In-situ synchrotron study on the thermal stability of the microstructure in Fe40Mn40Co10Cr10 multi-principal element alloy processed by high-pressure torsion

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The thermal stability of an Fe 40 Mn 40 Co 10 Cr 10 multi-principal element alloy (MPEA) was studied in the temperature range between 295 and 1273 K by in situ synchrotron X-ray diffraction (XRD). Two samples were investigated: an initial wrought material and a sample processed by high-pressure torsion (HPT) technique. The variations of the phase composition and the lattice constants of the main phases were determined and the corresponding coefficients of (linear) thermal expansion (CTEs) were calculated. In addition, the crystallite size and defect density evolution in the HPT-processed sample during annealing were determined. In the wrought sample, the major phase was a face-centered cubic (FCC) structure while the HPT-processed specimen contained mostly a hexagonal close-packed (HCP) phase. Between 400 and 680 K, the HCP phase gradually transformed back to FCC structure. An α-Mn type phase was observed in both the wrought and HPT-processed MPEA, even during heating up to ∼1050 K, in contrast to the previously reported σ-phase. The CTE of the FCC phase was (2.5–2.8) × 10 −5 K −1 for both states irrespective of the temperature. On the other hand, in the HPT sample the HCP phase exhibited CTE values of 3.0 × 10 −5 K −1 and 4.8 × 10 −5 K −1 in directions a and c , respectively. The dislocation density in the HPT-processed sample was about 56 × 10 14 m −2 and the FCC phase also contained twin faults with the probability of 2%. The microstructure remained stable up to 680 K, while above that temperature recovery and recrystallization of the severely deformed microstructure occurred.