Mechanically Robust Compositionally Complex Alloys

Abstract

Compositionally complex alloys (CCAs), including high- and medium- entropy alloys and steels, defy conventional alloy design rules by including multiple principal elements in the alloy composition. This has unlocked the possibility to synthesise an endless list of compositionally unique alloys with unfathomable properties, which could be applied in many industries such as power generation, manufacturing, and aerospace. Among the myriad of attractive properties, much research has been undertaken into their mechanical properties, including investigations into in how these materials are challenging the frontiers of strength-ductility limitations. This requires deep analysis into the structure-property relationship of these alloys, focussing on the role and evolution of nano-structural features, grain boundaries, and crystal structure, in response to deformation. The aim of this research work is to characterise the deformation mechanisms of CrCoNi and FeMnCoCr-based CCAs, and develop insight into how materials can be engineered to exhibit optimum mechanical properties. The first chapter of this thesis introduces the field of CCAs, highlighting the key developments and innovations in the field thus far, provides a summary of strengthening mechanisms, and introduces aims and objectives of the thesis. The second chapter presents an overview of the methodology applied, including preparation, fabrication, structural and chemical characterisation, and mechanical testing techniques. In the third chapter, an analysis of the deformation mechanisms of hierarchical nanostructured CrCoNi with dual-phase face-centred cubic (FCC) and hexagonal closed-packed (HCP) phases is conducted. The results suggest that multiple deformation pathways could be activated in CrCoNi with assistance of growth defects, thereby imparting this technically important alloy with appreciable ductility. The fourth chapter focusses on a body-centred cubic (BCC) FeMnCoCr-based interstitial high entropy alloy (iHEA) which incorporates B, C and O. The unusual combination of hardening effects brought about by interstitial strengthening, grain boundary segregation engineering, compositional fluctuations, and fine grain size, greatly strengthened the alloy by inhibiting dislocation motion. Deformation induced HCP/FCC nanolaminates enhanced plasticity via strain partitioning. Taken together, the newly developed BCC-structured iHEA affords not only high strength, but also confers remarkable ductility through multiple deformation pathways. In the fifth chapter, a Fe72.4Co13.9Cr10.4Mn2.7B0.34 high entropy steel is investigated. The distribution of iron and chromium shows an unusual, characteristic spinodal-like pattern at the nanometre scale, where compositions of Fe and Cr show strong anticorrelation and vary by as much as 20 at.%. The impressive plasticity is accommodated by the formation and operation of multiplanar, multicharacter dislocation slips, mediated by coherent interfaces, and controlled shear bandings. The excellent strength-ductility combination is thus enabled by a range of distinctive strengthening mechanisms, rendering the new alloy a potential candidate for safety critical, load-bearing structural applications. In the sixth chapter, the effect of deformation on hierarchical compositional fluctuations is investigated. As plastic strain increases, the alloy is able to prolong its ductility via a lattice strain relaxation mechanism. This phenomenon is rationalised in terms of the dislocation behaviour exhibited during glide plane softening. In the last chapter, major conclusions are drawn from this research. Some possible future work is proposed as extensions of what has been achieved.