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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Characterization of compositional variation and solidification condition on rapid solidification behavior of beta stabilized titanium

Rapid solidification (RS) is a field of study that has increased in importance due to the rise of additive manufacturing (AM) techniques. The breakdown of equilibrium at the solid-liquid interface results in an inability to use traditional solidification models that have been applied to great success in casting. As such, there is a current need for a high-throughput testing method to screen alloys at rapid solidification conditions without the significant cost for powder atomization. In this work, twin hammer splat quenching (SQ) is utilized and theoretically developed as a technique uniquely placed to test alloy systems for AM applications. Using analytical thermal spray models and numerical heat transfer simulations, the cooling rate and solidification conditions of splat quenching are compared across several parameters to evaluate the key parameters for generating cooling rates relevant to AM process. Once a framework for the cooling rates and relevancy of SQ are developed, SQ was used to process alloy compositions in the Ti5553 alloy space (AMS 7026) to study the formation of detrimental secondary phases occurring upon rapid quenching from the melt. A martensitic phase is found to occur at the lower end of the standard, but the composition listed in the standard were found to be safe. Finally, the rapid solidification behavior of Ti-5553 is evaluated by comparing splat quenching, laser welding, and plasma atomization that offer a wide window of solidification conditions. The segregation and solidification structure are analyzed and compared to models to provide a deeper understanding of the solidification behavior of beta stabilized titanium alloys.

Williamson, Christopher J.↗
DOE OSTI2016

Determination of crystal growth rates during rapid solidification of polycrystalline aluminum by nano-scale spatio-temporal resolution in situ transmission electron microscopy

In situ investigations of rapid solidification in polycrystalline Al thin films were conducted using nano-scale spatio-temporal resolution dynamic transmission electron microscopy. Differences in crystal growth rates and asymmetries in melt pool development were observed as the heat extraction geometry was varied by controlling the proximity of the laser-pulse irradiation and the associated induced melt pools to the edge of the transmission electron microscopy support grid, which acts as a large heat sink. Experimental parameters have been established to maximize the reproducibility of the material response to the laser-pulse-related heating and to ensure that observations of the dynamical behavior of the metal are free from artifacts, leading to accurate interpretations and quantifiable measurements with improved precision. Interface migration rate measurements revealed solidification velocities that increased consistently from ∼1.3 m s{sup −1} to ∼2.5 m s{sup −1} during the rapid solidification process of the Al thin films. Under the influence of an additional large heat sink, increased crystal growth rates as high as 3.3 m s{sup −1} have been measured. The in situ experiments also provided evidence for development of a partially melted, two-phase region prior to the onset of rapid solidification facilitated crystal growth. Using the experimental observations and associated measurements as benchmarks, finite-element modeling based calculations of the melt pool evolution after pulsed laser irradiation have been performed to obtain estimates of the temperature evolution in the thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Interface morphology and dislocation-mediated processes during rapid solidification of thin films

Rapid solidification experiments have, in recent years, revealed a wealth of new microstructural phenomena that suggest a strong connection between the kinetics of solidification and the crystalline structures that emerge as a result. In this work, we investigate the interplay between interface morphology and defect-mediated processes during rapid solidification conditions using a Phase Field Crystal (PFC) model, enabling us to simultaneously and efficiently explore the physics of solidification and elasto-plasticity in the formalism of a single-field theory. We predict that there are two mechanisms by which dislocations emitted directly from the solid–liquid interface induce orientation gradients as well as the formation of subgrain boundaries within a single solidifying cell. We relate these mechanisms to the morphology of the moving solid–liquid interface and identify a suitable control parameter in the PFC model with which we can go between said morphologies by effectively changing the relative strength of the capillary length and kinetic coefficients of the solid–liquid interface. Thus, we are able to provide mechanistic explanations for several microstructural features (with an emphasis on orientation gradients and subgrain boundaries) observed during the rapid solidification of pure materials. We also provide a simple explanation for the formation of “jagged” subgrain boundaries, which is consistent with our experimental observations in rapidly solidified samples of Aluminum, whose mechanisms have thus far been unknown.

Interface morphology↗
DOE OSTI2022

USE OF TWO-PISTON SPLAT QUENCHING TO INVESTIGATE & CHARACTERIZE THE IMPACT OF COMPOSITIONAL VARIATIONS ON RAPID SOLIDIFICATION MICROSTRUCTURES & SUB-MICROSCALE FEATURES IN STAINLESS STEEL ALLOY.

The objective of this dissertation was to use two-piston splat quenching (SQ) to investigate the impact of compositional modifications on the solidification and microstructure of rapidly solidified austenitic stainless steels (SS) and to demonstrate the ability of SQ to quickly and effectively simulate rapid solidification conditions similar to those found in powder bed fusion (PBF) additive techniques. PBF techniques like laser powder bed fusion (LPBF) are being implemented across a breadth of research and industrial applications to create parts with complex geometries and performance capabilities while pushing the current limits of processing conditions and understanding of material systems. In this work, SQ was used to experimentally produce rapid solidification in 20+ unique austenitic SS compositions with systematic variations of the chrome and nickel equivalency ratio (Cr/Nieq) through targeted compositional modifications. From the targeted change of Cr, Ni, and Mo concentrations in rapidly solidified SS alloys, the ferrite solidification mode was found to be the primary solidification mode at significantly lower Cr/Nieq than previously predicted for RS. Also, decreasing concentrations of Fe at a constant Cr/Nieq ratio (i.e., different Fe isopleths), or increased Mo concentrations at a constant Cr/Nieq ratio were found to suppress the ferrite to austenite massive transformation when compared to alloys with lower concentrations at the same Cr/Nieq. Using an established empirical relationship between cell size and cooling rate, the SQ technique was estimated to produce cooling rates between 106 and 108 K/s. Thermal gradients were extracted from 2-D heat transfer simulations of the SQ solidification event and used with these cooling rates to produce solidification rate estimates for SQ which were between 0.4-1.6m/s. The primary solidification mode was observed to be the determining factor in which elements segregated to the cell boundaries during RS, for which Cr and Mo were the main elements to segregate during primary austenite solidification and Ni during primary ferrite solidification. Finally, the solidification rates and conditions produced by SQ experiments resulted in similar microstructures, features, and microsegregation to what was found in LPBF samples of the same feedstock.

Hasenbusch, Zachary↗
DOE OSTI2026

Splat Quenching as a High Throughput Rapid Solidification Testing Method for Developing Additive Manufacturing Alloys

Splat quenching as a high throughput rapid solidification screening tool is evaluated. Samples were made using known alloys (SS316, IN625, Ti-5553) to evaluate and establish the microstructures for a variety of alloy systems as well as the predicted cooling rates. The samples are inductively heated and levitated prior to being struck between two platens that produce a high contact pressure and a thin sample resulting in cooling rates on the order of 10 6 to 10 7 K/s. Process parameters were evaluated with analytical models in addition to numerical simulations to provide an effect of process variables such as substrate material, melt superheat, and platen velocity on the resulting solidification. The splat thickness was found to be controlled by platen velocity, melt superheat, as well as the feedstock volume. The sample thickness is the key controlling factor for varying the average cooling rate experienced by the splat quenched sample. The splat quenching techniques can reach regions of rapid solidification space that meet and exceed laser and electron beam techniques across the sample. In conclusion, the results of the study provide a useful foundation in understanding the splat quenching technique and its potential as a low effort tool to explore rapid solidification effects on alloys.

Alloy Design↗
DOE OSTI2023

Three-dimensional morphology of an ultrafine Al-Si eutectic produced via laser rapid solidification

Al-Si alloys processed by laser rapid solidification yield eutectic microstructures with ultrafine and interconnected fibers. Such fibrous structures have long been thought to bear resemblance to those formed in impurity-doped alloys upon conventional casting. Here, we show that any similarity is purely superficial. By harnessing high-throughput characterization and computer vision techniques, we perform a three-dimensional analysis of the branching behavior of the ultrafine eutectic and compare it against an impurity-modified eutectic as well as a random fractal (as a benchmark). Differences in the branching statistics point to different microstructural origins of the impurity- and quench-modified eutectic. Finally, our quantitative approach is not limited to the data presented here but can be used to extract abstract information from other volumetric datasets, without customization.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Transmission electron microscopy of the rapid solidification microstructure evolution and solidification interface velocity determination in hypereutectic Al-20at.%Cu after laser melting

The evolution of rapid solidification (RS) microstructure and solidification interface velocity has been studied experimentally by in-situ transmission electron microscopy and postmortem characterization for hypereutectic Al-20at.%Cu (37 wt.%Cu) after laser melting. Four morphologically distinct regions formed via growth modes changing from θ-Al 2 Cu phase dendrites to eutectic cell growth, possibly α-Al dendrite growth (α-cell), banded growth, and α-Al plane front growth. Tendency for faceting and low capacity for solute trapping limited the θ-phase dendrite growth to solidification interface velocity v < 0.07 m/s. Consequently, formation of pro-eutectic micro-constituent was suppressed, and RS microstructure formation was dominated by eutectic, α-cell, and banded morphology grains for the Al-20at.%Cu alloy. Eutectic growth operated for interface velocity of 0.1 m/s ≤ v < 0.3 m/s, with a transition from regular lamellar, 2-λ and 1-λ mode to a dense irregular morphology dominated by α-phase at v = 0.3 m/s. Interface temperature calculations indicated feasibility of α-cell growth mode for 0.3 m/s ≤ v < 0.7 m/s. Banded growth occurred for 0.7 m/s ≤ v < 1.3 m/s. Plane front α-phase growth was evident for interface velocities v ≥ 1.3 m/s. Previous work on rapid solidification microstructure development in hypereutectic Al-Cu alloys reported a regime of α-cell growth subsequently to eutectic and prior to transition to banded growth for Al-19at.%Cu (36 wt.%Cu), while for Al-22at.%Cu (40 wt.%Cu) eutectic growth transitioned directly to α-plane front growth without emergence of a banded regime. Finally, based on the current study the disappearance of the banded growth regime occurs for a composition larger than 20at.%Cu.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Rapid Solidification in Bulk Ti-Nb Alloys by Single-Track Laser Melting

Single-track laser melting experiments were performed on bulk Ti-Nb alloys to explore process parameters and the resultant macroscopic structure and microstructure. The microstructures in Ti-20Nb and Ti-50Nb (at.%) alloys exhibited cellular growth during rapid solidification, with average cell size of approximately 0.5 µm. Solidification velocities during cellular growth were calculated from images of melt tracks. Measurements of the composition in the cellular and intercellular regions revealed nonequilibrium partitioning and its dependence on velocity during rapid solidification. Furthermore, experimental results were used to benchmark a phase-field model to describe rapid solidification under conditions relevant to additive manufacturing.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Composition and automated crystal orientation mapping of rapid solidification products in hypoeutectic Al-4 at.%Cu alloys

Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used in this paper to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8 m s - 1 < v < v a = 1.8 m s - 1 , two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al 2 Cu-based θ'-phase, which can be described as {110} θ ∥ {001} α , [001] θ ∥ [110] α and {001} θ ∥ {001} α , [100] θ ∥ [100] α . These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. Finally, this endows a kinetic advantage to the thermodynamically less stable θ'-Al 2 Cu phase relative to the more stable equilibrium θ-Al 2 Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ'-Al 2 Cu phase.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermoelectric Properties of TiNiSn Half Heusler Alloy Obtained by Rapid Solidification and Sintering

Thermoelectric TiNiSn-based half Heusler compound reveals high Seebeck coefficient and electrical conductivity, leading to high power factor. As a consequence of the solidification path, TiNiSn single phase cannot be obtained directly from the liquid phase. Thus, a long annealing step is needed to homogenize the alloy. In this work, we present a new processing route for half Heusler compound formation, combining arc melting of pure elements, rapid solidification of the molten alloy and sintering. Rapid solidification of the molten alloy allows to obtain almost single TiNiSn phase, limiting the formation of the primary TiNi{sub 2}Sn phase as a consequence of the deep undercooling of the liquid. The rapidly solidified alloy was ground to powder and sintered by open die pressing. As-sintered samples show a density around 95% of the theoretical value. Thermal cycling of the sintered samples shows evolution of the phases, suggesting that after sintering the sample is not in equilibrium yet. After the second thermal cycle thermoelectric properties become reproducible, indicating the attainment of the equilibrium. In conclusion, the proposed processing route allows to obtain dense TiNiSn in bulk form avoiding the time-consuming annealing step, typically used to homogenize this alloy after solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A novel interface strengthening layer: Nanoscale AuCu super-structure formed during Au80Sn20/Cu rapid solidification soldering process

Highlights: • The nanoscale AuCu super-structure formed in Au80Sn20/Cu soldering joint. • The phase-transformation diffusion mechanism of Au80Sn20/Cu soldering process was studied. • The orientation relationship of Au80Sn20/Cu intermetallic compounds was investigated. • The interfacial strengthening mechanism of nanoscale AuCu layer was discussed. - Abstract: Phase composition and orientation of intermetallic compounds layers determine the functional and mechanical properties of solder joints. In this paper, interfacial reaction and strengthening mechanism of Au80Sn20/Cu joint by rapid solidification reflow soldering were studied. The Au80Sn20/Cu solder joint by rapid solidification reflow soldering exhibit a superior shear property, and the shear strength reaches up to 90 MPa. It is found that a new nanoscale AuCu super-structure formed in the interfacial intermetallic compounds layer, which is coherent with the [001] Cu substrate. The orientation relationship of Cu matrix/interfacial intermetallic compounds is determined to be [001]{sub Cu}∥[001]{sub AuCu}∥[ 2-bar 4 2-bar 3]{sub (AuCu){sub 5Sn}} and (200){sub Cu}∥(200){sub AuCu}∥(10 1-bar 0){sub (AuCu){sub 5Sn}}. Nanoscale AuCu super-structure influences the interface slip direction and provides interface misfit dislocation, hinder the dislocation movement, lead to the interface strengthening effect. Our finding on nanoscale AuCu super-structure formed during Au80Sn20/Cu soldering process not only sheds light on future research direction but also provides practical guidance to the engineers in solder industry.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Rapid Solidification and Phase Transformations in Additive Manufactured Materials

Within the past few years, additive manufacturing (AM) has emerged as a promising manufacturing technique to enable the production of complex engineering structures with high efficiency and accuracy. Among the important factors establishing AM as a sustainable manufacturing process is the ability to control the microstructures and properties of AM products. In most AM processes, such as laser sintering (LS), laser melting (LM), and laser metal deposition (LMD), rapid solidification and high-temperature phase transformations play primary roles in determining nano- and microstructures, and consequently the mechanical and other properties of AM products. This topic of JOM is dedicated to summarizing the current research efforts in the area of rapid solidification and phase transformations in additively manufactured materials. Finally, a brief summary follows below of 10 journal articles in this topic.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Control of Carbide Precipitation During Electroslag Remelting-Continuous Rapid Solidification of GCr15 Steel

The effect of the electroslag remelting-continuous rapid solidification (ESR-CRS) process on element segregation and carbide precipitation in GCr15 bearing steel was investigated. The results showed that a microstructure with fewer primary carbides and less segregation can be achieved via the ESR-CRS process. In the specimen subjected to ESR, the morphology of primary carbide changed from angular to lumplike. After the ESR-CRS process, the dimension of primary carbide and the mass fraction of Cr in primary carbide decreased. With the increase of cooling intensity during the ESR-CRS process, the microstructure of ESR ingot became more refined and uniform and the size of large primary carbides in ESR ingot gradually decreased. Suppressing the formation of grain boundary cementite and primary carbide during ESR of GCr15 steel is beneficial to inhibiting the presence of large secondary carbide formation after annealing. With the increase of cooling intensity, the mass fraction of carbides in ESR ingot decreased and the mass fraction of Cr in carbides increased, whereas the types of carbides did not change; all the M{sub 3}C, M{sub 3}C{sub 2}, and M{sub 7}C{sub 3} exist before and after ESR-CRS. Al{sub 2}O{sub 3} inclusions promoted the formation of Ti(N,C) by serving as preferred heterogeneous nucleation sites, whereas the formation of Ti(N,C) was suppressed through the refinement of Al{sub 2}O{sub 3} inclusions by increasing the cooling intensity.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Interfacial free energy and stiffness of aluminum during rapid solidification

Using molecular dynamics simulations and the capillary fluctuation method, we have calculated the anisotropic crystal-melt interfacial free energy and stiffness of aluminum in a rapid solidification system where a temperature gradient is applied to enforce thermal non-equilibrium. To calculate these material properties, the standard capillary fluctuation method typically used for systems in equilibrium has been modified to incorporate a second-order Taylor expansion of the interfacial free energy term. The result is a robust method for calculating interfacial energy, stiffness and anisotropy as a function of temperature gradient using the fluctuations in the defined interface height. This work includes the calculation of interface characteristics for temperature gradients ranging from 11 to 34 K/nm. The captured results are compared to a thermal equilibrium case using the same model and simulation technique with a zero gradient definition. We define the temperature gradient as the change in temperature over height perpendicular to the crystal-melt interface. The gradients are applied in MD simulations using defined thermostat regions on a stable solid-liquid interface initially in thermal equilibrium. The results of this work show that the interfacial stiffness and free energy for aluminum are dependent on the magnitude of the temperature gradient, however the anisotropic parameters remain independent of the non-equilibrium conditions applied in this analysis. As a result, the relationships of the interfacial free energy/stiffness are determined to be linearly related to the thermal gradient, and can be interpolated to find material characteristics at additional temperature gradients.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Matching time and spatial scales of rapid solidification: Dynamic TEM experiments coupled to CALPHAD-informed phase-field simulations

A combination of dynamic transmission electron microscopy (DTEM) experiments and CALPHAD-informed phase-field simulations was used to study rapid solidification in Cu–Ni thin-film alloys. Experiments—conducted in the DTEM—consisted of in situ laser melting and determination of the solidification kinetics by monitoring the solid–liquid interface and the overall microstructure evolution (time-resolved measurements) during the solidification process. Modelling of the Cu–Ni alloy microstructure evolution was based on a phase-field model that included realistic Gibbs energies and diffusion coefficients from the CALPHAD framework (thermodynamic and mobility databases). DTEM and post mortem experiments highlighted the formation of microsegregation-free columnar grains with interface velocities varying from ~0.1 to ~0.6 m s –1 . After an 'incubation' time, the velocity of the planar solid–liquid interface accelerated until solidification was complete. In addition, a decrease of the temperature gradient induced a decrease in the interface velocity. The modelling strategy permitted the simulation (in 1D and 2D) of the solidification process from the initially diffusion-controlled to the nearly partitionless regimes. Lastly, results of DTEM experiments and phase-field simulations (grain morphology, solute distribution, and solid–liquid interface velocity) were consistent at similar time (μs) and spatial scales (μm).

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Naturally Deposited Film on the Sub-rapid Solidification of Medium Manganese Steel by Using Droplet Solidification Technique

In this study, the effect of naturally deposited film on the interfacial heat transfer during the sub-rapid solidification of medium manganese steel was studied by means of a droplet solidification technique. The heat transfer rates were calculated by the Inverse Heat Conduction Program (IHCP), through computing the responding temperatures’ gradient inside the cooling mold. The results showed that the contact angle between the mold and solidified droplet becomes lower (from 117.539 to 82.217 deg) with the repeat of the dropping tests, which means the wettability improves after the deposition of the natural film. Meanwhile the liquid time (the time from the start of ejection to the initiation of solidification) decreases from 1.76 to 1.28 seconds during the tests, suggesting that the steel keeps in the liquid state shorter. Therefore, the initiation of solidification is advanced, such that there would be less time for matching the scale of the shell and the mold. Consequently, the maximum heat flux increases from 2.90 to 6.35 MW/m{sup 2} with the deposition of the natural film, as the wettability is getting better leading to the reduction of the interfacial thermal resistance. However, the final maximum heat flux was decreasing to 4.98 MW/m{sup 2} due to the increased thickness of the film and corresponding interfacial thermal resistance. Moreover, it was found that the secondary dendrite arm spacing decreased from 10.84 to 5.01 μm at first, and then it increased to 8.11 μm with the variations of heat flux and cooling rate.

36 MATERIALS SCIENCE↗