Phase Transformations In Metals Research Articles

Prediction of Secondary‐Dendrite Arm Spacing for Binary Alloys by Means of a Phase‐Field Model

The mechanical properties and performance of metal materials depend on the intrinsic microstructures in these materials. In order to develop engineering materials as expected and to enable design with multifunctional materials, it is essential to predict the microstructural patterns, such as size, shape, and spacing of the dendritic structures observed in solidified metals. In materials science and related areas, the phase‐field model is widely used as one of the powerful computational methods to simulate the formation of complex microstructures during solidification and phase transformation of metals and alloys. In the present study, the secondary‐arm spacing for Fe–C binary alloys is numerically predicted using a phase‐field model in a two‐dimensional domain. When compared both with data by Ode et al. and with experimental data, the arm spacing predicted in the present work showed excellent agreement. Our estimates are performed at the late stage of growth. The change in arm spacing is examined both by changes of cooling rates and of local solidification time. A relation between material properties and model parameters is presented. Two‐dimensional simulations produced dendrites that are similar to the ones found in experiments reported in the literature. Through numerical examples, applicability of the phase‐field model to the problems of secondary‐dendrite arm spacing in Fe–C alloys is demonstrated.

Can a carbon nano-coating resist metallic phase transformation in silicon substrate during nanoimpact?

Nanomechanical response of a silicon specimen coated with a sp3 crystalline carbon coating (1.8nm thickness) was investigated using MD simulation. A sharp conical rigid tip was impacted at the speed of 50m/s up to a depth of ~80% of the coating thickness. Unlike pure silicon specimen, no metallic phase transformation was observed i.e. a thin coating was able to resist Si-I to Si-II metallic phase transformation signifying that the coating could alter the stress distribution and thereby the contact tribology of the substrate. The stress state of the system, radial distribution function and the load–displacement curve were all aligned with above observations.11Readers are requested to refer to the web based version for correct interpretation of the color legends.

Heat Transfer in Semitransparent Gas-Permeable Materials with the Spectral Dependence of Optical Properties

Introduction. Radiative heat transfer in porous insulating materials at high temperatures was investigated in [1–3], and compound heat transfer under unsteady-state conditions in a material was investigated in [4]. Radiation transfer in the gray medium approximation substantially limits the adequacy of the model used in [5]. The present work, which is a continuation of [5], was performed by us with a view to account for the dependence of the optical properties of gas-permeable materials on radiation wavelength. The article describes a computational investigation of compound heat and mass transfer in a semitransparent gas-permeable material in the presence of phase transition and oxygen chemisorption in pores. An innovation in the formulation of the problem is accounting for the interaction of the radiation fi elds of optical inhomogeneities (pores and the particles intruded into a material) when calculating the material�c s optical properties. The optical properties of particles are usually calculated by the Mie theory [6], but at high concentrations the radiation fi elds of individual particles overlap, which is not taken into account in the Mie theory and leads to a large error in the calculated properties. To avoid the occurrence of such an error, in the present work the optical properties at a high particle concentration are calculated by means of the Maxwell–Garnett approximation [7] that accounts for the interaction of the radiation fi elds of the indicated inhomogeneities. Physical Model. We studied quartz that contained pores fi lled with air and vanadium dioxide particles in which 2 nd -order dielectric–metal phase transformations occur with heat liberation during cooling and heat absorption upon heating. Heat exchange with the surrounding medium proceeds by radiation and convection. We consider the processes of heat and mass transfer in a thin gas-permeable plane layer in one-dimensional approximation under conditions where radiative heat transfer is comparable with heat conduction in the solid skeleton and gas convection in the pores. Under conditions of heating or cooling, 2 nd -order dielectric–metal phase transition occurs in the vanadium dioxide particles, which is accompanied by oxygen chemisorptions and by a change in the crystalline structure of particles, with simultaneous liberation or absorption of heat in the material. These processes can be modeled by volumetric heat and mass sources that are introduced into the effective heat capacity of the material [5]. The absence of large pressure gradients in the gas leads to a relatively slow gas fi ltration, which allows us to assume that thermal equilibrium was achieved between the solid skeleton and the gas. The problem formulated is solved in the following approximation: the material under study is considered as a set of three continua inserted into one another. The components of this set are the continua of air in the pores, quartz (the material skeleton), and of the vanadium dioxide particles introduced into the skeleton. The thermophysical properties of continuous components are the effective properties in relation to the corresponding properties of a real material. Gas fi ltration in a material is modeled as the motion of a continuous air component characterized by a velocity fi eld and by the fi eld of specifi c air fl ow rates. Radiative transfer in the system considered occurs through the solid skeleton and gas in the pores. In modeling the radiative transfer, we took into account the processes of the radiation absorption, emission, and scattering. We assumed that the optical properties of a material were dependent on the wavelength. The effective heat capacity of the material depends on temperature and

Identification of Weld Residual Stresses Using Diffraction Methods and their Effect on Fatigue Strength of High Strength Steels Welds

It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.

Copper Sludge from Printed Circuit Board Production/Recycling for Ceramic Materials: A Quantitative Analysis of Copper Transformation and Immobilization

The fast development of electronic industries and stringent requirement of recycling waste electronics have produced a large amount of metal-containing waste sludge. This study developed a waste-to-resource strategy to beneficially use such metal-containing sludge from the production and recycling processes of printed circuit board (PCBs). To observe the metal incorporation mechanisms and phase transformation processes, mixtures of copper industrial waste sludge and kaolinite-based materials (kaolinite and mullite) were fired between 650 and 1250 °C for 3 h. The different copper-hosting phases were identified by powder X-ray diffraction (XRD) in the sintered products, and CuAl2O4 was found to be the predominant hosting phase throughout the reactions, regardless of the strong reduction potential of copper expected at high temperatures. The experimental results indicated that CuAl2O4 was generated more easily and in larger quantities at low-temperature processing when using the kaolinite precursor. Maximum copper transformations reached 86% and 97% for kaolinite and mullite systems, respectively, when sintering at 1000 °C. To monitor the stabilization effect after thermal process, prolonged leaching tests were carried out using acetic acid with an initial pH value of 2.9 to leach the sintered products for 20 days. The results demonstrated the decrease of copper leachability with the formation of CuAl2O4, despite different sintering behavior in kaolinite and mullite systems. This study clearly indicates spinel formation as the most crucial metal stabilization mechanism when sintering copper sludge with aluminosilicate materials, and suggests a promising and reliable technique for reusing metal-containing sludge as ceramic materials.

Cases of the coincidence between the plasticity drop and the temperature ranges of superplasticity and its elements in iron and steels

The natures of a plasticity drop and superplasticity, which are usually studied separately, are analyzed. Both anomalies are shown to occur often at the temperatures of first- and second-order phase transformations in metals. These transformations in iron and steels bring metal atoms into an activated state. In materials free of embrittling impurities, this state initiates superplasticity or its elements; in the presence of impurities (oxygen, hydrogen), this state enhances the interaction between impurity and iron atoms and, thus, causes a plasticity drop.

Electronic structure and optical properties of CuInO2 under equibiaxial strain

An investigation on the electronic and optical properties of CuInO2 has been carried out under in-plane equibiaxial strain using ab initio calculations based on density functional theory with plane wave basis set as employed in the CASTEP simulation package code. Decrements of the band gap and semiconducting to metallic phase transformation have been observed due to strain. Our study reveals that the variation of band gap is very much asymmetric with respect to sign of the strain. Various optical properties of CuInO2 including dielectric constant, absorption coefficient, electron energy loss function and reflectivity were calculated as a function of incident photon energy and the effects of strain on these properties have been discussed. Our calculations also show that the in-plane and out-of-plane components of optical properties are very much asymmetric that depends on the equibiaxial strain.

Distinctive finite size effects on the phase diagram and metal–insulator transitions of tungsten-doped vanadium(iv) oxide

The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid–solid phase transitions such as those involved in metal–insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution WxV1−xO2. Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal → insulator and insulator → metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk WxV1−xO2 of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W6+ doping in the WxV1−xO2 nanobelts.

Weld metal phase transformation of as welded 6Mo superaustenitic stainless steel after cross-weld fatigue

The cyclic plastic deformation of the weldment of superaustenitic stainless steel with 6% molybdenum (UNS S31254) demonstrates that the strain rate does not affect the strength, but affects the ductility significantly. The response of tensile peak stress versus number of cycles for the weldment shows the cyclic hardening followed by the cyclic softening. A secondary hardening that appears at 1·5% strain amplitude is related to the strain induced martensite formation. The dislocations piled-up at the δ-ferrite grain boundary due to the essential rigid property of ferrite have a higher content of chromium and molybdenum. With a high fatigue strain rate, the austenite matrix of the weld metal exhibits persistent slip bands after fatigue at 0·6% strain amplitude and evolves to dislocation cell structures at 1·5% strain amplitude. The martensite band or the martensite sheath is formed at 1·5% strain, depending on the magnitude of the strain rate.

Reflectance changes during shock-induced phase transformations in metals

In performing shock wave experiments to study the characteristics of metals at high pressures, wave profiles (i.e., velocity measurements of the surface of the sample) are an established and useful way to study phase transformations. For example, a sudden change in the velocity or its slope can occur when the phase transformation induces a large volume change leading to a change in particle velocity. Allowing the shock to release into a transparent window that is in contact with the sample surface allows the study of conditions away from the shock Hugoniot. However, in cases where the wave profile is not definitive, an additional phase-transformation diagnostic is often useful. Changes in the electronic structure of the atoms in the crystal offer opportunities to develop new phase-change diagnostics. We have studied optical reflectance changes for several shock-induced phase transformations to see whether reflectance changes might be a generally applicable phase-transformation diagnostic. Shocks were produced by direct contact with explosives or with impacts from guns. Optical wavelengths for the reflectance measurements ranged from 355 to 700 nm. We studied samples of tin, iron, gallium, and cerium as each passed through a phase transformation during shock loading and, if observable, a reversion upon unloading. In addition to metals with complicated phase diagrams, we also measured dynamic, pressure-induced changes in the reflectivity of aluminum. For rapid solid-solid phase changes in tin and iron, we saw small changes in the surface scattering characteristics, perhaps from voids or rough areas frozen into the surface of the sample as it transformed to a new crystal structure. For melt in gallium and cerium, we saw changes in the wavelength dependence of the reflectance, and we surmise that these changes may result from changes in the crystal electronic structure. It appears that reflectance measurements can be a significant part of a larger suite of diagnostics to search for difficult-to-detect phase transformations.

Phase transitions in crystals with a BCC structure

A theory of bcc-hcp and bcc-fcc structural phase transformations in metals with a high-temperature bcc lattice has been constructed on the basis of a pseudo-spin Ising Hamiltonian, which describes cooperative oscillations of atomic planes in a two-well potential with allowance for interactions between nearest neighbors. The picture of diffuse scattering and of the combined rearrangement of original Bragg reflections and diffuse scattering into Bragg reflections upon transitions into low-temperature phases has been calculated at all temperatures. It is shown that the bcc-hcp and bcc-fcc phase transformations occur in a temperature interval rather than at a point. In the case of the bcc-hcp transformation, as the temperature decreases, the bcc structure first transforms jumpwise into an orthorhombic structure close to the hcp structure and then, with a further decrease in temperature, this structure smoothly changes into the strictly close-packed hexagonal structure at T → 0. In the case of the bcc-fcc transition, there occurs an analogous transformation into a strictly close-packed face-centered cubic structure at T → 0, but now through a monoclinic structure.

Nanoindentation-induced phase transformation in relaxed and unrelaxed ion-implanted amorphous germanium

We have investigated nanoindentation-induced plastic deformation in amorphous germanium (a-Ge) prepared by high-energy self-ion implantation. Using cross-sectional transmission electron microscopy, micro-Raman spectroscopy, and force-displacement curve analysis, we find strong evidence for a pressure-induced metallic phase transformation during indentation. Crystalline diamond-cubic Ge-I is observed in residual indents. Relaxed and unrelaxed structural states of a-Ge exhibit similar behavior on loading, but transform at different pressures on unloading. Both forms are markedly softer mechanically than crystalline Ge. These results assist in furthering the understanding of the intriguing phenomenon known as “explosive crystallization.”

Electronically driven instabilities in metals

Electronically driven instabilities of the conduction electrons in metals are manifested as spin density waves, charge density waves accompanied by periodic lattice distortions, or in structural phase transformations and are thought to result from a divergence in X(q). the generalized static electronic susceptibility. Examples of these instabilities are discussed by means of accurate calculations of the noninteraction conduction electron susceptibility X(q) using results of energy-band calculations and our recently developed analytic tetrahedron linear energy method, notably, the occurrence of change density waves in the layered dichalcogenides and the body-centered cubic to omega phase transformation in metals like Zr.

Interface conditions during mixed-mode phase transformations in metals.

A fast three-dimensional phase transformation model is formulated for the transformation from ferrite to austenite in low-carbon steel. The model addresses the parent microstructure, the nucleation behaviour of the new phase and the growth of the new phase. During the growth, the interface velocity of the ferrite grains is calculated using a mixed-mode growth model. The simulated transformation kinetics is compared with experimental kinetics for an Fe–C–Mn steel for four different cooling rates. In general, the model predicts the kinetics quite well. In addition, the mixed-mode character of the transformation is shown for the different cooling rates.

3DXRD Characterization and Modeling of Solid-State Transformation Processes

AbstractThree-dimensional x-ray diffraction (3DXRD) allows nondestructive characterization of grains, orientations, and stresses in bulk microstructures and, therefore, enables in situ studies of the structural dynamics during processing. The method is described briefly, and its potential for providing new data valuable for validation of various models of microstructural evolution is discussed. Examples of 3DXRD measurements related to recrystallization and to solid-state phase transformations in metals are described. 3DXRD measurements have led to new modeling activity predicting the evolution of metallic microstructures with much more detail than hitherto possible. Among these modeling activities are three-dimensional (3D) geometric modeling, 3D molecular dynamics modeling, 3D phase-field modeling, two-dimensional (2D) cellular automata, and 2D Monte Carlo simulations.

Comprehensive Analysis for Microstructure Evolution in Welding

Unidirectional solidification for low-carbon steel weld metal was characterized by using Time-Resolved X-Ray Diffraction (TRXRD) system. Solid-state phase transformation was also insitu observed in reciprocal lattice space. It was shown that TRXRD analysis had a potential as a comprehensive characterization technique for solidification and phase transformation process in welding. It made for the growth behavior of dendrites in unidirectional solidification and α γ δ − − phase transformation in steel weld metal to be characterized.

Strain Gradient Hardening and Pressure Induced Phase Transformation of Metals by HPT

Two unique characteristics of HPT as a SPD process, namely deformation with strain gradient and deformation under high pressure, were investigated. The effect of strain gradient was examined using low carbon steel. HPTed samples showed hardening at the center of the specimen where shear strain is principally zero and saturation hardness was higher than those without strain gradient such as ECAP or ARB. HPT experiments using specimen with different radial strain gradient in one sample also showed strain gradient hardening. The effect of high pressure during HPT was examined using pure Ti which transformed from α phase to ω phase when HPTed at high pressure. The volume fraction of ω phase increases with strain and with decreasing oxygen content. The ω phase is metastable at ambient conditions and it transforms back to α phase during heating.

Experimental constraints on the phase diagram of titanium metal

The phase transformations of titanium metal have been studied at temperatures and pressures up to 973 K and 8.7 GPa using synchrotron X-ray diffraction. The equilibrium phase boundary of the α–ω transition has a d T/d P slope of 345 K/GPa, and the transition pressure at room temperature is located at 5.7 GPa. The volume change across the α–ω transition is Δ V=0.197 cm 3/mol, and the associated entropy change is Δ S=0.57 J/mol K. Except for Δ V, our results differ substantially from those of previous studies based on an equilibrium transition pressure of 2.0 GPa at room temperature. The α–ω–β triple point is estimated to be at 7.5 GPa and 913 K, which is comparable with previous results obtained from differential thermal analysis and resistometric measurements. An update, more accurate phase diagram is established for Ti metal based on the present observations and previous constraints on the α–β and ω–β phase boundaries.

Development of in-Situ Microstructure Observation Techniques in Welding

Unidirectional solidification for low-carbon steel weld metal was characterized by the using Time-Resolved X-Ray Diffraction (TRXRD) system. Solid-state phase transformation was also observed in situ in reciprocal lattice space. It was shown that TRXRD analysis has a potential as a comprehensive characterization technique for solidification and phase transformation processes in welding. Furthermore, it was suggested that nucleation sites in solid-state phase transformation can be observed by high-temperature laser scanning confocal microscopy (HLSCM) in order to classify the ferrite microstructure. These techniques have been designed for the dendrite alignment in unidirectional solidification and the δ-β-α phase transformation in steel weld metal to be characterized.

Application of Statistical Moment Method to Thermodynamic Properties and Phase Transformations of Metals and Alloys

The thermodynamic properties and phase transformations of metals and alloys are studied using the statistical moment method, going beyond the quasi-harmonic approximations. Including the power moments of the atomic displacements up to the fourth order, the Helmholtz free energies and the related thermodynamic quantities are derived explicitly in closed analytic forms. The thermodynamic quantities, like thermal lattice expansion coefficients, specific heats, Grüneisen constants, elastic constants calculated by using the SMM are compared with those of other theoretical schemes and the experimental results. The hcp-bcc structural phase transformations observed for IVB elements, Ti, Zr and Hf, are discussed in terms of the anharmonicity of thermal lattice vibrations. The equilibrium phase diagrams are calculated for the refractory Ta-W and Mo-Ta bcc alloys. In addition, the temperature dependence of the elastic moduli C11, C12 and C14 and those of the ideal tensile and shear strengths of the bcc elements Mo, Ta and W are studied: We also discuss the melting transitions of metals and alloys within the framework of the SMM and estimate the melting temperatures through the limiting temperature of the crystalline stability.