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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

Transient Effects in Planar Solidification of Dilute Binary Alloys

The initial transient during planar solidification of dilute binary alloys is studied in the framework of the boundary integral method that leads to the non-linear Volterra integral governing equation. An analytical solution of this equation is obtained for the case of a constant growth rate which constitutes the well-known Tiller's formula for the solute transient. The more physically relevant, constant ramping down temperature case has been studied both numerically and analytically. In particular, an asymptotic analytical solution is obtained for the initial transient behavior. A numerical technique to solve the non-linear Volterra equation is developed and the solution is obtained for a family of the governing parameters. For the rapid solidification condition, growth rate spikes have been observed even for the infinite kinetics model. When recirculating fluid flow is included into the analysis, the spike feature is dramatically diminished. Finally, we have investigated planar solidification with a fluctuating temperature field as a possible mechanism for frequently observed solute trapping bands.

Mazuruk, Konstantin↗
NASA NTRS2009

Stability of Detached Solidification

Bridgman crystal growth can be conducted in the so-called "detached" solidification regime, where the growing crystal is detached from the crucible wall. A small gap between the growing crystal and the crucible wall, of the order of 100 micrometers or less, can be maintained during the process. A meniscus is formed at the bottom of the melt between the crystal and crucible wall. Under proper conditions, growth can proceed without collapsing the meniscus. The meniscus shape plays a key role in stabilizing the process. Thermal and other process parameters can also affect the geometrical steady-state stability conditions of solidification. The dynamic stability theory of the shaped crystal growth process has been developed by Tatarchenko. It consists of finding a simplified autonomous set of differential equations for the radius, height, and possibly other process parameters. The problem then reduces to analyzing a system of first order linear differential equations for stability. Here we apply a modified version of this theory for a particular case of detached solidification. Approximate analytical formulas as well as accurate numerical values for the capillary stability coefficients are presented. They display an unexpected singularity as a function of pressure differential. A novel approach to study the thermal field effects on the crystal shape stability has been proposed. In essence, it rectifies the unphysical assumption of the model that utilizes a perturbation of the crystal radius along the axis as being instantaneous. It consists of introducing time delay effects into the mathematical description and leads, in general, to stability over a broader parameter range. We believe that this novel treatment can be advantageously implemented in stability analyses of other crystal growth techniques such as Czochralski and float zone methods.

Mazuruk, K.↗
NASA NTRS2010

Linear Stability of Binary Alloy Solidification for Unsteady Growth Rates

An extension of the Mullins and Sekerka (MS) linear stability analysis to the unsteady growth rate case is considered for dilute binary alloys. In particular, the stability of the planar interface during the initial solidification transient is studied in detail numerically. The rapid solidification case, when the system is traversing through the unstable region defined by the MS criterion, has also been treated. It has been observed that the onset of instability is quite accurately defined by the "quasi-stationary MS criterion", when the growth rate and other process parameters are taken as constants at a particular time of the growth process. A singular behavior of the governing equations for the perturbed quantities at the constitutional supercooling demarcation line has been observed. However, when the solidification process, during its transient, crosses this demarcation line, a planar interface is stable according to the linear analysis performed.

Mazuruk, K.↗
NASA NTRS2012

ISS-Experiments of Columnar-to-Equiaxed Transition in Solidification Processing

The main topic of the research project CETSOL in the framework of the Microgravity Application Promotion (MAP) programme of the European Space Agency (ESA) is the investigation of the transition from columnar to equiaxed grain growth during solidification. Microgravity environment allows for suppression of buoyancy-driven melt flow and for growth of equiaxed grains free of sedimentation and buoyancy effects. This contribution will present first experimental results obtained in microgravity using hypo-eutectic AlSi alloys in the Materials Science Laboratory (MSL) on-board the International Space Station (ISS). The analysis of the experiments confirms the existence of a columnar to equiaxed transition, especially in the refined alloy. Temperature evolution and grain structure analysis provide critical values for the position, the temperature gradient and the solidification velocity at the columnar to equiaxed transition. These data will be used to improve modeling of solidification microstructures and grain structure on different lengths scales.

Sturz, Laszlo↗
DOE OSTI2024

From irregular to regular eutectic growth in the Al-Al 3 Ni system: In situ observations during directional solidification

Here we investigate the irregular eutectic growth dynamics of the Al-Al 3 Ni alloy, in which one of the solid phases (Al 3 Ni) grows faceted from the liquid. Leveraging in situ optical microscopy and synchrotron transmission x-ray microscopy, we address the question of the degree of coupling between Al and Al 3 Ni at the growth front and that of the shape of the microstructures left behind in the bulk solid during directional solidification. Real-time optical observations bring evidence for a morphological transition from a eutectic-grain dependent, irregular eutectic growth at low solidification velocity V (typically 1 μm s -1 ), to a weakly anisotropic, regular growth at higher V ( reaching 10 μm s -1 ). Unprecedented x-ray nano-imaging of the solid–liquid interface, and 3D characterization of the growth patterns, were made possible by a new DS setup at Brookhaven National Laboratory’s NSLS-II. At low V, the leading tips of partly faceted Al 3 Ni crystals are observed to grow not far ahead of the Al growth front. Correlating in situ images and postmortem 3D tomographic reconstructions reveals that the presence of faceted and non-faceted regions of Al 3 Ni crystals in the solid is a direct consequence of coupling and decoupling during DS, respectively. Upon increasing V, the lead distance of Al 3 Ni vanishes, and the shape of Al 3 Ni ceases to be governed by faceted growth. These observations cast light onto the basic mechanisms (faceted growth, diffusive coupling, and the dynamics of trijunctions) governing a faceted to rod-like transition upon increasing V in the Al 3 Ni system, with broad implications to a large class of irregular eutectics.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Revisiting solidification microstructure selection maps in the frame of additive manufacturing

Understanding microstructural development in additive manufacturing under highly non-equilibrium cooling conditions and the consequent effects on mechanical properties of the final component is critical for accelerating industrial adoption of these manufacturing techniques. In this study, simple but effective theoretical solidification models are recalled to evaluate their ability to predict of microstructural features in additive manufacturing applications. As a case study, the resulting solidification microstructure selection maps are created to predict the stable growth modality and the columnar to equiaxed transition (CET) of an Al-10Si-0.5Mg alloy processed via Selective Laser Melting. The potential of this method in microstructural predictions for additively manufactured products, as well as outstanding challenges and limitations, are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Comparison of Solidification Structures and Submicroscale Cellular Segregation in Rapidly Solidified Stainless Steels Produced via Two-Piston Splat Quenching and Laser Powder Bed Fusion

Fusion-based additive manufacturing techniques leverage rapid solidification (RS) conditions to create parts with complex geometries, unique microscale/nanoscale morphological features, and elemental segregation. Three custom composition stainless steel alloys with varying chromium equivalence to nickel equivalence ratio (Cr eq /Ni eq ) between 1.53 and 1.95 were processed using laser powder bed fusion (LPBF) and/or two-piston splat quenching (SQ) to produce solidification rates estimated between 0.4 and 0.8 m/s. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to collect high-resolution images, electron backscatter diffraction (EBSD) phase identification, and measure cellular segregation. Similar features were observed in both LPBF and SQ samples including phase and microstructure, nanoscale oxide particles, cell size, and segregation behavior. However, dislocation pileup was observed along the cell boundaries only in the LPBF austenite solidified microstructure. Targeted adjustment of the SQ feedstock Cr and Ni concentrations, within the ASTM A240 specification for 316L resulted in no observable impact on the cell size, oxide particle size, or magnitude of segregation. Also, the amount of Ni segregation in the ferrite solidified microstructures did not significantly differ, regardless of Cr/Ni eq or processing technique. Here, SQ is demonstrated as capable of simulating RS rates and microstructures similar to LPBF for use as an alternative screening tool for new RS alloy compositions.

42 ENGINEERING↗
DOE OSTI2017

From Solidification Processing to Microstructure to Mechanical Properties: A Multi-scale X-ray Study of an Al-Cu Alloy Sample

We follow an Al-12 at. pct Cu alloy sample from the liquid state to mechanical failure, using in situ X-ray radiography during directional solidification and tensile testing, as well as three-dimensional computed tomography of the microstructure before and after mechanical testing. The solidification processing stage is simulated with a multi-scale dendritic needle network model, and the micromechanical behavior of the solidified microstructure is simulated using voxelized tomography data and an elasto-viscoplastic fast Fourier transform model. This study demonstrates the feasibility of direct in situ monitoring of a metal alloy microstructure from the liquid processing stage up to its mechanical failure, supported by quantitative simulations of microstructure formation and its mechanical behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A Review of Permanent Magnet Stirring During Metal Solidification

Rather than using conventional electromagnetic stirring (EMS) with three-phase alternating current, permanent magnet stirring (PMS), based on the use of sintered NdFeB material which has excellent magnetic characteristics, can be employed to generate a magnetic field for the stirring of liquid metal during solidification. Recent experience with steel casting indicates that PMS requires less than 20 pct of the total energy compared with EMS. Despite the excellent magnetic density properties and low power consumption, this relatively new technology has received comparatively little attention by the metal casting community. This paper reviews simulation modeling, experimental studies, and industrial trials of PMS conducted during recent years. With the development of magnetic simulation software, the magnetic field and associated flow patterns generated by PMS have been evaluated. Based on the results obtained from laboratory experiments, the effects of PMS on metal solidification structures and typical defects such as surface pinholes and center cavities are summarized. The significance of findings obtained from trials of PMS within the metals processing sector, including the continuous casting of steel, are discussed with the aim of providing an overview of the relevant parameters that are of importance for further development and industrial application of this innovative technology.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Solidification Behavior of Calcium Ferrite Under Ultrasonic Vibration

Ultrasonic vibration was applied to the solidification process of calcium ferrite (CF) to modify the grain size and improve the homogeneity of the solidified microstructure. The effects of ultrasound on microstructure, density, compressive strength, and reducibility were studied. The acoustic pressure decreased and then increased along the axis symmetry due to the attenuation of acoustic pressure and reflection of ultrasonic waves in the melt under a fixed power. When the ultrasonic power was increased, the general acoustic pressure increased, which was beneficial to the avoidance of macropores and to improvement of the microstructure. Coarse acicular CF grains were significantly refined; the grain size decreased from 1967 to 442 μm when the ultrasonic power was increased from 0 to 300 W. The density of the samples increased from 4370 to 4680 kg/m{sup 3} due to cavitation and acoustic streaming effects. Ultrasonic vibration greatly improved the ultimate compressive strength of CF; the values were 41.8 ± 2.1 to 67.3 ± 3.4 MPa for ultrasonic powers of 100 to 300 W, respectively. Without ultrasonic treatment, it was only 37.4 ± 1.9 MPa. Ultrasonic vibration also influenced the reduction time of CF due to the composition segregation in the slag during solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Recalescence and Segregation Phenomena During Equiaxed Dendritic Solidification of Fe-C Alloy

Recalescence and segregation are two characteristic phenomena for the equiaxed dendritic solidification of alloys. The present work developed a two-dimensional dendritic model with cellular automaton (CA) method to investigate influence mechanisms of thermal conditions on recalescence and segregation behaviors of Fe-0.82wt pct C alloy. The released latent heat reduces the undercooling around the equiaxed dendrite, and thus eases its growth velocity. The predicted steady growth velocity agrees well with the analytical results as the melt undercooling is 9 K. Additionally, the present CA model can ensure the growth consistence of equiaxed dendrites in the undercooled melt. With improving the convective heat transfer coefficients applied around the domain boundaries, the temperature recalescence in the domain center becomes more significant, and the corresponding solid fractions enhance. It is because that the stronger cooling promotes the solidification, resulting in more latent heat released. Accordingly, the heat dissipation can be neutralized. Moreover, a deeper undercooling for the temperature recalescence is needed under a stronger cooling condition. With the increase of both the cooling rate and the convection coefficient, secondary arms of the equiaxed dendrite become more developed. At the lower cooling rate range, the segregation ratio in the domain enlarges with the improvement of the cooling rate. However, it gets weaker under the condition with super cooling intensity due to the expansion of the low concentration region. As the convection coefficient is enhanced, the solute segregation in the domain gets less pronounced.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Solidification and Liquation Cracking Behavior of Dual-Phase Steel DP600

In this work, the solidification and liquation cracking behavior of a dual-phase steel DP600 was examined using longitudinal Varestraint testing and hot ductility testing. Varestraint tests showed only limited cracking even at 4 pct augmented strain. Hot ductility tests showed that the nil-strength temperature of the steel was 1459 °C, while the nil-ductility and the ductility-recovery temperatures were 1435 °C and 1420 °C, respectively. The brittleness temperature range was found to be 39 °C, and the DP600 steel was highly resistant to both weld metal solidification cracking and heat-affected zone liquation cracking. The reasons for the high hot cracking resistance of dual-phase steel DP600 are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Controlled Solidification of Liquids Within the SFC Primary Phase Field of the “Fe{sub 2}O{sub 3}”-CaO-SiO{sub 2} System in Air

To provide fundamental information on the phases and microstructures formed during sintering, a liquid with a bulk composition within the silico ferrite of calcium (SFC) primary phase field in the ternary “Fe{sub 2}O{sub 3}”-CaO-SiO{sub 2} system in air was solidified at a controlled rate. Samples of a bulk composition with a CaO/SiO{sub 2} ratio of 4.00 and 69.24 wt pct Fe{sub 2}O{sub 3}, were cooled from 1623 K (1350 °C) at 2 K/s, with samples quenched at temperatures between 1513 K (1240 °C) to 1453 K (1180 °C). The silico ferrite of calcium and aluminium I (SFCA-I) and Ca{sub 7.2}Fe{sub 0.8}{sup 2+}Fe{sub 30}{sup 3+}O{sub 53} phases were observed to form an intergrowth (‘SFC-I’) rather than the anticipated SFC phase. Solidification was found to occur in three stages, Liquid + ‘SFC-I’, Liquid + ‘SFC-I’ + C{sub 2}S + CF{sub 2}, and C{sub 2}S + CF{sub 2} + CF, where C{sub 2}S denotes dicalcium silicate, CF denotes calcium ferrite and CF{sub 2} denotes calcium diferrite. The phases formed and the solidification sequence differ from those predicted under equilibrium and Scheil–Gulliver Cooling. Although not directly applicable to industrial operations, this research clearly shows that the formation of both the SFCA and SFCA-I phase in iron ore sinters is controlled by kinetic processes rather than equilibrium conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formation of Al{sub 15}Mn{sub 3}Si{sub 2} Phase During Solidification of a Novel Al-12%Si-4%Cu-1.2%Mn Heat-Resistant Alloy and Its Thermal Stability

The formation of Al{sub 15}Mn{sub 3}Si{sub 2} phase in Al-12Si-4Cu-1.2Mn (wt.%) alloy during solidification was investigated by adopting CALPHAD method and microstructural observation by optical microscopy, SEM–EDS, TEM–EDS/SAD and XRD analysis; SEM fixed-point observation method was applied to evaluate its thermal stability. As-cast microstructural observation consistently demonstrates the solidification sequence of the studied alloy predicted by phase diagram calculation. Based on the phase diagram calculation, SEM–EDS, TEM–EDS/SAD and XRD analysis, as well as evidences on Al-Si-Mn-Fe compounds from the literature, the primary and eutectic Mn-rich phases with different morphologies in the studied alloy are identified to be Al{sub 15}Mn{sub 3}Si{sub 2} that has a body-centered cubic (BCC) structure with a lattice constant of a = 1.352 nm. SEM fixed-point observation and XRD analysis indicate that Al{sub 15}Mn{sub 3}Si{sub 2} phase has more excellent thermal stability at high temperature than that of CuAl{sub 2} phase and can serve as the major strengthening phase in heat-resistant aluminum alloy that has to face a high-temperature working environment. Results of tension test show that addition of Mn can improve the strength of Al-Si-Cu alloy, especially at elevated temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Development of Graphene Nanoplatelet-Reinforced AZ91 Magnesium Alloy by Solidification Processing

It is a challenging task to effectively incorporate graphene nanoplatelets (GNPs) which have recently emerged as potential reinforcement for strengthening metals into magnesium-based matrices by conventional solidification processes due to their large surface areas and poor wettability. A solidification processing which combines mechanical stirring and ultrasonic dispersion of reinforcements in liquid matrix was employed to develop AZ91 magnesium alloy matrix composites reinforced with 0.25 and 0.5 wt.% GNPs. The microstructural studies conducted with scanning and transmission electron microscopes revealed that fairly uniform distribution and dispersion of GNPs through the matrix were achieved due to effective combination of mechanical and ultrasonic stirring. The GNPs embedded into the magnesium matrix led to significant enhancement in the hardness, tensile strength and ductility of the composites compared to those of unreinforced AZ91 alloy. The strength enhancement was predominantly attributed to the grain refinement by the GNP addition and dislocation generation strengthening due to the coefficient of thermal expansion mismatch between the matrix and reinforcement. The improved ductility was attributed to the refinement of β eutectics by transforming from lamellar to the divorced eutectics due to the GNP additions. In addition, the strengthening efficiency of the composite with 0.25 wt.% GNP was found to be higher than those of the composite with 0.5 wt.% GNP as the agglomeration tendency of GNPs is increased with increasing GNP content. These results were compared with those of the GNP-reinforced magnesium composites reported in the literature, indicating the potential of the process introduced in this study in terms of fabricating light and high-performance metal matrix composites.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Role of Titanium, Carbon, Boron, and Zirconium in Carbide and Porosity Formation during Equiaxed Solidification of Nickel-Based Superalloys

This work analyses the effect of Ti in the MC carbide formation and also in the γ/γ´ eutectic promotion in two nickel-based alloys melted under vacuum conditions and solidified into ceramic molds. The role of boron, carbon and zirconium on the porosity level after solidification of both alloys was also investigated; since the alloys were vacuum melted, it is assumed that the observed porosity is due to micro-shrinkage. The chemical compositions for two alloys were: 20.2Cr-13.7Co-4.78Mo-1.19Al-0.69Ti-0.08C-2.34Fe for Alloy 1, and 20.5Cr-15.6Co5.03Mo-1.57Al-3.42Ti-0.15C-1.64Fe-0.076Zr-0.01%B for Alloy 2. Results showed a higher volume fraction of MC carbides and the γ/γ´ eutectic for the Alloy 2 with higher titanium content since this element is a carbide and γ/γ´ eutectic promoter. The porosity level in the Alloy 2, with higher amounts of carbon, boron and zirconium was considerably lower than that for the Alloy 1. These results are discussed in terms of different phenomena caused by the segregating effects of boron, carbon and zirconium towards the interdendritic regions and the formation of zirconium carbides that produce an expansion in volume when they are formed at the end of the solidification process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Study of Rare Earth Additions Effect on the Solidification and Properties of 4130 Steel

This paper covers a set of experiments where two different rare earth additions were made to 4130 steel. A thermal analysis system was employed to measure solidification reactions and determine whether changes from the baseline occurred. Keel block castings were also poured and then machined into tensile bars. Mechanical testing found an increase in strength. Electron microscopy found RE oxysulfides had formed in the steel. Examination of the cooling rate curves noted the formation of additional phases during the solidification of the RE treated steels. The peritectic and solidus temperatures of the treated steels increased, indicating that the RE inclusions acted as heterogeneous nuclei for austenite during freezing.

36 MATERIALS SCIENCE↗
DOE OSTI2023

High fidelity model of directed energy deposition: Laser-powder-melt pool interaction and effect of laser beam profile on solidification microstructure

Metal additive manufacturing technologies keep receiving a great deal of interest as well as strong requests to develop methods to link the process science to printed parts performance and understand how to overcome inherent limitations. Here, a high-fidelity model based on the multiphysics ALE3D code was developed to reproduce the directed energy deposition process down to the powder scale. This includes resolving the laser-powder-melt pool interactions (powder impingement and incorporation into melt pool, hydrodynamics flow condition and laser absorption inefficiencies) as well as the resulting solidification microstructure. This micrometer scale digital twin captured the effect of powder incorporation process and powder flow rate on porosity. Furthermore, it was used to explore how a ring laser beam profile instead of the standard Gaussian laser profile could decrease the thermal gradient along the solidification front in the melt pool, which in turn can increase propensity for more desirable equiaxed grains.

36 MATERIALS SCIENCE↗