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DOE OSTI2024

Accelerate microstructure evolution simulation using graph neural networks with adaptive spatiotemporal resolution

Abstract Surrogate models driven by sizeable datasets and scientific machine-learning methods have emerged as an attractive microstructure simulation tool with the potential to deliver predictive microstructure evolution dynamics with huge savings in computational costs. Taking 2D and 3D grain growth simulations as an example, we present a completely overhauled computational framework based on graph neural networks with not only excellent agreement to both the ground truth phase-field methods and theoretical predictions, but enhanced accuracy and efficiency compared to previous works based on convolutional neural networks. These improvements can be attributed to the graph representation, both improved predictive power and a more flexible data structure amenable to adaptive mesh refinement. As the simulated microstructures coarsen, our method can adaptively adopt remeshed grids and larger timesteps to achieve further speedup. The data-to-model pipeline with training procedures together with the source codes are provided.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Electron Backscattered Diffraction Study of Microstructural Evolution During Isothermal Deformation of High-N Mn18Cr18 Alloy

In order to understand the dynamic recrystallization (DRX) mechanism of Mn18Cr18N austenitic stainless steel during hot deformation, the investigation of microstructure evolution is significant. Isothermal compression tests were conducted on a Gleeble-3500 thermomechanical simulator in the temperature range of 950 °C to 1150 °C and strain rate range between 0.001 and 1 s{sup −1}. Optical microscopy (OM) and electron backscattered diffraction (EBSD) were employed to study the resultant microstructures. More grain boundary information of hot-deformed specimens was provided by EBSD. It was found that the fraction of low-angle grain boundaries (LAGBs) increases with the decrease of deformation temperature or the increase of strain rate and can be expressed as a function of the Zener–Hollomon parameter. DRX easily occurs at the high deformation temperature and low strain rate. From the flow stress curves, the occurrence of single peak behavior and multiple transient steady-state (MTSS) behavior illustrated the existence of different DRX mechanisms. The EBSD analysis indicated that both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) existed during hot deformation. For the low fraction of 10 to 15 deg misorientation, CDRX is the minor DRX mechanism while DDRX is the major DRX mechanism for the studied steel.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Mechanical Properties, Microstructural Evolution, and the Effect of Friction on the Plastic Flow of the AISI 321 Austenitic Stainless Steel Tube During Cold Pilgering: An Experimental and Simulation Analysis

The mechanical properties, microstructural evolution, and the effect of friction on the plastic flow of the AISI 321 austenitic stainless steel (ASS) tube were investigated during the cold pilger process. The elastic–plastic behavior of as-received tube was simulated by the Johnson-cook model. The model parameters were obtained by the compression and tensile tests. The mechanical properties of the material were examined by the tensile and microhardness tests. Based on the ring compression test results, three different situations for friction conditions were considered. The stress and strain states at these conditions were examined for the outer and inner surfaces of the tube. Numerical evaluation of plastic shearing due to the cold pilgering of the AISI321 tube was performed. The X-ray diffraction, ferrite scope tests, and optical microscope were also used for the microstructure evaluation and verification of the simulation results, respectively. The Latham-Cockcraft damage was calculated for different conditions by introducing the new subroutine, showing that it was strongly dependent on the friction conditions and the shear strain, ε{sub zr}, respectively. It was shown that the friction coefficient of 0.3 resulted in the minimum damage of the tube and the different friction conditions between the two surfaces of the tube enhanced the damage function. Also, the strain induced ά-martensite increased the work-hardening capacity and affected the ductility of the tube.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Microstructural evolution and transmutation in tungsten under ion and neutron irradiation

This study aims to compare the effects of neutron and self-ion irradiation on the mechanical properties and microstructural evolution in W. Neutron irradiation at the HFR reactor to 1.67 dpa at 800 °C resulted in the formation of large Re and Os rich clusters and voids. The post-irradiation composition was measured using APT and verfified against FISPACT modelling. The measured Re and Os concentration was used to create alloys with equivalent concentrations of Re and Os. These alloys were exposed to self-ion irradiation to a peak dose of 1.7 dpa at 800 °C. APT showed that self-ion irradiation leads to the formation of small Os clusters, wheras under neutron irradiation large Re/Os clusters form. Voids are formed by both ion and neutron irradiation, but the voids formed by neutron irradiation are larger. By comparing the behaviour of W-1.4Re and W-1.4Re-0.1Os, suppression of Re cluster formation was observed. Irradiation hardening was measured using nanoindentation and was found to be 2.7 GPa, after neutron irradiation and 1.6 GPa and 0.6 GPa for the self-ion irradiated W-1.4Re and W-1.4Re-0.1Os. The higher hardening is attributed to the barrier strength of large voids and Re/Os clusters that are observed after neutron irradiation.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Microstructural evolution, defect mitigation, and precipitation behavior in AA6061 via laser powder bed fusion with high-temperature substrate heating

Defects, particularly solidification cracking, remain persistent challenges in the laser powder bed fusion (LPBF) processing of AA6061 aluminum alloy. This study systematically investigates defect mitigation, microstructural evolution, and mechanical properties associated with high-temperature substrate preheating at 500 °C. Comprehensive microstructural analyses, including characterization of defects, grain structures, and precipitation behavior, were performed on samples in both as-built and T6 heat-treated states. Elevated preheating substantially reduced solidification cracking across a wide processing window, while demonstrating decreased crack sensitivity to laser parameters. Columnar cracks along the build direction were observed despite substrate preheating. Lack-of-fusion and keyhole porosity were effectively eliminated, though gas-induced microporosity persisted at higher powers. In-depth characterization of two distinct laser power and speed conditions confirmed the formation of micron-sized, non-coherent Mg 2 Si precipitates under heated substrate conditions, alongside α-AlFeCrMnSi intermetallic phases indicative of in-situ thermal effects during fabrication. Subsequent T6 heat treatment revealed the formation of fine, coherent needle-shaped β″ strengthening precipitates. Despite substantial differences in processing parameters, comparable mechanical properties were measured in the as-built samples (∼52 MPa yield strength, ∼130 MPa tensile strength), primarily due to reduced strain hardening effects and consistent precipitation characteristics. Meanwhile, the T6 heat treatment led to significant improvement in properties, enhancing yield strength by over 400%, aligning closely with the performance of conventional wrought AA6061-T6. These findings underscore that high-temperature substrate preheating offers an effective means to suppress cracks, control precipitation, and enhance mechanical performance in LPBF-processed AA6061.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Microstructural evolution of thin polycrystalline metallic films under extreme conditions

Thin films are materials systems that are widely used in applications ranging from electronics and optical devices to industrial and biomedical ones. However, these systems are unstable against various homogenization processes and aging mechanisms (grain growth, coarsening, surface evolution, and diffusion of species) even at low service temperatures. In this work, we examine the role of various aspects of microstructure (grain boundary types and characters, free surfaces, surface diffusion, and thermal grooves) on the thermal aging of such systems. Existing experimental tools will be leveraged to characterize thin films, i.e., in-situ quantitative thermal annealing, and provide direct comparisons to predications emerging from a recently developed meso-scale model via Precession Electron Diffraction (PED). Parametric studies will be conducted to gain insights on the role of each of the aforementioned aspects of microstructure on the dynamics. Herein, we will focus on “hard” gold thin films (with alloying elements like Co, Ni, or Fe) as they are materials of choice in a wide range of applications at Sandia. Initially, pure gold systems are examined and future studies will focus on microstructure evolution in the presence of alloying elements and second-phase particles.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Understanding Microstructural Evolution During Rapid Heat Treatment of Microalloyed Steels Through Computational Modeling, Advanced Physical Simulation, and Multiscale Characterization Techniques

An AISI 1045 steel modified with vanadium (V) and niobium (Nb) was studied to evaluate microstructural conditioning prior to and throughout a rapid heat treat process. In order to accomplish this, both computational and physical simulation techniques have been employed with the goal of assessing the microstructural evolution in a medium-carbon bar steel during the rapid austenitization and quenching procedures involved in an induction hardening process. The appropriate thermal profiles for induction hardening were obtained through finite element modeling using Flux 2D software. Physical simulations of the induction hardening process were carried out using a Gleeble{sup ®} 3500. Analysis of prior austenite grain size is complemented by observation of nanoscale carbonitride precipitation via transmission electron microscopy, scanning transmission electron microscopy, and high-energy synchrotron small-angle x-ray scattering. Through a combination of characterization techniques, this study presents a deeper understanding of nano- and microstructural changes occurring in a microalloyed steel during an induction hardening process.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Geometry-Induced Spatial Variation of Microstructure Evolution During Selective Electron Beam Melting of Rene-N5

High gamma prime (γ') nickel-based alloys produced by selective electron beam melting are of interest to the turbine industry which requires control of microstructure in relation to loading conditions within complex component geometry. Welding literature predicts cracking and microstructure evolution as a function of alloy composition and process parameters in this class of alloys. In addition, Additive manufacturing causes variations in the above conditions due to the interaction of geometry on processing and heat transfer. The influence of geometry on processing conditions was explored for alloy Rene N5 by characterizing the solidification grain microstructure and solid-state precipitation. A Semi-Analytical Heat Transfer Model was employed to explain the resulting variation in solidification grain morphology that occurred due to the part geometry. Finally, the as-built precipitation structure was found to vary as a function of build height and had no correlation to the solidification grain structure or the layer geometry.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Microstructural Evolution and the Precipitation Behavior in X90 Linepipe Steel During Isothermal Processing

Thermomechanical controlled processing of 560-MPa (X90) linepipe steel was simulated in the laboratory using a thermomechanical simulator to study the microstructural evolution and precipitation behavior during isothermal holding. The results indicated that martensite was obtained when the steels were isothermally held for 5 s at 700 °C. Subsequently, granular bainite and acicular ferrite transformation occurred with increased holding time. Different amount of polygonal ferrite formed after isothermally holding for 600-3600 s. Pearlite nucleated after isothermally holding for 3600 s. Precipitation occurred after isothermal holding for 5 s and continuous precipitation occurred at grain boundaries after isothermally holding for 600 s. After isothermally holding for 3600 s, large Nb/Ti carbide precipitated. The presence of MX-type precipitates was confirmed by diffraction pattern. The interphase precipitation (IP) occurred between 5 and 30 s. Maximum hardness was obtained after isothermally holding for 600 s when IP occurred and rapidly decreased to a low value, mainly because polygonal ferrite dominated the microstructure after isothermally holding for 3600 s.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Effect of thermal annealing on microstructure evolution and mechanical behavior of an additive manufactured AlSi10Mg part

The powder-bed laser additive manufacturing (AM) process is widely used in the fabrication of three-dimensional metallic parts with intricate structures, where kinetically controlled diffusion and microstructure ripening can be hindered by fast melting and rapid solidification. Therefore, the microstructure and physical properties of parts made by this process will be significantly different from their counterparts produced by conventional methods. This work investigates the microstructure evolution for an AM fabricated AlSi10Mg part from its nonequilibrium state toward equilibrium state. Special attention is placed on silicon dissolution, precipitate formation, collapsing of a divorced eutectic cellular structure, and microstructure ripening in the thermal annealing process. These events alter the size, morphology, length scale, and distribution of the beta silicon phase in the primary aluminum, and changes associated with elastic properties and microhardness are reported. In conclusion, the relationship between residual stress and silicon dissolution due to changes in lattice spacing is also investigated and discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Effect of Different Heat Treatments on the Microstructural Evolution and Mechanical Properties of Ni-Cr-Si Alloy Fabricated by Laser Additive Manufacturing

Ni-Cr-Si alloy samples were fabricated on mild steel via laser additive manufacturing technology. The microstructural evolution and mechanical properties of the samples were studied systematically in as-deposited conditions and under three different types of heat-treated conditions. The scanning electron microscopy, energy-dispersive x-ray spectroscopy and x-ray diffraction results showed that the structures of the as-deposited samples were mainly dendritic matrices of γ(Ni, Fe), continuous interdendritic eutectics of γ + Ni{sub 3}B and discrete Cr-rich carbides and borides. After solid solution treatment, carbides (Cr{sub 7}C{sub 3}) and borides (CrB) precipitated on the boundary of the eutectic structure, distributing discretely in the matrix. In the solution plus aging heat-treated conditions, the interdendritic eutectics formed Ni{sub 3}Fe intermetallic compounds with a layered distribution. After annealing, the grains were refined, and the hard phases (CrB) were uniformly dispersed along the grain boundary. Among the tested samples, the as-deposited specimens exhibited the highest microhardness and good wear ability at room temperature; however, the wear resistance of the as-deposited sample obviously decreased at 250 °C. The mechanical properties of the specimens were not greatly improved by solution treatment or solution plus aging treatment. The sample subjected to annealing treatment exhibited excellent wear resistance both at room temperature and at a service temperature of 250 °C.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Phase stability, porosity distribution and microstructural evolution of amorphous Al{sub 50}Ti{sub 50} powders consolidated by electrical resistance sintering

The effect of intensity and duration of the electrical resistance sintering process on the phase stability, porosity distribution and microstructural evolution of Al{sub 50}Ti{sub 50} amorphous powders is studied. The phase transformations during the consolidation process were determined by X-ray diffraction. The porosity distribution was observed by optical and scanning electron microscopy. The amorphous phase is partially transformed to the crystalline phase during the sintering process, and formation of AlTi and AlTi{sub 3} intermetallic compounds occurs for temperatures higher than 300 °C. Finally, it is observed that the compacts core have lower porosity and a higher tendency to the amorphous-crystalline phase transformation than the periphery.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Microstructural evolution and grain growth kinetics of GZ31 magnesium alloy

Grain growth behavior of Mg–3Gd–1Zn (GZ31) magnesium alloy was studied in a wide range of annealing time and temperature to clarify the kinetics of grain growth, microstructural evolution and related metallurgical phenomena. This material exhibited typical normal grain growth mode under annealing conditions with annealing temperature of lower than 300 °C and soaking time of lower than 240 min. However, the abnormality in grain growth was also evident at annealing temperature of 400 °C and 500 °C. The dependence of abnormal grain growth (AGG) at mentioned annealing temperatures upon microstructural features such as dispersed precipitates, which were rich in Zn and Gd, was investigated by optical micrographs, X-ray diffraction patterns, scanning electron microscopy images, and energy dispersive X-ray analysis spectra. The bimodality in grain-size distribution histograms also signified the occurrence of AGG. Based on the experimental data on grain growth obtained by annealing treatments, the grain growth exponent and the activation energy were also figured out.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Microstructural evolution of diamond films from CH{sub 4}/H{sub 2}/N{sub 2} plasma and their enhanced electrical properties

The influence of N{sub 2} concentration in CH{sub 4}/H{sub 2}/N{sub 2} plasma on microstructural evolution and electrical properties of diamond films is systematically investigated. While the diamond films grown in CH{sub 4}/H{sub 2} plasma contain large diamond grains, for the diamond films grown using CH{sub 4}/H{sub 2}/(4%)N{sub 2} plasma, the microstructure drastically changed, resulting in ultra-nanosized diamond grains with Fd3m structure and a{sub 0} = 0.356 nm, along with the formation of n-diamond (n-D), a metastable form of diamond with space group Fm3m and a{sub 0} = 0.356 nm, and i-carbon (i-C) clusters, the bcc structured carbon with a{sub 0} = 0.432 nm. In addition, these films contain wide grain boundaries containing amorphous carbon (a-C). The electron field emission (EFE) studies show the best EFE behavior for 4% N{sub 2} films among the CH{sub 4}/H{sub 2}/N{sub 2} grown diamond films. They possess the lowest turn-on field value of 14.3 V/μm and the highest EFE current density value of 0.37 mA/cm{sup 2} at an applied field of 25.4 V/μm. The optical emission spectroscopy studies confirm that CN species are the major criterion to judge the changes in the microstructure of the films. It seems that the grain boundaries can provide electron conduction networks to transport efficiently the electrons to emission sites for field emission, as long as they have sufficient thickness. Whether the matrix nano-sized grains are 3C-diamond, n-D or i-C is immaterial.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Microstructural Evolution of Tantalum During Deformation and Subsequent Annealing

Microstructure-aware models are necessary to predict the behavior of material based on process knowledge or to extrapolate mechanical properties of materials to environmental conditions which are not easily reproduced in the laboratory, e.g., nuclear reactor environments. Elemental Ta provides a relatively simple BCC system in which to develop a microstructural understanding of deformation processes which can then be applied to more complicated BCC alloys. In situ neutron diffraction during compressive deformation and subsequent heat treatment have been used to monitor the evolution of microstructural features in Ta throughout simulated processing steps. Crystallographic texture and dislocation density are determined as a function of first plastic strain, then temperature. Lattice strains are determined and attributed to stresses at macroscopic, grain and dislocation length scales. The increase of the dislocation density through deformation and subsequent recovery during heat treatment is monitored through the changing diffraction line profile. Also, randomization of the texture is used as a signature of recrystallization. The recovery of dislocations through annihilation is not observed to depend on the initial dislocation density in the range studied here. In contrast, recrystallization is observed to depend strongly on the initially dislocation density.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Contrasting thermally-induced structural and microstructural evolution of alumino-silicates with tubular and planar arrangements: Case study of halloysite and kaolinite

Materials with the similar chemical structures but different morphological organization exhibit considerable differences in thermally induced chemo-morphological evolution. With recent advancements in multi-scale X-ray scattering measurements, it is now possible to non-invasively probe the structural and microstructural evolution of such materials. In this study, we investigate the chemo-morphological evolution of halloysite and contrast the results with those for kaolinite. Halloysite and kaolinite have tubular and planar morphologies, respectively. Both materials are alumino-silicates comprising silica tetrahedra (T) and alumina octahedra (O) organized in a TO structure. Four distinct stages in the structural evolution were identified. Stages I, II, III, and IV correspond to temperature ranges of 25 to 125 °C, 125 to 400 °C, 400 to 625 °C, and 625 to 875 °C. Major structural changes correspond to the removal of interlayer/adsorbed water in stage I, the existence of the halloysite structure without interlayer water in stage II, dehydroxylation of halloysite in stage III, and the conversion of ordered halloysite to amorphous meta-halloysite in stage IV. Additionally, heating halloysite up to 875 °C resulted in slight widening of the nanotubes, as the average pore radius increased from 6.4 nm to 6.6 nm. Heating also resulted in an increase of wall thickness of the nanotubes from ~120 nm (25 °C) to 161 nm (875 °C). The increase in the halloysite nanotube diameter was attributed to the expansion of the structure and an increase in the surface roughness. The pore sizes in halloysite nanotubes were also confirmed using N 2 adsorption-desorption and nano-X-ray computed tomography (nano-XCT) measurements. The interlayer basal spacing in halloysite changed from 9.8 Å to 7.2 Å after the removal of interlayer water. At temperatures in the range of 625 to 875 °C, heating halloysite causes a small widening of nanotube pores and a minor increase in the surface area. In contrast, the interlayer spacing in kaolinite collapses on heating which reduces the nanoscale porosity. Furthermore, these studies demonstrate the differences in the chemo-morphological evolution of alumino-silicates with tubular and planar morphologies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

The effect of postprocessing on tensile property and microstructure evolution of friction stir welding aluminum alloy joint

Friction stir welding is an efficient manufacturing method for joining aluminum alloy and can dramatically reduce grain size conferring excellent plastic deformation properties. Consequently, friction stir welding is used to manufacture tailor welded blanks to optimize weight or performance in the final component. In the study, the microstructural evolution and mechanical properties of friction stir welding joint during plastic forming and subsequent heat treatment were investigated. The microstructural characteristics of the friction stir welding joints were studied by Electron Backscattered Diffraction and Transmission Electron Microscopy. The mechanical properties were evaluated by tensile and microhardness tests. It is found that the tensile and yield strengths of friction stir welding joints are significantly improved after severe plastic deformation due to the grain refinement. Following heat treatment, the strength of the friction stir welding joints significantly decrease due to the obvious abnormal grain growth. Careful attention must be given to the processing route of any friction stir welding joint intended for plastic forming, especially the annealing between forming passes. Severe plastic deforming of the friction stir welding joint leads to a high level of stored energy/dislocation density, which causes the abnormal grain growth during subsequent heat treatment, and consequently reduce the mechanical properties of the friction stir welding joint. - Highlights: • Great changes are observed in the microstructure of FSW joint after postprocessing. • Postprocessing shows great effect on the microstructure stability of FSW joint. • The weld shows more significant decrease in strength than the BM due to the AGG. • Attention must be given to the processing route of FSW joint for plastic forming.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Microstructural evolution of neutron-irradiated T91 and NF616 to ~4.3 dpa at 469 °C

Ferritic-martensitic steels such as T91 and NF616 are candidate materials for several nuclear applications. Here, this study evaluates radiation resistance of T91 and NF616 by examining their microstructural evolutions and hardening after the samples were irradiated in the Advanced Test Reactor to ~4.3 displacements per atom (dpa) at an as-run temperature of 469 °C. In general, this irradiation did not result in significant difference in the radiation-induced microstructures between the two steels. Compared to NF616, T91 had a higher number density of dislocation loops and a lower level of radiation-induced segregation, together with a slightly higher radiation-hardening. Unlike dislocation loops developed in both steels, radiation-induced cavities were only observed in T91 but remained small with sub-10 nm sizes. Lastly, other than the relatively stable M 23 C 6 , a new phase (likely Sigma phase) was observed in T91 and radiation-enhanced MX → Z phase transformation was identified in NF616. Laves phase was not observed in the samples.

36 MATERIALS SCIENCE↗