Negative Heat Of Mixing Research Articles

Effect and mechanism of similar elements on glass forming abilities of Hf-based bulk metallic glasses

ABSTRACT Compared to Zr-based bulk metallic glasses (BMGs), glass forming abilities (GFAs) of Hf-based counterparts limit practical applications. X-ray diffraction and transmission electron microscopy analysis revealed that only HfNiAl and CuHf2 crystal phases existed in Hf47Cu29Ni10Al14 master ingots. However, when partially substituting Ni with Ag or Co, the GFAs were remarkably improved. Particularly, alloying 3 at.% Ag resulted in an almost fully amorphous master ingot with a maximum thickness of 10.75 mm. Our experimental results conflict with conventional theories such as the deep eutectics theory. A large negative heat of mixing (HM) between constituent elements has been widely accepted as a key factor in enhancing GFAs. Our results posed a puzzling problem due to the non-negative HM between two similar elements either Ag and Cu or Co and Ni. To address the contradiction, a similar competition mechanism was proposed to elucidate the apparent difference in GFAs between quaternary and quinary alloys. Competition is triggered between similar elements when interacting with another element due to their analogous properties, which undermines the interaction between each of similar elements and another element and then suppresses crystal phases containing one similar element. To quantitatively describe this mechanism, we introduced a dimensionless parameter, χ.

Crystal-to-amorphous phase transformation in ternary shape memory alloys

This systematic study evaluates the role of important factors in crystal-to-amorphous phase transformation during severe plastic deformation in ternary shape memory alloys. Experiments coupled with thermodynamic calculations to obtain comprehensive interpretation on the amorphization in ternary shape memory alloys. Equiatomic binary NiTi together with ternary Ni–45Ti–5Hf and Ti–45Ni–5Cu (at%) alloys as alloy models were processed by high-pressure torsion (HPT) at room and cryogenic temperatures. The binary and Hf-added alloys revealed an amorphous structure with a few remaining nanocrystals, however, Cu-added ternary alloy showed resistance to the amorphization after HPT processing. The results revealed two key factors of preventing martensite-to-austenite transformation during deformation and the intrinsic potential of the material for amorphization drive crystal-to-amorphous phase transformation. Higher transformation temperatures, enthalpy and thermal hysteresis prevented martensite-to-austenite transformation, which therefore encouraged amorphization. In addition, a large negative heat of mixing, Gibbs energy for amorphization and a large atomic size mismatch are important factors in implementing crystal-to-amorphous phase transformation. This investigation is a step forward to a fundamental understanding of ternary or even compositionally complex shape memory alloys for microstructural engineering by providing a homogeneous ultrafine-grained structure due to amorphization and subsequent heat treatment to achieve superior shape memory effect and superelasticity.

Turing pattern and chemical medium-range order of metallic glasses

The formation of bulk metallic glass requires the constituent elements to have a negative heat of mixing but has no restrictions on its magnitude. An understanding of this issue is lacking due to the absence of a valid method for describing chemical ordering of metallic glasses. For example, the radial distribution function is ineffective in identifying the elemental preferences of packed atoms. Here, we show that using molecular-dynamics simulation, the chemical medium-range ordering of liquid alloys can be evaluated from persistent homology. This inherently arising chemical medium-range order in metallic glasses is exclusively regulated by the activation and inhibition of the constituent components, making the topology of metallic glasses a Turing pattern. The connecting schemes of atoms of the same species form three distinct regions, reflecting different correlations at the short and medium length scales, while the difference in the schemes corresponds to chemical ordering. By changing the elemental types, it is demonstrated that the chemical medium-range order strongly depends on the relative depth of the interatomic-potential wells. The study separates metallic glasses from crystals under the condition of negative heat of mixing by emphasizing their fundamental difference in interatomic potentials.

Amorphous/crystalline Zn60Zr40 alloys lattice structures with improved mechanical properties fabricated by mechanical alloying and selective laser melting

ABSTRACT In the present study, mechanical alloying (MA) was employed for synthesising non-equilibrium Zn60Zr40 amorphous powders, and then consolidated into amorphous/crystalline Zn60Zr40 alloys using selective laser melting (SLM). The results showed that the MA process destabilised the atomic periodicity of Zn and Zr powders and induced crystalline-to-amorphous transformation due to atomic size mismatch and negative heat of mixing. Moreover, the amorphisation trend of as-milled powders was intensified with increasing milling time and attained almost fully amorphous structure after 30 h of milling. During SLM, the ultra-high cooling rate restricted the long-range atomic diffusion of the amorphous powders and enabled successful survival of amorphous phase, leading to amorphous/crystalline Zn60Zr40 alloys. The alloys exhibited a maximum compressive yield strength and microhardness of 160.9 ± 9.1 MPa and 3.73 ± 0.8 GPa, respectively. These findings demonstrated that the developed MA-SLM process might be a promising strategy for the preparation of amorphous/crystalline alloys with superior properties.

Core-shell structure and stability of nanocrystalline precipitates in Fe83.3Si4B8P4Cu0.7 soft magnetic amorphous alloy

The core-shell nanostructure of α-Fe precipitates was investigated in a recently developed high-performance Fe83.3Si4B8P4Cu0.7 nanocrystalline soft magnetic alloy utilizing small-angle neutron scattering (SANS) and atom probe tomography (APT). SANS measurements under a saturating magnetic field provided evidence of core-shell structures at the nanoscale. Moreover, the core-shell structure was directly visualized from APT analysis, where P-enriched shells were found around Fe-enriched cores due to the large negative heat of mixing between Fe and P elements and the slow diffusion of P atoms. The core-shell nanostructures induced by nanoscale partitioning are effective in stabilizing the residual amorphous matrix and suppressing the crystal growth. Our findings provide insight into the microstructure of Fe-based amorphous alloys and have direct implications in alloy design for the improvement of the soft magnetic properties.

Multi-scale electrochemical thermal model of Electric Double Layer Capacitor under galvanostatic cycling

Electric Double Layer Capacitors (EDLC) have a wide spectrum of application with internal heat generation which may lead to thermal runaway or accelerated aging. This study proposes a multi-scale electrochemical thermal model of EDLCs. Energy equation is solved for cylindrical EDLC device with averaged reversible heat generation rate calculated from local heat generation rate near single carbon nanoparticles derived from first principle. The numerical heat generation and temperature profile resemble experimental results with a prediction error less than 15%. The temperature is uniform across the jelly roll but shows a major drop across the air gap. The reversible heat generation profile is asymmetric and features a sharper peak for exothermic process compared with endothermic process. This is attributed to the relative fast heat dissipation from the EDLC device to the surrounding during exothermic process. Furthermore, detailed contribution of heat generation rate near the electrode/electrolyte interface is analyzed. Negative heat of mixing is observed at the start of charging. This can be attributed to the deviation of flux direction due to the existence of a curved surface. This comprehensive model can be used to study the detailed effect of nanoparticle phenomena on the heat generation and temperature profiles of commercial EDLC devices.

Blends Containing Amphiphilic Biopolymers and their Compatibility Behavior

Abstract: This mini-review deals with the miscibility behaviour of two biopolymers, chitosan and alginate. It is well known that the miscibility in multifunctional polymer blends is favoured due to specific interactions, which origin a negative heat of mixing. Particular interest is focused on functionalized polymers because they are the most suitable way to obtain interacting polymers, producing a single-phase material. Due to the polyfunctionality of chitosan (CS) and other biopolymers, they can be taken into account as a basis of a strongly interacting polymer. They would allow obtaining compatible polymeric materials. For this reason, blends containing CS with different vinyl polymers have been studied. The most significant polymeric blends with these natural polymers will be analyzed in this review. Chitosan is obtained from the biopolymer chitin through sequential processes of demineralization, deproteinization and deacetylation. The native chitin is obtained by direct separation from the marine crustaceans shell, which is abundant on the sea coasts. Some classic results that relate to the polymeric blends containing amphiphilic polymers will be discussed. Another biopolymer of the coast is Sodium Alginate (SA). Alginate also allows the formation of compatible polymer blends. Results in this regard will also be analyzed in this review.

Developing a multilateral-based neural network model for engineering of high entropy amorphous alloys

Due to a great number of composition-processing factors, it is very difficult to design high entropy amorphous alloys without performing manifold trial-and-error experimentations. To solve this problem, in this study we developed a machine learning-based approach, namely multilateral-based neural network, which is able to predict new high entropy amorphous compositions through estimating the highest glass forming ability and the critical casting thickness. In this approach, the entropy parameters were individually correlated to each input, which leads to the improvement of predictive model in evaluating the high entropy glassy alloys. As a case study, Ti20Zr20Hf20Be20Co20 high entropy metallic glass (MG) was considered and the effects of added elements such as Y, Ni, Cr and V and Cu on the glass formation and critical casting thickness were investigated. According to the results, it is determined that the Y addition acts as a microalloying process in the base composition, while other elements improve the configurational entropy and the total negative heat of mixing, which lead to the engineering of equi-atomic high entropy MGs.

CRYSTALLIZATION PROCESSES OF SOME AMORPHOUS ALLOYS

A series of Fe40Co40Zr8M2B10 (M=Nb, V, Cr, Ti, W, Al) alloys were prepared using melt-spinning. The thermal curve, structure and magnetic property of alloys are examined. Because of different negative heat of mixing between elements, only Fe40Co40Zr8M2B10 (M=Nb, V, Cr, Ti) alloys form amorphous structure. These amorphous alloys are annealed at different temperatures under vacuum conditions. The crystallization processes of four amorphous alloys are similar. In the primary stage of crystallization process, only α-Fe (Co) phase precipitates and Co element mainly distributes in the residual amorphous. For the four alloys after annealing at 550°C, there is a few differences in saturation magnetization (Ms) and coercivity (Hc) due to their different microstructures.

Crystallization processes and magnetic properties of FeCoZrMB (M=Nb, V, Cr, Ti) amorphous alloys

A series of Fe40Co40Zr8M2B10 (M=Nb, V, Cr, Ti, W, Al) alloys were prepared using melt-spinning. The thermal curve, structure and magnetic property of alloys are examined. Because of different negative heat of mixing between elements, only Fe40Co40Zr8M2B10 (M=Nb, V, Cr, Ti) alloys form amorphous structure. These amorphous alloys are annealed at different temperatures under vacuum conditions. The crystallization processes of four amorphous alloys are similar. In the primary stage of crystallization process, only α-Fe(Co) phase precipitates and Co element mainly distributes in the residual amorphous. For the four alloys after annealing at 550°C, there is a few differences in saturation magnetization (Ms) and coercivity (Hc) due to their different microstructures.

Nanometer-scale phase separation in Al60Ge30Mn10 amorphous alloy

Nanometer-scale phase separation has been explored in an Al60Ge30Mn10 amorphous alloy, where all atomic pairs have negative heats of mixing. Here we use a combination of experimental (X-ray diffraction, X-ray absorption fine structure, small angle X-ray scattering and transmission electron microscopy) and simulation (Ab initio molecular dynamics and reverse Monte Carlo) techniques to resolve the atomic- and micrometer-level structures in the phase-separated Al60Ge30Mn10 amorphous alloy. Three characteristic peaks appear in the structure factor, linking with three kinds of composition fluctuation regions with different heterogeneous structures. The origin of three characteristic peaks is investigated in details and attributed to different atomic pairs. By analyzing the local chemical ordering between main polyhedra, we find that various phases exhibit differences in atomic structure, composition and bond-orientational order. The results obtained here provide atomic-scale understanding of the nanometer-scale phase separation phenomenon in Al60Ge30Mn10 amorphous alloy.

On the metastable liquid miscibility gap in Pd-Ni-P

Based on the results of experiments performed at temperatures T ≳ 613 K, a thermodynamic model was recently proposed to explain: (i) the origin of a metastable liquid miscibility gap (MLMG) in Pd-Ni-P, an alloy system with a negative heat of mixing. It originates from the superposition of two distinct liquid Gibbs free energy blades; and (ii) a liquid-liquid phase transition (LLT) just outside the MLMG. Since Gibbs free energy is a function of T, the MLMG was revisited in this work for its T dependence, but this time with T ≲ 613 K. It was found that the MLMG has the shape of a truncated right angle prism, instead of a right angle prism as suggested previously. It is similar to, but not the same as an immiscibility loop in a binary liquid mixture. The revised MLMG is in good agreement with the predictions of the thermodynamic model.

Effect of Substrate Chemistry on Prenucleation

Understanding atomic ordering in a metallic liquid adjacent to a solid substrate is of both scientific and technological importance. In this study, we used ab initio molecular dynamics (MD) simulations to investigate systematically the effect of substrate chemistry on atomic ordering in the liquid adjacent to artificially created substrates that have the same crystal structure as the solid phase upon solidification but different chemical characteristics. We found that for a given liquid, an attractive chemical interaction (negative heat of mixing) between the liquid and the substrate strengthens atomic ordering in the liquid at the interface, while a repulsive interaction (positive heat of mixing) weakens atomic ordering. It is realized that although both structure and chemistry of the substrate affect atomic ordering in the liquid adjacent to the liquid/substrate interface, the structural effect dominates the atomic ordering process, while the chemical effect is but a secondary factor and affects atomic ordering through either strengthening or weakening the structural templating. Such knowledge of atomic ordering may help us to understand both prenucleation and heterogeneous nucleation, and to develop more effective approaches to grain refinement during solidification through effective manipulation of the interplay between structural and chemical effects of the substrate on atomic ordering in the liquid adjacent to the interface.

Nanosized Cu-Li glass

Amorphous metallic alloy (also known as metallic glass) was firstly discovered in the 1960s as a young member of the amorphous material family. Their unique metastable disordered structure imparts metallic glasses many fascinating properties and features such as high elasticity and high strength, excellent wave-absorption ability and ultralow elastic moduli. In general, the formation of an amorphous alloying phase requires a relatively large negative heat of mixing between the constituent elements, such that they have got the tendency to spontaneously alloy on atomic scale due to the reduction of Gibbs free energy upon intermixing. The systems with heat of mixing being near zero or even positive, however, is an systems, for which the formation of the amorphous alloying phase is difficult to achieve. Cu-Li is a well-known immiscible system, which means that Cu and Li have little tendency to spontaneously alloy on atomic scale in thermodynamic equilibrium . It is for this reason that Cu foils are widely used as the anodic current collectors in worldwide lithium ion batteries, to ensure that the current collector does not undergo electrochemical lithiation during battery cycling. However, in a very recent in-situ transmission electron microscopy (TEM) study, Dr. Muhua Sun, Prof. Wenlong Wang and Prof. Xuedong Bai from the Institute of Physics, Chinese Academy of Sciences, show that ultrafine Cu nanocrytallines can, unexpectedly, be electrochemically lithiated to form amorphous alloying phases.

Unusual Crystallization Behavior Close to the Glass Transition

Using molecular simulations, we shed light on the mechanism underlying crystal nucleation in metal alloys and unravel the interplay between crystal nucleation and glass transition, as the conditions of crystallization lie close to this transition. While decreasing the temperature of crystallization usually results in a lower free energy barrier, we find an unexpected reversal of behavior for glass-forming alloys as the temperature of crystallization approaches the glass transition. For this purpose, we simulate the crystallization process in two glass-forming Copper alloys, Ag_{6}Cu_{4}, which has a positive heat of mixing, and CuZr, characterized by a large negative heat of mixing. Our results allow us to identify this unusual behavior as directly correlated with a nonmonotonic temperature dependence for the formation energy of connected icosahedral structures, which are incompatible with crystalline order and impede the development of the crystal nucleus, leading to an unexpectedly larger free energy barrier at low temperature. This, in turn, promotes the formation of a predominantly closed-packed critical nucleus, with fewer defects, thereby suggesting a new way to control the structure of the crystal nucleus, which is of key importance in catalysis.

Structures and physical properties of two magnetic Fe-based metallic glasses

Significant differences in structural, thermal, mechanical and magnetic properties were observed between traditional FeSiB metallic glass and newly developed FeMB (M = mixture of Zr, Nb and Y) metallic glass. The origin of the differences is attributed to the more negative heat of mixing between boron and the metallic alloying elements. This leads to the transition of local structure and bonding nature of FeB elucidated by the measured extended X-ray absorption fine structure (EXAFS) spectra functions that were analyzed employing ab initio multiple scattering calculations using the FEFF8.4 code and by the electronic structures of Fe as well. Such dramatic change sheds light on the understanding of the correlations between structure and physical properties in metallic glasses. The results were discussed based on electronic structures and they provide a new route to design magnetic Fe-based metallic glasses.

Phase evolution and microstructure characteristics of Mo-based Tb2O3-Dy2O3 composites synthesized by ball milling and sintering

Mo-based Tb2O3-Dy2O3 composites used as neutron absorbers in nuclear power reactor were synthesized by powder metallurgy. The comparative studies of Mo-based Tb2O3 and Mo-based Dy2O3 composites were carried out to deeply understand the phase evolution and microstructure characteristics of Mo-based Tb2O3-Dy2O3 composites. Ball milling induced terbium oxide and dysprosium oxide in the powder mixtures to be first fined, nano-crystallized, amorphized and finally dissolved into Mo matrix to form the supersaturated nanocrystalline solid solution that was driven by mechanical work, not by negative heat of mixing. Mo lattice parameter increased with increasing ball-milling time, opposite for Mo grain size. A phase transformation of Dy2O3 crystal from cubic to monoclinic and then to amorphous was observed during ball milling. The microhardness of sintered bulks was first increased and then decreased with increasing sintering time. The maximum value was obtained at the bulks sintered for 8 h. The microhardness and bulk density were increased with increasing sintering temperature before 1600 °C. The mechanism of ball milling and sintering was also discussed.

High Entropy Alloys, Miscibility Gaps and the Rose Geometry

Recent review articles on high entropy alloys (HEAs) provide little information about miscibility gaps in multicomponent systems, especially about how to respond with alloying should they be found. Also, there is a lack of information about how miscibility gaps might appear on calculated or measured multicomponent phase diagrams. In this work concepts concerning miscibility gaps that form in binary and ternary systems are reviewed. Then the work is extended to alloys with more components including HEAs. The previous work predicts that there are significant differences between binary systems and those with three or more components. For example, miscibility gaps do not form in binary systems that have a negative heat of mixing, but they do form in ternary systems. Also, ternary systems with a positive heat of mixing can have their stability temperature lowered by adding ternary components that add positive heats of mixing. The morphology and topology of multicomponent/multiphase miscibility gaps differ from typical phase diagrams, as well. For example, one type of miscibility gap is said to have the rose geometry, because of its floral design. Normally only 2-phase miscibility gaps can form in binary and ternary systems. However using the Graph Method it is suggested that 3-phase miscibility gaps might form in HEA systems, even while trying to avoid them. A conclusion of this investigation is that with additional computational and experimental work it may be possible to expand the boundaries of where HEAs can be found.

Microstructural Evolution, Thermodynamics, and Kinetics of Mo-Tm2O3 Powder Mixtures during Ball Milling

The microstructural evolution, thermodynamics, and kinetics of Mo (21 wt %) Tm2O3 powder mixtures during ball milling were investigated using X-ray diffraction and transmission electron microscopy. Ball milling induced Tm2O3 to be decomposed and then dissolved into Mo crystal. After 96 h of ball milling, Tm2O3 was dissolved completely and the supersaturated nanocrystalline solid solution of Mo (Tm, O) was obtained. The Mo lattice parameter increased with increasing ball-milling time, opposite for the Mo grain size. The size and lattice parameter of Mo grains was about 8 nm and 0.31564 nm after 96 h of ball milling, respectively. Ball milling induced the elements of Mo, Tm, and O to be distributed uniformly in the ball-milled particles. Based on the semi-experimental theory of Miedema, a thermodynamic model was developed to calculate the driving force of phase evolution. There was no chemical driving force to form a crystal solid solution of Tm atoms in Mo crystal or an amorphous phase because the Gibbs free energy for both processes was higher than zero. For Mo (21 wt %) Tm2O3, it was mechanical work, not the negative heat of mixing, which provided the driving force to form a supersaturated nanocrystalline Mo (Tm, O) solid solution.

Ab initio molecular dynamics simulations of short-range order in Zr50Cu45Al5 and Cu50Zr45Al5 metallic glasses

Comparative analysis between Zr-rich Zr50Cu45Al5 and Cu-rich Cu50Zr45Al5 metallic glasses (MGs) is extensively performed to locate the key structural motifs accounting for their difference of glass forming ability. Here we adopt ab initio molecular dynamics simulations to investigate the local atomic structures of Zr50Cu45Al5 and Cu50Zr45Al5 MGs. A high content of icosahedral-related (full and distorted) orders was found in both samples, while in the Zr-rich MG full icosahedrons is dominant, and in the Cu-rich one the distorted icosahedral orders, especially and , are prominent. And the polyhedra in Cu50Zr45Al5 MG mainly originate from Al-centered clusters, while the in Zr50Cu45Al5 derives from both Cu-centered clusters and Al-centered clusters. These difference may be ascribed to the atomic size difference and chemical property between Cu and Zr atoms. The relatively large size of Zr and large negative heat of mixing between Zr and Al atoms, enhancing the packing density and stability of metallic glass system, may be responsible for the higher glass forming ability of Zr50Cu45Al5.