressing (HIP) and field assisted hot pressing (FAHP). Several post-processing heat treatments were carried out in both CC and PM samples. A thorough characterization of the microstructures thus generated was performed by scanning and transmission electron microscopy. In situ mechanical tests were then carried out in selected samples according to specific microstructures in a scanning electron microscope (SEM) aided by electron backscatter diffraction (EBSD) at 700 °C. In particular, constant strain rate (=10⁻³ s⁻¹) and constant stress (creep) (σ=250-450 MPa) tensile tests were performed and the microstructural evolution of selected areas was periodically imaged by SEM. The main findings of this research are summarized below.First, in lamellar centrifugally cast microstructures deformed under creep conditions colony boundary cracking was observed to be the main fracture mechanism. It occurred at low and high stresses, during the secondary and the tertiary creep stages, respectively. The same phenomenon has been observed to predominate along the grain boundaries in finer duplex powder metallurgy microstructures under creep conditions. The occurrence of grain/colony boundary cracking reveals the activation of grain/colony boundary sliding (G/CBS) during creep deformation of lamellar and duplex microstructures, which leads to the nucleation of cracks at triple points. Moreover, in lamellar microstructures creep tested at high stresses (σ>400 MPa) and tensile tested at constant strain rate, the appearance of interlamellar ledges was observed, revealing that interlamellar areas become weaker as the stress increases.Furthermore, the results obtained suggest that, in lamellar microstructures tested at high temperature and constant strain rate, true twin lamella boundaries constitute the weakest obstacles to dislocation motion. Thus, the relevant length scale might be influenced by the distance between non-true twin boundaries. Crystallographic slip is also observed to contribute to deformation under creep conditions. The slip activity during creep deformation was evaluated by trace analysis and a methodology to estimate the relative activity of ordinary and superdislocations, as well as the corresponding critical resolved shear stresses (CRSS), is proposed. This work showed the presence in both lamellar and duplex microstructures of a significant dislocation activity that does not comply with the Schmid law with respect to the applied stress and which thus seems to be a response to local stresses. Intragranular slip is suggested to be an active accommodation mechanism for GBS during creep of duplex microstructures. -------------------------------------------------------