In-situ heating observations on microstructure relaxation of ultrafine-grained high-entropy alloys using neutron diffraction and laser-scanning confocal microscopy

The thermal stability of ultrafine-grained metals can be fully understood when observing time-resolved microstructural changes over multiple-length scales. The global microstructural relaxation behavior upon heating of an ultrafine-grained (UFG) CoCrFeNi high-entropy alloy (HEA) was characterized by in-situ heating neutron diffraction measurements. Before heating, the nanocrystalline microstructure was introduced by applying high-pressure torsion (HPT), leading to severe lattice distortion by excess dislocations and defects. The sequential information on the structural relaxation of recovery, recrystallization, and grain growth are identified by in-situ heating neutron diffraction analysis defining the texture development, linear thermal lattice expansion, and stress relaxation behaviors of the UFG HEA with increasing temperature up to 1300K. By contrast, nanocrystalline metals processed by HPT are often inhomogeneous microstructurally and compositionally. The influence of such inhomogeneity on the macro-scale microstructural relaxation is monitored using an HPT-processed CoCrFeNiMn high-entropy alloy through in-situ heating laser-scanning confocal microscopy. This study emphasizes the importance of characterization techniques for further in-depth exploration of the SPD-processed ultrafinegrained structure.