Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Chemical and Materials Engineering
Dr. Thomas John Balk
High-entropy alloys (HEAs) are a class of multicomponent alloys based on an innovative alloying strategy that employs multi-principle elements in relatively high concentrations. Commonly defined as alloys that contain at least five principal elements, each with a concentration between 5 and 35 at %. The term entropy refers to the excess configurational entropy associated with HEAs, which is thought to facilitate the formation of solid solutions. The design strategy results in vast compositional space for exploration and innovative potential triggering a renaissance in physical metallurgy. These alloys may have favorable properties compared to conventional dilute solid solutions, but their preeminent complexity and relative novelty mean that they are difficult to design and explore. Numerous studies in this field have explored and developed these alloys motivated by the primary HEA concept, which postulates that maximum configurational entropy can be achieved through equiatomic ratios, which, in turn, will stabilize single-phase solid solutions. However, a growing number of studies have shown that entropic stabilization alone is insufficient, and the optimal balance may be found in non-equiatomic mixtures.
The primary objective of this work is to develop and evaluate single-phase non-equiatomic HEAs with unique compositions that will improve fundamental understanding and/or raise new questions and challenges. The findings in this work address multiple aspects of HEA development, focusing on methodology, discovery, and physical properties.
For the first part of this work, the association between the thermal history and the resultant phases and microstructures is investigated for the equiatomic CrMnFeCoNiCu system. Motivated by the natural phenomena of crystal growth and conditions of equilibrium, we introduced a method that is applicable to HEA development, where controlled processing conditions decide the most probable and stable composition. This is demonstrated by cooling an equiatomic CrMnFeCoNiCu from the melt within 3 days. This results in large Cr-rich precipitates and almost a Cr-free matrix with compositions within the MnFeCoNiCu system. From this juncture, it is argued that the most stable composition is within the MnFeCoNiCu system and not within the CrMnFeCoNi system. With further optimization and evaluation, a unique non-equiatomic alloy, Mn17Fe21Co24Ni24Cu14 is derived. The alloy solidifies and recrystallizes into single-phase FCC phase and can be used in fundamental studies that contrast the equiatomic counterpart.
The second part of this work utilizes a thin-film combinatorial approach to develop a compositional and structural library for the OsRuWCo alloy system. A total of 24 unique compositions were produced, representing a structural library in which amorphous hexagonal closed-pack structures hexagonal closed-pack structures and single phase hexagonal close-pack (HCP) structures are identified. From a selected film composition, a new high-entropy bulk alloy with OsRuWCo in nonequiatomic portions was synthesized. The alloy exhibited a single-phase HCP structure in the as-cast state. Three derivatives from this system were also produced considering heats of mixing, atomic size, and binary solubility. These derivatives are OsRuWCoIr, OsRuWCoFe, and OsRuWCoMoRe and all exhibit single-phase HCP as-cast structures, based on x-ray diffraction and electron microscopy. Additionally, this large compositional space was utilized to evaluate conventional parameters that describe high-entropy alloys. Trends illustrating the evolution from amorphous to crystalline phases are discussed.
A further part of this work evaluates the strengthening due to grain size reduction for the newly developed Mn17Fe21Co24Ni24Cu14. Tensile tests were performed on samples with microstructure with grain size ranging from ~7 um to 120 µm. The study addresses a significant challenge in HEA research in which the available sample size in laboratory settings hinders mechanical testing and evaluation of HEAs in tension. This is overcome by developing a furnace casting method that produces ingots large enough to produce multiple tensile specimens. The alloy exhibits excellent strengthening tendencies with an increase in yield stress based on square root scaling taking the form and the form with an unconstrained scaling exponent. Furthermore, the strengthening phenomena and the physical interpretation of the observed strengthening in HEAs are evaluated with discussions aimed at answering the fundamental question: “Do HEAs exhibit exceptional size effects?”
Digital Object Identifier (DOI)
This study was supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0019402 from 2019 to 2021
The Kentucky Science and Engineering Foundation, under the ward KSEF-148-502-15- 363 in 2016
Ter-Isahakyan, Artashes, "DERIVATION, EXPLORATION AND EVALUATION OF NON-EQUIATOMIC HIGH ENTROPY ALLOYS" (2022). Theses and Dissertations--Chemical and Materials Engineering. 146.