Including Phase Transformation Research Articles

Wavelet analysis of microscale strains

Recent improvements in experimental and computational techniques have led to a vast amount of data on the microstructure and deformation of polycrystals. These show that, in a number of phenomena, including phase transformation, localized bands of deformation percolate in a complex way across various grains. Often, this information is given as point-wise values arrayed in pixels, voxels and grids. The massive extent of data in this form renders identifying key features difficult and the cost of digital storage expensive. This work explores the efficiency of wavelets in storing, representing and analyzing such data on shape-memory polycrystals as a specific example. It is demonstrated how a compact wavelet representation captures the essential physics contained in experimental and simulated strains in superelastic media.

Formation and stability of small well-defined Cu- and Ni oxide particles

Well-defined and -structured Cu/Cu2O and Ni/NiO composite nanoparticles have been prepared by physical-vapor deposition on vacuum-cleaved NaCl(001) single crystal facets. Epitaxial growth has been observed due to the close crystallographic matching of the respective cubic crystal lattices. Distinct particle morphologies have only been obtained for the Ni/NiO particles, comprising truncated half-octahedral, rhombohedral- and pentagonal-shaped outlines. Oxidation of the particles in the temperature range 473–673 K in both cases led to the formation of well-defined CuO and NiO particles with distinct morphologies. Whereas CuO possibly adopts its thermodynamical equilibrium shape, NiO formation is accompanied by entering a Kirkendall-like state, that is, a hollow core–shell structure is obtained. The difference in the formation of the oxides is also reflected by their stability under reducing conditions. CuO transforms back to a polycrystalline mixture of Cu metal, Cu2O and CuO after reduction in hydrogen at 673 K. In contrast, as expected from theoretical stability considerations, the formation of the hollow NiO structure is reversed upon annealing in hydrogen at 673 K and moreover results in the formation of a Ni-rich silicide structure Ni3Si2. The discussed systems present a convenient way to tackle and investigate various problems in nanotechnology or catalysis, including phase transformations, establishing structure/activity relationships or monitoring intermetallic particles, starting from well-defined and simple models.

Research Progress on the Preparation of Ceramic Hollow Nanofibers by Electro-spinning

Ceramic hollow nanofibers possess unique one-dimensional tubular nanostructure, which have potential application prospects in the fields of microelectronics, catalysts, gas sensors and photoelectric transducers. The re- search progress on ceramic hollow nanofibers prepared by electrospinning is reviewed. The tendency of studies on fabricating ceramic hollow nanofibers through coaxial electrospinning, single capillary electrospinning and postpro- cessing is mainly introduced. And the discussion on the formation mechanism of hollow structure of ceramic nanofi- bers is presented with emphasis on single capillary electrospinning method, including phase transformation mechanism, gas evaporating mechanism and Kirkendall effect. The application prospect and disadvantages are also summarized.

Strong Solutions of the Navier–Stokes Equations for a Compressible Fluid of Allen–Cahn Type

In this work, we study the “Navier–Stokes–Allen–Cahn” system, a combination of the compressible Navier–Stokes equations with an Allen–Cahn phase field description. This model describes two-phase patterns in a flowing liquid including phase transformations. Our purpose is to show the existence and uniqueness of local strong solutions for arbitrary initial data. Part of the proof is based on methods used in a companion paper which investigated the compressible Navier–Stokes equations.

Coarse-grained atomistic modeling and simulation of inelastic material behavior

This paper presents a new methodology for coarse-grained atomistic simulation of inelastic material behavior including phase transformations in ceramics and dislocation mediated plasticity in metals. The methodology combines an atomistic formulation of balance equations and a modified finite element method. With significantly fewer degrees of freedom than those of a fully atomistic model and without additional constitutive rules but the interatomic force field, the new coarse-grained (CG) method is shown to be feasible in predicting the nonlinear constitutive responses of materials and also reproducing atomic-scale phenomena such as phase transformations (diamond → β-Sn) in silicon and dislocation nucleation and migration, formation of dislocation loops and stacking faults ribbons in single crystal nickel. Direct comparisons between CG and the corresponding full molecular dynamics (MD) simulations show that the present methodology is efficient and promising in modeling and simulation of inelastic material behavior without losing the essential atomistic features. The potential applications and the limitations of the CG method are also discussed.

Constitutive model for shape memory alloys including phase transformation, martensitic reorientation and twins accommodation

This paper deals with the thermomechanical modeling of the macroscopic behavior of NiTi shape memory alloys (SMAs). A phenomenological 3D-model, based on thermodynamics of irreversible processes is presented. Three main physical mechanisms are considered: the martensitic transformation, the reorientation of martensite and the inelastic accommodation of twins in self-accommodated martensite. The description of such strain mechanisms allow an accurate analysis of SMA behavior under complex thermomechanical paths, especially when transformation occurs at low stress level. Moreover, some key characteristics such as tension–compression asymmetry and internal loops inside the major hysteresis loop are taken into account. Numerical simulations for various theromechanical loading paths are presented to illustrate the present model capability to capture the complex behavior of SMAs

Computer aided interpretation of results of the Jominy test

Jominy end quench test is considered in the paper. Finite element simulations of this test were performed including phase transformations modelling. To demonstrate the capability of the model, the numerical simulations results for two bainitic steels with different chemical composition are presented. Phase transformation model for these steels was developed on the basis of the dilatometric tests. These models were implemented in the FE code and kinetics of transformations during the Jominy test was calculated. Distributions of the structural constituents in the sample after the tests were determined. Comparison of the hardenability of the two investigated steels was made on the basis of the results of simulations.

Influence of Heat Treatment Conditions on Crystallization and Thermal Expansion of Las Glass-Ceramics

The LAS glass containing P2O5has been prepared by conventional molten quenching method. The influence of heat treatment conditions on crystallization behavior, including phase transformation and microstructure, and thermal expansion coefficient (TEC) of Li2O–Al2O3–SiO2(LAS) glass-ceramics were investigated. DSC, XRD, SEM and TEC were used to detect the microstructure and properties of glass-ceramics under the different heat-treatment conditions. The results show the virgilite crystalline separated firstly from the matrix glass when heat treatment temperature was 850 °C. As the heat treatment temperature increased from 850 °C to 1050 °C, virgilite and β-spodumene were identified as main crystal phases. The TEC of glass-ceramics ranges from 0.5×10-6 °C-1to 2.8×10-6 °C-1, which is much lower than that of matrix glass.

Prediction of residual stress distributions for single weld beads deposited on to SA508 steel including phase transformation effects

The sensitivity of residual stress distributions in bainitic–martensitic steel welds to the transformation strains that arise when austenite decomposes on cooling has been assessed by examining the predictions of three models for a simple bead-on-plate configuration. These cover the following scenarios: case I, no phase transformations; case II, transformations with volume change effects only; case III, transformations with volume change effects and associated Greenwood–Johnson transformation plasticity. Austenite decomposition was predicted by implementing Kirkaldy's reaction rate equations as a subroutine in the finite element code Sysweld, eliminating the need for a continuous cooling transformation diagram. Predicted residual stresses were then compared and rationalised alongside measurements obtained by neutron diffraction and the contour method. It was found that serious errors in predicting the location and magnitude of the peak stresses occurred if transformations were not included, while cases II and III gave similar results generally in agreement with the stress maps. Indeed, the trends in the experimental results were intermediate between cases II and III. Differences between the models and the potential for further improvements are discussed.

IDS: Thermodynamic–kinetic–empirical tool for modelling of solidification, microstructure and material properties

IDS (InterDendritic Solidification) is a thermodynamic–kinetic–empirical tool for simulation of solidification phenomena of steels including phase transformations from melt down to room temperature. In addition, important thermophysical material properties (enthalpy, thermal conductivity, density, etc.) are calculated. The model has been developed in the Laboratory of Metallurgy, Helsinki University of Technology, Finland, since 1984. IDS includes two main modules, the IDS module and the ADC (Austenite DeComposition) module. IDS module simulates the solidification phenomena from liquid down to 1000 °C and ADC the austenite decomposition down to room temperature. Both modules have their own recommended composition ranges. The IDS module is based on the so-called sharp interface concept. The ADC is mainly statistical based on empirical CCT (Continuous Cooling Transformation) diagrams. IDS tool is also coupled with the thermodynamic programmer's library, called ChemApp, developed by a German company, GTT-Technologies. This coupled package is used to simulate among other things multiphase inclusions during solidification. The present paper summarises the features of the IDS tool including the coupling with the ChemApp library.

A new constitutive model of austenitic stainless steel for cryogenic applications

Abstract A series of uniaxial tensile test under cryogenic temperature was carried out for AISI 304 and 316 austenitic stainless steels (ASS) in this study. Typical non-linear hardening phenomena under the cryogenic environment, such as transformation induced strain hardening and threshold strain for the 2nd hardening, has been observed in a quantitative manner. The important factors affecting the non linear material behavior of austenitic stainless steel including phase transformation, discontinuous yielding and micro-damage are modeled using constitutive equations system based on strain decomposition at the small strain formulation. A strong nonlinearity of strain hardening is described using the coupling of modified Bodner’s plasticity model and phase transformation induced strain model. The strain (threshold strain for onset of 2nd hardening) dependent plasticity model was proposed in the hardening function of Bodner’s model. In order to explicitly express the phase transformation induced strain, TI model (Tomita and Iwamoto model [Y. Tomita, T. Iwamoto, Constitutive modeling of TRIP steel and its application to the improvement of mechanical properties, International Journal of Mechanical Sciences 37 (1995) 1295–1305.]), which is a function of accumulated plastic strain and volume fraction factor of martensite, is selected in this study. Also the unified damage model, which can be connected with elasto-plastic constitutive equation developed in this study, is suggested, and the utility of proposed model was validated by the comparison between experiments and numerical evaluations.

Finite element simulation of welding processes using a solid-shell element

A solid-shell finite element (FE), suitable for simulation of arc-welding processes, is developed. The element formulation is based upon the Taylor series expansion of the strain matrix, and the so-called hyperelastic formulation is adopted for FE formulation. The B-bar approach is employed and the shear-strain operator is modified to eliminate various locking modes. The welding constitutive equation, including phase transformation and transformational plasticity, is implemented into the solid-shell element. This shows an excellent performance for large aspect ratios, so that this element surmounts the limitation of the regular solid element with regard to welding, particularly when specimens are subject to welding under mechanical loadings.

Mechanisms of Radiation Damage and Properties of Nuclear Materials

AbstractRadiation damage effects in ceramics, e.g., nuclear waste forms, transmutation targets, and inert matrix fuels, may have important implications for the physical and chemical stability of these materials as the cumulative radiation dose increases over time. A key aspect of scientific research in this area is the ability to understand the fundamental damage mechanisms through the combination of experimental and atomistic modelling techniques. In this paper, we review some of the lessons learned from the significant body of data now available for pyrochlore-defect fluorite based materials, followed by an illustration of the advantages of working on simple compounds with well established interatomic potentials. We conclude the paper with a description of radiation damage processes in the LaxSr1-1.5xTiO3 defect perovskites, a system that includes phase transformations, short-range order effects, and complex defect behavior.

A thermo-elastoplastic constitutive equation including phase transformation and its applications

Heat-processed materials undergoing phase transformation are considered to be a mixture of several phases, and each of them possesses thermo-elastoplastic property. A thermo-elastoplastic constitutive equation involving phase transformation is put forward based on the physics of continuous media and the irreversible thermodynamics with internal variables. The material parameters in the constitutive equation and their variations with temperature are determined by a series of short-time-tension tests at different temperatures. The heat conduction equation including phase transformation and the transformation kinetics equation including stresses are also presented. The numerical algorithm and the finite element procedure related to these equations are developed and applied to the investigation of the distribution of the residual stress in the welding slot area of 1Cr12WMoV stainless steel pipe. The obtained results of the residual stress are in good agreement with the measurements by X-ray diffraction.

Dust modification under photon, electron and ion irradiation

Irradiation is one of the main processes which influences the evolution of dust in astrophysical environments. This article reviews the basics of particle-matter interactions and gives an introduction to the induced modifications including phase transformation, sputtering, implantation, charging effects and transmutation. We also discuss the role of the nature of the incident particle (energetic photons, electrons and ions) as a function of its energy. Finally, we briefly review the main results obtained by irradiation studies on ices, carbonaceous matter and silicates.

Effect of Calcination Temperature of Kaolin Microspheres on the In situ Synthesis of ZSM-5

ZSM-5 zeolite has been successfully synthesized in-situ on calcined kaolin microspheres by the hydrothermal method using n-butylamine as a template. The supported ZSM-5 was characterized by X-ray diffraction and scanning electron microscopy. The effect of calcination temperature of kaolin microspheres on the in-situ synthesis of ZSM-5 was investigated. The influence of the pretreatment temperature on the properties of kaolin microspheres including phase transformation, amounts of active SiO2 and Al2O3, and pore structures, was studied using fourier transform infrared (FT-IR), nitrogen adsorption and chemical analysis. The results showed that when the calcination temperature increased from 300 to 900 °C, the amount of active SiO2 in the kaolin microspheres increased slightly and the amount of active Al2O3 initially increased rapidly and then decreased steadily. The surface area and pore volume of the kaolin calcined at both low and high temperatures was less than those of kaolin calcined at a medium temperature. The property changes of kaolin caused the relative crystallinity of in situ synthesized ZSM-5 to vary.

Modifying residual stress levels in rail flash-butt welds using localised rapid post-weld heat treatment and accelerated cooling

AbstractFlash-butt welding is used in the manufacture of continuously-welded rails. Finished welds typically exhibit high tensile residual stresses in the rail web and at the upper surface of the rail foot, which may increase the risk of fatigue failure in service. An understanding of the influence of the welding process, including post-weld cooling, on the residual stress distribution is necessary to improve the performance of flash-butt welds by post-weld heat treatment (PWHT), since incorrect treatment may have adverse effects on both residual stress and weld material characteristics. A finite element model has been developed to simulate post-weld cooling in flash-butt welded AS60 kg m–1 rail. Computed thermal histories for normal (air) cooling, rapid PWHT, and accelerated cooling (water spray) were used as inputs to calculate sequentially coupled stress–time histories, including phase transformations. In addition, the localised influence of the initiation time for rapid PWHT, after final upset, on the...

Simulation of the distortion of cylindrical shafts during heat treatment due to segregations

A conventional low-alloy steel (SAE 5120, EN 20MnCr5) was extensively investigated by dilatometer experiments and at heat-treated cylindrical shafts to determine the three-dimensional anisotropic change in length and curvature during heat treatment due to segregations. The change in length due to the transformations during heating as well as during cooling can be detected as the main reason for distortion. Segregations are too small in relation to a whole part to simulate them for the entire part. This is the main problem with including segregations in a simulation model. Due to this a mean substitution had to be used. Segregations were taken into account by three-dimensional anisotropic transformation strains, which were implemented into the FEM-simulation program Sysweld ® . Thereby, heating as well as cooling - including phase transformations and mechanical behaviour - were considered, because anisotropy occurs in both cases and its overlapping leads to the final distortion.

The Structural and Electrochemical Properties of Thermally Aged Li[Co0.1Ni0.15Li0.2Mn0.55]O2Cathodes

As a cathode material of lithium rechargeable batteries, charged Li[Co0.1Ni0.15Li0.2Mn0.55]O2 electrodes, which were aged thermally at 25 oC and 90 oC respectively, were characterized by means of charge/discharger, impedance spectroscopy, and X-ray diffraction. The discharge capacity diminution of the electrodes aged at 25 oC and 90 oC for 1 week was 4% and 23%, respectively. The cell aged at 25 oC was recovered on cycling. However, the capacity loss after ageing at 90 oC was not recovered in a subsequent cycling test, which demonstrates that the reaction occurring during ageing at 90 oC is irreversible. A significant impedance increase of aged electrode at 90 oC is associated with irreversible capacity loss. The structural changes including phase transformation were not detected by XRD analysis, because it could be due to out of detection limit. After ageing, impedance was slightly decreased during subsequent cycling test. It could be explained the cyclic performance of aged sample is stable. The thermal stability was not deteriorated by ageing even at the high temperature of 90 oC.

Phase transformation of Li xNi 1−yCo yO 2 along with lithium deintercalation

A systematic investigation of the structural change of Li x Ni 1− y Co y O 2, a possible cathode material in lithium ion secondary battery, has been performed during lithium deintercalation. Three representative models, LiNiO 2, LiCoO 2 and LiNi 0.8Co 0.2O 2, are constructed initially and then the molecular simulations according to the lithium content are performed using the modified universal force field. The phase transformations from rhombohedral to monoclinic and finally to rhombohedral have occurred for all three models by successively decreasing the lithium content. The ranges of lithium content where the phase transformation occurred agree well with the experimental results. This simple force field-based calculations are successfully applied to the structural changes including phase transformation of Li x Ni 1− y Co y O 2 system.