In-situ transmission electron microscopy investigation of deformation-induced microstructural evolution of a FeCoCrNiMn high-entropy alloy

High-entropy alloys (HEAs) are alloys with multiple (usually ≥5) principal elements. HEAs are attracting increasing interest because of their promising mechanical properties and phase stability, which can be used for various applications, such as high-speed cutting tools, anticorrosive high-strength parts in chemical plants and deep-sea exploration due to the excellent mechanical properties at cryogenic temperatures. Thorough understanding of the structural evolution during deformation of HEAs is a prerequisite for understanding and further improving their superior mechanical properties, which is critical for future production of high-entropy alloys with desirable properties. Many HEAs form a stable single solid-solution phase. However, phase transformation could occur in some HEAs under certain circumstances, including high stress. Because stress concentration usually occurs at crack tips during deformation, it is interesting to check if any deformation-induced phase transformation would occur at crack tips during the deformation processes of HEAs. This thesis aims at using various transmission electron microscopy techniques to investigate the structural evolution of HEAs at room temperature. Research results showed surprisingly crystalline to amorphous phase transformation at crack tips in a FeCoCrNiMn HEA with an ultrafine-grained structure. Details of the phase transformation process was video recorded. The mechanism responsible for the phase transformation is discussed based on the observed microstructural evolution. Toughening introduced by nanobridging and the phase transformation is also briefly discussed