Heterogeneous Phase Transformation Research Articles

Surface structure reconstruction to suppress heterogeneous phase transformation for air-stable single crystalline O3-type sodium oxide

Surface structure reconstruction to suppress heterogeneous phase transformation for air-stable single crystalline O3-type sodium oxide

MXene-Integrated Solid-Solid Phase Change Composites for Accelerating Solar-Thermal Energy Storage and Electric Conversion.

The high thermal storage density of phase change materials (PCMs) has attracted considerable attention in solar energy applications. However, the practicality of PCMs is often limited by the problems of leakage, poor solar-thermal conversion capability, and low thermal conductivity, resulting in low-efficiency solar energy storage. In this work, a new system of MXene-integrated solid-solid PCMs is presented as a promising solution for a solar-thermal energy storage and electric conversion system with high efficiency and energy density. The composite system's performance is enhanced by the intrinsic photo-thermal behavior of MXene and the heterogeneous phase transformation properties of PCM molecular chains. The optimal composites system has an impressive solar thermal energy storage efficiency of up to 94.5%, with an improved energy storage capacity of 149.5Jg-1, even at a low MXene doping level of 5wt.%. Additionally, the composite structure shows improved thermal conductivity and high thermal cycling stability. Furthermore, a proof-of-concept solar-thermal-electric conversion device is designed based on the optimized M-SSPCMs and commercial thermoelectric generators, which exhibit excellent energy conversion efficiency. The results of this study highlight the potential of the developed PCM composites in high-efficiency solar energy utilization for advanced photo-thermal systems.

Heterogeneous phase transformation pathways in additively manufactured Al-Ce-Mn alloys

Heterogeneous phase transformation pathways in additively manufactured Al-Ce-Mn alloys

Stress-induced heterogeneous transformation and recoverable behavior of laser powder bed fused Ni-rich Ni50.6Ti49.4 alloys without post treatment

In comparison with the conventional processes, laser-based additive manufacturing can induce a different mechanical response due to the formation of highly non-equilibrium microstructure. In this paper, the heterogeneous phase transformation and recoverable behavior of Ni50.6Ti49.4 samples fabricated by laser powder bed fusion (LPBF) during the compression were reported. Based on the indirect in-situ X-ray diffraction method, the residual lattice strain and relative volume fraction of B2 and B19′ phases were evaluated and analyzed. It was found that the change trend of residual lattice strain of B19′ phase with the applied strain was basically consistent with that of the macro-scale residual strain. But the evolution of residual lattice strain of B2 phase demonstrated a complicated case. Besides, the evolution curves of relative volume fraction of B2 and B19′ phases showed a typical ‘X’ shape with an apparent plateau stage, which indicated considerable reversible transformations still occurred even at a severely deformed condition. The maximum recoverable strain could reach 0.079 at room temperature. A different strain stage division was therefore proposed, including B2-phase dominated stage, reversion stage, balance stage and B19′-phase dominated stage.

Study of the influence of δ-ferrite on the TRIP effect of a low-density high aluminum steel by using full-field crystal plasticity modeling

Abstract Low-density high aluminum transformation induced plasticity (TRIP) steel is a new developed light-weight structural material. In the conventional casting (CC) TRIP steel, however, the presence of high Al content results in the formation of coarse dendritic δ-ferrite, which seriously deteriorates the performance of the material. The spray forming process show good potential to refine and homogenize the microstructure. It was found that the δ-ferrite in the spray formed (SF) TRIP steel is refined remarkably and its distribution is more homogeneous. Uniaxial tension tests demonstrate that the ductility of the SF TRIP steel is increased by 5% compared with the CC TRIP steel. The difference of their properties is mainly due to the different morphologies of the δ-ferrite. We further employed a full-field crystal plasticity (CP) modeling to probe the micromechanical mechanisms in details, and a new simplified model was proposed to describe the TRIP effect and phase transformation. The material parameters of three individual phases δ-ferrite (δ), austenite (γ), and martensite (α′) for the CP model were identified from the loading curves of nanoindentation tests. Through the simulation, it was found that compared with SF TRIP steel, there are large deformation bands formed in the CC TRIP steel during the deformation. These areas are the regions in which austenite has the priority to transform into martensite. It is the initial heterogeneity of deformation in the CC TRIP steel that results in the heterogeneous phase transformation, and thus intensifies the heterogeneous deformation at the phase boundaries, which ultimately accelerates the generation of cracks.

Propagation topography of redox phase transformations in heterogeneous layered oxide cathode materials

Redox phase transformations are relevant to a number of metrics pertaining to the electrochemical performance of batteries. These phase transformations deviate from and are more complicated than the conventional theory of phase nucleation and propagation, owing to simultaneous changes of cationic and anionic valence states as well as the polycrystalline nature of battery materials. Herein, we propose an integrative approach of mapping valence states and constructing chemical topographies to investigate the redox phase transformation in polycrystalline layered oxide cathode materials under thermal abuse conditions. We discover that, in addition to the three-dimensional heterogeneous phase transformation, there is a mesoscale evolution of local valence curvatures in valence state topographies. The relative probability of negative and positive local valence curvatures alternates during the layered-to-spinel/rocksalt phase transformation. The implementation of our method can potentially provide a universal approach to study phase transformation behaviors in battery materials and beyond.

Bending deformation and fracture characterisation in quenching and partitioning steel

ABSTRACTThe bending deformation recorded and fracture characterisation seen during the process of quenching and partitioning (Q&P) steel are investigated as a function of bending angle. According to experimental results, the mechanically induced martensitic transformation of the retained austenite occurred heterogeneously in the direction of thickness. Results from electron backscattered diffraction and finite element analysis confirmed that the heterogeneous phase transformation was caused by the stress difference in the direction of thickness. The results prove fracture behaviour is dominated by martensite cracking. The results provide a new insight into deformation characterisation of the Q&P steel under bending, which allows the design and optimisation of mechanical properties.

(Invited) Phase Transformation and Chemomechanical Breakdown of Alkali Metal Ion Cathode Materials

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium and sodium batteries. We found that chemomechanical properties of cathode materials are associated with phase transformation behaviors and can promote the cathode–anode crosstalk in full cells. In this presentation, we will first discuss an integrative approach of mapping valence states and constructing chemical topographies to investigate the redox phase transformation in layered oxide cathode materials at the secondary particle level under thermal abuse conditions. We discover that, in addition to the three-dimensional heterogeneous phase transformation, there is a mesoscale dynamic evolution of local valence curvatures in valence state topographies. The relative probability of negative and positive local valence curvatures alternates during the phase transformation. Such a behavior is likely associated with local chemical environments (e.g., grain boundaries, compositional variation) and can determine chemomechanical properties and ionic transport in cathode materials. Then we will discuss a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge. We observe that the mechanical breakdown of charged cathode materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles; i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, we will highlight some methods that we develop to overcome chemomechanical breakdown.

Quantitative Analysis of Phase Transformation in Ni-Ti Shape Memory Alloys

Abstract Researchers at the University of Michigan are investigating the complex heterogeneous phase transformations associated with deformation in shape memory alloys by means of in-situ tensile loading in combination with SEM, EBSD, and digital image correlation. This article describes the general approach and presents and interprets test results.

Microstructure Evolution of Sputtered Bi-Te Films during Post-Annealing: Phase Transformation and Its Effects on the Thermoelectric Properties

This study examined the microstructure evolution of sputtered Bi-Te films during post-annealing. Combining X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses, we unraveled the fundamental aspects of the materials process: The as-deposited film was not in the Bi2Te3-type phase but in a metastable phase based on a rock-salt structure. Annealing at an elevated temperature allowed the nucleation and growth of the stable phase from the film of the metastable phase in a typical manner of the heterogeneous phase transformation. In addition, the growth of the stable phase is found accountable for the remarkable improvements of the electrical and thermoelectric properties of the films observed after post-annealing.

Heterogeneous Phase Transformation Texture Evolution in Low Alloyed ULC Steel Sheets

AbstractIn the present study the cold rolled sheet of ultra low carbon steel alloyed with manganese and aluminium has been investigated after α − γ − α transformation annealing in vacuum. The texture analysis was carried out on the surface, sub‐surface and in the bulk of the annealed sheet. A <100> // ND and <110>//ND texture with weak intensity is observed on the sheet surface and a strong <111>//ND fibre in the bulk of the transformed sheet. In depth micro‐texture analysis by orientation contrast microscopy reveals that two different types of orientation selection mechanisms could be active. The texture at the metal‐vapour interface is controlled by surface energy anisotropy (producing a local <100>, <110>//ND texture at the surface) and a texture memory mechanism generating strong <111>//ND fibre is active in the bulk of the sheets.

Failure of Kissinger(-like) methods for determination of the activation energy of phase transformations in the vicinity of the equilibrium phase-transformation temperature

The activation energies of heterogeneous equilibrium phase transformations have been frequently determined by Kissinger(-like) analysis. This work demonstrates that the applicability of Kissinger(-like) method(s) to determine activation energies of heterogeneous phase transformations is justified if the phase transformation occurs far away from the equilibrium phase-transformation temperature but not if the phase transformation is investigated in the vicinity of the equilibrium phase-transformation temperature. Experimental results on the kinetics of the order–disorder transformation in Ni3Sn2, obtained both near the equilibrium phase-transformation temperature and considerably below it (using quenched samples), expose the non-validity and confirm the validity, respectively, of activation-energy values obtained by Kissinger(-like) analyses applied in the corresponding temperature ranges. Application of the Kissinger(-like) analysis in a temperature range close to the equilibrium phase-transformation temperature leads to erroneous, abnormally high values for the activation energy.

Full-Field Measurements of Fracture Initiation and Crack Growth in Superelastic Nitinol

Abstract We present results for full-field measurements of crack tip deformation fields in Nitinol using high sensitivity moiré interferometry. Three fatigue loading scenarios are examined illustrating the effect of stress induced martensite transformations at the tip of the crack. Understanding the behavior of fatigue crack growth is paramount to the successful use of this highly complex material response, especially for such application as implantable medical devices. The results demonstrate the sensitivity of heterogeneous phase transformations on the crack growth behavior.

Methods of stoichiography in geochemistry and mineralogy

Abstract Reported is a novel reference-free technique, based on stoichiography, for the molecular and phase analysis of complex mixtures with unknown compositions. The concept of stoichiography is based on the stoichiometry of mass transfer of the constituents in transient, homogeneous and heterogeneous phase transformation processes. The technique utilizes both the separation of components according to their differential dissolution characteristics as well as the dynamics of the changes in stoichiometry. The detailed study of the dissolution processes has been made possible by the use of a specially designed instrumental system, termed a Stoichiograph, which consists of a high sensitive inductively coupled plasma atomic emission spectrometer as the detector–analyzer. Fundamentals, methodology, and instrumentation of the technique are discussed and its application for the phase analysis of multielement-multiphase samples such as minerals, aerosols, and archaeological materials is demonstrated.

Oxidation and hydrogenation of quasicrystals

Abstract Quasicrystals are often regarded as Hume-Rothery phases stabilized at different electron concentrations e/a . Concentration changes during oxidation or hydrogenation are expected to displace the electron concentration out of the stability range. Therefore, these processes might be accompanied by phase transformations. Oxidation experiments with Al 63 Cu 25 FeCu 12 icosahedral quasicrystals reveal only in the early stages the formation of a smooth oxide layer accompanying reconstruction of the surface. No phase transformation due to oxygen diffusion into the quasicrystalline structure could be detected. During further oxidation the formation of oxide nodules has been observed rather than a homogeneous thickening of the oxide layer. Heterogeneous phase transformation into λ-Al 13 Fe 4 indicates additional segregation effects. i -Zr 69.5 Ni 12 Cu 11 Al 7.5 quasicrystals can absorb large amounts of hydrogen which leads to an increase of the quasilattice constant by 10% and microstructural changes of so far unknown character: weakening of the contrast of quasicrystals in transmission electron microscopy (TEM) as well as a disappearance of the weak diffraction spots in the corresponding diffraction pattern.

Kinetics of hydration‐dehydration reactions considered as solid transformations

AbstractA mechanism is proposed for the dehydration‐rehydration process in solid inorganic salts, and model rate equations are derived and applied to the observed behavior of potassium carbonate. Quantitative expressions for the effect of pressure on the reaction rates are derived using basic principles from nucleation and heterogeneous phase transformation theory. Model equation predictions agree with experimental dehydration and rehydration rate data at all but extreme pressures. The basic rate equation is also used to interpret the data of Eckhardt and Flanagan (1964) for the effect of pressure on the dehydration of manganous formate dihydrate. The mechanism on which the model equations are based is also consistent with the observed effects of cycling and of high temperature pretreatment on the K2CO3 rehydration rate.

Paraequilibrium and other Restricted Equilibria

ABSTRACTAfter a review of the historic development of the concept of paraequilibrium in the field of alloyed steels, the basic principles of restricted thermodynamic equilibrium at a migrating interface during phase transformations are outlined. As a demonstration they are first applied to classical nucleation. In addition to the case of paraequilibrium they are then applied to adiabatic and to isochoric conditions. It is argued that a full equilibrium model, with a spike of the intensive property in front of the migrating interface may be more realistic. It is shown that the full equilibrium conditions require about twice as much undercooling or underpressure as the restricted equilibrium conditions because of the consumption of driving force in the spike. The concept of restricted equilibrium only applies to heterogeneous phase transformations. For homogeneous reactions, the same experimental conditions will give full equilibrium.

Nucléation sur la surface d'un fluide de petites dimensions

The nucleation on liquid surfaces may be considered as a general case of heterogeneous phase transformation. In the present paper two special cases of nucleation on a curved liquid surface are studied theoretically. 1) Nucleation from the vapour phase on foreign drops. Crystallization of small liquid droplets if the wetting of the nuclei by the melt is incomplete. In the first case it results that foreign liquid particles are more efficient in stimulating the nucleation, than solid ones (by equal adhesion energy). The influence of droplet size on the crystallization shows some particularities, e.g. the possibility of formation of crystal nuclei above the melting point and the existence of a droplet size effect also for crystallization without volume change.