Coarsening Effects Research Articles

Heat Treatment Post-Processing for the Improved Mechanical Properties of Scalmalloy® Processed via Directed Energy Deposition

As high-strength aluminum alloys present several processability issues with additive manufacturing (AM), Scalmalloy®, an Al-Mg-Sc-Zr-based alloy, has been developed. This alloy is age-hardenable, allowing it to precipitate out a strengthening precipitate phase, Al3(Sc,Zr). The manufacturer recommends a single-stage aging treatment at 325 °C for 4 h; however, the majority of the literature studies utilize a powder bed processing known as selective laser melting (SLM) over powder-fed processing directed energy deposition (DED). This study addresses the lack of information on heat treatments for DED fabrication by exploring the application of artificial aging temperatures of 300–400 °C for 2, 4, and 6 h to: 1. determine the impact on the microstructural evolution and mechanical performance and 2. determine whether the recommended treatment for Scalmalloy® is appropriate for DED fabrication. Tensile testing determined that low-temperature treatments exhibited no visible dependence on time (2–6 h); however, time becomes influential at higher temperatures starting at 350 °C. The temperature plays a considerable role in the mechanical and microstructural behaviors of DED Scalmalloy®. The highest tensile strength was noted at 300 °C (384 MPa, 21.6% increase), but all heat-treated cases resulted in an improvement over the as-built case. This investigation established that increasing the treatment temperature resulted in a decreasing trend for the tensile strength that held over time. Elongation at 2 h displayed a near parabolic trend that peaks at 350 °C (20%) and falls with higher temperatures. At the 4 h treatment, a slight decreasing trend was noticed for elongation. No visible change was observed for elongation at 6 h, with elongation values remaining fairly consistent. The microstructural evolution, including micron-sized and nano-sized Al3(Sc,Zr) and grain size, was examined, and coarsening effects were noted with the increase in the temperature. It is recommended that treatment be conducted at 300 °C to achieve the precipitation of the strengthening Al3(Sc,Zr) phase while minimizing coarsening.

Coarsening effects on the liquid permeability in foam-filled porous media

Coarsening effects on the liquid permeability in foam-filled porous media

Hierarchically heterogeneous microstructure and mechanical properties in laser-directed energy deposition of γ-TiAl alloy through intrinsic cyclic heat treatment

The complex thermal effects of laser direct energy deposition (L-DED) provide significant advantages for production of high-performance γ-TiAl alloy, whereas challenges still exist in microstructure tailoring and mechanical properties. This work focuses on the influence of intrinsic cyclic heat treatment (ICHT) on the evolution of microstructures and mechanical properties in different building height regions of an L-DED TiAl alloy thin-wall sample. The results indicate that ICHT induces discontinuous coarsening (DC) effects by consuming the lamellar interface energy of the initial lamellae, resulting in grain refinement. With the increase in building height, the thermal accumulation leads to an increase in the initial lamellar spacing, which weakens the DC effect. At the top, insufficient thermal cycles prevent fine grain growth and erode coarse grains, hindering overall grain refinement. Ultimately, the hierarchically heterogeneous microstructure forms in the L-DED TiAl alloy, including refinement of hierarchical lamellar colony structure decorated with fine (α2+γ) lamellar structure and (α2+6H-LPS) lamellar structure (6H-LPS refers to 6H long-period structure phase). Specifically, the bottom region exhibits the best combination of ultimate tensile strength (706 ± 33 MPa) and strain (0.68 ± 0.05 %). The high tensile strength can be attributed to the hindrance of dislocation movement by the fine lamellae and the 6H-LPS phase ,caused by the high cooling rate of L-DED, while the elongation is attributed to the reduction in crack propagation energy due to the presence of fine grains.

Metallurgical, mechanical, and corrosion behavior of AISI 304L joints welded using variable arc energy inputs and subjected to thermal aging

The effect of three levels of welding arc energy input on the metallurgical, mechanical, and corrosion behavior of AISI 304L stainless steel was studied with the aim of investigating and recommending the most useful range of process parameters for improved performance of engineering structures made from this industrially important alloy. The gas tungsten arc welding process was employed, and all the joints were butt-welded using a single-V groove design. With an increase in heat input, the δ-ferrite phase changed from lathy to vermicular, besides the heat-affected zones of all the welds exhibiting grain coarsening effects. Postweld thermal aging led to the preferential occurrence of carbides in the interdendritic regions, which degraded their corrosion performance. Thermal aging did not affect the tensile strength of these welds, but their ductility was significantly impacted. The extent of segregation of Cr and C, especially, was observed to be the least in the case of low heat input welds. This study establishes that while designing welding procedures for AISI 304L structures, low welding arc energy should be used as it promotes better mechanical as well as corrosion properties.

Change in Mechanical Properties of Laser Powder Bed Fused AlSi7Mg Alloy during Long-Term Exposure at Warm Operating Temperatures.

Al-Si-Mg alloys are most commonly used to produce parts by laser powder bed fusion for several industrial applications. A lot of papers have already focused on the effects induced by conventional heat treatments on the microstructure and mechanical properties of AlSi10Mg alloys, rather than on AlSi7Mg. Nobody has investigated thermal stability during long-term direct and artificial aging heat treatments of AlSi7Mg. This study investigates the changes in mechanical properties induced by long-term exposure (512 h) at 150 and 175 °C (the operating temperature of AlSi7Mg) after (i) the laser powder bed fusion process performed on a pre-heated build platform (150 °C), and (ii) heat treatments to the solution at 505 °C per 0.5 and 4 h. Thermal stability was evaluated through both Vickers microhardness measurements to obtain the aging profiles, and tensile tests to evaluate the mechanical properties in specific conditions. An optical microscope was used to investigate the microstructure. It was found that aging at 175 °C confers the same effects induced by a secondary aging heat treatment on as-built samples and, simultaneously, the worst effects on the solution heat treated AlSi7Mg alloy after long-term exposure. The AlSi7Mg DA at both 150 °C and 175 °C showed the same Vickers microhardness (~95 HV0.5), UTS (~300 MPa), and YS (~200 MPa) values for the longest exposure times because the fine and cellular α-Al matrix confers higher stiffness and strength despite the over-aged conditions. On the other hand, the coarsening effects that affected the precipitates during aging at 175 °C, as well as the formation of the precipitate-free zones along the grain boundaries, justified the highest detrimental effects induced on the SHTed samples.

(Keynote) Degradation Processes in Solid Oxide Cell Ni-YSZ Electrodes

Solid oxide cells (SOCs) can have a significant impact on climate change over the next decade and beyond, in applications such as balancing renewable grid electricity via electrolytic fuel production, and producing electricity from bio-fuels combined with CO2 product sequestration. However, long-term performance degradation remains a key variable that may limit further implementation of SOCs. This talk focuses on the Ni-YSZ fuel electrode that is widely used but is known to be an important contributor to SOC degradation. Various processes that cause Ni-YSZ degradation are discussed. Results on 3D tomography measurements of accelerated Ni coarsening are described and a quantitative model is developed to predict long-term degradation. Although the results indicate that coarsening effects can be minimized in well-designed Ni-YSZ microstructures, degradation can still occur, especially during high-current-density electrolysis operation. For operation under low H2O/H2 conditions, high electrolysis current density can yield reduction of zirconia to form Ni-Zr compounds, along with substantial microstructural damage. For operation under high H2O/H2conditions, high electrolysis current density can yield Ni migration away from the electrolyte. A phase-field simulation is described that predicts this Ni migration, using actual Ni-YSZ microstructures measured using 3D tomography as the starting point, and compared with experimental observations. The model assumes that Ni transport is driven by a spatial gradient in surface tensions, i.e., a decrease in the Ni/YSZ contact angle with increasing distance from the electrolyte. Recent results on electrolysis and reversibly operated SOCs, making use of Ceria-infiltrated Ni-YSZ to improve stability, are described.

Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-δ during reactive air brazing

Reactive air brazing of the ceramic oxygen transport membrane material BSCF causes microstructural modifications. The trend towards minimizing of membrane thicknesses requires a physical understanding of these modifications since they may influence both oxygen permeation and mechanical properties. To this purpose, wetting samples with variations in the Ag-xCuO braze alloy (1 < x < 25 at.-%) and the brazing time (0 < t < 120 min) were characterized by quantitative image analysis. We found that the pore size in reactive air brazed BSCF increases with increased brazing time as well as CuO-content in the braze. A local porosity minimum is always observed at the end of the reaction zone close to the unaffected bulk BSCF. Additionally, a zone with unidirectionally elongated grains at the end of which silver residues were found is observed. The observed coarsening effects are explained by liquid melt film penetration along the grain boundaries which increases the grain boundary mobility. Phase field simulations qualitatively confirmed the experimentally observed elongated grain structure.

Effect of HIP post-processing at 850 °C/200 MPa in the fatigue behavior of Ti-6Al-4V alloy fabricated by Selective Laser Melting

Hot Isostatic Pressing(HIP) is a thermomechanical post-processing technique widely used in Additive Manufacturing parts to reduce internal defects, such as entrapped-gas-pores or lack-of-fusion, which have a great influence on the mechanical and fatigue properties of the material. In this paper, the effect of a non-conventional HIP-cycle on the fatigue behavior of a Ti-6Al-4V alloy manufactured by Selective Laser melting (SLM) is studied. The HIP-cycle examined in this study is carried out at pressure of 200 MPa and a temperature of 850 °C for 2 h. Moreover, the cooling process is faster than that obtained from conventional furnace cooling rates, with the aim to limit the microstructural coarsening effects that affect the fatigue behavior. For the study, an extensive experimental fatigue program was carried out which included a first batch of SLM specimens tested under as-built conditions, a second batch of SLM specimens subjected to the present HIP process, and a third batch of specimens of a reference wrought processed material obtained by rolling and annealing processes. The microstructure of the material, before and after HIPping, is analyzed and a fractographic analysis is carried out to study the mechanism of crack initiation and its relation to the fatigue behavior. The results show that the present HIP-process allows for very good material densification, a microstructure that shows minimal coarsening effects, and good fatigue properties comparable to the conventional wrought processed material.

The geometric axial surface profiles of granular flows in rotating drums

SYNOPSIS A mechanistic description of axial segregation in rotating drum flows remains an open question. Consequently, optimal mixing of grinding balls and rocks for efficient breakage, maximum production of fines, and slurry transport is seldom achieved. Experimental and numerical studies of granular mixtures in rotating drums identify alternating axial bands that eventually coarsen in the long-term limit. Most models of axial segregation are limited to binary mixtures and cannot always predict the logarithmic coarsening effects observed experimentally. A key missing factor is a robust description of the axial free surface profile that is valid across a wide range of flow regimes. We present a practical model of the axial free surface profile by linking it to readily-derived geometric features of the cross-sectional S-shaped free surface profile. A parametric study shows good agreement with experimental measurements reported in the literature and heuristically valid trends. Keywords: rotating drum, granular flow, axial profile, comminution, mixing, segregation.

Microstructure evolution and mechanical properties of the dissimilar joint between IN718 and STS304

Modern fuel injectors for gasoline, diesel, and gaseous fuel use dissimilar joints of Inconel 718 and STS 304. To better understand the microstructural evolution and mechanical properties of these types of dissimilar joints, the joints of these alloys were fabricated by employing laser beam welding process. The joint was characterized by electron microscopy and hardness measurement techniques. By employing the laser beam welding, a crack-free deep penetrating joint was obtained. The weld metal was primarily composed of an austenite phase with a distribution of Laves and Ti/Nb carbides in the interdendritic regions. The low heat inputs and fast cooling rates suppressed the Laves precipitation, as its volume fraction was less than that of most IN718 joints fabricated by traditional welding techniques. Partially melted zones (PMZ) were formed on both sides of the joints. The PMZ of IN718 base metal was complex with Nb-rich precipitates detected as δ-Ni3Nb, while that of STS 304 was free of precipitates. No significant grain coarsening or negative metallurgical effects were observed at the heat-affected zones. The fusion zone (FZ) recorded a lower hardness, compared to the parent metal, which is mainly due to the depletion of strengthening elements (Nb, Mo, and Ti) and lack of hardening γ′′ particles. The joint showed a low strength with failure occurring at the FZ. Post-weld solution treatment at 980 °C resulted in the partial dissolution of the Laves phase and complete disappearance of the dendrites, followed by the precipitation of plate- and needle-like δ phase. Post weld heat treatment increased the hardness of the joint. The solution treated + double aged joint showed much higher strength than the directly aged joint. The increased hardness reflects the precipitation of γ′′ in the weld.

High Entropy Alloys: Ready to Set Sail?

Over the past decade, high entropy alloys (HEAs) have transcended the frontiers of material development in terms of their unprecedented structural and functional properties compared to their counterpart conventional alloys. The possibility to explore a vast compositional space further renders this area of research extremely promising in the near future for discovering society-changing materials. The introduction of HEAs has also brought forth a paradigm shift in the existing knowledge about material design and development. It is in this regard that a fundamental understanding of the metal physics of these alloys is critical in propelling mechanism-based HEA design. The current paper highlights some of the critical viewpoints that need greater attention in the future with respect to designing mechanically and functionally advanced materials. In particular, the interplay of large compositional gradients and defect topologies in these alloys and their corresponding impact on overall mechanical response are highlighted. From the point of view of functional response, such chemistry vis-à-vis topology correlations are extended to novel class of nano-porous HEAs that beat thermal coarsening effects despite a high surface to volume ratio owing to retarded diffusion kinetics. Recommendations on material design with regards to their potential use in diverse applications such as energy storage, actuators, and as piezoelectrics are additionally considered.

Effects of isothermal storage on grain structure of Cu/Sn/Cu microbump interconnects for 3D stacking

The crystal orientation and grain distribution of Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) in miniaturized solid-liquid interdiffusion (SLID) interconnects for 3D stacking were investigated. Therefore Cu/Sn microbumps with a diameter of 15 μm on top die (metal height 5.4 μm/3.6 μm) and Cu microbumps with a diameter of 25 μm on bottom die (metal height 9.5 μm) were used for bonding and subsequent thermal storage. The effect of the storage time (varied from 10 min to 96 h) and storage temperature (150, 240 and 260 °C) on the grain structure formation was investigated by Electron Backscatter Diffraction (EBSD). After the initial Cu6Sn5 scallops have grown together, the corresponding Cu6Sn5 layer only consists of one or two grains, which are orientated with 〈10−11〉 and 〈2−1−12〉 directions parallel to the IMC growth direction (perpendicular to substrate or Cu layer). The Cu3Sn IMC showed small columnar grains in its early growth stage, which develop into grains with a polygonal shape due to coarsening effects. Cu3Sn grains are orientated randomly at the early growth stage and tend to be orientated mostly with 〈10−10〉 and 〈2−1−10〉 parallel to the IMC growth direction at higher temperatures and longer storage times.

Effect of Infiltration on Performance of Ni-YSZ Fuel Electrodes

Ni-YSZ electrodes for solid oxide cells (SOCs) can have a diverse range of compositions, porosities, and particle sizes – factors that impact electrochemical performance. A typical Ni-YSZ structure in an anode-supported cell fired at 1400oC has feature sizes of ~ 0.5 μm that yield desirably low polarization resistance values < 0.1 Ω cm2 at 800oC in H2-H2O fuel, along with good stability. Decreasing feature size increases three-phase boundary density, thereby reducing polarization resistance and improving low-temperature cell performance. However, the feature size that can be reached in anode-supported cells is limited by the relatively high co-firing temperature. Furthermore, decreased feature sizes can exacerbate coarsening effects that degrade performance. This paper discusses an alternative method for enhancing the low-temperature performance of Ni-YSZ anodes – infiltration of Gd-doped Ceria (GDC). Since GDC is introduced after the high-temperature firing, nano-scale particles can be achieved. A single-step GDC infiltration into Ni-YSZ is studied with different solution concentrations. The optimal infiltration is found to reduce polarization resistance by a factor of 3 times at 600oC.

A thermodynamically consistent model for elastoplasticity, recovery, recrystallization and grain coarsening

We present a model for the thermomechanical behaviour and microstructure evolution of metallic materials during thermomechanical processing. Special emphasis is put on its derivation based on the Müller–Liu procedure which ensures consistency with fundamental physical principles. The model represents in principle the strong coupling between elastoplastic deformation, recovery, recrystallization, grain coarsening and related thermal effects. Furthermore, it takes the dragging effect of precipitates on grain boundaries and dislocations into account by which a higher strength can be achieved and the grain size is stabilised. For the microstructure description, we use a mean-field approach. In several numerical examples, we demonstrate the capability of the model to consistently predict the interplay between elastoplastic deformation, microstructure evolution, dynamic hardening and softening and the related temperature change.

Performance Degradation Predictions Based on Microstructural Evolution Due to Grain Coarsening Effects in Solid Oxide Fuel Cell Electrodes

Due to the time required to test fuel cell degradation experimentally, a physics-based model which can predict degradation can be a valuable tool. Grain coarsening and the resulting microstructure evolution is a primary mode of degradation. In this study, a multi-physics model of a fuel cell is presented which can predict performance loss as a function of time caused by coarsening in the electrodes of an LSM-YSZ/YSZ/Ni-YSZ SOFC. Microstructural properties are updated as a function of time from their initial values using functional relations derived from experimental data. Specie and charge transport equations are solved to predict performance changes with time from the microstructural changes. The model is first calibrated such that it correctly captures the performance of a button cell. Performance change over time predicted by the model is compared to experimental data for a cell operated for 775 hours. It is found that the model predicts a slower degradation rate than the rate which occurs experimentally. This is reasonable as other forms of degradation are not being accounted for. The model is then used to make long term predictions of cell performance loss due to grain coarsening out to 40,000 hours for various operating temperatures and initial microstructures.

Prediction of Performance Degradation Due to Grain Coarsening Effects in Solid Oxide Fuel Cells

Long term degradation of solid oxide fuel cells is one of the biggest impediments to commercialization. Due to the inordinate amount of time required to properly test degradation experimentally, physics based models which can predict long term degradation can be crucial time and money saving tools. At the cell level, one of the primary modes of degradation comes from grain coarsening and the resulting changes in microstructure properties. In this study a multi-physics model of a single fuel cell is presented which aims to predict performance loss as a function of time caused by coarsening in the electrodes of an LSM/YSZ/Ni SOFC. Microstructural properties are changed as a function of time from their initial values using functional relations derived from data from experiments and phase field models. Specie and charge transport equations are solved using these parameters to predict performance changes with time which result from the aforementioned microstructural changes. The model is first calibrated such that it correctly captures the performance and impedance response of a particular button cell at a given time without any degradation. This same model is run for a duration of 750 hours with performance data being measured for the same cell including impedance measurements being taken intermittently. Performance change over time predicted by the model are compared to experimental data. It is found that the model predicts a slower degradation rate than the rate which occurs experimentally. This is deemed reasonable as other forms of degradation are not being accounted for. The model is then used to make long term predictions of cell performance loss due to grain coarsening. The resulting degradation rate is considered to be the theoretical minimum which occurs when all other modes of degradation are not present. Work is underway to include other forms of degradation in the model so that a complete and robust degradation prediction tool is available for design analysis without the need for costly long term degradation measurements.

A Flux-Corrected Phase-Field Method for Surface Diffusion

AbstractPhase-field methods with a degenerate mobility have been widely used to simulate surface diffusion motion. However, apart from the motion induced by surface diffusion, adverse effects such as shrinkage, coarsening and false merging have been observed in the results obtained from the current phase-field methods, which largely affect the accuracy and numerical stability of these methods. In this paper, a flux-corrected phase-field method is proposed to improve the performance of phase-field methods for simulating surface diffusion. The three effects were numerically studied for the proposed method and compared with those observed in the two existing methods, the original phase-field method and the profile-corrected phase-field method. Results show that compared to the original phase-field method, the shrinkage effect in the profile-corrected phase-field method has been significantly reduced. However, coarsening and false merging effects still present and can be significant in some cases. The flux-corrected phase field performs the best in terms of eliminating the shrinkage and coarsening effects. The false merging effect still exists when the diffuse regions of different interfaces overlap with each other. But it has been much reduced as compared to that in the other two methods.

Prediction of Performance Degradation Due to Grain Coarsening Effects in Solid Oxide Fuel Cells

Long term degradation of solid oxide fuel cells (SOFCs) is one of the biggest impediments to commercialization. Physics based models which can predict long term degradation can be crucial time and money saving tools. At the cell level, one of the primary modes of degradation comes from grain coarsening and the resulting changes in microstructure properties. In this study, a multi-physics model of a single fuel cell is presented which aims to predict performance loss as a function of time and temperature caused by coarsening in the electrodes of an LSM-YSZ/YSZ/Ni-YSZ SOFC. Microstructural properties are updated as a function of time from their initial values using functional relations derived from data obtained experimentally and from phase field models. Performance change over time predicted by the model is compared to experimental data and a study is performed on the effect temperature has on the degradation rate due to coarsening.

Tip-assisted directional growth of atomically thin Ag islands

The growth and decay of metal nanostructures are important in many areas of physical science and catalytic chemistry. Here, we show the directional growth of Ag island stripes at room temperature via tip-assisted coarsening of hexagonal islands on Ag(111) using scanning tunneling microscope (STM). The mechanism of this process is attributed to the alignment of the adatom and vacancy islands along the scanning direction via tip-sample interaction during the process of the intrinsic decay of the metastable Ag islands. We find that this tip-assisted island growth is not directly related to the tunneling current and the bias voltage. Experiments performed using Ag tips and W tips show similar coarsening effects. The chamber pressure and the O2 exposure also do not have systematic effects on the rate of island growth. The tip-assisted coarsening strongly depends on the tip morphology.

A benchmark for the surface Cahn–Hilliard equation

We study the surface Cahn–Hilliard equation, an often used model for spinodal decomposition and coarsening effects on surfaces. As the width of the diffuse interface shrinks to zero one expects a convergence towards a surface Hele-Shaw model. While results from formal matched asymptotic expansions confirm this conjecture, a numerical reproduction of this asymptotic behavior has not been provided, yet. It is the purpose of this contribution, to fill this gap and present a rotationally symmetric example on the standard sphere, where analytic expressions for the surface Hele-Shaw model allow a comparison with numerical results for the surface Cahn–Hilliard equation.