Defect energetics and deformation mechanisms in refractory high entropy alloys

Abstract

High entropy alloys (HEAs) are a novel class of metallic materials composed of multiple principal elements in roughly equal atomic proportions. HEAs exhibit remarkable properties such as high strength, hardness, wear resistance, thermal stability, and corrosion resistance, surpassing many conventional alloys. Numerous investigations have been conducted on HEAs using both experimental and theoretical approaches shedding light on the complex interplay of various factors in HEA systems leading to these unique properties. However, there remains a significant knowledge gap in our general understanding of how the local chemical environments impact the structural properties of HEAs. Using atomistic simulations, this thesis explores the defect energy landscapes and deformation mechanisms in refractory High Entropy Alloys (RHEA). The gamma surfaces, representing the energy landscape for dislocation motion, in the NbMoTaW body centered cubic (BCC) RHEA were significantly influenced by changes in the local chemical environment. Interaction strengths within the NbMoTaW HEA system were formulated using orbital projections, which provided insights into the underlying mechanisms governing the behaviour of this alloy. In the NbMoTaW system, sluggish grain shrinkage behaviour was observed using a cylindrical grain model. During the annealing process, low entropy islands predominantly composed of high melting elements were found to form along the grain boundaries. Short-range ordering was observed, with heavy elements segregating along the grain boundaries, further slowing down the grain shrinkage process. In the equimolar HfNbTaTiZr RHEA system under uniaxial strain, the transformation-induced plasticity (TRIP) deformation mechanisms exhibit orientation dependence. Reduction in BCC stabilizer elements, Nb and Ta, led to a metastable BCC system undergoing TRIP to Hexagonal Close Packing (HCP) along different crystal orientations.