Including Phase Transformation Research Articles

Experimental–numerical analysis of silicon micro-scratching

The continuous decrease in feature size in the production of electronic chips has two major consequences for silicon wafers: (i) processing conditions for slicing and grinding operations need to be optimized and (ii) surface damage imposed by wafer handling and transport needs to be minimized. Cultivating a deeper understanding of the subsurface implications of contact, in particular during scratching, is necessary to achieve these goals. The active subsurface micro-deformation mechanisms, including phase transformations and crack formation, can only be measured post-scratch. On the other hand, numerical models are free of experimental constraints and can therefore serve an instrumental role in uncovering the active deformation mechanisms during scratching. However, model identification and validation with experimental data is essential for reliable simulation results. Therefore, in this paper, a numerical scratch model consisting of a continuum particle methodology combined with a large strain inelastic constitutive model is exploited, whereby the parameters are identified from indentation data. The numerical–experimental results are next compared for a range of micro-scratch tests. The experimental micro-scratch device was mounted inside a scanning electron microscope to perform in situ scratch experiments. The real-time observations at the scratched surface allowed to assess and exclude possible experimental artifacts. The simulations demonstrate the ability to investigate the evolution of the phase transformations underneath the scratched surface and relate this to the hydrostatic pressure. Notably, at the simulated subsurface phase boundary, hydrostatic tension was found which is expected to play an important role in median crack formation, as often reported in the literature. For the simulation and experiments, the steady-state segments are compared and it was found that the model accurately reproduces the substantial surface recovery due to the Si-II → Si-a phase transformation. Additionally, the simulated and experimental surface cross-sectional profiles for normal scratching loads of 30, 40 and 50 mN are in adequate agreement.

Advanced TiO2/Al2O3 Bilayer ALD Coatings for Improved Lithium-Rich Layered Oxide Electrodes.

Surface modification is a highly effective strategy for addressing issues in lithium-rich layered oxide (LLO) cathodes, including phase transformation, particle cracking, oxygen gas release, and transition-metal ion dissolution. Existing single-/double-layer coating strategies face drawbacks such as poor component contact and complexity. Herein, we present the results of a low-temperature atomic layer deposition (ALD) process for creating a TiO2/Al2O3 bilayer on composite cathodes made of AS200 (Li1.08Ni0.34Co0.08Mn0.5O2). Electrochemical analysis demonstrates that TiO2/Al2O3-coated LLO electrodes exhibit improved discharge capacities and enhanced capacity retention compared with uncoated samples. The TAA-5/AS200 bilayer-coated electrode, in particular, demonstrates exceptional capacity retention (∼90.4%) and a specific discharge capacity of 146 mAh g-1 after 100 cycles at 1C within the voltage range of 2.2 to 4.6 V. The coated electrodes also show reduced voltage decay, lower surface film resistance, and improved interfacial charge transfer resistances, contributing to enhanced stability. The ALD-deposited TiO2/Al2O3 bilayer coatings exhibit promising potential for advancing the electrochemical performance of lithium-rich layered oxide cathodes in lithium-ion batteries.

Computationally efficient simulation methodology for railway repair welding: Cyclic plasticity, phase transformations and multi-phase homogenization

The in-situ railway repair welding process consists of multiple weld passes, which makes it significantly different from other rail welding processes. In this study, finite element simulations of repair welding are performed to predict the resulting microstructure and residual stresses. To accurately simulate the material behavior, the modeling includes phase transformation kinetics, cyclic hardening plasticity, transformation induced plasticity, and multi-phase homogenization. More specifically, four different homogenization methods are investigated: isostrain, isostress, self-consistent and linear mixture rule. The performance of the material modeling is demonstrated by simulating multiple weld passes using a classical three-bar welding experiment. Based on the results, the self-consistent method and linear mixture rule are used in a 3D full-scale railhead repair weld simulation, in which the former generates a more realistic mechanical response. The immense computational cost associated with 3D full-scale, full-detail multi-pass welding simulations is addressed by exploring different model reduction schemes. From this study, a 2D generalized plane strain model, extended with out-of-plane axial and bending stiffness, is found to replicate the full-scale model at a mere fraction of the computational cost. Finally, the longitudinal residual stress distribution obtained from the reduced model is shown to correlate well with experimental measurements.

An Explicit Modeling for the Pseudoelastic Response of Porous SMA Thick-Walled Cylinders under Internal and External Pressure

Shape-memory alloy (SMA) structures, either dense or porous, are widely used because of their functional characteristics arising from shape-memory effect and pseudoelasticity. Hence, the precise modeling of these materials is essential due to design issues. In this study, the solution algorithm based on an explicit phenomenological constitutive model is investigated to analyze pressurized cylinders. The proposed method is extracted following return mapping scheme to find the evolution of phase transformation in these structures. The formulation is validated by comparing uniaxial tension–compression curves during loading, unloading and thermal actuation against published literature. By the aid of this model, assuming elastic and phase transformation strains, a comprehensive study is presented to inspect stress, strain and displacement fields along the thickness of internally and externally pressurized thick-walled cylinders. To this end, the governing nonlinear equation in elastic–inelastic zones is discretized by the efficient method of generalized differential quadrature (GDQ) and the resultant relations are solved numerically. The influences of internal and external pressure on stress distribution along the cylinder wall during loading–unloading for different temperatures including phase transformation and saturation states are discussed. Also, the closed-form solution for critical pressure at the onset of the phase transformation is studied.

FBG-LPFG-Based Sensor to Monitor 3-D Strain in Ice During Freezing–Melting Processes

Three-dimensional strain (3D strain) and the principal strain in ice are very important for glacier movement research and glacier-related disaster prediction. The internal 3D strain of ice is sensitive to temperature, and it is very complex during the whole freezing and melting process including phase transformation. In this study, we present a 3D strain sensor (3D-S Sensor) based on fiber Bragg grating (FBG) and long period fiber grating (LPFG) that can measure the 3D strain in ice. The LPFG (the temperature sensitivity coefficient is 56.3 pm/°C) was fabricated by the laser etching method, which was used for the temperature compensation of FBGs to improve the accuracy of strain measurement. The 3D principal strain can be calculated by the value of 6 direction strains from FBGs. A freeze-melt experiment is designed to verify the performance of 3D-S Sensor. The experimental results show that the sensor can measure the complex 3D deformation of ice from a more precise angle. Among the 6 directions of strain measured by the sensor, the strain value in the Z direction is the largest (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>z</i></sub> = 6232.9 με). The first principal strain in ice is close to the negative direction of the Z axis (θ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>Z</i></sub> = 160.7°), and the maximum value is 6638.2 με. The measured results are consistent with the actual deformation of ice, indicating that the sensor can effectively measure the 3D strain and principal strain inside the ice.

Reversible and irreversible transformations of Ni-based electrocatalysts during the oxygen evolution reaction

Nickel-based catalysts for the alkaline oxygen evolution reaction (OER) demonstrate excellent catalytic performance and stability. However, a lack of fundamental understanding of the dynamic electronic and structural changes that occur under OER conditions inhibits the rational design of new materials. Recent advances in operando spectroscopy and computational modelling techniques have helped to elucidate the electrochemically-driven transformations of Ni-based materials. For reversible transformations, this encompasses an increased understanding of the redox transformations of Ni/Fe centers, the adsorption and desorption of reaction intermediates, oxygen vacancy dynamics, phase transformations, and the mechanism of dissolution and redeposition of surface atoms. Likewise, there have been great advances in scientific understanding of irreversible transformations including phase transformations related to ageing, as well as operando surface reconstruction which involves the growth of new OER active phases.

Comprehensive structural changes in nanoscale-deformed silicon modelled with an integrated atomic potential

In spite of remarkable developments in the field of advanced materials, silicon remains one of the foremost semiconductors of the day. Of enduring relevance to science and technology is silicon's nanomechanical behaviour including phase transformation, amorphization and dislocations generation, particularly in the context of molecular dynamics and materials research. So far, comprehensive modelling of the whole cycle of events in silicon during nanoscale deformation has not been possible, however, due to the limitations inherent in the existing interatomic potentials. This paper examines how well an unconventional combination of two well-known potentials - the Tersoff and Stillinger-Weber - can perform in simulating that complexity. Our model indicates that an irreversible deformation of silicon (Si-I) is set in motion by a transformation to a non-diamond structure (Si-nd), and followed by a subsequent transition to the Si-II and Si-XII phases (Si-1→Si-nd→Si-II→Si-XII). This leads to the generation of dislocations spreading outwards from the incubation zone. In effect, our simulations parallel the structural changes detected experimentally in the deformed material. This includes both the experimentally observed sequence of phase transitions and dislocation activity, which - taken together - neither the Tersoff nor Stillinger-Weber, or indeed any other available Si interatomic potential, is able to achieve in its own right. Notably, the Si-XII phase was not discerned by any of the previous computational models, which points towards the effectiveness of our integrated approach to forecasting novel phenomena discovered by advanced structure examinations. Last not least, our method satisfies the demand for a quick means to construct potentials by opening up the huge library of existing models to new applications in various branches of materials science.

Frontiers, challenges, and solutions in modeling of swift heavy ion effects in materials

Since a few breakthroughs in the fundamental understanding of the effects of swift heavy ions (SHIs) decelerating in the electronic stopping regime in the matter have been achieved in the last decade, it motivated us to review the state-of-the-art approaches in the modeling of SHI effects. The SHI track kinetics occurs via several well-separated stages and spans many orders of magnitude in time: from attoseconds in ion-impact ionization depositing an extreme amount of energy in a target to femtoseconds of electron transport and hole cascades, to picoseconds of lattice excitation and response, to nanoseconds of atomic relaxation, and even longer times of the final macroscopic reaction. Each stage requires its own approaches for quantitative description. We discuss that understanding the links between the stages makes it possible to describe the entire track kinetics within a hybrid multiscale model without fitting procedures. The review focuses on the underlying physical mechanisms of each process, the dominant effects they produce, and the limitations of the existing approaches, as well as various numerical techniques implementing these models. It provides an overview of the ab initio-based modeling of the evolution of the electronic properties, Monte Carlo simulations of nonequilibrium electronic transport, molecular dynamics modeling of atomic reaction including phase transformations and damage on the surface and in the bulk, kinetic Mote Carlo of atomic defect kinetics, and finite-difference methods of track interaction with chemical solvents describing etching kinetics. We outline the modern methods that couple these approaches into multiscale and combined multidisciplinary models and point to their bottlenecks, strengths, and weaknesses. The analysis is accompanied by examples of important results, improving the understanding of track formation in various materials. Summarizing the most recent advances in the field of the track formation process, the review delivers a comprehensive picture and detailed understanding of the phenomenon. Important future directions of research and model development are also outlined.

Effect of deep cryogenic treatment, tempering temperature and time on hardness of Nimonic-90

The present study has been done to determine the effect of deep cryogenic treatment (DCT) on the hardness of Nimonic-90. From the study, it has been found that the hardness of the material is significantly affected with cryogenic treatment which includes soaking the samples at subzero temperature (−196 °C) for 24 h and 48 h and tempering operation which includes heating the samples in temperature range of 100–200 °C. Also, it has been found that as the tempering temperature decreases from 200–100 °C, the hardness of the samples increases. The number of tempering cycles affected the hardness of the samples as well. The lowest hardness value came out for the untreated sample as 284 HV. Among all the cryo-treated samples, the highest value of hardness was for the single tempering at a tempering temperature of 100 °C, soaking period of 48 h, and tempering time of 1 h which was 435.76 HV. The minimum hardness in DCT samples was for the case of 24 h soaking period, tempering temperature of 200 °C and tempering time of 3 h. which was 376.9 HV. The tremendous improvement in hardness of Nimonic-90 is because of microstructural changes at different soaking period and tempering temperature which includes phase transformation and morphological changes that includes grain size which decreased from 43 to 4.418 µm, precipitation of Cr23C6, NiC, and Cr2Ti at respective soaking period, tempering temperature, and the number of tempering cycles in each treatment.

Linear friction welding of Ti60 near-α titanium alloy: Investigating phase transformations and dynamic recrystallization mechanisms

Linear friction welding (LFW) is a solid-state welding process used primarily in manufacturing of aeroengine blisks. Ti60 near-α titanium alloy was developed for blisks operating at 600 °C, and shows excellent thermal stability and high-temperature oxidation resistance. In order to promote the wider use of such blisks, the mechanisms of microstructure evolution including phase transformation and dynamic recrystallization of welded Ti60 joint were studied. The effects of phase transformation and dynamic recrystallization on microhardness and tensile strength were investigated as well. Results show that the microstructure of the thermo-mechanically affected zone (TMAZ) deforms severely along the friction direction, forming {0001}[11 2 ¯ 0] α texture. β → metastable β then → α + β transformations occur, leading to lamellar α precipitating from the deformed β. Sufficient continuous dynamic recrystallization makes the WZ almost composed of fine equiaxed grains of 10 μm size. In addition, α → β with the subsequent αˊ transformation occurs in the weld zone (WZ), producing extensive distribution of acicular αˊmartensite inside the recrystallized grains. Under the effects of strain hardening, precipitation strengthening and fine grain strengthening, the hardness in WZ increases significantly (i.e., 381 HV) compared to base metal (BM) (335 HV), with the tensile strength of the joint (943 MPa) also increasing over that of the BM (914 MPa). However, because of inhomogeneous deformation, the elongation of the joint (13.3%) is slightly lower than that of BM (14.1%). • β → metastable β then → α + β transformations occur in TMAZ, leading to lamellar α precipitating from deformed β; α → β with the subsequent αˊ transformation occurs in WZ, producing extensive distribution of acicular αˊmartensite. • Adequate continuous dynamic recrystallization occurs in weld zone, accompanying by violent dislocation motion and formation of many subgrains and low-angle grain boundaries. • The tensile strength of the joint (943 MPa) increases to that of the base metal (914 MPa), but the elongation of the joint (13.3%) is slightly lower than that of the base metal (14.1%) because of inhomogeneous deformation.

The effect of submicron SiC particles on the thermal stability of nanocrystalline AZ91 alloy

Nanocrystalline (NC) magnesium alloys and composites present potential for improving energy efficiency in transportation applications due to outstanding strength. Unfortunately, softening owing to grain growth at elevated temperature would induce fatal invalidity. The NC SiCp/AZ91 composite was isothermally annealed. A study on the thermal stability, including phase transformation, grain growth, softening, and strengthening mechanisms was carried out. After annealing treatment at 673 K for 240 min, the average grain size of AZ91-15vol.%SiCp composite increased from 29.0 nm to 53.1 nm. Coherent interface between nano-scale Mg17Al12 precipitates and Mg matrix was certified by HRTEM. Pinning effect due to the dispersing submicron SiC particles with size of 164 nm and nano-scale Mg17Al12 precipitates and dragging effect due to dissolving Al element were the reasons for excellent thermal stability. After annealing treatment, the hardness values of AZ91-xvol.%SiCp (x = 5, 10 and 15) composites were 1.61 GPa, 1.48 GPa and 1.78 GPa. Taking annealed AZ91-15vol.%SiCp composite as an example, there was no evident softening phenomenon and the decreasing ratio was ∼2% in comparison with milled composite with hardness of 1.82 GPa. Non-thermal sensitive SiC particles and high thermal stability Mg matrix played a key role in non-softening of composites.

Mechanically assisted chemical interaction of doped bismuth oxide with silver

Oxygen selected membranes on the base of cermet δ-Bi2O3/Ag with an interpenetrating structure has the maximum potential air separation efficiency. However, degradation processes, including phase transformations of δ-Bi2O3 and recrystallization/segregation of silver with a loose of interpenetrating structure, will not allow to create a membrane with the required perfection and durability. The hypothesis about the possible chemical interaction of doped bismuth oxide with silver arose when studying the processes of membrane degradation. In this work, the mechanochemical and thermally activated chemical interaction of mechanically activated doped bismuth oxide with silver was proved by direct experiments with powder XRD on selected samples of solid electrolytes with the compositions Bi0.78Dy0.12Er0.1O1.50 and Bi0.76Dy0.12Er0.1W0.02O1.53. The results obtained influence the choice of the composition of disordered fluorite, the conditions for its synthesis, and the creation of oxygen selective membranes based on cermet δ-Bi2O3/Ag.

Smoothed Boundary Method Electrochemical Simulation Framework for Complex Electrode Microstructures

Battery and fuel cell electrodes possess highly complex microstructures: tortuous interparticle space, irregular particle surfaces, and various particle sizes. Additionally, coupled physical mechanisms, such as mass transport and electron transport, heat generation, and phase transformations, simultaneously occur during the electrode’s operations. All these combined complexity makes modeling electrochemical processes with explicit considerations of electrode microstructures very challenging. As such, electrode designs are still heavily relied on experimental trial-and-error methods even though modern computational resources have grown rapidly. Furthermore, experimental technologies have become very mature to reconstruct three-dimensional (3D) microstructures. The abundant microstructure data open a window for directly simulating the physical processes in complex microstructures. This talk will introduce an innovative simulation method using a continuous function to define complex microstructures. Since the irregular complex microstructure surfaces are implicitly described, this method no longer requires meshes conformal to the complex microstructures as in the conventional simulations. Thus, it allows us to simulate detailed electrochemical phenomena in complex electrode microstructures in an unprecedented pace with ease, especially for image-based, reconstructed microstructures. We will showcase several recent simulations to demonstrate the presented method, including phase transformations in porous graphite anode, hybrid electrodes, electrochemical impedance spectroscopy, and electrochemical processes in NMC-separator-graphite full-cell. The presented method is applicable to other electrochemical systems, and can be extended to include other physical mechanisms for further studying cycling-induced phenomena such as stress, heat, and Li-plating. Figure 1

Homogenization based macroscopic model of phase transformations and cyclic plasticity in pearlitic steel

In this contribution macroscopic modeling of phase transformations and mechanical behavior of low alloy steels are developed and investigated. Such modeling is of importance in simulations of transient thermo-mechanical processes which can cause phase transformations, examples from the railway industry include train braking induced frictional heating as well as rail grinding and welding operations. We adopt a modeling approach which includes phase transformation kinetics and individual constitutive models for the phases in combination with different homogenization methods. Algorithmic implementations of the isostrain, isostress and self-consistent homogenization methods are presented and demonstrated in finite element simulations of a laser heating experiment. Stress field results from the different homogenization methods are compared against each other and also against experimental data. The importance of including transformation induced plasticity in the modeling is highlighted, as well as the multi-phase stages of the heating and cooling.

成形-相変態連成CAEによるホットスタンプ部材の形状予測

The hot stamping method is established for manufacturing high-strength automotive parts with good shape fixability. Recently, the hot stamping method has been applied to higher performance parts with tailored properties. CAE is indispensable in designing such parts. For designing hot-stamped parts, the forming simulation must be coupled with thermal analysis considering the temperature effect on formability. The final shape need not necessarily be predicted because of its good fixability. However, in high-performance parts with tailored properties, it is important to predict how the final shape is affected by phase transformation. To predict the final shape of hot-stamped parts, a CAE method that takes into account phase transformation effects has been developed. The accuracy of the calculation of the final shape by the CAE method was investigated. Hot-stamping experiments were carried out to obtain parts of different shapes by cooling control, and the CAE analysis of the final shape for each experimental condition was conducted. It was clarified that the final shape was varied by changing the cooling process, and that the results of CAE including phase transformation were in good agreement with the tendency of shape change in cross section.

Constitutive modeling of viscoplasticity including phase transformations for graded thermo‐mechanical processing

AbstractTailored materials are of particular interest in modern engineering applications due to their potentially high strength and residual ductility. During hot forming processes, such as press hardening, microstructure transformations in low carbon steels are considered to achieve the production of tailored components. The relevant microstructure mechanisms during thermo‐mechanical processing are visco‐plastic deformation and phase transformations. Thus, we present an extension to an existing thermodynamically consistent continuum mechanical model. The evolution of phase transformations under inhomogeneous temperature loading in a homogeneous tensile specimen are illustrated.

Evaluation method for hot rolling & run out table cooling parameters

A new hot rolling & Run Out Table cooling simulator is constructed. The simulator permits continuous collation of process data including in-situ temperature measurement at numerous positions of the strip steel. Tensile properties and microstructural analysis of the hot rolled strip are correlated to the collated process data. A numerical model including phase transformation and latent heat evolution is constructed and calibrated against the experimental data. The resulting numerical model can be used to efficiently predict the optimal process parameters for manufacturing hot rolled strip steels, while the new hot rolling & Run Out Table cooling simulator can be used to efficiently validate the numerical simulation results before up-scaling to the industrial hot mill.

Brittle-to-ductile transition in Ti–Pt intermetallic compounds

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

Application of an Advanced Friction Model in Hot Stamping Simulations: A Numerical and Experimental Investigation of an A-Pillar Reinforcement Panel from Volvo Cars

Although hot stamping is now an established method, numerical analysis of the process is still a challenge. In industrial applications, process engineers use sophisticated models which consider complex physical effects including phase transformation, phase dependent flow curves, temperature dependent r-values and temperature dependent FLC’s. However, in most of the industrial applications the tribological system is still oversimplified by using a constant Coulomb friction coefficient. Currently, there is enough experimental evidence in literature showing that friction coefficients evolve during the process, depending on contact pressure, temperature and sliding velocity. This work focuses on accurate modelling of friction in hot stamping processes. The advanced modelling technique presented in this paper tracks each material point and calculates the corresponding coefficient of friction depending on the current process conditions. This technique was used on simple part geometries in order to generate fundamental knowledge. In this analysis, part geometry, process conditions and tool kinematics are varied. In the next step, an A-Pillar reinforcement panel from Volvo Cars was analyzed experimentally and numerically. It was seen that accurate friction modelling enhances the quality of the simulation results and evolving friction conditions have a direct effect on the feasibility analysis of industrial parts.

Microstructural Evolution in 42SiCr Steel in a High-Temperature Chamber Under Microscope Objective

Thanks to today’s computer programs, engineers can derive a large amount of information from chemical com-position of a material. This information includes phase transformation temperatures Ac1, Ac3, Ms and Mf. CCT and TTT diagrams for austenite decomposition can be determined as well. When steel is heated, austenite grains nu-cleate and grow in the initial microstructure. On cooling, the grains decompose into pearlite, ferrite, cementite and hardening phases. Transformations of this kind are thoroughly described in literature: the ways the individual phases form and the speed of their formation and temperatures involved. However, in-situ visual recordings of such transformations are relatively rare. With Linkam TS1400XY high-temperature stage and chamber integrat-ed in an optical microscope, one can observe phase transformations in situ during heating and cooling. This paper explores microstructural evolution in 42SiCr steel in the course of heat treatment in a high-temperature chamber on an optical microscope stage which offers observation of changes in the material right under the microscope objective.