Solid Phase Transformation Research Articles

Phase-field modelling of mechanical wave propagation in polycrystalline materials: Validation study

With all the great advances in phase-field modelling combined with continuum mechanics, a detailed accuracy and convergence analysis about the influence of the diffuse interface upon the dynamic mechanical energy is still missing. Based on the previous work, i.e. deriving the homogenization scheme on the basis of balance conditions and embedding the high-order discontinuous Galerkin method into the phase field method, the influence of the diffuse interface, especially the multiphase/multigrain interface, on the dynamic mechanical energy is studied by considering the factors, such as the wave types, the material properties, the phase/grain size, the interface width and the normal vector formula. The numerical results are compared with the sharp-interface results to show the accuracy and stability, and with the linear homogenization method to demonstrate the advantages of the proposed scheme. The research work in this paper will build the foundation for the future simulation of rapid solid phase transformation.

Concentration of transformation-induced plasticity in pseudoelastic NiTi shape memory alloys: Insight from austenite–martensite interface instability

Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined with in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid–solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite–martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves “smoothly” with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite–martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved.

Heating Techniques of Shape Memory Alloy (SMA) - A Review

Shape Memory Alloy (SMA) possess memory capability to revert its original shape when exposed to load or temperature changes. This unique thermomechanical property occurs during solid state phase transformations which corresponds to functional properties of SMA, shape memory effect (SME) and super elasticity (SE). The significant coupling behaviour of SMA can be utilized as actuator in aerospace, automotive, electrical and civil fields. However, in practical applications, the coupling behaviour of SMA are non-linear and hysteretic. The control mechanism of SMA coupling behaviour is complex. Therefore, in order to achieve good control of thermomechanical properties, a highly controllable and promising heating technique is required. Thus, this paper reviewed the existing heating techniques for the SMA intending to find a controllable heating technique for the SMA. Besides that, this review suggested the promising induction heating for SMA thermomechanical characterization which offer temperature controllability and faster heating capability. However, till date most of research works are purely empirical. The present paper is able to provide an insight on the experimental approaches toward induction heating of SMA. Thus, the main aim of this paper is to provide a better understanding on the heating mechanism of SMA to develop an optimized utilization of SMA as an actuator.

Synthesis of NaA zeolite via the mesoscale reorganization of submolten salt depolymerized kaolin: A mechanistic study

Deep understanding of the formation mechanism is crucial to the rational design and controllable synthesis of zeolites with desired pore architecture and acidity. Herein, we take the synthesis of NaA zeolite from a submolten salt depolymerized kaolin (SMS-K) as an example to elucidate the formation mechanism of NaA zeolite. By using SMS-K as sole Si and Al sources, a highly pure and crystalline NaA zeolite with superior ion exchange ability was synthesized. By using various ex-situ and in-situ characterizations and studying the crystallization kinetics, a plausible formation mechanism of NaA zeolite via the mesoscale reorganization of SMS-K was proposed. The results indicate that the reorganization of SMS-K into NaA zeolite follows the solid-phase transformation route. Upon being mixed with H2O, SMS-K that is mainly composed of silicate monomers and chain/ring microstructures is in-situ transformed rapidly into mesoscale intermediates containing double four-membered rings and β cages, sufficient stirring during aging generates more mesoscale intermediates. During the subsequent crystallization process, the mesoscale intermediates are further self-assembled into NaA nanoparticles when theirs amount reaches a maximum, and finally the nanoparticles grow into well-shaped cubic NaA zeolite particles with smooth surfaces through layer spreading. Our work provides the theoretical foundation for the design and green synthesis of zeolites directly from the natural minerals.

A Multi-Phase Modeling Framework Suitable for Dynamic Applications

Under dynamic loading conditions and the associated extreme conditions many metals will undergo phase transformations. The change in crystal structure associated with solid–solid phase transformations can significantly alter the subsequent mechanical response of the material. For the interpretation of experiments involving dynamic loading it is beneficial to have a modeling framework that captures key features of the material response while remaining relatively simple. We introduce a candidate framework and apply it to the metal tin to highlight a range of behaviors that are captured by the model. We also discuss potential extensions to capture additional behaviors that could be important for certain materials and loading scenarios. The model is useful for analysis of results from dynamic experiments and offers a point of departure for more complex model formulations.

A level-set formulation to simulate diffusive solid/solid phase transformation in polycrystalline metallic materials - Application to austenite decomposition in steels

Numerous full-field numerical methods exist concerning the digital description of polycrystalline materials and the modeling of their evolution during thermomechanical treatments. However, these strategies are globally dedicated to the modeling of recrystallization and grain growth for single-phase materials, or to the modeling of phase transformations without considering recrystallization and related phenomena. A generalized numerical framework capable of making predictions in a multi-phase polycrystalline context while respecting the concomitance of the different microstructural mechanisms is thus of prime interest. A novel finite element level-set based full-field numerical formulation is proposed to principally simulate diffusive solid–solid phase transformation at the mesoscopic scale in the context of two-phase metallic alloys. A global kinetic framework, capable of accounting for other concomitant mechanisms such as recrystallization and grain growth is considered in this numerical model. The proposed numerical framework is shown to be promising through a couple of illustrative 1D and 2D test cases in the context of austenite decomposition in steels and compared with ThermoCalc estimations.

A Simulation Study on the Effect of Residual Stress on the Multi-Layer Selective Laser Melting Processes Considering Solid-State Phase Transformation.

The selective laser melting (SLM) manufacturing process is a complex process involving moving a molten pool, rapid non-equilibrium solidification and solid phase transformation. If the thermal residual stress is too large, it may lead to warping, cracking and failure of the structures. The present work aims to establish a thermo-mechanical framework to predict temperature evolutions, molten pool configurations and residual stresses of materials in the SLM process, based on the toolpath-mesh intersection method. Moreover, the influences of the laser power, process parameters and mesh size have been discussed. The stress concentration occurred at the interface between the melt layer and substrate results in warping deformation of the materials. This work provides a novel method to reveal the resulting physical mechanism inside the molten pool in terms of residual stresses and distortions.

Rapid Microstructure Homogenization of a Laser Melting Deposition Additive Manufactured Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Alloy by Electropulsing.

The typical microstructure of the laser melting deposition (LMD) additive-manufactured Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy (TC11) contains the heat-affected bands (HABs), the narrow bands (NBs) and the melting pools (MPs) that formed due to the reheating and superheating effects during the layer-by-layer manufacturing process. Characterization results indicated that the coarse primary α lath (αp) and transformed β (βt) structures were located in the HABs, while the fine basketweave structure was formed inside the MPs. The rapid modifications of microstructure and tensile properties of the LMD-TC11 via electropulsing treatment (EPT) were investigated. The initial heterogeneous microstructure transformed into a complete basketweave structure and the HABs vanished after EPT. Thus, a more homogeneous microstructure was achieved in the EPT sample. The ultrafast microstructural changes were mainly attributed to the solid state phase transformation during electropulsing. The tensile properties of the sample were basically stable, except that the yield strength decreased as EPT voltage increased. This study suggests that EPT could be a promising method to modify the microstructure and mechanical properties of the additive-manufactured alloys in a very short time.

Cenosphere-Based Zeolite Precursors of Lutetium Encapsulated Aluminosilicate Microspheres for Application in Brachytherapy.

Coal fly ash hollow aluminosilicate microspheres (cenospheres) of stabilized composition (glass phase—95.4; (SiO2/Al2O3)glass—3.1; (Si/Al)at. = 2.6) were used to fabricate lutetium-176 encapsulated aluminosilicate microspheres as precursors of radiolabeled microspheres applied for selective irradiation of tumors. To incorporate Lu3+ ions into cenosphere’s aluminosilicate material, the following strategy was realized: (i) chemical modification of cenosphere globules by conversion of aluminosilicate glass into zeolites preserving a spherical form of cenospheres; (ii) loading of zeolitized microspheres with Lu3+ by means of ion exchange 3Na+ ↔ Lu3+; (iii) Lu3+ encapsulation in an aluminosilicate matrix by solid-phase transformation of the Lu3+ loaded microspheres under thermal treatment at 1273–1473 K. Two types of zeolitized products, such as NaX (FAU) and NaP1 (GIS) bearing microspheres having the specific surface area of 204 and 33 m2/g, accordingly, were prepared and their Lu3+ sorption abilities were studied. As revealed, the Lu3+ sorption capacities of the zeolitized products are about 130 and 70 mg/g Lu3+ for NaX and NaP1 microspheres, respectively. It was found that the long-time heating of the Lu3+-loaded zeolite precursors at 1273 K in a fixed bed resulted in the crystallization of monoclinic Lu2Si2O7 in both zeolite systems, which is a major component of crystalline constituents of the calcined microspheres. The fast heating–cooling cycle at 1473 K in a moving bed resulted in the amorphization of zeolite components in both precursors and softening glass crystalline matter of the NaX-bearing precursor with preserving its spherical form and partial elimination of surface open pores. The NaX-bearing microspheres, compared to NaP1-based precursor, are characterized by uneven Lu distribution over the zeolite-derived layer. The precursor based on gismondin-type zeolite provides a near-uniform Lu distribution and acceptable Lu content (up to 15 mol.% Lu2O3) in the solid phase.

Engulfment of a Particle by a Growing Crystal in Binary Alloys

Under quasi-steady particle pushing conditions in an alloy, fresh liquid has to flow to the gap separating a particle and an advancing solid–liquid interface of a crystal to feed the volume change associated with the liquid–solid phase transformation. In the meantime, solute rejected by the growing crystal has to diffuse out of the gap against the physical feeding flow. An inequality equation was derived to estimate the pushing-to-engulfment transition (PET) velocity of the crystal under which the particle is pushed by the growing crystal. Experiments were performed in an Al-4.5 wt.%Cu-2 wt.% TiB2 composite under isothermal coarsening conditions. TiB2 particles were indeed engulfed by the growing aluminum dendrites as predicted using the inequality equation. Predictions of the inequality equation also agreed reasonably well with literature data from the solidification of distilled water containing particles obtained under minimal convection conditions. The inequality equation suggests that the PET velocity is much smaller in a binary alloy than that in a pure material. Without the influence of fluid flow or other factors that put a particle in motion in the liquid, the particle should be engulfed by the growing crystal in alloys solidified under normal cooling rates associated with convectional casting conditions.

Features of microstructure, phase composition and strengthening capability of stainless steels with 13 – 17 % Cr

The paper considers the study of the features of structure and phase transformations in high-strength, resistant to carbon dioxide corrosion, complex alloyed steels of martensitic, austenitic-martensitic and martensitic-ferritic classes with 13 – 17 % Cr. Influence of the alloying on crystallization and solid state phase transformations was revealed in the temperature range of hot deformation and heat treatment using thermodynamic modeling and experimental study. The effect of quenching temperature on the phase composition and microstructure was analyzed as a result of X-ray diffraction phase analysis, optical and transmission electron microscopy. It was found that increase of nickel content leads to growth of retained austenite fraction resulting in significant decrease of yield strength along with high tensile strength and elongation. To obtain predominantly martensitic microstructure in martensitic-austenitic steel, the multistage heat treatment is proposed including quenching, intermediate annealing for precipitation of dispersed carbides and tempering forming final mechanical properties. The composition of precipitated carbides was evaluated by X-ray microanalysis. The results of the tensile test for steels with martensitic and martensitic-ferritic microstructure showed that required strength grade (σ0.65 ≥ 862 MPa; σв ≥ 931 MPa) was reached after heat treatment including quenching and tempering. Multistage heat treatment including quenching, intermediate annealing and final tempering was resulted in required strength properties of high-nickel martensitic-austenitic steel with 15 % Cr.

Study of microstructure and compressive strength of a novel Ti-0.75Al-9.5V alloys manufactured by laser deposition melting

A novel Ti-0.75Al-9.5 V alloy is designed based on the typical Ti-6Al-4V alloy and prepared by laser melting deposition in this study. The microstructure, texture, and properties of the different scanning and building planes of the LMDed samples under a serpentine reciprocating scanning strategy are characterized and tested. It is found that the more rapid cooling rate on the building planes leads to a partial morphology transformation of the α phase from the original rod-like shape to a needle-like shape. Furthermore, different maximum thermal stress gradients result in the formation of (110)β texture and (001)β texture on different building planes while The (110)β // (0001)α crystallographic orientation relation leads to the formation of the (21¯1¯0)α texture on both two building planes during solid-phase transformation. The texture evolution on two different building planes also affects corresponding surface slip and mechanical properties. The (001)β texture leads to parallel surface slip tracks on its building plane. The strong (110)β and (21¯1¯0)α textures increase the Vickers hardness from 348.6 ± 2.6 to 371.5 ± 2.7 HV and the compressive yield strength from 1188.0 ± 1.6 to 1276.5 ± 7.3 MPa but suppress the plastic strain from 16.7 ± 0.9% to 12.6 ± 1.0%. All the samples have their compressive fractural strength nearing 1730 MPa, which is higher than that of the reported LMDed Ti-6Al-4V alloy (1690 MPa).

Schmid factor crack propagation and tracking crystallographic texture markers of microstructural condition in direct energy deposition additive manufacturing of Ti-6Al-4V

Metallic additive manufacturing (AM) provides a customizable and tailorable manufacturing process for new engineering designs and technologies. The greatest challenge currently facing metallic AM is maintaining control of microstructural evolution during solidification and any solid state phase transformations during the build process. Ti-6Al-4V has been extensively surveyed in this regard, with the potential solid state and solidification microstructures explored at length. This work evaluates the applicability of previously determined crystallographic markers of microstructural condition observed in electron beam melting powder bed fusion (PBF-EB) builds of Ti-6Al-4V in a directed energy deposition (DED) build process. The aim of this effort is to elucidate whether or not these specific crystallographic textures are useful tools for indicating microstructural conditions in AM variants beyond PBF-EB. Parent β -Ti grain size was determined to be directly related to α -Ti textures in the DED build process, and the solid state microstructural condition could be inferred from the intensity of specific α -Ti orientations. Qualitative trends on the as-solidified β -Ti grain size were also determined to be related to the presence of a fiber texture, and proposed as a marker for as-solidified grain size in any cubic metal melted by AM. Analysis of the DED Ti-6Al-4V build also demonstrated a near complete fracture of the build volume, suspected to originate from accumulated thermal stresses in the solid state. Crack propagation was found to only appreciably occur in regions of slow cooling with large α + β colonies. Schmid factors for the basal and prismatic α -Ti systems explained the observed crack pathway, including slower bifurcation in colonies with lower Schmid factors of both slip systems. Colony morphologies and localized equiaxed β -Ti solidification were also found to originate from build pauses during production and uneven heating of the build edges during deposition. Tailoring of DED Ti-6Al-4V microstructures with the insight gained here is proposed, along with cautionary insight on preventing unplanned build pauses to maintain an informed and controlled thermal environment for microstructural control.

Solid–solid phase transformation of aspirin at high pressures and room temperatures

Drug molecules undergo changes to their intermolecular binding patterns under extreme conditions, leading to structural phase transitions which produce different polymorphs. Polymorphism of aspirin (acetylsalicylic acid), one of the most widely consumed medications, has attracted many scientists, chemists and pharmacologists to identify its stable polymorphs and phase transformations at ambient temperatures and pressures. Here, density functional theory at the ωB97XD/6-31G* functional level is utilized to calculate the lattice constants, volumes, Gibbs free energies, vibrational spectra, stabilities and phase transitions of aspirin forms I and II at different pressures and temperatures. These computations confirm that phase transformation occurs between these two forms of aspirin at higher pressures (from 3 to 5 GPa) and near room temperatures. Taking aspirin as a case study, this work can help design, produce and store drugs, guiding scientists, chemists and pharmacologists to perform further experiments.

Solid phase transformation effects on stress and strain in the thick plate EH40 welded butt joint

To accurately predict welding-induced residual stress and strain of high strength steel thick plate EH40 butt joint in single-pass full penetration laser welding, a complete solid phase transformation model according to the actual weld microstructure was proposed through material constitutive development. At the same time, the Case 1 considering the traditional thermo-elastoplastic were used as comparative study. The simulation calculation results and experimental results of each phase content were compared and verified. The stress and strain distribution of welded joints and the evolution mechanism of nodes in three zone including fusion zone, heat-affected zone and base metal were deeply studied and compared. The results indicated that the solid phase transformation had an obvious effect on the peak and distribution of longitudinal, transverse, normal and equivalent residual stress, as well as the stress gradient at the junction of heat-affected zone and BM. It increased the peak value of longitudinal residual compressive stress from –103 MPa to –768 MPa, with an increase of 645.6%. The stress gradient at the junction of heat-affected zone and base metal was up to 1299 MPa. The state distribution of longitudinal and transverse stress in the weld and its vicinity were completely opposite to the Case 1. The peak value of transverse plastic compressive strain increased from –0.059 to –0.111, with an increase of 88.1%. The fusion zone and heat-affected zone of the plastic strain in welded joints were more easily observed. It was found that martensitic transformation was the main reason for the obvious difference of stress and strain distribution and the evolution mechanism of stress and strain in the later stage of cooling stage.

Effect of multiple thermal cycles on the microstructure evolution of GA151K alloy fabricated by laser-directed energy deposition

The microstructure and mechanical properties of high-performance Mg-RE (Magnesium rare earth) alloys prepared through laser-directed energy deposition (LDED) are worthy of an in-depth study, due to their excellent application perspectives in the aviation industry. In this work, a nominal Mg-15Gd-1Al-0.4Zr wt% (GA151K) alloy was fabricated by LDED, and the solid phase transformation induced by the application of multiple thermal cycles was investigated in detail. Moreover, in the last deposited layer, fine α-Mg grains, particle-like secondary phases, and intergranular island-like phases were observed. In addition, in the second layer from the top, α-Mg grains grew slightly by 1.4 ± 1.0 µm and the fraction of the intergranular island-like phase was significantly decreased by 9.3 ± 2.1%. As the deposition process progressed, the pre-solidified layers experienced additional thermal cycles, and two solid phase transformations occurred. One of the recorded transformations was that of the intergranular phase from Mg 3 Gd to Mg 5 Gd, while the other one was the precipitation of β′-Mg 7 Gd within the α-Mg matrix from the supersaturated solid solution. Here, the thermal cycles acted as an incomplete solution and aging treatment, leading to the unique morphology of the as-built GA151K alloy. The microhardness was improved by 6.9 ± 5.0 HV 0.2 with the application of consecutive thermal cycles. This work established a simplified relationship between the process parameters, the microstructure, and the mechanical properties of LDED Mg-Gd alloys, which is expected to promote the development and application of LDED high-performance Mg-RE alloys. • The transformation of intergranular phase from Mg 3 Gd to Mg 5 Gd and precipitation of β′ in AM Mg-Gd alloys are firstly reported. • The microstructure evolution principle of LDED GA151K under the influence of thermal cycles is studied in detail. • The importance of in situ precipitation strengthening to AM Mg-RE alloy is proposed.

Atomic-Scale Investigation of the Pattern Formation of Quasicrystal Growth by Phase-Field Crystal Simulations

How quasicrystal nuclei grow into diversified patterns is of great significance for both quasicrystal research and crystal growth theory. Here, using the phase-field crystal model, we investigate the pattern formation of decagonal quasicrystal growth on the atomic scale. The results show that as the growth driving force increases, the obtained patterns change from equilibrium decagonal polyhedral and dendritic patterns to circular shapes, accompanying the transition from the slow-growth mode into the rapid growth mode. Meanwhile, the growth mechanisms undergo transitions from perfect matching growth and defect-repairing growth to decoupling growth, leading to the variation of liquid–solid phase transformation paths. Based on these results, we unravel the atomic-scale mechanisms for the morphological evolution processes from small 10-fold nuclei to diversified patterns. Our study not only contributes to a systematic understanding of the pattern formation of quasicrystal growth but can also enrich crystal growth theory by clarifying the common points and differences in the pattern formation and growth mechanisms between crystal and quasicrystal growth.

Relationship between Austenite‐Stabilizing Elements and Austenite Fraction in Near‐Rapidly Solidified Fe–Mn–Al–C Lightweight Steel

A peritectic reaction (liquid + δ‐ferrite → γ‐austenite) and eutectoid reaction (γ‐austenite → α‐ferrite + κ carbide) occur in the Fe–Mn–Al–C lightweight steel. Experimental results show that the main constituent phase of the near‐rapidly solidified Fe–Mn–Al–C lightweight steel is γ‐austenite and/or δ‐ferrite in this study, and the eutectoid reaction is restrained by the rapid cooling rate. With the increase of C content, the γ‐austenite fraction of near‐rapidly solidified medium‐Mn and high‐Mn Fe–Mn–Al–C lightweight steels increase significantly. However, in the early study, the increased Mn content had little effect on the γ‐austenite fraction of the near‐rapidly solidified Fe–(8/12/16)Mn–9Al–0.8C steel strips. This study attempts to reveal the reasons for the different effects of Mn and C on Fe–Mn–Al–C lightweight steel strip under near‐rapid solidification. It is found that the γ‐austenite fraction of the near‐rapidly solidified Fe–Mn–Al–C steel strip is closely related to the γ‐austenite fraction at the end of equilibrium solidification. This suggests that the main constituent phase of lightweight steel strip depends on its liquid–solid phase transformation under near‐rapid solidification and is seldom affected by solid phase transformation.

Nanocrystals with metastable high-pressure phases under ambient conditions.

The ambient metastability of the rock-salt phase in well-defined model systems comprising nanospheres or nanorods of cadmium selenide, cadmium sulfide, or both was investigated as a function of composition, initial crystal phase, particle structure, shape, surface functionalization, and ordering level of their assemblies. Our experiments show that these nanocrystal systems exhibit ligand-tailorable reversibility in the rock salt-to-zinc blende solid-phase transformation. Interparticle sintering was used to engineer kinetic barriers in the phase transformation to produce ambient-pressure metastable rock-salt structures in a controllable manner. Interconnected nanocrystal networks were identified as an essential structure that hosted metastable high-energy phases at ambient conditions. These findings suggest general rules for transformation-barrier engineering that are useful in the rational design of next-generation materials.

ExaCA: A performance portable exascale cellular automata application for alloy solidification modeling

Modeling the as-solidified grain structures that form during alloy processing is a critical component in understanding process-property relationships, particularly for additive manufacturing (AM) where grain structure is very sensitive to processing conditions. While cellular automata (CA)-based models have proven able to predict aspects of microstructure for several alloys and AM process conditions, long run times and large resource sets required limit the utility and the problem size to which existing CA models can be applied. As part of the ExaAM project, an initiative within the Exascale Computing Project (ECP) to develop, test, and optimize an exascale-capable coupled and self-consistent model of AM parts, we developed ExaCA (https://github.com/LLNL/ExaCA) for the liquid–solid phase transformation in the wake of AM melt pools. The CA-based code is parallelized using MPI and the Kokkos programming model, the latter enabling simulation on both CPUs and GPUs within a single-source implementation. We detail the steps taken to transform a baseline, MPI-based CA code into one that is performant on CPUs and GPUs. Performance testing of ExaCA on Summit (a pre-exascale machine at Oak Ridge National Laboratory) was used to quantify CPU–GPU speedup comparing with equal numbers of nodes. Testing showed comparable CPU performance to the MPI-only CA code and a 5-20x speedup when running AM-based test problems using GPUs. The improved performance of CA through GPU utilization and the performance portable nature of ExaCA will enable accurate part-scale modeling by harnessing the power of current and future generations of high performance computing resources. Future work will include improving the strong scaling of ExaCA on GPUs by reducing load imbalance associated with the locality of the problem, and continuing performance optimization across exascale hardware.