Abstract text (incl. figure legends and references)

The current state-of-the-art, single-crystal nickel-based superalloys operate close to 80 to 90% of their melting temperatures. There is a continuous need to design newer materials that can withstand higher operating temperatures above 1100 °C, as the operation of turbine engines at higher operating temperatures leads to improved energy efficiency in the power generation and aerospace sectors [1]. On the other hand, during operation at higher temperatures, the materials undergo higher creep deformation.

The recently developed eutectic MoSiTi alloy (Mo-20Si-52.8Ti in at.%) exhibits a good combination of pesting oxidation and creep properties [2]. It consists of two phases: body centred cubic (bcc) solid solution and hexagonal Ti-rich silicide, M5Si3 (where M-Ti, Mo). The accumulation of creep strain in the two phases, particularly the deformation behavior of the two phases and lattice defects generated in response to the applied stress has to be thoroughly understood. Compressive creep tests were performed under vacuum at 1200 °C and 100 MPa stress to assess the creep behavior. The samples are tested at various true strains of 1.3, 10, 20, and 40% to understand the deformation mechanism of the alloy.

The crept samples were characterized using scanning electron microscopy (SEM)- backscattered electron (BSE) imaging to analyse the microstructure of phases, their arrangement and detect precipitation. Conventional transmission electron microscopy (CTEM) techniques such as bright field (BF), dark field (DF) and weak beam dark field (WBDF) are employed to image dislocations and analyse dislocation substructure in all crept conditions [3]. Deformation mechanisms can be drawn from the type of dislocation (edge, screw, or mixed) and the dominant slip plane in the bcc phase. In addition, dislocations in the silicide phase are imaged and the operative slip system (basal, prismatic or pyramidal) is identified. Scanning transmission electron microscopy (STEM)-energy dispersive spectroscopy (EDS) is used to determine the composition of the precipitates and analyse any segregation of the alloying elements to the dislocations in the crept samples. The high angle annular dark field (HAADF) STEM image in Fig. 1a shows the precipitation of Ti-rich silicides in the bcc matrix, that are semi-coherently embedded in the matrix. Based on diffraction analysis (Fig. 1b), the precipitates are hexagonal and dislocations are observed in the silicide phase of 10% crept samples (Fig. 1c).

Fig.1: Transmission electron microscopy (TEM) investigations of the crept eutectic MoSiTi alloy a) Scanning transmission electron microscopy (STEM)-high angle annular dark field (HAADF) image of Ti-rich silicide precipitates in body centered cubic (bcc) solid solution b) Diffraction pattern of precipitates in the bcc phase aligned to [001] zone axis in 1.3% true strain and c) Weak beam dark field (WBDF) image of dislocations in hexagonal silicide phase of 10% crept sample. The dislocations are marked with alphabets: A represents long dislocation line and B denotes dislocations inclined to the TEM foil.

[1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068–1069.

[2] Schliephake et al., Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys, Intermetallics. 104 (2019) 133–142.

[3] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, Springer US, Boston, MA, 2009.