In-situ electron microscopy investigation of the thermal stability of a nanocrystalline CrMnFeCoNi high-entropy alloy

High-entropy alloys (HEAs), which are defined as the alloys that combine with multiple principal elements in equal or near equal atomic ratios, have been in vogue over the past decade. The single face-centred cubic (FCC) phase Cantor alloy comprising Cr, Mn, Fe, Co and Ni in equiatomic ratios is one of the most extensively and thoroughly studied HEAs. Their most astonishing mechanical behaviours are in the cryogenic range, including high strength, excellent ductility and toughness, which has seen them being utilised in many applications under cryogenic environments such as fuel tanks, hydrocarbon explorations and some specific aerospace applications. However, the strength of the coarse-grained Cantor alloy is not ideal at the ambient temperature, which is even not comparable to some advanced steels. Nanocrystallisation, as one effective approach to enhance the strength, is proposed and can extend the application of the Cantor alloy at the ambient temperature. However, with regard to applying the nanocrystalline Cantor alloy at intermediate temperatures and high temperatures, existing reports are not enough to reveal their properties at higher temperatures, which in turn impedes their applications. Thermal stability, including microstructure evolutions and elemental distributions, plays a vital and decisive role in the mechanical properties of materials. Therefore, this thesis aims to investigate the thermal stability of a nanocrystalline Cantor alloy by mainly focussing on the phase instability at intermediate temperatures using various electron microscopy techniques. Apart from the phase stability, the results obtained also experimentally verify the other two vital conclusions. Firstly, diffusion-induced amorphisation existed in the Cantor alloy under heat treatments involving chemical disordering. Secondly, such amorphisation can inversely transform back to crystalline structure if increasing the heating temperature. There are 4 Chapters in this thesis. Chapter 1 briefly introduces the concept and excellent behaviours of HEAs and the Cantor alloy, followed by the reported thermal stability of the Cantor alloy and the outstanding issues that need to be solved in the future. Chapter 2 firstly introduces various characterisation techniques used in this thesis, followed by the in-situ technique, sample manufacturing and processing and sample preparation steps. In Chapter 3, the characterisation results of the Cantor alloy before and after the nanocrystallisation process using high pressure torsion (HPT), the nanocrystalline Cantor alloy after in-situ heating and ex-situ annealing experiments and the ultrafine-grained Cantor alloy before and after an in-situ heating experiment are provided. They are followed by a discussion on the mechanisms, including the inducement and development process of the amorphisation, intermetallic phases and the inverse phase transformation that occurred in the results. In Chapter 4, major conclusions in this thesis are drawn followed by the proposal of future perspectives