Phase Transformation Boundary Research Articles

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO2.5 and the oxygen ordered phase perovskite SrCoO3. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.

Grain boundary mediated plasticity: On the evaluation of grain boundary migration - shear coupling

The solutions of the Frank-Bilby equation for the dislocation content of symmetric tilt grain boundaries were analyzed for a number of grain and twin boundaries. Based on this analysis, the geometrical model of grain boundary migration - shear coupling was extended to evaluate the coupling factor (amount of shear) for twin and grain boundaries in various crystal structures. The coupling factors calculated according to the proposed general formula are in excellent agreement with respective experimental and simulation data known from literature. For a given grain/twin boundary the coupling factor was found to result from a combination of two elementary coupling modes, the balance between which is constituted by an introduced weighting factor. Hence, the coupling factor (or the twinning shear) depends not only on the tilt angle of the grain/twin boundary, but also on the weighting factor, which may vary for crystallographically equivalent boundaries. With respect to deformation twinning, the weighting factor is intimately linked to the amount and direction of twinning shear produced by the boundary. A variation of the weighting factor for the same boundary can be interpreted as a result of a structural change of the grain/twin boundary, i.e. grain boundary phase transformation.

Grain boundary wetting-related phase transformations in Al and Cu-based alloys. Review

The phase transformations can proceed not only between bulk phases but also in free surfaces, grain boundaries and interphase boundaries. In this review we consider the grain boundary phase transformations in Cu- and Al-based alloys. In particular, among those transformations are the transitions between compete and incomplete grain boundary wetting. The wetting phase can be either liquid or solid. If the wetting phase is solid, the portion of wetted grain boundaries can increase also with decreasing temperature. The transition itself can be discontinuous (of the first order) or continous (of the second order). The thin layers of grain boundary phases (called also the grain boundary complexions) can occur in the conditions (temperature, pressure and concentration) where only one volume phase can exist. The phenomenon of the pseudo-incomplete (or pseudo-partial) grain boundary wetting is also discussed. In this case the non-zero grain boundary contact angle coexists with thin layer of grain boundary phase. The new lines of respective grain boundary phase transformations appear in the conventional phase diagrams for three-dimensional phases. The grain boundary phase transitions can strongly influence the properties of grain boundaries themselves and those of polycrystals as a whole. For example, the presence of grain boundary layers can increase the plasticity (if the phase is ductile) or decrease it (of the grain boundary phase is brittle). The influence of grain boundary phase transitions increases with decreasing grain size and becomes critical in nanograined materials.

Grain boundary phase transformations in PtAu and relevance to thermal stabilization of bulk nanocrystalline metals

There has recently been a great deal of interest in employing immiscible solutes to stabilize nanocrystalline microstructures. Existing modeling efforts largely rely on mesoscale Monte Carlo approaches that employ a simplified model of the microstructure and result in highly homogeneous segregation to grain boundaries. However, there is ample evidence from experimental and modeling studies that demonstrates segregation to grain boundaries is highly non-uniform and sensitive to boundary character. This work employs a realistic nanocrystalline microstructure with experimentally relevant global solute concentrations to illustrate inhomogeneous boundary segregation. Experiments quantifying segregation in thin films are reported that corroborate the prediction that grain boundary segregation is highly inhomogeneous. In addition to grain boundary structure modifying the degree of segregation, the existence of a phase transformation between low and high solute content grain boundaries is predicted. In order to conduct this study, new embedded atom method interatomic potentials are developed for Pt, Au, and the PtAu binary alloy.

GLAG theory for superconducting property variations with A15 composition in Nb3Sn wires

We present a model for the variation of the upper critical field Hc2 with Sn content in A15-type Nb-Sn wires, within the Ginzburg-Landau-Abrikosov-Gor’kov (GLAG) theory frame. Hc2 at the vicinity of the critical temperature Tc is related quantitatively to the electrical resistivity ρ, specific heat capacity coefficient γ and Tc. Hc2 versus tin content is theoretically formulated within the GLAG theory, and generally reproduces the experiment results. As Sn content gradually approaches the stoichiometry, A15-type Nb-Sn undergoes a transition from the dirty limit to clean limit, split by the phase transformation boundary. The H-T phase boundary and pinning force show different behaviors in the cubic and tetragonal phase. We dipict the dependence of the composition gradient on the superconducting properties variation in the A15 layer, as well as the curved tail at vicinity of Hc2 in the Kramer plot of the Nb3Sn wire. This helps understanding of the inhomogeneous-composition inducing discrepancy between the results by the state-of-art scaling laws and experiments.

Inverse Thermal Analysis of Titanium GTA Welds Using Multiple Constraints

Inverse thermal analysis of titanium gas-tungsten-arc welds using multiple constraint conditions is presented. This analysis employs a methodology that is in terms of numerical-analytical basis functions for inverse thermal analysis of steady-state energy deposition in plate structures. The results of this type of analysis provide parametric representations of weld temperature histories that can be adopted as input data to various types of computational procedures, such as those for prediction of solid-state phase transformations. In addition, these temperature histories can be used to construct parametric function representations for inverse thermal analysis of welds corresponding to other process parameters or welding processes whose process conditions are within similar regimes. The present study applies an inverse thermal analysis procedure that provides for the inclusion of constraint conditions associated with both solidification and phase transformation boundaries.

Inverse Thermal Analysis of a Titanium Laser Weld Using Multiple Constraint Conditions

Inverse thermal analysis of a titanium laser weld using multiple constraint conditions is presented. This analysis employs a methodology that is in terms of numerical-analytical basis functions for inverse thermal analysis of steady-state energy deposition in plate structures. The results of this type of analysis provide parametric representations of weld temperature histories that can be adopted as input data to various types of computational procedures, such as those for prediction of solid-state phase transformations. In addition, these temperature histories can be used to construct parametric-function representations for inverse thermal analysis of welds corresponding to other process parameters or welding processes whose process conditions are within similar regimes. The present study extends an inverse thermal analysis procedure applied in previous studies. This extension provides for the inclusion of constraint conditions associated with both solidification and phase transformation boundaries.

Water in the slab: A trilogy

Abstract The geological and geophysical phenomena associated with water in the slab can be grouped into three different categories: those related to 1) the storage of water in the oceanic lithosphere at the surface, 2) the subduction of a hydrated slab and 3) its dehydration that ultimately leads to mantle regassing. Widespread hydration of the uppermost oceanic lithosphere occurs at mid-ocean ridges and more pervasively at the trench–rise system in response to bending-related fracturing and faulting. The amount of water stored in the upper lithosphere appears to be proportional to the spreading slowness and amount of bending. Hydrothermal alteration of the oceanic plate may also occur more locally along transform margins, fracture zones and volcanic ridges. Subduction of a hydrated slab should be observed from a reduction of seismic velocities, anomalous Vp/Vs ratios and seismic anisotropy in its uppermost portions, and shift of the main phase transformation boundaries characteristic of the mantle. Because of the increased buoyancy and weakness, hydrated or wet slabs should tend to stagnate over the 660 km discontinuity, favoring layered mantle convection patterns. Slab dehydration takes place according to the plate thermal regime function of the slab age and sinking velocity. Hydrous minerals in the oceanic crust and mantle are stable down to a maximum of 300 km and 1200 km depth, respectively, after which minor amounts of water can be retained in nominally anhydrous minerals. There is abundant geophysical evidence for dehydration of the slab crust and sub-Moho mantle, while fragmentary and often indirect evidence supports the presence of water in the lower plane of the Double Seismic Zone and at depths > 300 km.

On the effect of boron on the mechanical properties of a new polycrystalline superalloy

Boron is used as a grain boundary strengthener in the nickel-based superalloys, but the reasons for its effect and the optimum quantities which need to be added are not well understood. Recently, some of the authors have developed a new corrosion-resistant single crystal superalloy for power generation applications which has a good balance of mechanical properties and resistance to environmental degradation. Here, this same alloy system is studied but in the polycrystalline state with additions of boron and carbon. The influence of boron on creep behaviour has been quantified and rationalised, with an emphasis on the grain boundary phase transformations which occur. NanoSIMS is utilised to investigate the segregation of boron at grain boundaries, with SEM used to characterise the effect of boron on the precipitation of Cr 23 C 6 type carbides at grain boundaries. When the boron content increases, the agglomeration of M 23 C 6 carbides at grain boundaries is inhibited. Uniformly distributed discrete M 23 C 6 carbides are observed at higher boron content, whereas a deleterious film of M 23 C 6 along the grain boundaries arises as the boron concentration is reduced. Boron promotes also the formation of γ′ layers at the grain boundaries.

Elasticity behavior, phonon spectra, and the pressure–temperature phase diagram of HfTi alloy: A density-functional theory study

The pressure-induced phase transition, elasticity behavior, thermodynamic properties, and P–T phase diagram of α, ω, and β equiatomic HfTi alloy are investigated using first-principles density-functional theory (DFT). The simulated pressure-induced phase transition of the alloy follows the sequence of α→ω→β, in agreement with the experimental results of Hf and Ti metals. Our calculated elastic constants show that the α and ω phases are mechanically stable at ambient pressure, while the β phase is unstable, where a critical pressure of 18.5GPa is predicted for its mechanical stability. All the elastic constants, bulk modulus, and shear modulus increase upon compression for the three phases of HfTi. The ductility of the alloy is shown to be well improved with respect to pure Hf and Ti metals. The Mulliken charge population analysis illustrates that the increase of the d-band occupancy will stabilize the β phase under pressure. The phonon spectra and phonon density of states are studied using the supercell approach for the three phases, and the stable nature of α and ω phases at ambient pressure are observed, while the β phase is only stable along the [110] direction. With the Gibbs free energy calculated from DFT-parametrized Debye model as a function of temperature and pressure, the phase transformation boundaries of the α, ω, and β phases of HfTi are identified.

Effect of Barium on Formation and Metastable Structure of Tricalcium Silicate

The effect of barium on the formation, structure and phase transition of tricalcium silicate (C3S) has been studied by X-ray powder diffraction combined with Rietveld method, Differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the low contents of barium was favorable the formation of C3S with up to 1.5% BaO. Higher amounts of BaO inhibited the C3S crystallization and lead to preferential formation of dicalcium silicate solid solution. Three triclinic forms T1, T2 and T3 of C3S were stabilized. The lattice parameters changed linearly to the barium content, however the discontinuity appeared at the phase transformation boundary, which follows the Vegards law. The crystallographic structure of T1 and T2 was similar to that of T3. The SiO44- tetrahedra were distorted in T1 and T2, while that was approximately regular tetrahedron in T3. The phase transitions of C3S were influenced by the incorporation of barium. It was believed that the stabilization of high temperature polymorphs of C3S was related to the lattice distortion caused by ionic substitution.

Hydrogen-Vacancy Effects in Pu-2 at. % Ga Alloys

ABSTRACTPlutonium and Pu-Ga alloys have been observed to have anomalous hydrogen solubility behavior, including a significant concentration dependence of hydrogen diffusivity in the dilute regime, a sharp drop off in the hydrogen solubility constant in the dilute regime, and a near complete absence of change in the Sieverts’ constant as the alloys are heated across phase transformation boundaries. We are investigating the possibility that a vacancy mechanism is responsible for this behavior. X-ray diffraction measurements show a 0.14% lattice contraction in Pu-2 at. % Ga alloys when they are charged with ~2 at. % hydrogen. The lattice re-expands when the hydrogen is removed. Density functional calculations show that increasing the number of hydrogen atoms associated with a vacant lattice site in Pu lowers the energy of the hydrogen-vacancy complex. These observations support the idea that vacancies are stabilized by hydrogen in the Pu lattice well beyond their thermal equilibrium concentration and could be responsible for the anomalous hydrogen response of Pu.

Grain boundary phase transformation in Cu–Co solid solutions

The present work examined the creep behavior of copper based solid solutions with cobalt at temperatures between 980°C and 1080°C and stresses lower than 0.2MPa. The samples were made from 18μm foil and were formed into cylinders. After the pre-annealing at 1000°C during about 30h the samples had a parquet structure. The experiments were performed in the hydrogen atmosphere. New equipment was designed for these measurements.The activation energy of pure copper creep was close to the activation energy of copper volume self-diffusion. Cu–Co solid solution creep rate was always lower than that of pure copper. It was shown that the creep activation energy in the relatively low temperature region was higher than in the high temperature region. The same behavior was typical for all studied solid solutions. The transition temperature was about 1030°C.It was proposed that such creep behavior was connected with grain boundary phase transformation and explanation was made in terms of interface controlled diffusional creep.

Coupled phase transformations and plasticity as a field theory of deformation incompatibility

The duality between terminating discontinuities of fields and the incompatibilities of their gradients is used to define a coupled dynamics of the discontinuities of the elastic displacement field and its gradient. The theory goes beyond standard translational and rotational Volterra defects (dislocations and disclinations) by introducing and physically grounding the concept of generalized disclinations in solids without a fundamental rotational kinematic degree of freedom (e.g. directors). All considered incompatibilities have the geometric meaning of a density of lines carrying appropriate topological charge, and a conservation argument provides for natural physical laws for their dynamics. Thermodynamic guidance provides the driving forces conjugate to the kinematic objects characterizing the defect motions, as well as admissible constitutive relations for stress and couple stress. We show that even though higher-order kinematic objects are involved in the specific free energy, couple stresses may not be required in the mechanical description in particular cases. The resulting models are capable of addressing the evolution of defect microstructures under stress with the intent of understanding dislocation plasticity in the presence of phase transformation and grain boundary dynamics.

Effects of Sintering Additives on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics by GPS

Silicon nitride ceramics were prepared by gas pressure sintering (GPS) with different sintering additives, including La2O3, Sm2O3 and Al2O3. Effect of sintering additives on the phase-transformation, microstructure and mechanical properties of porous silicon nitride ceramics was investigated. The results show that the reaction of sintering additives each other and with SiO2 had key effects on the phase-transformation, grain growing and grain boundaries. With 9MPa N2 atmosphere pressure, holding 1h at 1850°C, adding 10wt% one of the La2O3, Sm2O3, Al2O3, porous silicon nitride was prepared and the relative density was 78%, 72%, 85% respectively. The flexural strength was less than 500MPa, and the fracture toughness was less than 4.8MPam1/2. Dropping compounds sintering additives, such as La2O3+Al2O3, Sm2O3+Al2O3 effectively improves the sintering and mechanical properties. The relative density was 99.2% and 98.7% with 10wt% compounds sintering additives. The grain ratio of length to diameter was up to 1:8. The flexural strength was more than 900MPa, and the fracture toughness was more than 8.9MPam1/2.

Thermal evolution and grain boundary phase transformations in severely deformed nanograined Al–Zn alloys

The structure, phase composition and thermal evolution of binary Al–Zn alloys were studied before and after high-pressure torsion (HPT) in Bridgman anvils. On heating of HPT-deformed samples from room temperature to 300 °C, Zn grains dissolved, and a relatively fine-grained (Al) equilibrium solid solution formed. Differential scanning calorimetry curves reveal two-stage melting in the Al–Zn alloys studied, i.e., the melting of the (Al) solid solution starts 10–25 °C below the bulk solidus line. The effect is more pronounced in fine-grained samples. It is explained by the presence of layers of liquid-like phase in the (Al) grain boundaries (GB) between bulk and GB solidus lines. The new metastable GB solidus line appears in the (Al) single-phase region of the Al–Zn phase diagram, it can be compared with the metastable solvus lines for the formation of GP zones and α phases in the (Al)+Zn two-phase area.

Atomistic simulation of the effect of Ga on crack tip opening in Al bicrystals

Liquid metal embrittlement (LME) in the Al–Ga system is studied using molecular dynamics simulations. Crack tip opening loads are exerted on symmetric tilt Al bicrystals in the presence of liquid Ga. In general, the speed of crack propagation increases in the presence of Ga. However, there is significant variation in the dynamic behavior of the different boundaries studied here. We observe the formation of a sub-grain by grain boundary dissociation (grain boundary phase transformation) in [1 1 0] Σ9 and [1 1 0] Σ11 grain boundaries; this results in crack blunting even in the presence of Ga. In the Σ3 twin, the absence of alternating compressive and tensile stresses present in other boundaries results in fast Ga penetration after the crack path is formed. The structure of grain boundaries apart from thermodynamic considerations significantly affects LME.

Laser-heating diamond anvil cell studies of simple molecular systems at high pressures and temperatures

We report the application of new laser-heating techniques and sample preparation procedures for simple molecular materials (diatomic molecules and water) under high pressure in the diamond anvil cell (DAC). Both continuous and pulsed laser heating was employed. We probed the materials using Raman spectroscopy and also by analyzing the time evolution of the temperature of the metallic coupler that is used to absorb laser radiation and heat the sample. Raman measurements of H 2, D 2, N 2, H 2O and O 2 show a broadening of intramolecular vibrations at high P− T conditions, indicating a decreasing molecular lifetime, and hence suggest an increasing molecular dissociation. In diatomic molecules the intramolecular bonding can be further probed by observations of sidebands corresponding to vibrational transitions from excited states; the energies of these sidebands imply intramolecular potentials that become increasingly less anharmonic as pressure is increased. We also show that the pulsed heating technique combined with instantaneous radiative temperature measurements provides a useful tool for studies of thermochemical properties and phase transformation boundaries.

Study of the Ti-Al-Nb phase diagram

Differential thermal and X-ray diffraction analyses as well as optical and transmission electron microscopy were used to study orthorhombic Ti2AlNb-based alloys, such as Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb. These alloys in the initial state contain an orthorhombic O phase (Ti2AlNb) ordered in terms of all three elements. The differential thermal analysis was performed at different heating and cooling rates, namely, 40 and 80 K/min. The solidus and liquidus temperatures of the Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb alloys have been measured; these are T S = 1707 and T L = 1737°C, T S = 1703 and T L = 1730°C, and T S = 1694 and T L = 1728°C, respectively. The temperature boundaries of phase transformations occurring in these alloys have been determined. The activation energy of phase transformations has been calculated using the Johnson-Mehl-Avrami equation. The results obtained are compared with the available literature data. Causes of the discrepancy between equilibrium phase diagrams of the Ti-Al-Nb system reported by different authors are analyzed.

Structural phase transformation and phase boundary∕stability studies of field-cooled Pb(Mg1∕3Nb2∕3O3)–32%PbTiO3 crystals

Structural phase transformations in (001)-oriented (1−x)Pb(Mg1∕3Nb2∕3O3)–32%PbTiO3 crystals have been investigated by x-ray diffraction. A C→T→MC sequence was observed in both the field-cooled and zero-field-cooled conditions. Most interestingly, an anomalous increase in the C→T phase boundary with increasing field has been observed, which is seemingly a common characteristic of crystals whose compositions are in the vicinity of the morphotropic phase boundary, irrespective of the width of the T and MC phase regions.