Structure of some CoCrFeNi and CoCrFeNiPd multicomponent HEA alloys by diffraction techniques

The structure of CoCrFeyNi (y = 0, 0.8 and 1.2) and CoCrFeNi-Pdx (x = 0.0, 0.5, 0.8, 1.0, 1.2 and 1.5) High Entropy Alloys has been investigated by neutron and standard X-ray as well as by high-energy X-ray diffraction techniques. The alloys were produced by arc melting and afterwards heat treated under several different conditions. It has been concluded that the CoCrFeNi alloy in as-cast condition is, contrary to what is claimed in the literature, not single-phase but consists of at least two different phases, both of fcc type. The difference in lattice constant between the two phases is close to 0.001 Å. Diffraction patterns measured by X-ray and neutron diffraction have shown that the structure of the alloy is not affected by 3 h heat treatment up to 1100 °C. Changing the amount of Fe has no drastic effect on alloy structure. The Pd-containing alloys have also all been found not to be single-phase but to consist of at least four different phases, all being of fcc type. The lattice constants for all phases increase with Pd content. The relative amounts of the different phases depend on Pd concentration. Furthermore, heat treatments of 3 h duration at different temperatures have a significant effect on the alloy phase composition. It is suggested that HEAs should be considered as multicomponent alloys presenting “simple” diffraction patterns, e.g. consisting of one or several lattices of fcc, hcp or bcc type with very close lattice parameters

Machine Learning-Based Classification, Interpretation, and Prediction of High-Entropy-Alloy Intermetallic Phases

The large compositional space of high-entropy alloys (HEA) poses considerable challenges in designing materials with desirable properties. To date, various machine learning (ML) models can essentially predict specific HEA solid-solution phases (SS). Although high-entropy intermetallic phases (IM) are also known to have outstanding properties, the predictions for IM are underdeveloped due to limited datasets and lack of appropriate ML-features. This article presents feature engineering-assisted ML models with detailed phase classification and high accuracy. A combination of phase-diagram-based and physics-based features sequentially train the ML models, which achieve the classification of individual SS and several common IM (Sigma, Laves, Heusler and refractory B2 phases) with accuracies of 85 - 95 %. Meanwhile, the machine-learned features facilitate the interpretation of IM formation. The model is validated by synthesizing 86 new alloys. The present approach provides a robust and practical framework for guiding HEA phases design, particularly the technological important intermetallic phases

High-entropy alloys: a critical assessment of their founding principles and future prospects

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.One of the authors (NGJ) would like to acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding under EP/M005607/1

Molecular dynamics study of the binary Cu_(46)Zr_(54) metallic glass motivated by experiments: Glass formation and atomic-level structure

We have identified a binary bulk metallic glass forming alloy Cu_(46_Zr_(54) by analyzing the structure and thermal behaviors of copper mold cast samples using x-ray diffraction, transmission electron microscopy, and differential scanning calorimeter. Motivated by these experimental results, we fitted the effective Rosato-Guillope-Legrand-type force field parameters for the binary Cu-Zr alloy system and the atomistic description of glass formation and structure analysis of the Cu_(46)Zr_(54) alloy based on molecular dynamics simulation will be also presented

Production and characterization of the Cr35Fe35V16.5Mo6Ti7.5 high entropy alloy

The microstructure, thermal stability, and mechanical properties of a novel Cr35Fe35V16.5Mo6Ti7.5 high-entropy alloy were studied. The mechanical properties were mapped by nanoindentation, and the results correlated with the microstructure and the Vickers microhardness measurements. The alloy was produced by arc melting in a low pressure He atmosphere. Thermal treatments were performed to study the thermal stability of the alloy. The as-cast microstructure of the alloy exhibited a body-centered cubic phase with morphology of dendrites, outlined by a very thin interdendritic phase with a crystallographic structure compatible with Fe2Ti. The presence of the intermetallic particles was predicted by a free-energy based model, in contrast with the single solid solution alloy predicted by a parameter-based model. The volume fraction of the dendrites in the alloy is ∼ 94 % after arc melting. A small fraction of sparse Ti-rich particles, ∼0.4 vol%, was observed. The thermal treatments produced an increase of the population of Ti-rich particles, the formation of a σ-phase and nucleation of precipitates enriched with Fe and Ti into the previous dendrites. The material in as-cast condition exhibited a microhardness value of 6.2 ± 0.3 GPa, while the alloy aged at 960 °C resulted in 7.1 ± 0.4 GPa. Nanoindentations maps showed an excellent correlation with the microstructure, and their statistical analyses yielded a nanohardness mean value of 8.2 ± 0.4 GPa in the dendritic BCC regions of the as-cast and thermal treated samples and 14.1 ± 0.6 GPa for the σ-phase. The onset of the plastic behavior has been studied by analyzing the pop-in phenomenon observed in the nanoindentation loading curves. For the as-cast alloy, this analysis showed that the elastic-to-plastic transition seems to be triggered by dislocation nucleation. The alloy has a low thermal diffusivity in the measured temperature range that increases on increasing temperature.This research has been supported by the Agencia Estatal de Investigación of Spain (PID2019-105325RB-C33/AEI/10.13039/501100011033) and by the Regional Government of Madrid through the program TECHNOFUSIÓN(III)CM (S2018/EMT-4437), project cofinancing with Structural Funds (ERDF and ESF). The support of the Madrid Government (Comunidad de Madrid-Spain) through the Multi-annual Agreement with UC3M ( Excellence of University Professors – EPUC3M14 and in the context of the V PRICIT - Regional Programme of Research and Technological Innovation) is also acknowledge

Revealing structural changes at glass transition via radial distribution functions.

Transformation of glasses into liquids is discussed in terms of configuron (broken chemical bond or transformation of an atom from one to another atomic shell) percolation theory with structural changes caused. The first sharp diffraction minimum (FSDM) in the pair distribution function (PDF) is shown to contain information on structural changes in amorphous materials at the glass transition temperature (Tg). A method to determine the glass transition temperature is proposed based on allocating Tg to the temperature when a sharp kink in FSDM occurs. The method proposed is more sensitive compared with empirical criterion of Wendt-Abraham; e.g., for amorphous Ni the kink that determines Tg is almost twice sharper. Connection between the kink in fictive temperature behavior of PDF and Wendt-Abraham criterion is discussed

DERIVATION, EXPLORATION AND EVALUATION OF NON-EQUIATOMIC HIGH ENTROPY ALLOYS

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?

엔트로피 제어 비정질 형성 합금 시스템에서 비정질 형성능 및 기계적 특성 변화 고찰

학위논문(석사)--서울대학교 대학원 :공과대학 재료공학부,2019. 8. 박은수.Herein, we systematically investigated the effect of configuration entropy (CE) on the glass-forming ability (GFA) and the mechanical response in a series of equiatomic binary Cu50Zr50 to denary (CuNiBeCoFe)50(ZrTiHfTaNb)50 metallic glasses (MGs) with similar atomic size difference and heat of mixing through alloy design involving careful selection of elements chemically and topologically similar to Cu and Zr and subsequent substitution of pre-constituent elements. Interestingly, the senary (CuNiBe)50(ZrTiHf)50 MG with relatively medium CE value of 1.79R exhibited the maximum GFA among the investigated MGs, implying that the CE is not the dominant factor for GFA. The mechanical response analysis was comprehensively performed using nanoindentation test including deformation dynamics of shear avalanche through statistical analysis of pop-in behavior and the analysis result was compared to the atomic-level structure data obtained by high energy X-ray scattering experiment. The overall trend of the nanohardness and the Youngs modulus (E) was shown to outwardly increase which is dominantly due to the increased 3 atom-connection of polyhedra as well as lower fragility. However, the severe local structural irregularity and compositional complexity in MG with higher CE facilitate the chaotic deformation behavior that results in the unanticipated local softening of amorphous phase and ultimately modulate the response towards ductile deformation. Consequently, it can be concluded that the CE could be one of the crucial factors in designing an MG to alter its characteristics towards achieving desirable properties such as optimized GFA and enhanced ductility.본 연구에서는 구성 엔트로피가 비정질 형성능 및 기계적 거동에 미치는 영향을 알아보기 위해 이성분계 Cu50Zr50 부터 십성분계 (CuNiBeCoFe)50(ZrTiHfTaNb)50에 이르기 까지 다양한 금속원소들이 등가원소비로 이루어진 비정질 합금 시리즈의 기계적, 구조적 특성에 대해 분석하였다. 첨가 원소들간 원자 반경 차이, 혼합열 등의 특성이 비슷해 구성 엔트로피 효과를 확인하기 적합한 등가원소비 비정질 합금 시스템을 제조하기 위해 Cu 및 Zr과 화학적, 구조적 특성이 유사한 금속들을 선별하여 등가원소비로 첨가한 뒤 비정질 합금을 제조하였다. 특이하게도 비교적으로 중간 구성 엔트로피를 값을 (1.79R) 갖는 육성분계 (CuNiBe)50(ZrTiHf)50 비정질 합금이 가장 높은 비정질 형성능을 보이며 구성 엔트로피가 비정질 형성능의 우성인자가 아닌 것을 알 수 있었다. 구성원소 개수에 따른 구성 엔트로피 변화가 기계적 거동에 미치는 영향을 확인하기 위해 나노압입 시험을 통한 pop-in 거동의 통계적 분석을 통해 수행하였으며, 이를 이해하기 위해 고 에너지 X-선 산란 실험 (high energy X-ray scattering) 에 의해 얻어진 원자 단위 구조 데이터와 비교 하여 논하였다. 그 결과, 원자 클러스터 간 결합 패턴의 비율이 면결합 (3 atom-connection)으로 증가 및 fragility는 감소할수록 합금의 경도 및 탄성계수 변화는 전반적으로 증가함을 확인할 수 있었다. 또한 원소 첨가에 따른 구성 엔트로피 증가는 국부적으로 구조적 불규칙성 및 조성적 복잡성을 유발 하여 chaotic 변형 거동을 촉진시켜 국부적 softening 현상이 나타나게 하였으며 궁극적으로 연성 변형에 대한 반응을 조절 시킬 수 있었다. 결과적으로, 구성 엔트로피는 최적의 비정질 형성능 및 향상된 연성과 같은 우수한 특성을 달성하기 위해 비정질 합금 설계에 중요한 요소임을 알 수 있었다.Chapter 1. Introduction 1 1.1. Metallic glass 1 1.1.1. Definition and characteristics 1 1.1.2. Glass-forming ability parameter 4 1.1.3. Fragility 9 1.1.4. Deformation mechanism 11 1.1.5. Shear avalanche in metallic glass 13 1.2. High entropy alloy 16 1.2.1. Definition and characteristics 16 1.2.2. Four core effects of HEA 19 1.3. High entropy metallic glass 22 1.3.1. Properties and the current understanding 23 1.4. Thesis objective and research strategy 27 Chapter 2. Experimental procedure 29 2.1. Sample preparation 29 2.1.1. Fabrication of metallic glass ribbon 29 2.2. Structural analysis 30 2.2.1. X-ray diffraction 30 2.2.2. Transmission electron microscopy 30 2.2.3. High energy X-ray scattering 30 2.3. Thermal analysis 34 2.3.1. Differential scanning calorimetry 34 2.4. Mechanical testing 35 2.4.1. Nanoindentation test 35 Chapter 3. Results 37 3.1. Alloy design 37 3.2. Structural analysis 42 3.2.1. X-ray diffraction analysis 42 3.2.2. Transmission electron microscopy analysis 42 3.2.3. High energy X-ray scattering analysis 46 3.3. Glass-forming ability evaluation 48 3.4. Fragility evaluation 53 3.5. Nanoindentation test 55 Chapter 4. Discussion 60 4.1. Influence of configuration entropy on the atomic-level structure 60 4.2. Anomalous modulus variation in high entropy metallic glass 65 4.3. Anomalous deformation dynamics of high entropy metallic glass 70 Chapter 5. Conclusion 78Maste

The role of local structure in dynamical arrest

Amorphous solids, or glasses, are distinguished from crystalline solids by their lack of long-range structural order. At the level of two-body structural correlations, glassformers show no qualitative change upon vitrifying from a supercooled liquid. Nonetheless the dynamical properties of a glass are so much slower that it appears to take on the properties of a solid. While many theories of the glass transition focus on dynamical quantities, a solid's resistance to flow is often viewed as a consequence of its structure. Here we address the viewpoint that this remains the case for a glass. Recent developments using higher-order measures show a clear emergence of structure upon dynamical arrest in a variety of glass formers and offer the tantalising hope of a structural mechanism for arrest. However a rigorous fundamental identification of such a causal link between structure and arrest remains elusive. We undertake a critical survey of this work in experiments, computer simulation and theory and discuss what might strengthen the link between structure and dynamical arrest. We move on to highlight the relationship between crystallisation and glass-forming ability made possible by this deeper understanding of the structure of the liquid state, and emphasize the potential to design materials with optimal glassforming and crystallisation ability, for applications such as phase-change memory. We then consider aspects of the phenomenology of glassy systems where structural measures have yet to make a large impact, such as polyamorphism (the existence of multiple liquid states), aging (the time-evolution of non-equilibrium materials below their glass transition) and the response of glassy materials to external fields such as shear.Comment: 70 page