Stage Of Solidification Research Articles

Experimental and constitutive modelling studies of type 316L stainless steel based on internal stress under low cycle fatigue and creep-fatigue interaction

Low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of 316L stainless steel at 550 °C were investigated using experimental and modelling approaches. Results demonstrate that friction and back stress contribute to the initial hardening stage, whereas the former is primarily responsible for the subsequent softening stage. Besides, friction stress also accounts for the non-masing feature and varied softening rate. Increasing holding time induces a rise followed by a decline in friction stress, while the back stress decreases steadily to a saturated value. Moreover, a short tensile holding duration leads to planar structures and higher dislocation density. However, a longer tensile holding time promotes dislocation annihilation and subgrains growth, and elongates dislocation cells. Finally, an improved viscoplastic constitutive model was proposed based on the internal stress analysis. The kinematic hardening rule was improved by considering the cycle-dependent plastic modulus and stress relaxation, and the non-masing and variable softening rate were captured by modifying the isotropic hardening rule. In addition, holding time-related factors were also coupled into the model. The proposed model is robust for describing the cyclic features under both LCF and CFI.

Microstructure and mechanical properties of WRe/GH3128 alloy electron beam welded joint

This study investigates electron beam welding of WRe/GH3128 alloy, focusing on the microstructure, defects, mechanical properties, and fracture characteristics of the welded joint. Competitive grain growth within the weld, attributed to solidification rate and grain orientation, as well as the influence of electron beam keyhole oscillation on grain morphology, is analyzed. The study discusses the uneven distribution of solute concentration within the solid phase during the non-equilibrium solidification stage of the weld, along with dendritic segregation and phase transformation processes. Segregation of Cr, Re, and C elements in the interdendritic region results in abundant precipitation of σ phase and M6C. Non-equilibrium solidification phase fractions and phase transformation processes within the weld are calculated using JMartPro software. Furthermore, the potential relationship between the uneven diffusion of elements within the reaction layer and the formation of intermetallic compounds is discussed. XRD testing confirms the presence of Cr7Re3, Ni4W, Ni4Mo, and M6C within the reaction layer. The microhardness and tensile properties of the joints are evaluated, with a tensile strength of 600 MPa, representing 77% of GH3128's strength, while the elongation is only 2.5%. The increase in weld microhardness is attributed to changes in grain morphology and precipitation of M6C and σ phases, with M6C reducing joint plasticity. Cracks and defects are observed in the WRe heat-affected zone, and the role of metallurgical bonding and solid-state healing in crack repair is elucidated. Finally, an analysis is conducted on factors such as crystal microstructure defects that can lead to crack nucleation and propagation, providing separate explanations for intergranular fracture in the WRe base material and brittle fracture in the reaction layer.

Solidification microstructure characteristics and their formation mechanism of K447A nickel-based superalloy for dual-performance blisk

In this paper, the effects of cooling rate after casting and stirring process on γ’ phase, γ / γ’ eutectic phase, MC type carbide morphology and grain structure of K447A alloy turbine blisk were studied systematically. The quantitative relationships between cooling rate, stirring process and γ’ phase, γ / γ’ eutectic phase, MC type carbide and grain size were described in detail. The results show that with the increase of cooling rate, the distribution region of γ / γ ‘eutectic phase from the center of blisk to blade decreases gradually and MC type carbide changes from skeleton to block due to the shorter formation time of eutectic and carbide in the later stage of solidification. Besides, with the decrease of cooling rate, the size of γ’ phase increases under the influence of interfacial strain energy and elastic strain energy, and the morphology of γ’ phase gradually evolves from cuboid to concave cuboid, octet and dendritic. At the same time, due to the synergistic effect of cooling rate and stirring process, the blade part is columnar grain and the central part of the blisk is equiaxed grains. In this paper, the evolution mechanism of γ ‘phase, γ / γ’ eutectic phase, MC type carbide and grain size with cooling rate and stirring process was proposed. The research results would play a key supporting role in precise control during solidification for microstructures and properties of dual-performance superalloy blisk.

Evolution of Dendritic Austenite in Parallel With Eutectic in Compacted Graphite Iron Under Three Cooling Conditions

Shrinkage defects are common problems in industrially produced metal cast components. Local density changes occur during freezing, which demand material transport between parts of the casting, often involving flow of liquid through partially solid regions. Cast alloys typically freeze with a dendritic morphology, which large interface against the liquid restricts liquid flow. Recent research also indicates that this dendritic structure has an impact on the mechanical properties of the final material. For these reasons it is important to understand and predict the evolution of this structure through the solidification of cast alloys. In this work, the evolution of the dendritic austenite structure is investigated in a near-eutectic compacted graphite iron solidified under three different cooling conditions. The solidification was interrupted by water quenching, enabling characterization of the dendritic austenite structure at different stages of solidification. Higher cooling rate was found to promote a more coherent dendritic austenite structure which constituted a larger volume fraction. In parallel with growth of the eutectic, the amount of dendritic austenite in extra-eutectic regions continued to rise. This rise was associated with both tip growth of new dendrites and with growth by thickening of existing dendrites.

Patterns of localized deformation at pre-fracture stage in carbon steel – stainless steel bimetal

The work is devoted to the study of strain localization at macroscale level during parabolic mechanical hardening and pre-fracture under quasi-static loading of a carbon steel – stainless steel bimetal. The problem of estimating the scale of the phenomena that determine plasticity is decisive in the development of any theories of plastic deformation, in particular, dislocation theories. The main difficulty in constructing such theories is the reconciling the dislocation scales, characteristic for most deformation and mechanical hardening mechanisms, with macroscopic parameters of deformation processes. In the framework of the autowave model of localized plastic deformation, this problem can be reduced to the possibility of obtaining parameters from the results of macroscale observations of localized plastic flow development. During the experiments, it was confirmed that in a bimetal at any forming stage, a specific pattern of localization centers distribution is spontaneously generated - a pattern of localized plastic flow. The shape of such patterns is determined by the law of mechanical hardening acting in the material. It is shown that the observed localization patterns can be used as an informative feature in predicting the plasticity margin. In the process of uniaxial tension at the stage of parabolic mechanical hardening of the bimetal, the deformation mode is realized with the formation of several potential fracture centers. It was established that at the pre-fracture stage, during the time evolution of the wave pattern of deformation localization, the zone of active plastic deformation narrows, but the number of centers in it either remains the same with a decrease in the distance between them, or even increases. The result of this process is the formation of a macroscopic neck, and then fracture. At the pre-fracture stage, the collapse point indicates the place of future fracture and signals the need to stop the deformation process in order to avoid the fracture of the bimetallic material. Thus, the well-known manifestation of deformation macroscopic localization – formation of a neck – is preceded by complex phenomena of mutually coordinated motion of localized plasticity centers at the pre-fracture stage.

Mechanical properties and microstructure evolution of NbSiZr based alloy

To improve the mechanical properties of Nb-Si-Zr based superalloys at room temperature, the effect of a trace amount of Cr, C, Hf, Ta and Sc elements on the solidification processes, phase composition, mechanical properties and fracture behavior of Nb-Si-Zr based superalloys are studied by vacuum non-consumable arc-melting. The results show that the based alloy consists Nbss phase, γ-(Nb,X)5Si3 phase and Zr-rich γ-(Nb,X)5Si3 phase. However, there is no Zr-rich γ-(Nb,X)5Si3 phase in 20Zr-2C and 20Zr-4Hf alloys, indicating that a trace amount of C or Hf elements accelerates the diffusion of Zr element, and improves the segregation of the Nb-Si-Zr based alloys for the later stage of the superalloy solidification. The Nbss fraction in the matrix alloy (20Zr alloy) is 60%. C element increases the Nbss phase fraction by 4%, and Cr and Sc elements reduce the Nbss phase fraction by 4% and 6%, Hf and Ta elements reduce the Nbss phase by 1% and 2% compare with the matrix alloy (20Zr alloy). C element is dissolved in Nbss phase and γ-(Nb,X)5Si3 phase, while Cr element is more dissolved in Nbss phase and Sc element is more dissolved in Zr-rich γ-(Nb,X)5Si3 phase. The toughness of 20Zr-2 C alloy is the highest, and its KQ value reaches 14.05 MPa·m1/2, compared with the matrix alloy (20Zr alloy) increased by 23.46%. The enhancement of Nbss phase fraction and petal shaped microstructure make the crack propagation more difficult, thus increasing the KQ value of the 20Zr-2C alloy. The compressive properties of 20Zr-4Cr alloy are the best, and its compressive strength reaches 1807.3 MPa, compared with the matrix alloy (20Zr alloy) increased by 17.09%. The reason is that a trace amount of Cr element increases the fraction of γ-(Nb,X)5Si3 phase.

Highly oriented microtube arrays of decagonal quasicrystal approximants in Al20Si20Mn20Fe20Ga20 high-entropy alloy

The unique structures of micro/nanotubular materials have received extensive attention, while the micro/nanotubular structures of quasicrystals or quasicrystal-related phases, especially oriented arrays, are rarely found. Here, oriented microtube arrays of high-entropy decagonal quasicrystal approximants are prepared in Al20Si20Mn20Fe20Ga20 alloy by simple melt spinning. Transmission electron microscopy revealed microtubes are mostly composed of alternating arrangements of (1/0, 2/1) quasicrystal approximant domains (lattice constants: a = 0.73 nm, b = 1.23 nm, and c = 2.24 nm) and defective (1/0, 2/1) domains. However, heat treatment leads to the change of the structure and morphology of microtubes. At 873 K, a primitive cubic phase existed, and at 973 K, microtube morphology disintegrated and the (1/0, 2/1)-related domain transformed into complex domains, showing instability. By nanoindentation testing, microtubes show the hardness of 10.9 GPa and the elastic modulus of 189.3 GPa. However, after heat treatment at 873 K, these values decreased to 5.3 GPa and 128.4 GPa, and to 3.2 GPa and 107.9 GPa at 973 K. We demonstrate that the formation of microtubes is mainly related to the temperature gradient, twinning growth of approximant domains, and also the insufficient supply of raw materials in the later stages of solidification.

Microstructure characteristics of a γ single-phase Ti-55Al-7.5Nb alloy fabricated via additive manufacturing

Developing a new ordered single-phase TiAl alloy, with no solid-state transformation operated at a long-term high temperature, is important in the aerospace industry. Here, the preparation of newly Ti-55Al-7.5Nb alloy via direct laser deposition (DLD) is proposed and studied. The oriented grain growth in Ti-55Al-7.5Nb alloy is successfully achieved by using laser additive manufacturing (AM) without interlayer pause during the laser scaning back and forth. AM features a rapid cooling rate while Ti-55Al-7.5Nb alloy owns a narrow L → α and α → γ phase transformation temperature window, and then enabling the TiAl liquid phase directly transform into a γ single-phase, which is confirmed by the combination of experiments and thermodynamic calculations. The heat treatment in the AM thermal cycles induces a thermal gradient, promoting the new recrystallized grains growing base on the preferred growth of [111]γ along the heat flow during the DLD solidification stage. The AM Ti-55Al-7.5Nb alloy exhibits continuous γ single-phase columnar grains, meanwhile, accompanied by the twins growing along the building direction.

Predicting solidification cracking in directed energy deposition of Hastelloy X alloys based on thermal-mechanical model

Directed energy deposition (DED) has received extensive attention in recent years owing to its advantages of near-net forming and high design freedom. However, the thermal stress induced by rapid heating and cooling during fabrication of DEDed Hastelloy X (HX) alloys usually results in solidification cracking. To suppress or even eliminate solidification cracking by optimizing processing parameters, it is necessary to establish a correlation between stress in the last stage of solidification and solidification cracking sensitivity (SCS) of DEDed HX alloys. In this study, we first confirmed the critical role of stress in solidification cracking by comparing the simulation results calculated by a thermal-mechanical model with an experimental crack distribution and then proposed a stress-related index for describing the SCS. By calculating the index in different deposited regions and with different processing parameters, we successfully predicted the crack-prone regions at the part scale and the relationship between processing parameters and SCS. The results show that there are almost no cracks in the edge region of the DEDed HX alloy sample, while cracks are mainly generated in the region within a quarter of the width from the edge of the DEDed HX alloy sample. Moreover, solidification cracking can be effectively suppressed under a high heat input. All these simulated results are in good agreement with the experimental results. These findings are of significance to understand the solidification cracking mechanism in DED, which can be used to guide the fabrication of DED crack-free HX alloys.

Investigating build orientation-induced mechanical anisotropy in additive manufacturing 316L stainless steel

Build orientation-induced mechanical anisotropy is critical for additive manufacturing metals. To leverage the existing research on anisotropy between Z and X/Y build orientations, this work focuses on the anisotropic behavior within 2D planes. Therefore, tensile coupons of 316L stainless steel (316L-SS) were fabricated in five different build orientations (X, Y, XY45°, Z, and ZX45°) using the laser powder bed fusion process with a pulsed laser. Different characterization techniques at various length scales were employed to investigate the mechanisms of mechanical anisotropy. According to the electron back-scattered diffraction (EBSD) study, X and Y samples had a larger fraction of <001> and <110> grains, whereas XY45° coupons had a larger fraction of <111> grains along the tensile axis. In contrast, Z samples exhibited a dominant <110> texture parallel to the tensile axis. X-ray diffraction analysis revealed that all the samples had a single austenitic phase but different dislocation densities. The tensile results of all samples showed higher yield strength and comparable ductility with those in the literature. Among all the build orientations, XY45° coupons had the highest yield strength due to the large fraction of <111> grains oriented along the tensile axis and the highest dislocation density of 2.2 × 1015 m−2. In contrast, the highest ductility of Z samples was caused due to twinning-favored <110> crystallographic texture along the tensile axis. The onset and termination of different stages of strain hardening were mainly affected by the cellular sub-grain structure and crystallographic texture, as well as their interactions with dislocation generation and evolution. These findings indicated that crystallographic texture and dislocation density were the dominant contributions to the mechanical anisotropy followed by grain morphology.

Effectively mitigated macro-segregation and improved tensile properties of twin-roll cast Al–Mg–Si (6022) alloy strips via Zr addition

It is a great challenge to avoid the central macro-segregation during the solidification process of twin-roll casting (TRC), which seriously limits the mechanical properties of 6xxx series Al alloys. In this study, we find that the trace addition of 0.08 wt% Zr to Al-1.0Mg-1.2Si (6022) alloy can effectively promote the columnar-to-equiaxed transition and refine the solidification microstructure (particularly α-Al dendrites and π-AlFeMgSi phase). More importantly, the macro-segregation of the as-cast TRC strip is well mitigated with Zr addition. The refinement of α-Al dendrites and π-AlFeMgSi phase may be attributed to the formation of (Al, Si)3Zr phases when introducing Zr element, which can act as heterogeneous nucleation sites for both of them. The secondary dendrite arm spacing (SDAS) in the central region of strips is sharply decreased from ∼33 to ∼16 μm, which facilitates the incorporation of more solute elements into the Al matrix, thus increasing the partition coefficients of Mg (kMg) and Si (kSi) elements. As a result, there are fewer solute atoms in the residual liquid phase during the final stage of solidification, leading to the alleviation of central macrosegregation. The tensile properties of the as-cast, solid solution treated (T4) and paint baking treated (T6) alloys are synchronously optimized with Zr addition. In particular, the fracture elongation of the T4 alloy is increased from ∼30.9% to ∼33.4%, indicating excellent formability. The yield strength of the T6 alloys is increased from ∼227 MPa to ∼239 MPa, satisfying the more demanding service requirements. This work can provide a strategy for the effective control of macro-segregation and produce alloy strips with high performance by the TRC process.

Formation Mechanism of Secondary Inclusions in Fe-36mass%Ni Alloy Using a Novel Combination Analysis Technique

Controlling the size, number, and composition of secondary inclusions is vital in the production of high-quality steels. In this study, experimental and computational investigation of the relationship between secondary inclusion formation in Fe-36mass%Ni alloy and cooling rate was carried out. Assuming the case of large ingots, solidification experiments using various cooling rates (0.17 to 128 K/min) were employed and the size, number, composition, and distribution of inclusions were analyzed by SEM-EDS automatic inclusion analysis. Like previous studies, inclusion number density increased with increasing cooling rate, while inclusion size decreased with increase of cooling rate. On the contrary, oxide inclusion area fraction was found to have little relationship with the cooling rate and was instead found related with oxygen content of the sample. As a new attempt to investigate the relationship between microsegregation and secondary inclusion formation, a combination of SEM-EDS analysis and EPMA mapping analysis was carried out. By superimposing information of microsegregation and inclusions, it was found that high-Al2O3 inclusions formed during the early stage of solidification, whereas low-Al2O3 inclusions formed during the later stage of solidification. These findings suggest that Al2O3 inclusions formed in the early stage of solidification reacted with the remaining Si-enriched liquid steel and changed into low-Al2O3 inclusions. Experimental results were also confirmed by thermodynamic calculations. Present work made it possible to understand deeper the relationship between microsegregation and secondary inclusion formation.

A Study of Sliver in C-Shaped Grain Selectors during Investment Casting of Single-Crystal Superalloy

In this study, the formation mechanism of sliver defects in C-shaped 2D grain selectors during investment casting of single-crystal superalloy was investigated by analyzing the effects of the C-shaped 2D grain selectors on the solidification behavior of superalloy. The physical field properties of the sliver formation and solidification characteristics of CM247LC nickel-based superalloy were determined. The temperature and stress fields were simulated using ProCAST. The results showed that the stress fields of solidification played an essential role in the formation of sliver, indicating that the solidification interval characteristics of the alloy and stress based on the geometry of the C-shaped 2D grain selector are crucial for the formation of sliver. In addition, the origin of sliver depended upon tensile stress during solidification, relying on the constraints of dendrite boundaries. The findings suggested that the joint sections of the starter block—i.e., selector and selector-casting joint of C-shaped selector sections—are stress-sensitive areas where sliver can form readily. Furthermore, sliver is formed in the final stages of solidification and especially originates in the grain selection part where the accumulated thermal stress is high, and there is only a small quantity of liquid phase with a low melting point between the dendrites. Therefore, the solidification and stress conditions generate thermal cracks, which can also cause sliver defects.

Mineral chemistry and genesis of monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna pegmatite system at Piława Górna, Lower Silesia, Poland

AbstractMonazites are one of the most interesting groups of accessory mineral components of crystalline rocks due to the information on geochemical evolution of the crystallisation environment coded in their chemical compositions, in addition to comprising one of the most valuable objects for geochronology studies. This paper presents monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna system of rare-element pegmatites at Piława Górna, Lower Silesia, Poland. These monazites are unique due to their unusually high Sm and Nd contents, reaching 33.22 wt.% Sm2O3 and 34.12 wt.% Nd2O3, respectively. We consider the most significant factors of the enrichment in Sm and Nd to be the occurrence of highly fractionated pegmatite-forming melts during the final stages of solidification and associated hydrothermal fluids that were strongly enriched in rare earth element REE–Cl and REE–F complexes. Local disequilibria allowed for the rapid growth of accessory phases under supercooling conditions associated with the scavenging of selected elements, leading to their local depletion, which was not balanced by diffusion processes. As a consequence, the depletion of light rare earth elements (LREE) led to the incorporation of available middle rare earth elements (MREE, Sm–Dy) in the case of Sm and Nd, which could occupy an acceptable structural position in minerals of the monazite group.

Elastoplastic Analysis of Shear Behavior of Multicell Shaped Concrete-Filled Steel Tubular Connection Based on Unified Theory

Connection between multicell shaped concrete-filled steel tube (MCFST) columns and steel beams gives full play of strength and properties of steel tube and core concrete and has many advantages in mechanical and seismic performance. Shear behavior of the connection is crucial to damage performance of special-shaped CFST structure under seismic load. However, few studies focused on shear behavior of the MCFST connection. This study investigates elastoplastic shear behavior of the MCFST connection. Nine specimens were tested under constant axial load and lateral low cyclic loading. The test results showed that as height to thickness ratio of column section increased, the plastic status of the stresses of the flange limb steel webs was obviously different from those of the web limb steel webs, and initial shear stiffness of the panel zone increased. In order to give analytical methods for shear behavior of the MCFST connection under compressive and shear force at the material level to engineers, according to unified theory the analytical model of MCFST composite material in the panel zone is equivalent to that of concrete-filled circular steel tube of web and flange limbs considering the difference of the stresses of the steel webs and interaction between the web and flange limbs. Based on the previous research and analysis of test results, migration and evolution are carried out from analytical models for shear behavior of the MCFST connection at the material level to that at the member level. The analytical expression of initial shear stiffness of the MCFST connection is derived, and the theoretical results correlate well with the experimental results. In addition, the analytical expression of shear stiffness of the MCFST connection in plastic hardening stage is obtained. The calculation formulas are proposed to predict the shear stiffness of the MCFST connection in the elastic and plastic hardening stages by geometrical parameters, material property parameters, and stress state of the steel webs.

Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by In Situ Positron Annihilation Lifetime Experiments

Using the newly developed positron annihilation lifetime spectroscopy (PALS) facility with a high count rate up to 3000 cps, in situ PALS experiments were performed for the first time on the continuous stretching process of polymers to quantitatively analyze the minute-scale evolution of free-volume holes. According to the stress–strain relationship and PALS results of four types of polyethylenes with different crystallinities, the tensile process could be divided into four distinct stages: elastic, initial nonlinear (until yield point), postyield, and strain hardening stages. The increase of o-Ps (orthopositronium) lifetime in the first three stages exhibits an enlargement of free-volume hole size with increasing strain. The decrease of the o-Ps lifetime in the last stage is most probably due to the increasing anisotropy of free-volume holes. The relative fractional free volume FFVr (derived from hole radius R (calculated from the Tao–Eldrup model) and o-Ps intensity) generally increases in the first two stages but remains nearly unchanged in the other two stages. This work demonstrates a new feasibility to disclose minute-scale evolution of microstructure of materials through in situ PALS experiments in the future.

Cracking behavior of newly-developed high strength eutectic high entropy alloy matrix composites manufactured by laser powder bed fusion

A novel AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) composite doped with SiC particles was designed and fabricated by laser powder bed fusion (LPBF). Its microstructure characteristic, tensile properties, and metallurgical defects, with an emphasis on cracking behavior, have been investigated. The results showed that the addition of SiC particles into the AlCoCrFeNi2.1 matrix enabled the development of a {100} texture and highly elongated columnar grains, which were the main contributors to mechanical behavior anisotropy. The ultimate tensile strength of 1466 ± 26 MPa and elongation of 9% ± 3% achieved in the as-deposited EHEA composite surpassed those of advanced metal alloys subjected to additive manufacturing processes. Unfortunately, severe horizontal and longitudinal cracks, as well as a few micro-cracks were observed in the as-deposited bulk samples. Micro-cracks were verified to be associated with the aggregation of carbon and oxide particles. They formed in the final stage of solidification owing to insufficient liquid feeding ability and solidification contraction. The formation of macroscopic cracking was induced by the tensile stress accumulations at sample edges, and the stress concentration areas where microcracks and pores were located were the predominant propagation location. This work provides guidelines for defect control in SiC-reinforced EHEA, assisting in the high-performance design and integrated manufacturing of EHEA composite components.

Influence of whitening additives on the properties of decorative slag-alkali cements and mortars

The paper shows a comparative study of the influence of whitening additives (kaolin, TiO2 and СаСО3) on the production of decorative alkali-activated slag cement and mortars with a degree of whiteness of at least 70%; as well as their influence on the structure formation and evolution of physico-mechanical properties. According to results obtained, kaolin provides chemical bonding of Na+ into insoluble zeolite-like compounds; and CaCO3 densifies the structure and reduces shrinkage deformations. At the early stages of hardening (up to 7 days), the additions of kaolin and calcite, due to their significant amount (15 and 24%), reduces the compressive strength of the cement paste; nevertheless, at later ages (until 90 days) the difference in strength almost disappears. The high colourfastness and weather resistance of pigmented cements under the influence of ultraviolet radiation and freeze/thaw cycles has been established. A comparative assessment of the economic efficiency has shown that СаСО3 is the best cost-effective additive.

Substructure development, martensitic transformation and rapid work-hardening in an as-cast interstitial substituted N atom in FeMnCoCr high-entropy alloy

The microstructural evolution and room-temperature mechanical properties of nitrogen-added Fe50Mn30Co10Cr10 as-cast high-entropy alloy were investigated. Interestingly, an excellent strength-ductility trade-off with a rapid hardening stage was achieved in the as-cast structure (tensile strength: 740 MPa, ductility: 42 %) that was comparable with those counterparts holding wrought structure. These excellent mechanical properties were attributed to the capability of metastable FCC microstructure enabling deformation-induced martensitic transformation even at low imposed strain at room temperature. Simultaneously, the occurrence of substructure development independent of martensitic transformation, significantly contribute to increase the capability of the material for strain accommodation.

Multi-scale analysis for solidification of phase change materials (PCMs): Experiments and modeling

We present a multi-scale solidification framework for pure substances using laboratory experiments and mathematical modeling. The multi-scale phenomena of solidification are experimentally captured by the state-of-the-art thermal control chamber and optical devices. In particular, the transient temperature over five solidification stages is observed under various cooling rates, while the nucleation site and crystal evolution are photographed and further processed via image analysis. A unified mathematical model is also developed to quantitatively examine the solidification process at three scales: (i) the supercooling, freezing, and subcooling stages with a time-dependent boundary at the macroscale are analytically solved by Duhamel’s theorem and perturbation method; (ii) the two-dimensional (2D) dendritic growth at the mesoscale is computed by the Allen-Cahn equation through phase field modeling; and (iii) the heterogeneous nucleation at the microscale is numerically simulated. The results demonstrated that the experimental data and modeling were in close agreement with the freezing curve, nucleation temperature, nucleation time, 2D crystal evolution, and freezing time.