Diffusionless Process Research Articles

Rapid Phase Transitions of Thermotropic Glycolipid Quasicrystal and Frank-Kasper Mesophases: A Mechanistic Rosetta Stone.

Experimental results are presented that serve to lower the barrier for developing the science and technology of non-classical thermotropic glycolipid mesophases, which now include dodecagonal quasicrystal (DDQC) and Frank-Kasper (FK) A15 and σ mesophases that can be produced under mild conditions from a versatile class of sugar-polyolefin conjugates. By employing "alloys" comprised of mono- and disaccharide-polyolefin conjugates, and optionally with vitamin E as a small molecule phase modulator, we report the spontaneous formation of stable A15 mesophases at ambient temperature. We further document a rich thermotropic phase map that includes DDQC, A15, and σ mesophases of tunable periodicity that are connected through rapid thermotropic phase transitions as a function of increasing temperature in the order: liquid-like packing (LLP)→DDQC → A15→σ→ disorder. This first direct observation of a rapid thermotropic A15→σ phase transition provides support for a diffusionless martensitic process proceeding through strain-induced introduction of planar defects into the A15 lattice.

Rapid Phase Transitions of Thermotropic Glycolipid Quasicrystal and Frank‐Kasper Mesophases: A Mechanistic Rosetta Stone

AbstractExperimental results are presented that serve to lower the barrier for developing the science and technology of non‐classical thermotropic glycolipid mesophases, which now include dodecagonal quasicrystal (DDQC) and Frank–Kasper (FK) A15 and σ mesophases that can be produced under mild conditions from a versatile class of sugar‐polyolefin conjugates. By employing “alloys” comprised of mono‐ and disaccharide‐polyolefin conjugates, and optionally with vitamin E as a small molecule phase modulator, we report the spontaneous formation of stable A15 mesophases at ambient temperature. We further document a rich thermotropic phase map that includes DDQC, A15, and σ mesophases of tunable periodicity that are connected through rapid thermotropic phase transitions as a function of increasing temperature in the order: liquid‐like packing (LLP)→DDQC → A15→σ→ disorder. This first direct observation of a rapid thermotropic A15→σ phase transition provides support for a diffusionless martensitic process proceeding through strain‐induced introduction of planar defects into the A15 lattice.

Ultrafast olivine-ringwoodite transformation during shock compression

Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions. These collisions were expected to occur between kilometre-sized asteroids, generating transient high-pressure states lasting for several seconds to facilitate mineral transformations across the relevant phase boundaries. However, their mechanisms in such a short timescale were never experimentally evaluated and remained speculative. Here, we show a nanosecond transformation mechanism yielding ringwoodite, which is the most typical high-pressure mineral in meteorites. An olivine crystal was shock-compressed by a focused high-power laser pulse, and the transformation was time-resolved by femtosecond diffractometry using an X-ray free electron laser. Our results show the formation of ringwoodite through a faster, diffusionless process, suggesting that ringwoodite can form from collisions between much smaller bodies, such as metre to submetre-sized asteroids, at common relative velocities. Even nominally unshocked meteorites could therefore contain signatures of high-pressure states from past collisions.

Electron Transfer Kinetics at Polysulfone-Copper Oxide Metalloplastic Nanocomposite Surface

The study evaluated the electrochemical charge transfer kinetics at two polysulfone-copper oxide polymeric semi-conducting nanocomposite electrode surfaces (compact and thin-film) in the presence of phosphate ions. The materials were characterized using scanning electron microscopy (SEM), energy dispersive x-ray (EDS) and cyclic voltammetry (CV). The results showed that both materials followed Laviron’s prediction of the diffusionless electrochemical process leading to variable estimation of transfer coefficients and rate constants. Furthermore, the rate of electron transfer at each surface varied with the scan rate while the thin-film electrode showed better anodic transfer rate than the compact electrode. Significantly, the materials showed the transition from the adsorbed species electrolysis to diffused species electrolysis in a vice versa manner when the change in the peak potentials was varied with the logarithm of the scan rates. Transfer coefficients between 0.8 and 1.0 and rate constants between 6.0 x 10-3 - 3.5 x 100 s-1 were obtained within the experimental conditions. The materials generally showed the capability to split hydrogen peroxide to oxygen gas at room temperature. The significance of the study on the application of the materials for various functions such as sensors and fuel cell fabrication was discussed. Figure 1

Fast crystal growth at ultra-low temperatures

It is believed that the slow liquid diffusion and geometric frustration brought by a rapid, deep quench inhibit fast crystallization and promote vitrification. Here we report fast crystal growth in charged colloidal systems under deep supercooling, where liquid diffusion is extremely low. By combining experiments and simulations, we show that this process occurs via wall-induced barrierless ordering consisting of two coupled steps: the step-like advancement of the rough interface that disintegrates frustration, followed by defect repairing inside the newly formed solid phase. The former is a diffusionless collective process, whereas the latter controls crystal quality. We further show that the intrinsic mechanical instability of a disordered glassy state subject to the crystal growth front allows for domino-like fast crystal growth even at ultra-low temperatures. These findings contribute to a deeper understanding of fast crystal growth and may be useful for applications related to vitrification prevention and crystal-quality control.

Looking deeper: Decoding the core structure of a micron-sized S-ZVI particle

• The diffusion of sulfur to the core of the micron ZVI was confirmed for the first time. • The inner core composition of the micron ZVI was confirmed and quantified. • XRPD and XANES analysis pointed out that about 12.5% iron was transformed to FeS x . Although it has been widely accepted that the sulfidation of zero-valent iron (ZVI) represents a typical diffusionless process, which mainly happens on the surface of the ZVI particle and results in a FeS x @Fe core-shell structure, such an assumption has never been experimentally proved due to the difficulties in probing the interior domains of the large particles, especially those in micro size. In this work, we demonstrate that the sulfidation process not only takes place on the surface but also synchronizes throughout the entire body of the micron-sized ZVI, which was confirmed by the close inspection of the cross-section of a finely sliced ZVI particle. Coupled with the X-ray powder diffraction and X-ray absorption near-edge structure analysis, it is revealed that the whole S-ZVI particle consists of a mixed lattice of the dominant metallic iron (87.5%) and minor iron sulfides (12.5%), according to the established averaged oxidation state for iron. This work is the first to unambiguously reveal the compositional information of the micron-sized S-ZVI particles, especially the inner components that are typically difficult to reach. It clarified a long-time misunderstanding that the sulfidation process on ZVI is initiated from the surface and continuously undergoes a diffusion pathway, reaching to the center of the particle.

The Instability of Monolayer-Thick PbSe on VSe2

Two-dimensional monolayers derived from 3D bulk structures remain a relatively unexplored class of materials because of the challenge of stabilizing nonepitaxial interfaces. Here, we report an unusual reconstruction during the deposition of precursors when targeting the synthesis of heterostructures with an odd number of PbSe monolayers. Multilayer elemental precursors of Pb|Se + V|Se were deposited to have the correct number of atoms to form [(PbSe)1+δ]q(VSe2)1 where q is the number of PbSe monolayers in the heterostructure. Structural analysis of the self-assembled precursor via X-ray reflectivity, X-ray diffraction, and HAADF-STEM suggests three different behaviors upon deposition. Precursors with q ≥ 7 and even values of q have the targeted nanoarchitectures after deposition, which are maintained as the products are self-assembled through a near diffusionless process. Significant lateral surface diffusion occurred during the deposition of precursors with q = 1, 3, and 5, resulting in the precursor to have a different nanoarchitecture than targeted. Additional perpendicular long-range diffusion occurs during self-assembly of these precursors, resulting in different final products than targeted. Density functional theory (DFT) calculations of PbSe blocks show that the odd-numbered layers are less stable than the even-numbered layers, which suggests an energetic driving force for the observed rearrangement. This work highlights the importance of understanding the reaction mechanism when attempting to prepare 2D layers of constituents with bulk 3D structures.

Transformations in CrFeCoNiCu High Entropy Alloy Thin Films during In-Situ Annealing in TEM

In-situ TEM-heating study of the microstructural evolution of CrFeCoNiCu high entropy alloy (HEA) thin films was carried out and morphological and phase changes were recorded. Post annealing investigation of the samples was carried out by high resolution electron microscopy and EDS measurements. The film is structurally and morphologically stable single phase FCC HEA up to 400 °C. At 450 °C the formation of a BCC phase was observed, however, the morphology of the film remained unchanged. This type of transformation is attributed to diffusionless processes (martensitic or massive). From 550 °C fast morphological and structural changes occur, controlled by volume diffusion processes. Fast growing of a new intermetallic phase is observed which contains mainly Cr and has large unit cell due to chemical ordering of components in <100> direction. The surface of the films gets covered with a CrO-type layer, possibly contributing to corrosion resistance of these.

Resolving the diffusionless transformation process of twinning in single crystal plasticity theory

This paper outlines a single crystal plasticity theory for the mesoscale resolution of the diffusionless transformation process of deformation twinning. Unlike prevalent crystal plasticity models of lattice transformation that characterize volume fraction evolution of coexisting parent and product lattices at a material point, our model alternates between two deformation gradient decompositions, depending on whether or not the lattice is transforming, so that only a single lattice exists at a material point at all times. This approach permits a proper spatiotemporal resolution of the transformation, and further captures its associated residual lattice distortions. Our formulation was implemented as a VUMAT subroutine on the finite element solver ABAQUS\\Explicit and used to model the twinning of a magnesium single crystal that was loaded under compression at different angles. As verified against multiple experiments from the literature, our model successfully characterizes for moderate strain: (a) the anisotropic stress-strain behavior of the Mg crystal, (b) the resulting spatial cross-hatch pattern of spindle shaped twins, and (c) the natural completion times of the twinning transformation. These results indicate that our mesoscale theory is capable of an in-depth spatiotemporal investigation of twinning and can be potentially extended to similar diffusionless transformation processes in single crystals.

Modelling and design of new stainless-steel welding alloys suitable for low-deformation repairs and restoration processes

The plasticity associated with low-temperature martensitic transformation can be exploited to reduce the stresses developed due to thermal contraction of the weldments. The key feature of stress-mitigating welding alloys is their transformation from austenite to martensite at low temperatures e.g. ideally close to ambient temperature. Thermodynamics databases (MTDATA and SGTE) were used to model and design new welding alloys with low martensitic transformation temperatures (Ms) around 200 °C. The modelling, conducted in this work, was based on this assumption that martensitic transformation starts at a certain temperature when the free-energy change for austenite to transform to ferrite reaches a critical value. Since martensitic transformation is a diffusion-less process, the change in free-energy vs. temperature was calculated for the austenite and ferrite phases with the same composition.Three prototype welding alloys, CamAlloys 4 & 5 and OpenAlloy 1, were successfully designed and made in the University of Cambridge (UK) and the Open University (UK). The design of these alloys was purely based on thermodynamics equations. Comprehensive characterisation, examinations and mechanical tests showed this family of alloys could substantially reduce contraction-induced deformations in stainless steel weldments. One of the applications of these alloys is in the repair and restoration of damaged stainless-steel components.

Critical Driving Forces for Formation of Bainite

An empirical equation for predicting bainite start temperatures of steels was recently derived by starting from binary Fe-C alloys and continuing with ternary Fe-C-M alloys. This result is now illustrated with a family of BS lines in a T,C diagram for a series of constant Mn contents. The critical driving force for the formation of ferrite is calculated for diffusionless or diffusional processes, and these quantities are used as dependent variables with carbon content or temperature as independent variables. Negative critical driving forces are predicted for a diffusionless process in binary Fe-C alloys, showing that this process cannot apply to the formation of bainite. The critical driving force for a diffusional process increases strongly with decreasing temperature and increasing carbon content. Mn and Ni, contrary to Cr, Mo and Si, have remarkably small effects on this critical driving force. The results are discussed by imagining that the magnitude of the critical driving force is governed by the height of an energy barrier that must be surmounted during growth. It is modeled as completely determined by the alloy composition. It is represented with an equation evaluated by fitting to the recent empirical equation and describing the carbon dependence of the barrier.

A new high-pressure form of Mg2SiO4 highlighting diffusionless phase transitions of olivine

High-pressure polymorphism of olivine (α-phase of Mg2SiO4) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg2SiO4). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.

Griffith phase-induced ferromagnetic diffuse phase transition

A ferromagnetic transition is generally regarded as a diffusionless process, exhibiting a sharp change of magnetic properties with temperature. In this paper, however, we report a ferromagnetic diffuse phase transition (FDPT) induced by Griffiths phase in La1 − xSrxMnO3 (0·06 ≤ x ≤ 0·2). Such a FDPT experiences a broad transition temperature range, resulting in temperature-controlled ferromagnetic cluster owing to the existence of Griffiths phase during the diffusion process. The transition temperature range calculated from M–T curve is as large as 146 K for the FDPT sample. Further analysis suggests that this novel FDPT has the same scenario with that in the ferroelectric relaxor, where the ferroelectric clusters are embedded in the paraelectric matrix.

Calculations of Phase Transformations in Welding Simulations

In the paper the principle of welding simulation is presented and the methods of solution of phase transformation are described. The first part characterizes elementary equations of heat transient solution, boundary conditions during welding simulation (prescribing moving heat flux, convection, radiation). The methods of phase transformations’ solution are described for diffusion processes as well as diffusionless processes.

Molecular Dynamics Simulations of Shock Compressed Graphite

We present molecular dynamic simulations of the shock compression of graphite with the LCBOPII potential. The range of shock intensities covers the full range of available experimental data, including near-terapascal pressures. The results are in excellent agreement with the available DFT data and point to a graphite-diamond transition for shock pressures above 65 GPa, a value larger than the experimental data (20 to 50 GPa). The transition mechanism leads preferentially to hexagonal diamond through a diffusionless process but is submitted to irreversible regraphitization upon release: this result is in good agreement with the lack of highly ordered diamond observed in post-mortem experimental samples. Melting is found for shock pressures ranging from 200 to 300 GPa, close to the approximate LCBOPII diamond melting line. A good overall agreement is found between the calculated and experimental Hugoniot data up to 46% compression rate.

Domain Models of Ferromagnetic Shape Memory Materials

AbstractMagnetic shape‐memory materials are multiferroics that combine ferroelastic and ferromagnetic order. The diffusionless transformation processes in these materials unroll by the creation of crystallographic domains or twinned microstructures that are coupled to magnetic domain patterns. Miniaturization of the crystallographic domains to the scale of lattice unit cells is afforded by the concept of adaptive modulated martensite. A ferroelastic model for these crystallographic domain patterns is introduced for the exemplary case of the Ni2MnGa Heusler alloy. The pseudo‐microscopic model incorporates data from ab initio calculations and phonon dispersion. The twin‐boundary energy for the diffuse wall of this model are very low of the order 1 mJ · m−2 fulfilling a key requirement for the formation of adaptive martensite in this material. An extension of the Ginzburg–Landau phase‐field model for large strain gradients is introduced to model atomically sharp twin‐boundaries.

Biologically Inspired Molecular Assembly Lines

This paper proposes a biologically inspired 'molecular assembly line' — an externally programmable polymeric chain along which molecules are shuttled between chain sites along an arbitrary pathway. Our hope is to construct a scaffold that mimics biological enzymes known as polyketide synthases in their ability to physically hand-off molecules between assembly sites or domains, thereby making all chemical reactions domain-specific and diffusionless processes. It will be shown that a system with three wavelength selective chemical interactions allows a 'shuttle' on the assembly line to move bi-directionally and be bi-stable in the absence of inputs. Potential applications of the molecular assembly line in programmed assembly and molecular electronics will be discussed, as well as initial experimental work towards building a molecular assembly line driven by UV-light.

Martensitic transformations in ‘unfamiliar’ systems

Abstract In stiff engineering materials, martensitic transformations are shear-dominant diffusionless processes, and their crystallographic features can be successfully predicted using the phenomenological theory of martensite crystallography (PTMC). Recently, an alternative approach has been developed wherein the interface structure is modelled in terms of interfacial dislocations. This model, referred to as the topological model (TM), provides insight into the transformation process and identifies a set of five criteria that must be met for a transformation to be diffusionless. In the case of stiff engineering materials, the crystallographic predictions of the PTMC and TM are very similar. However, the TM enables a broader range of transformations to be treated, and two examples of ‘unusual’ martensites are presented here. The first is a diffusionless transformation in a small elastically soft protein crystal. The second is a transformation in a prospective high-temperature engineering alloy which exhibits the characteristic crystallographic features of martensite but where concomitant diffusion occurs. In this case, four of the five criteria mentioned above that relate to conservation of substitutional atomic sites are satisfied.

Martensitic transformations in ?unfamiliar? systems

In stiff engineering materials, martensitic transformations are shear-dominant diffusionless processes, and their crystallographic features can be successfully predicted using the phenomenological theory of martensite crystallography (PTMC). Recently, an alternative approach has been developed wherein the interface structure is modelled in terms of interfacial dislocations. This model, referred to as the topological model (TM), provides insight into the transformation process and identifies a set of five criteria that must be met for a transformation to be diffusionless. In the case of stiff engineering materials, the crystallographic predictions of the PTMC and TM are very similar. However, the TM enables a broader range of transformations to be treated, and two examples of ‘unusual’ martensites are presented here. The first is a diffusionless transformation in a small elastically soft protein crystal. The second is a transformation in a prospective high-temperature engineering alloy which exhibits the characteristic crystallographic features of martensite but where concomitant diffusion occurs. In this case, four of the five criteria mentioned above that relate to conservation of substitutional atomic sites are satisfied.

A model of martensite formation in terms of interfacial defect mechanisms

The objective of this work is to demonstrate the consistency between a new model of martensite formation and the classical theory of martensite crystallography. Recent developments in the understanding of interfacial defects have enabled their topological character, i.e. Burgers vectors and step heights, to be defined rigorously. In addition, the material flux associated with their motion along an interphase interface has also been quantified. On this basis, a defect model has been developed in which an initially strained nucleus evolves via a diffusionless process into a martensite plate. A bcc to hcp transformation is illustrated, demonstrating the crystallographic equivalence of the dislocation model and the phenomenological theory.