M.Sc Thesis

M.Sc StudentEssel Shai
SubjectThermal Stability of High Entropy Alloys
DepartmentDepartment of Materials Science and Engineering
Supervisors PROFESSOR EMERITUS Menachem Bamberger
DR. Alexander Katsman


High entropy alloys (HEAs) have recently become the focus of research attention in the field of metal alloying. A typical HEA exhibits thermal stability in the temperatures of the designed working envelope rather than optimized properties or microstructure under ambient conditions. 

To achieve thermal stability at higher temperatures, the HEA designer searches for high entropy phases of the materials that comply with thermodynamic stability as defined by the Gibbs free energy equation. Hence the name High Entropy Alloys and the understanding that the optimal alloy structures are indeed highly disordered solid solutions. Thus, most of the materials investigated consist of multiple components (up to six elements of roughly aqui-molar ratios). Presently, there are two primary HEA families under study, defined by the lattice structure of the optimal solid-solution phases, FCC-BCC.

The presented study investigates the thermal stability of BCC HEAs. Based mostly on refractory elements, this HEA family is characterized by high melting and working temperatures. The alloys investigated were VNbMoTaW, CrNbTiVZr and TaNbHfZrTi. 

The fabrication methodology?including arc melting, alloying and casting?was followed by heat treating at various temperatures, as will be described. The resulting microstructure? comprising mostly a solid-solution?demonstrates the precipitation of stable inter-metallic phases, usually the Laves phases (Zr,Nb,Ti)Cr2. The fracture failure mode of the tensile test specimens was found to be inter-granular, correlating well with grains of a strong solid solution surrounded by brittle Laves phases.

Measurements of hardness is presented and used to analysis the different thermal conditions, this proves to be similar to literature reference. However the compression strength is very different than the one presented in the literature background perhaps due to different sample production methodology. The tensile strength of several cast and HIPed TaNbHfZrTi samples (two casting methodologies) is presented and a strong correlation between the grain size and the tensile properties is distinguished.

 700 MPa tensile TaNbHfZrTi samples are presented and their correlation to literature available data is discussed.