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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Analyzing the microstructural evolution and hardening response of an Al-Si-Mg casting alloy with Cr addition

Highlights: • The formation of β-Al{sub 5}FeSi intermetallics was suppressed since impurity Fe has been consumed by Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics; • A large number of Al{sub 13}Cr{sub 4}Si{sub 4} dispersoids were observed in Al dendrites in the as-quenched and aged alloy; • The precipitation of Cr-bearing dispersoids harden the as-quenched alloy by over 10HV; • The current thermodynamic database underestimate the solubility of Fe in Al{sub 13}Cr{sub 4}Si{sub 4} intermetallics during solidification simulation. - Abstract: Chromium is added to Al-Si based casting alloys for the mitigation of die soldering. In the present work, EBSD, EDS and TEM characterizations are combined with thermodynamic simulations to study the microstructural evolution and hardening response of an Al-Si-Mg-Cr casting alloy during heat treatments. It is shown that the Fe-containing intermetallic compounds formed in the inter-dendritic region were acicular π-Al{sub 8}(Fe, Cr)Mg{sub 3}Si{sub 6} phase and polyhedral Al-(Cr, Fe)-Si phase which is structurally similar to face-centered cubic Al{sub 13}Cr{sub 4}Si{sub 4}. The polyhedral Al-(Cr, Fe)-Si intermetallics that crystallized prior to β-Al{sub 5}FeSi consumed impurity Fe, suppressing the formation of β phase during gravity die casting process. Also, Cr-addition enables the precipitation of nano-scale Cr-bearing dispersoids in Al dendrite during solution treatment at 813 K. Hardness increment of over 10 HV can be induced by the Cr-bearing dispersoids for as-quenched alloy. Though the dispersoids have a moderate hardening effect on the as-quenched alloy, they may impair the ageing hardening response by acting as heterogeneous nucleation sites for coarse Mg-Si rods and Si particles when quenching into water at 338 K.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Variation improvement of mechanical properties of Mg-9Al-1Zn alloy with melt conditioned high pressure die casting

Highlights: • The variation improvement of mechanical properties of AZ91D was achieved via application of high shear melt conditioning. • The elongation variation of AZ91D with MC-HPDC has a significant reduction from 31.2% for tradition HPDC to 19.7%. • High shear melt conditioning can result in grain refinement of primary α-Mg formed in both shot sleeve and die cavity. - Abstract: Melt conditioning treatment via high shearing on Mg-9Al-1Zn melt was employed to disperse the oxide inclusions, and to harness the well-dispersed particles as the effective heterogeneous nucleation sites for primary α-Mg grains during solidification in high pressure die casting. To reveal the mechanism of property improvement, Prefil pressure filtration cleanliness analyzer, field-emission scanning electron microscopy with energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy were employed in present study. The experimental results showed that the application of high shearing not only improved elongation, ultimate tensile strength and yield strength simultaneously, but also reduced the variation of the mechanical properties of Mg-9Al-1Zn alloy. The improvement was ascribed to the refinement of the primary α-Mg grains formed in both shot sleeve and die cavity, and to the reduction of porosity as well as refinement of Mg{sub 17}Al{sub 12} phases.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In-situ microstructural observation of Ti-Cu alloys for semi-solid processing

The semi-solid processing of metallic alloys strongly depends on the microstructural features exhibited by the raw material. Consequently, many characterization techniques have been used to elucidate semi-solid microstructures. In the present work, high-temperature laser-scanning confocal microscopy (HT-LSCM) was used to characterize Ti-Cu alloys intended for thixoforming. During the reheating stage, this in-situ technique enabled us to observe the Ti-β grains formed through the reverse eutectoid transformation. Furthermore, the results showed that thermomechanical treatment was primarily responsible for breaking up the dendritic microstructure. The liquid formation and peritectic temperature were easily determined via the HT-LSCM investigation. Prior to the reverse peritectic reaction, which corresponds to the onset of equilibrium melting, we obtained experimental evidence of the melting of non-equilibrium phases that originated via segregation during solidification. Finally, it was possible to qualitatively study the coarsening mechanisms that occurred in the semi-solid state. Irrespective of the alloy composition (liquid fraction), both Ostwald ripening and coalescence occurred during the isothermal heat treatments. Based on the obtained results, HT-LCSM can be considered a valuable technique for the characterization of thixotropic alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting

Highlights: • Single and multiple tracks on a loose bulk powder substrate were generated. • Understanding of the microstructure development during SLM was achieved. • Single tracks reveal a nearly symmetrical crystallography with elongated grains. • Multiple tracks show elongated grains perpendicular to the build direction. - Abstract: During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Scan strategies in EBM-printed IN718 and the physics of bulk 3D microstructure development

Three-dimensional (3D) characterization provides opportunities for understanding processing-structure relationships in additively manufactured (AM) materials. Bulk samples of Inconel 718 were fabricated via electron beam melting (EBM) in order to study microstructural development as a function of energy input and beam scan strategy. TriBeam tomography of bulk Inconel 718 microstructures built under steady-state growth conditions reveals the sensitivity of microstructure formation and evolution to machine process parameters. Here, samples manufactured using a narrow range of energy input per unit build area result in varied grain morphologies and crystallographic textures. Using TRUCHAS, a thermal simulation software, the thermal history of bulk scan strategies was predicted, and combined with a calibrated microstructure-processing map to accurately predict bulk grain morphologies. The solidification parameters and the 3D measured nucleation density are used to predict the transition between columnar and equiaxed grain morphologies, providing a process map to guide AM parameter choices to locally control as-printed microstructure. A two-dimensional metric for characterizing bulk grain morphology was also found to agree well with predictions from the process map calibrated by 3D data. Combined with 3D tomography and thermal modelling, the physics of structure development were understood at a new level of detail with respect to the competing processes of grain nucleation and epitaxial growth.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Microstructural characterization of dendritic evolution using two-point statistics

Characterizing large four-dimensional materials datasets is difficult due to the presence of complex mi- crostructures and time-varying length scales. We showcase the use of two-point statistics as an efficient and un-biased way of extracting materials parameters from an Al-Cu alloy during solidification. The evo- lution of dendrite primary arm thickness, average secondary arm spacing, and average tip-to-tip spacing were tracked using two-point Pearson auto-correlations of scaled mean curvatures. Insights into compet- itive side-branching are also reported. We show both visually and quantitatively that most length scales change rapidly during early stages of dendritic growth, but slow as diffusion fields of dendrites overlap.

Aluminum alloys↗
DOE OSTI2017

Formation and crystallisation of a liquid jet in a film exposed to a tightly focused laser beam{sup *}

This paper considers the effect of an ultrashort laser pulse on a thin gold film on a glass substrate at a focal spot size near 1 μm. We analyse the motion and thermal history of a film that has peeled off from the substrate in the heating spot as a consequence of melting. The detached zone is shown to form a dome-shaped bump whose motion is hindered by surface tension. After the dome stops and turns back, towards the substrate, a jet begins to grow on its top. Concurrently, because of the heat dissipation in the film, melt recrystallisation begins, involving first the dome and then the jet. The liquid part of the jet elongates and breaks up into droplets because of the Plateau – Rayleigh instability development. The formation of a neck and the detachment of the last droplet occur in the solidification zone between the crystalline and liquid parts of the jet. The propagation of the crystallisation zone in the jet leads the necking process, so neck disruption occurs in the solid phase under nonequilibrium crystallisation conditions (the melt temperature is hundreds of kelvins lower than the melting point), at limiting mechanical stress and at high deformation rates. As a result, the jet transforms into a high needle with an extremely small tip radius (a few nanometres). (paper)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Two-step nucleation of the Earth’s inner core

Significance Understanding the formation of the Earth’s inner core is essential to understanding the geodynamo and Earth's history. However, recent attempts to explain the initial solidification of the inner core have been unsuccessful. The supercooling necessary to form hcp iron is unrealistically large and creates the “inner core nucleation paradox.” Our work demonstrates that molten iron can crystallize via a two-step nucleation process involving the intermediate bcc phase under core conditions. This mechanism significantly reduces the required undercooling necessary to nucleate solid iron. This work also suggests that bcc and hcp iron have similar free energies at pressures near the inner core center.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Integrated modeling of cracking during deep penetration laser welding of nickel alloys

During deep penetration laser welding of nickel alloys, interactions between composition and processing conditions can lead to the formation of defects. Inconel 740H, for example, has demonstrated a susceptibility to horizontal fusion zone cracking at locations between 70% and 80% of the weld depth during laser welding at powers above 5 kW. Coupling three-dimensional heat transfer, fluid flow, and stress modeling tools allowed that both the strain rate and stress normal to the solidification direction to be calculated. In the Inconel 740H welds, cracks formed at locations where the strain rate and stress simultaneously reached critical levels. No cracking was observed in the Inconel 690 welds, since the strain rate and stress did not simultaneously reach these critical levels.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A Monte Carlo model for 3D grain evolution during welding

Welding is one of the most wide-spread processes used in metal joining. However, there are currently no open-source software implementations for the simulation of microstructural evolution during a weld pass. Here we describe a Potts Monte Carlo based model implemented in the SPPARKS kinetic Monte Carlo computational framework. The model simulates melting, solidification and solid-state microstructural evolution of material in the fusion and heat-affected zones of a weld. The model does not simulate thermal behavior, but rather utilizes user input parameters to specify weld pool and heat-affect zone properties. Weld pool shapes are specified by Bezier curves, which allow for the specification of a wide range of pool shapes. Pool shapes can range from narrow and deep to wide and shallow representing different fluid flow conditions within the pool. Surrounding temperature gradients are calculated with the aide of a closest point projection algorithm. Furthermore, the model also allows simulation of pulsed power welding through time-dependent variation of the weld pool size. Example simulation results and comparisons with laboratory weld observations demonstrate microstructural variation with weld speed, pool shape, and pulsed-power.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Rapid solidified ductile Cu-Al-Mn ribbon and its elastocaloric potential

Abstract Cu-Al-Mn alloys display martensitic transformation over a wide range of temperatures. In addition to low cost, this alloy is known for its low transformation stress with reasonable latent heat favoring elastocaloric applications. However, the ductility of Cu-Al-Mn can be limited owing to ordering and intergranular fracture. Through rapid solidification by melt spinning, we show that Cu-Al-Mn ribbon can be made highly ductile (greater than 8% tensile strain in the as-spun state and 10% tensile strain after heat treatment). The ductility of the melt-spun ribbon is related to the suppression of L2 1 ordering that is characterized through magnetic property measurement. Heat treatment of the ribbon promotes bamboo grain formation, and the latent heat is increased to 6.4 J g −1 . Under tensile conditions, we show that the ribbon exhibited about 4 °C temperature change (4.4 °C on heating and 4.2 °C on cooling from 6.3% strain).

36 MATERIALS SCIENCE↗
DOE OSTI2025

Defect-Free Growth of Decagonal Quasicrystals around Obstacles

Quasicrystals are aperiodic solids with exotic physical properties. The mechanisms driving their growth are far from understood, particularly in the presence of rigid obstacles. Here, using in situ synchrotron x-ray tomography and molecular dynamics simulations, we investigate the interaction between decagonal quasicrystals and obstacles (shrinkage pores) during solidification of an Al 79 ⁢Co 6 ⁢Ni 15 alloy. The results reveal defect-free quasicrystal growth around the pores, independent of pore size and geometry. Our findings point to a universal feature of aperiodic solids: their ability to accommodate structural disruptions due to additional (phasonic) degrees of freedom. This Letter opens new possibilities for synthesizing large-scale, single quasicrystals from a liquid metal precursor.

alloys↗
DOE OSTI2020

Comparison of Model Predictions and Performance Test Data for a Prototype Thermal Energy Storage Module

Although model predictions of thermal energy storage (TES) performance have been explored in previous investigations, relevant test data that enable experimental validation of performance models have been limited. This is particularly true for high-performance TES designs that facilitate fast input and extraction of energy. In this paper, we present a summary of experimental tests of a high-performance TES unit using lithium nitrate trihydrate phase change material as a storage medium. Performance data are presented for complete dual-mode cycles consisting of extraction (melting) followed by charging (freezing). These tests simulate the cyclic operation of a TES unit for asynchronous cooling in a variety of applications. Finally, the model analysis is found to agree reasonably well, within 10%, with the experimental data except for conditions very near the initiation of freezing, a consequence of subcooling that is required to initiate solidification.

25 ENERGY STORAGE↗
DOE OSTI2018

Mechanisms of Interaction of Molybdenum and Tungsten Polyoxide with Aluminum Oxide Melt under Reducing Conditions

The main chemical reactions between Mo and W polyoxides and Al{sub 2}O{sub 3} melt in a controlled Ar + H{sub 2} atmosphere (T = 2400 K, P = 1 bar) during sapphire growth by horizontal directional solidification have been investigated. Under these thermodynamic conditions, the melt and products of its dissociative evaporation may actively react with the tungsten heater and molybdenum thermal screens of the crystallization system. It is shown that the polyoxides formed during evaporation do not directly interact with the melt; this interaction occurs only with participation of reagents exhibiting pronounced reducing properties (Al, H{sub 2}, H, WO, Al{sub 2}O, AlH, AlH{sub 2}, AlH{sub 3}). It is established that most of processes occur with participation of aluminum hydrides. A particular role of Mo(W) dioxide–W(Mo) polyoxide functional pairs in the interaction with the melt is determined.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Inelasticity and Nanostructured Dislocation Deformation of Aluminum–Silicon Alloy with a Supermodified Eutectic Structure

Young’s modulus and the logarithmic decrement of oscillations at a frequency of ~100 kHz as well as the subgrain size and residual stresses in the strontium modified alloy of aluminum with 15 wt % silicon have been studied. The alloy was obtained with a solidification rate of 1 mm/s at the shifted eutectic point. The dependence of inelastic dislocation deformation on the applied oscillating stress has been obtained and analyzed. The effect of strontium modification on the microstrain diagram can be accounted for by transformation of the lamellar fiber structure of eutectic silicon into a superfine fiber structure.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Simulated Microstructures for Laser Powder Bed Fusion Additive Manufacturing Using Myna, AdditiveFOAM, and ExaCA

This dataset provides sample datasets containing voxelized, three-dimensional representations of simulated grain structures and crystallographic orientations that can result from laser powder bed fusion additive manufacturing. The six microstructure files each contain approximately 1 cubic millimeter of material (1 mm x 1 mm cross-section over 26 simulated layers of deposition). Some of the microstructures have columnar grains that extend across nearly the entire simulation domain, while others have more equiaxed or truncated columnar grains. The process conditions to generate these microstructures were from the Peregrine v2023-10 dataset (10.13139/ORNLNCCS/2008021). The codes used are publicly available and released under open-source licenses. Myna (https://github.com/ORNL-MDF/Myna) was used for configuration of the cases from the Peregrine v2023-10 HDF5 dataset and to run the simulation workflow. AdditiveFOAM (https://github.com/ORNL/AdditiveFOAM) was used to simulate the melt pool and generate solidification conditions. And ExaCA (https://github.com/LLNL/ExaCA ) was used to simulate the three-dimensional microstructures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Laser Weld Geometry and Microstructure of Cast Uranium-6 wt% Niobium Alloy

Laser welding was performed on a U6 wt% Nb uranium alloy using a 400W solid state laser with welding speeds from 20 to 2,500 mm/s. This speed range spanned melt pool sizes from traditional welding to surface modification and additive manufacturing. With increasing scan speed, the ratio of weld length relative to depth and width increased, with melt pool lengths being more than 5x greater than the width and 10x greater than the depth at the highest speeds. Keyhole mode welds were shown to occur at low speeds, while conduction mode welds occurred at 700 mm/s or higher as the weld depth dropped off more rapidly than width at higher speeds. Microstructures that form at the boundary between the fusion zone and base metal were observed to have a nonconventional appearance consisting of interpenetrating dark and light contrast phases before cells or dendrites appear. Dendrites with secondary arms form from this boundary in keyhole welds and refine to no visible secondary arms near the weld center. Primary and secondary dendrite arms, where present, were shown to refine in size inversely with cooling rate raised to the 0.465 and 0.376 powers respectively. Dendrites were largely absent from the conduction mode welds at higher speeds, and were replaced by a banded microstructure, that appears to form by an oscillatory solidification front mechanism.

36 MATERIALS SCIENCE↗