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At least 73 records · Page 4

Learning two-phase microstructure evolution using neural operators and autoencoder architectures

Abstract Phase-field modeling is an effective but computationally expensive method for capturing the mesoscale morphological and microstructure evolution in materials. Hence, fast and generalizable surrogate models are needed to alleviate the cost of computationally taxing processes such as in optimization and design of materials. The intrinsic discontinuous nature of the physical phenomena incurred by the presence of sharp phase boundaries makes the training of the surrogate model cumbersome. We develop a framework that integrates a convolutional autoencoder architecture with a deep neural operator (DeepONet) to learn the dynamic evolution of a two-phase mixture and accelerate time-to-solution in predicting the microstructure evolution. We utilize the convolutional autoencoder to provide a compact representation of the microstructure data in a low-dimensional latent space. After DeepONet is trained in the latent space, it can be used to replace the high-fidelity phase-field numerical solver in interpolation tasks or to accelerate the numerical solver in extrapolation tasks.

36 MATERIALS SCIENCE↗

Understanding the microstructural evolution of hypersaline cemented paste backfill with low-field NMR relaxation

Highlights: • Microstructural evolution of CPB, prepared with tap and hypersaline water, is monitored using low-field NMR relaxometry. • T{sub 1} and T{sub 2} NMR relaxation measurements compared with mechanical strength via UCS tests • Hypersaline mixing water results in slower structural evolution and lower mechanical strength. Cemented paste backfill (CPB) comprising mineral tailings, binders and mixing water is an important potential support material in the mining industry. As the mechanical properties of CPB are significantly influenced by its microstructural characteristics the development of measurement tools to better understand its pore structure evolution is important for its increased utilisation. This study reports the application of low-field nuclear magnetic resonance (NMR) relaxation time measurements to characterise the microstructural evolution of CPB materials over 56 days of hydration, contrasting common tap water and hypersaline water (~22 wt% salt) as mixing water. Distinct NMR relaxation time populations were evidenced within each CPB sample, revealing the presence of both capillary (T{sub 1,2} ≈ 10 ms) and gel pore water (T{sub 1,2} ≈ 300–500 μs), with time-dependent relaxation measurements facilitating characterisation of capillary pore structure evolution over the hydration period assessed. Hypersaline samples demonstrated a time-lag in this measured capillary pore evolution, relative to those hydrated with tap water, while pore structure evolution rates were observed to increase with increased CPB binder content. Further, both T{sub 1} and T{sub 2} NMR relaxation times were found to correlate with the uniaxial compressive strength of the CPB materials investigated, facilitating the formulation of a predictive correlation function between NMR relaxation characteristics and mechanical properties.

36 MATERIALS SCIENCE↗

Microstructural evolution of NARloy-Z at elevated temperatures

Microstructural evolution was studied in samples of wrought and vacuum plasma sprayed (VPS) NARloy-Z exposed to temperatures up to 970 C (1,780 F) for up to 60 h. Samples were heated in a vacuum furnace, followed by rapid quenching in helium (He) gas at a cooling rate of 166 C (300 F) per second. Microstructural analyses were conducted using optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). In both the wrought and VPS conditions, precipitates rich in silver (Ag) and zirconium (Zr) were present in the matrix and at the grain boundaries even after long exposure to elevated temperatures. Islands rich in oxygen (O2) and Zr were also observed, as well as incipient melting at the grain boundary triple points. Results indicated that the alloy cannot be homogenized by heat treatment at elevated temperatures.

Singh, J.↗

Microstructural Evolution and Mechanical Properties of Ti-22Al-25Nb (At.%) Orthorhombic Alloy with Three Typical Microstructures

Microstructural evolution, tensile and creep behavior of Ti-22Al-25Nb (at.%) orthorhombic alloy with three typical microstructures were investigated. The three typical microstructures were obtained by different solution and age treatment temperatures and analyzed by the BSE technique. The tensile strengths of the alloy at room temperature and 650 °C were investigated. The creep behaviors of the three typical microstructures were also studied at 650 °C/150 MPa for 100 h in air. The phase transformation mechanisms in creep deformation were also found. The experimental results showed that the formations of the three typical microstructures were decided by the isothermal forging and heat treatment. It was supposed that the high-temperature solution treatment might be dominant for the volume fraction and diameter of the equiaxed particles. While the double age treatment would lead to lamellar O phases. Due to grain refinement strengthening, the equiaxed microstructure presented the best tensile strength and ductility. The fully lamellar microstructure had the best creep resistance than that of other microstructures. In this paper, the phenomenon of creep-induced α{sub 2} phase decomposition was occurred during creep deformation of the equiaxed microstructure.

36 MATERIALS SCIENCE↗

Microstructure evolution in the conventional single side and bobbin tool friction stir welding of thick rolled 7085-T7452 aluminum alloy

Highlights: • Comparison of microstructure evolution via EBSD of SS-FSW and BB-FSW was investigated. • Friction heat, strain rate and degree of recrystallization affected grain size. • Discontinuously dynamic recrystallization occurred in the weld nugget zone. • The degree of recrystallization was mainly controlled by the strain rate. - Abstract: The microstructure evolution during the conventional single side friction stir welding (SS-FSW) and the bobbin tool FSW (BB-FSW) of 7085-T7452 alloy thick plate was extensively studied by using the state-of-the-art materials characterization techniques. The Electron backscattering diffraction results reveal that a significant grain refinement and a larger percentage of high angle grain boundaries (HAGBs) exist in the weld nugget zone (WNZ) and the thermal-mechanical affected zone (TMAZ) compared with the base material. Meanwhile, more obviously inhomogeneous refined grains and HAGBs in WNZ along the thickness direction and a larger percentage of low angle grain boundaries (LAGBs) were observed in SS-FSW joint than that in BB-FSW joint. The change of the average grain size is the result of friction heat and strain rate of plastic deformation combined with a certain degree of recrystallization. Moreover, the recrystallization is mainly controlled by the strain rate during the FSW process. The presence of Cube and Cube ND after welding shows that the discontinuously dynamic recrystallization (DDRX) is a major factor in the final microstructural response. The research and findings help the understanding of the mechanism of microstructure homogenization and further optimizing the welding process for application.

36 MATERIALS SCIENCE↗

Anisotropic physics-regularized interpretable machine learning of microstructure evolution

Anisotropic Physics-Regularized Interpretable Machine Learning Microstructure Evolution (APRIMME) is a general-purpose machine learning solution for grain growth simulations. In prior work, PRIMME employed a deep neural network to predict site-specific migration as a function of its neighboring sites to model normal, isotropic, grain growth behavior. This work aims to extend this method by incorporating grain boundary misorientation-based grain growth behavior. APRIMME is trained on anisotropic simulations created using the Monte Carlo-Potts (MCP) model. Furthermore, the results of this work are compared statistically using grain radius, number of sides per grain, mean neighborhood misorientations, and the standard deviation of triple junction dihedral angles, and are found to match in most cases. The exceptions are small and seem to be related to two causes: (1) the deterministic model of APRIMME is learning from the stochastic simulations of MCP, which seems to accentuate triple junction behaviors; and, (2) a bias against very small grains is made evident in a quicker decrease in grains than expected at the beginning of an APRIMME simulation. APRIMME is also evaluated for its general ability to capture anisotropic grain growth behavior by first investigating different test case initial conditions, including a circle grain, three grain, and hexagonal grain microstructures.

36 MATERIALS SCIENCE↗

Densification, Microstructural Evolution, Mechanical Properties and Oxidation Study of CrB{sub 2} + EuB{sub 6} Composite

This paper reports the results of investigation on densification, microstructural evolution, mechanical properties and oxidation study of CrB{sub 2} + EuB{sub 6} composite. CrB{sub 2} + EuB{sub 6} (10 and 20 wt.%) composites have been fabricated by hot pressing at a temperature of 1700 °C and 35 MPa pressure. The hardness and flexural strength were measured in the range of 21.25-24.48 GPa and 171-199 MPa, respectively. The fracture toughness increased from 3.3 to 4.01 MPa m{sup 1/2} by the addition of 20% EuB{sub 6}. Microstructural evolution revealed the uniform distribution of EuB{sub 6} and absence of any reaction product. Fracture surface analysis confirmed the presence of transgranular mode of fracture. Oxidation study at 1200 °C revealed that the developed composites have good oxidation resistance and followed the parabolic rate of oxidation.

36 MATERIALS SCIENCE↗

Microstructure evolution of wrought nickel based superalloy GH4037 in the semi-solid state

Highlights: • Semi-solid isothermal treatment of wrought superalloy (SSITWS). • Near-globular microstructure. • Carbide precipitates. - Abstract: Study on semi-solid processing of superalloy is little reported due to the difficulties in fabricating semi-solid billets. In this manuscript, a novel method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was firstly proposed in order to fabricate semi-solid billets of superalloy. The effects of isothermal temperature and soaking time on microstructure of Ni-based superalloy GH4037 in the semi-solid state were investigated. The results show that the near-globular microstructure of wrought GH4037 superalloy can be obtained by the semi-solid isothermal treatment. The average grain size increased and the roundness decreased with the prolonged soaking time at 1350 °C and 1360 °C. When the isothermal temperature was elevated to 1370 °C or 1380 °C, the microstructure evolution was complex due to the combination of breaking up mechanism and coarsening mechanism. At 1390 °C, the dendritic grains were obviously observed in the microstructure. With the increasing of isothermal temperature and soaking time, the liquid boundary film coarsening behavior occurred in the microstructure. With the increasing of soaking time, the size of intragranular liquid droplets increased, but the amount of them decreased. When the isothermal temperature was high, the intragranular liquid droplets disappeared. The coarsening rate constant at 1360 °C was lower compared to that at 1350 °C. However, when the isothermal temperature was higher than 1370 °C, the coarsening kinetics was not suitable for describing the microstructure evolution. The discrete carbide precipitates appeared in the grain boundaries. The distribution of Ti and Mo were higher in the grain boundaries than that in solid grains, and the distribution of other elements was homogenous in the grain boundaries and inside the grains.

36 MATERIALS SCIENCE↗

Microstructural Evolution and Properties of 24CrNiMoY Alloy Steel Fabricated by Selective Laser Melting

24CrNiMoY alloy steel samples were fabricated by selective laser melting (SLM), and a phase transformation model was established to study the alloy steel microstructural evolution. Meanwhile, microhardness and tensile properties of 24CrNiMoY alloy steel prepared by different laser energy densities (E{sub v}) were investigated. Results indicate that the microstructural evolution of 24CrNiMoY alloy steel is consistent with the phase transformation model. The main microstructure changed from martensite to bainite with the increase in thermal cycle numbers. In addition, a suitable E{sub v} plays an important role in refining the bainite structure and improving the alloy steel properties. When the E{sub v} decreases from 210 to 140 J/mm{sup 3}, the bainite lath width reduces from 1.7 to 0.6 μm. Simultaneously, the relative density, tensile strength and microhardness of the fabricated samples increase first and decrease later. 24CrNiMoY alloy steel sample prepared by 160 J/mm{sup 3} has fine mechanical properties: The tensile strength is 850 MPa and microhardness is 360 HV{sub 0.2}.

36 MATERIALS SCIENCE↗

Microstructure evolution and a new mechanism of B2 phase on room temperature mechanical properties of Ti-47Al-2Cr-2Nb alloy prepared by hot isostatic pressing

Highlights: • The microstructure evolution of Ti-47Al-2Cr-2Nb alolly was investigated in detail. • The foramtion of B{sub 2} phase can be adjusted in Ti-47Al-2Cr-2Nb alloy. • A new mechanism of B{sub 2} phase on room temperature mechanical properties of Ti-47Al-2Cr-2Nb alloy was proposed. - Abstract: The formation of B{sub 2} phase was adjusted via various hot isostatic pressing processes in Ti-47Al-2Cr-2Nb alloy. The microstructure evolution and a new mechanism of B{sub 2} phase on room temperature mechanical properties was investigated by BSE, EBSD and TEM in detail. The result shows that the formation of B{sub 2} phase leads to Cr element content decrease in γ matrix phase, which results in lattice parameter increase, namely the c/a ratio of γ matrix phase. Finally, at room temperature the mechanical properties of Ti-47Al-2Cr-2Nb alloy deteriorates.

36 MATERIALS SCIENCE↗

Investigation on Microstructural Evolutions and Mechanical Properties of P92 Steel During Thermal Processing

The evolution of the microstructure of P92 steel during heat processing has a significant effect on the final performance of steel products and is thus an area of critical interest. Accordingly, the grain growth behavior of austenite and δ-ferrite changes in P92 steel is comprehensively investigated in this study through thermodynamic calculations and physical simulation. The results show that the A{sub C1}, A{sub C3} and γ–δ transformation temperatures of P92 steel are 841 °C, 878 °C and 1095 °C, respectively, which are comparable to the measured values of 850 °C, 879 °C and 1175 °C, respectively. When the temperature exceeds the A{sub C1} temperature, the austenite grain size increases with increasing temperature and longer holding times, and the growth rate of austenite grains increases with higher temperatures but decreases with holding time at a certain temperature. A model that shows the austenite grain growth kinetics is established: D{sup n} − D{sub 0}{sup n} = 2.03 × 10{sup 16}exp(− 498,770/RT)t, in which the time exponent n decreases from 4.52 to 2.11 with an increase in temperature from 1000 °C to 1200 °C due to the reduced pinning effect resultant of the dissolution of precipitates, M{sub 23}C{sub 6}, VX and NbX at 887 °C, 1073 °C and 1200 °C, respectively. The formation of δ-ferrite at 1200 °C when held for 600 s and 1300 °C when held for 60 ~ 600 s causes a smaller austenite grain size. The relationship between the evolution of the microstructure of P92 steel and the mechanical properties is also studied. The results show that the prior austenite grain size has an inverse relationship with impact energy and ultimate tensile strength at 620 °C, while the presence of soft δ-ferrite leads to a reduction in the microhardness, impact energy and ultimate tensile strength at 620 °C.

36 MATERIALS SCIENCE↗

Phase-field modeling of aging-induced microstructure evolution in pentaerythritol tetranitrate thin films and ramifications for shock initiation

Aging of energetic materials may change performance and affect their safety and reliability, but the relationship between microstructure changes induced by aging and consequent performance changes has not been fully established. This work presents results of phase-field method simulations used to model microstructure evolution of vapor-deposited pentaerythritol tetranitrate (PETN) thin films. Simulated aging is shown to induce grain coarsening and substantial changes of the configuration of porosity in the film: Specifically, we show that porosity tends to concentrate in large pores to a greater degree in aged films, a state that is arrived at by closure or consolidation of small pores. To evaluate the performance of the as-deposited and aged films, we perform two-dimensional hydrocode flyer-film impact simulations that incorporate the phase-field output microstructures directly, permitting us to connect features therein to changes in reactivity, a key metric of energy output for shock initiation. The results demonstrate that declining sensitivity obtained for the simulated aged films can be correlated with the loss of fine-structured pores relatively early in the aging process, while long-term microstructure evolution that gradually alters the shape of large, branching pores is less impactful. Finally, we discuss commonalities and discrepancies between our simulation results and high-throughput initiation experiments on shock initiation of aged PETN thin films.

36 MATERIALS SCIENCE↗

Microstructural evolution and mechanical properties on an ARB processed IF steel studied by X-ray diffraction and EBSD

Accumulative Roll Bonding (ARB) is one of the so-called severe plastic deformation (SPD) processes, allowing the production of metals and alloys with ultrafine (micro-nano) structures. Materials with ultrafine grains present attractive properties like the simultaneous increase in strength and ductility. Our interest in these materials is focused on their microstructural evolution during ARB processing, eventually responsible for the enhancement of those mechanical properties. In the current work we follow the evolution of the microstructure in an interstitial-free (IF) steel deformed by ARB after consecutive processing cycles, by means of Electron BackScatter Diffraction (EBSD) and X-ray diffraction (XRD). Particularly, we present results related to texture, grain (GS) and domain sizes, grain boundary character, density of Geometrically Necessary Dislocations (GND), Grain Orientation Spread (GOS), lattice parameters, microstrain, dislocation density and their spatial arrangement. After 5 ARB cycles the system shows a microstructure constituted mainly by submicrometric grains with high angle boundaries and low presence of dislocations inside the grains. - Highlights: •The evolution of microstructure is followed simultaneously by using GAM, GOS and GND (EBSD) and XRD. •LAGBs and subgrains disappear after few cycles SSDs and HAGBs persist at the end. •Dynamic recrystallization counterbalances dislocation arrays and diminishes hardening rate. •Grain size stabilization is revealed as a mechanism for increasing ductility after few ARB cycles.

36 MATERIALS SCIENCE↗

The Prediction of Microstructure Evolution of 6005A Aluminum Alloy in a P-ECAP Extrusion Study

Finite element modeling (FEM) was applied for predicting the recrystallized structure in extruded 6005 aluminum alloy, and simulated results were experimentally validated. First, microstructure evolution of 6005 aluminum alloy during deformation was studied by means of isothermal compression test, where the processing parameters were chosen to reproduce the typical industrial conditions. Second, microstructure evolution was analyzed, and the obtained information was used to fit a dynamic recrystallization model implementing inside the DEFORM-3D FEM code environment. FEM of deformation of 6005 aluminum has been established and validated by microstructure comparison. Finally, the obtained dynamic recrystallization model was applied to tube extrusion by using a portholes-equal channel angular pressing die. The finite element analysis results showed that coarse DRX grains occur in the extruded tube at higher temperature and in the extruded tube at the faster speed of the stem. The test results showed material from the front end of the extruded tube has coarse grains (60 μm) and other extruded tube has finer grains (20 μm).

36 MATERIALS SCIENCE↗

Effect of cooling rate on microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during multi-step heat treatment

Highlights: • Novel ideas to control cooling rate during different phase transformation are proposed. • Cooling rates induced microstructure evolutions during the whole phase transformation are investigated. • Phase transformation texture and the characteristics of lamellar structures are related to cooling rate. - Abstract: Microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy (at.%) resulted from different cooling rates during the whole phase transformation process has been thoroughly investigated. The results show that the cooling rate during β → α transformation plays a crucial role in the modification of the crystallographic orientation relationship between the α grains and their parent β phase. The decrease of cooling rate can reduce the volume fraction of Burgers α grains and avoid the preferential distribution of lamellar interface traces. This could be attributed to the different nucleation and growth kinetics of α phase affected by the cooling rate. Moreover, the lamellar structure characteristics are closely related to the cooling rate during α → α{sub 2} + γ transformation and their responses to stabilization treatment shall vary accordingly. A fast cooling rate retains a high quantity of high temperature α/α{sub 2} phase. These α/α{sub 2} phase could decompose into ultrafine lamellar structure during subsequent stabilization treatment. While the average lamellar spacing of the slow cooled microstructure has a small increase by limited continuous growth of the pre-existing γ lamellae during stabilization treatment.

36 MATERIALS SCIENCE↗

Microstructure Evolution and Mechanical Property of Low-Alloy Steel Used for Armor Layer of Flexible Pipe During Thermomechanical Process and Hot Rolling Process

In this paper, the thermomechanical process and hot rolling process were carried out to study the microstructure evolution and mechanical property of low-alloy steel used for armor layer of flexible pipes by analyzing continuous cooling transformation curve, microstructure morphology, grain characteristics, and strength. The experimental results indicate that deformation process promotes phase transformation process of both ferrite and bainite and induces the formation of fine grain. The density of low-angle grain boundary rises with increasing cooling rate. The dislocation concentration in grain becomes larger with the extended cooling rate. The microstructure of hot-rolled specimen consists of polygonal ferrite and granular bainite, which is consistent with one of the deformed specimens with similar cooling rate in dilatometer. The microstructure evolution provides effective data fundament for hot rolling process.

36 MATERIALS SCIENCE↗

A Monte Carlo-finite element model for strain energy controlled microstructural evolution - 'Rafting' in superalloys

This paper presents a specialized microstructural lattice model, MCFET (Monte Carlo finite element technique), which simulates microstructural evolution in materials in which strain energy has an important role in determining morphology. The model is capable of accounting for externally applied stress, surface tension, misfit, elastic inhomogeneity, elastic anisotropy, and arbitrary temperatures. The MCFET analysis was found to compare well with the results of analytical calculations of the equilibrium morphologies of isolated particles in an infinite matrix.

Gayda, J.↗

Dynamic microstructural evolution of an Al–Zn–Mg–Cu alloy (7075) during continuous heating and the influence on the viscoelastic response

Highlights: • A TEM/HRTEM, FIB-SEM and APT analysis of an Al–Zn–Mg–(Cu) alloy is performed. • Microstructural evolution is analyzed during constant heating rate heat-treatment. • Composition, size, number density and vol. fraction of present phases are reported. • Vol. fraction and kinetic data from rate equations in the literature are validated. • Viscoelastic behavior of the alloy is associated to microstructural transformations. - Abstract: The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperature range of 298 to 648 K (25 to 375 °C) is characterized by focused ion beam-scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The chemical composition, dimensions, number density, and volume fraction of precipitates are measured comprehensively. Quantitative measurements of Guinier-Preston zones volume fraction and η′/η phase volume fraction are correlated to the viscoelastic response of the Al–Zn–Mg–Cu alloy with temperature in the studied range. This provides valuable evidence indicating that observed variations in the viscoelastic behavior of the Al–Zn–Mg–Cu alloy may be associated with phase transformations involving Guinier-Preston zones and phases η′ and η.

36 MATERIALS SCIENCE↗