Investigation of the fabrication, thermal stability and tensile properties of BCC HEA.

seminars
04.12.2016
14:30
אודיטוריום ע"ש דיוויד וואנג, קומה 3, בנין דליה מידן
Shai Essel, M.Sc candidate

Shai Essel, M.Sc candidate
Israel Institute of Metals
Technion Research and Development Foundation, 32000 Haifa, Israel
 
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.
In order to achieve thermal stability at higher temperatures, the HEA designer searches for high entropy
phases of the materials which comply with thermodynamic stability as defined by the Gibbs free energy
equation. Hence the name “High Entropy Alloys.” This foundation, leads to 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 6 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, FCCB-CC.
 
The present 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 Cr20
Nb20Ti20V20Zr20, Ta20Nb20 Hf20Zr20Ti20 and AlMo0.5NbTa0.5TiZr (nominal composition). The fabrication
methodology—including arc melting, alloying and casting—was followed by heat treating at various
temperatures as will be described. The resulting microstructure—comprised of a mostly 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 specimen was found to be inter-granular, correlating well with grains
of strong solid solution surrounded by brittle Laves phases.

BCC_HEA
(Left to right – Cast Cr20Nb20Ti20V20Zr20 , microstructure after heat treatment , SEM fractography)

 

Supervisor: Prof-em. Menachem Bamberger