Grain-size dependence of σ-phase precipitation kinetics in CrMnFeCoNi high-entropy alloys and its effect on mechanical behavior

• Applicant: Dr.-Ing. Aditya Srinivasan Tirunilai
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 512530786

The present project seeks to investigate the precipitation kinetics of a topologically close-packed σ-phase in an initially single-phase, Face-Centered Cubic (FCC) CrMnFeCoNi High-Entropy Alloy (HEA) and its impact on tensile properties. Three key scientific questions are considered in the present research proposal: (i) How does the mean grain size affect the σ-phase precipitation kinetics? Grain boundaries and triple points are potent nucleation sites, while grain boundary diffusion is orders of magnitude faster than volume diffusion. Due to these two effects, it is expected that the mean grain size should strongly affect the σ-phase precipitation kinetics as well as the distribution of allotriomorphs and in-grain precipitates. (ii) How do σ-allotriomorphs and in-grain precipitates contribute to the overall precipitation kinetics in terms of nucleation, growth, soft and hard impingement? (iii) How does the presence of the σ-phase affect mechanical properties? The σ-phase is a strong intermetallic compound that can increase strength but also decrease ductility. The trade-off between these two properties is not just restricted to the relative amount of σ-phase, but morphology and distribution at different nucleation sites, as well as grain size of the matrix phase. To answer these three questions, thermomechanical processing will be employed to obtain initially single-phase FCC Cr26-Mn20-Fe20-Co20-Ni14 HEA (composition in at.%) with different grain sizes. These will be annealed for varying durations, between 600-1000 °C to induce σ-phase precipitation. The resulting specimens will then be characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Chemical compositions will additionally be determined through Energy-Dispersive X-ray Spectroscopy (EDS) in the SEM and Atom Probe Tomography (APT). The results will help to answer the first two questions. Moreover, the data that will be obtained will help identify key conditions necessary to generate model microstructures in terms of grain size of the matrix, σ-phase volume fraction, and distribution at grain boundaries and within grains. Their tensile properties will then be tested both ex-situ and in-situ to understand macroscopic deformation behavior and crack propagation through the σ-precipitates respectively. In this way, the present project aims to provide a quantitative description of how the σ-phase affects mechanical properties.

• DFG Programme: WBP Position