Prof. Rimma Lapovok
27/11/2025
אודיטוריום ע"ש דויד וואנג, בניין מידן, קומה 3
13:30
A comparative investigation of two fundamentally different approaches for the synthesis, microstructure evolution, and mechanical properties of the refractory high-entropy alloys (RHEA) HfNbTaTiZr and HfNbTiZr is performed. The two methods comprise conventional arc (button) melting and a powder route based on mechanical alloying and consolidation via severe plastic deformation.
Initially, blended elemental powder is pre-compacted via equal channel angular pressing (ECAP) at 500 °C and then subjected to 10 revolutions of high pressure torsion (HPT) at room temperature to an effective strain between 4 and 40. Some samples are then annealed at 500 °C for 1 h to investigate the thermal stability of the phases. The ECAP has been performed for one to four passes. It was shown that the four ECAP passes at 500 °C do not result in the formation of the body-centered cubic (BCC) phase typical for the classical RHEAs despite the presence of interfacial zones between particles and defect-driven diffusion.
Moreover, a single ECAP pass is sufficient to create a solid bulk sample for subsequent HPT. After 10 HPT revolutions, in contrast to melting route resulting in a single BCC phase alloy, both alloys form new phases comprising a Nb-rich BCC phase and a ZrHf-rich HCP phase in both alloys.
Building upon the first stage of this research, the second stage achieved a breakthrough in alloy synthesis by directly producing equiatomic RHEAs: HfNbTaTiZr and HfNbTiZr, from blended elemental powders through ten revolutions of HPT at room temperature. This method has demonstrated its effectiveness and simplicity not only in producing solid bulk materials but also in manufacturing RHEAs. Unlike the melting route and first stage strategy of production, both systems formed new three-phase alloys. These phases were defined as the Zr-based hcp1 phase, the α-Ti-based hcp2 phase, and the Nb-based bcc phase. The volume fraction of the phases was dependent on the accumulated plastic strain. The thermal stability of the phases was studied by annealing samples at 500 ºC for one hour, which resulted in the formation of a mixed structure consisting of the two new hexagonal and cubic phases.