Coarse Microstructure Research Articles

Microstructure Evolution and Coarsening of θ′-Al2Cu Precipitates in a Creep Aged Al-Cu-Mn Alloy

This paper describes the microstructure evolution in a creep aged Al-Cu-Mn alloy using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show the sub-grains formation and accelerated coarsening of θ′ precipitates in creep aged samples. It is observed that at an applied stress of 50MPa, as the creep temperature increases the average length of θ′ precipitates increases and their aspect ratio first increases then decreases. It is also found that after 100h of creep exposure at 200°C/50MPa, θ′ precipitates have grown to an average length of 279nm; at 225°C/50MPa to an average length of 425nm and at 250°C/50MPa they have grown to an average length of 844nm. Coarsening/growth rate of the precipitates is described by Lifshitz-Slyozov-Wagner (LSW) model.

Property Improvement of Additively Manufactured Ti64 by Heat Treatment Characterized by In Situ High Temperature EBSD and Neutron Diffraction

Among various off-equilibrium microstructures of additively manufactured Ti-6Al-4V alloy, electron beam powder bed fusion, in which three dimensional metallic objects are fabricated by melting the ingredient powder materials layer by layer on a pre-heated bed, results in a specimen that is nearly free of the preferred orientation of the α-Ti phase as well as a low beta phase fraction of ∼1 wt%. However, when further heat treatment of up to 1050 ∘C was applied to the material in our previous study, a strong texture aligning the hexagonal basal plane of α phase with the build direction and about 6% β phase appeared at room temperature. In this study, to understand the mechanism of this heat treatment, the grain level microstructure of the additively manufactured Ti-6Al-4V was investigated using in situ high temperature EBSD up to 1000 ∘C, which allows the tracking of individual grains during a heat cycle. As a result, we found a random texture originating from the fine grains in the initial material and observed a significant suppression of α phase nucleation in the slow cooling after heating to 950 ∘C within the α and β dual phase regime but close to the the β-transus temperature at ∼980 ∘C, which led to a coarse microstructure. Furthermore, the texture resulting from phase transformation of the additively manufactured Ti-6Al-4V assuming nucleation at the grain boundaries was modeled, using the double Burgers orientation relationship for the first time. The model successfully reproduced the measured texture, suggesting that the texture enhancement of the α phase by the additional heat treatment derives also from the variant selection during the phase transformation and nucleation on grain boundaries.

Microstructural evolution and mechanical properties in Zr–Cu–Al–Nb bulk metallic glass composites prepared by laser metal deposition

In this work, laser metal deposition (LMD), a direct energy deposition (DED) additive manufacturing technology, is used to prepare Zr59.3Cu28.8Al10.4Nb1.5 bulk metallic glass composites (BMGCs). Phase constitutions and thermal behavior of LMD-processed Zr–Cu–Al–Nb samples are examined using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The microstructure evolution parallel to the build direction is characterized in detail utilizing scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microscopy analysis (EPMA) and electron backscatter diffraction (EBSD). Young's modulus and hardness in typical microstructures are evaluated employing nanoindentation. Within each deposited layer, a periodic microstructure, exhibiting crystalline features of three distinctly different morphologies, is observed, depending on the location within that layer. Along the build direction, the dominating microstructure of the melt pool evolves from a nearly featureless amorphous state over fine nanocrystals to coarse nanocrystals while the morphology of most micro-scale dendritic crystals changes from flower-shaped to columnar, simultaneously a coarse microstructure appearing in upper layers. The fine Al-rich phases gradually coarsen directionally parallel to the build direction. Furthermore, the correlation between microstructure and mechanical properties is investigated. This work enhances our knowledge about microstructural characteristics and mechanical properties of BMGCs in-situ fabricated by DED additive manufacturing.

Thermal aging of power module assemblies based on ceramic heat sink and multilayers pressureless silver sintering

An assembly based on the deposition and pressureless sintering of successive silver layers on an Aluminum Nitride heat sink is studied in this paper. Sintered silver layers act as die attach, current tracks and adhesion layer on the ceramic. Compared to previous studies where silver sintering is only used as die attach, an additional degree of freedom on the sintering temperature of the ceramic adhesion and track layers is obtained. This allows the investigation of silver sintering at non-conventional temperatures (>400 °C). The presented assembly is expected to endure high operating temperature since all its constituent materials are able to withstand such temperatures. The porosity of the sintered layer is controlled by the dwell time and the sintering temperature, and porosity values between 19% and 41% are obtained. The electrical and thermal conductivities of sintered layers as functions of joint porosity are in good agreement with the literature data and the assembly presents good mechanical properties with a shear strength value higher than 18 MPa. The assembly shows a high robustness during thermal storage at 200 °C for 1000 h. The microstructure and the electrical conductivity of track layers are both stable during the whole aging period. However, a slight improvement of the thermal response and the mechanical properties of the assembly is detected after 200 h of aging followed by a stabilization until the end of the aging tests (1000 h). Such observation is correlated to the microstructure coarsening of the sintered silver under the device and it is shown that this evolution is favored by the presence of the device.

Microstructural Coarsening and Mechanical Properties of Eutectic Sn-58Bi Solder Joint During Aging

Microstructural Coarsening and Mechanical Properties of Eutectic Sn-58Bi Solder Joint During Aging

Influence of Rotary Swaging on Creep Behaviour of P92 Steel

This work investigates the influence of rotary swaging on creep behaviour and microstructure heterogeneity of P92 martensitic steel. Its as-received coarse-grained state was processed by rotary swaging at room temperature. The bar of 30 mm in diameter was reduced at room temperature down to 15 mm by rotary swaging. The value of equivalent strain imposed during rotary swaging was about 1.4. Microstructures were investigated by scanning electron microscope Tescan Lyra 3 equipped with NordlysNano EBSD detector operating at an accelerating voltage of 20 kV with specimen tilted at 70°. It was found that the processing by rotary swaging led to the formation of heterogeneous microstructure containing a mixture of fine nearly equiaxed grains and large grains, which were significantly, elongated parallel to the swaging direction. Constant load tensile creep tests in tension and compression were conducted at 923 K under different applied stresses. In order to evaluate the influence of microstructural heterogeneity on the creep behaviour, the creep tests were performed both in longitudinal and also transverse directions to the rotary swaging. The results of tensile creep tests revealed that specimens manufactured in the direction parallel to the rotary swaging exhibited significantly higher creep life and strain to fracture in comparison with the results measured in the transverse direction. The creep results showed that creep tests performed at medium stresses exhibit a pronounced minimum creep rate. The microstructure results suggest that creep behaviour depends on the microstructure coarsening during creep testing and the orientation of the grain boundaries with respect to the applied stress.

On the role of input welding parameters on the microstructure and mechanical properties of Al6061-T6 alloy during the friction stir welding: Experimental and numerical investigation

The Taguchi method was employed to find the optimum values of friction stir welding parameters including welding speed, rotating speed, and tilt angle for joining AA6061-T6 aluminum alloys. The combined influences of these parameters were entirely analyzed. Statistical outcomes were investigated by the study of variances and signal-to-noise ratios. A Coupled Eulerian and Lagrangian technique is implemented to simulate and verify the optimal parameters during the friction stir welding. To verify results, a comparison between the welding process under optimized parameters with experimental and non-optimized parameters was simulated for the friction stir welding process. The material flow, strain rate, thermal behaviors, and mechanical properties of samples fabricated with optimal welding parameters are higher than those produced from the non-optimal parameters. It was also concluded that the grain size of the stir zone under optimal welding parameters (6–8 µm) is finer than that of non-optimal welding parameters (11–13 µm). Low uniform distribution of material element and coarse microstructure were some of the results of welding with non-optimized parameters. Based on residual stress analysis, the application of optimal joining conditions can decrease the peak tensile residual stress by about 38.3%. The much desirable results obtained in terms of microstructure and mechanical properties could be of great significance to the welding industry.

Additive manufacturing of thin-walled SS316L-IN718 functionally graded materials by direct laser metal deposition

Functionally graded materials (FGMs) are a good response to those advanced applications that service requirements are diverse and require high performance. Additive manufacturing (AM) technology, with its many advantages, including high flexibility for complex geometries and near-net-shape integration, has attracted special attention in the development of FGMs. In this research, the solidification behavior and microstructure evolution in the laser additive manufacturing of thin-walled stainless steel 316L-Inconel 718 graded materials have been studied with the help of solidification concepts in the welding metallurgy, according to the common principles of welding and additive manufacturing processes. For this purpose, optical and electron microscopy techniques, X-ray energy dispersive spectroscopy, and microhardness measurement were used along the build direction of FGMs with different transition designs. Microstructure evaluation showed that due to re-melting of layers, despite the increased undercooling in the build direction, morphological evolution occasionally occurred periodically between solidification modes, and due to thermal accumulation, a coarser microstructure is formed in the final layers. In addition, in the chemical analysis, it was observed that the mixing of adjacent layers caused by dilution led to a deviation of the composition distribution from the desired design. Also, the microsegregation of some elements during the non-equilibrium solidification of the process caused secondary phases such as carbides and intermetallic compound of Laves, which can have an adverse effect on the mechanical properties of the structure. However, microhardness variations along the cross-section of the samples showed that the gradation of the dissimilar thin-walled structure can effectively bring the properties and behavior of adjacent layers closer together and therefore be very useful in improving the service life.

Two-scale microstructure construction by statistical correlation functions

In this research, a new procedure is presented for two-scale construction of three-dimensional microstructures. Based on two-point correlation functions and phases recovery algorithm the fine and coarse scale microstructures are constructed separately for arbitrary volume fractions. All required correlation functions are calculated using an exponentially decreasing sinusoidal function that makes all constructed microstructures to be bicontinuous for a wide range of volume faction. Using the Boolean subtraction of fine scale microstructure from the coarse scale one, the intended multiscale microstructure is constructed. Connectivity evaluation of constructed microstructure reveals that even for low volume fractions, the connectivity of different phase regions will be established throughout the three-dimensional volume element that is crucial if conduction of heat, ion, electron, etc. is required. Another important property of the constructed microstructure is that, due to the periodic assumption, the final sections of the volume element gradually become similar to the initial ones in three dimensions and as a result it is possible to use the volume element as a unit cell to construct an arbitrary large structure with complete continuity, smoothness and isotropy.

Improvement of electrical and mechanical properties of In-48Sn solder bumps for flexible LED signage using Cu-Ag nanoparticles

This study aimed to enhance the mechanical strength of In-48Sn alloy solder bumps without the loss of electrical conductance for application of flexible electronic devices. As a simple method for controlling the mechanical strength, Cu nanoparticles were added to the In-48Sn solder paste during paste formulation. The addition of Ag-encapsulated Cu nanoparticles effectively improved the shear strength without loss of electrical conductance while the addition of Cu nanoparticles deteriorated both mechanical and electrical properties. The development of a dense or coarse microstructure during the soldering process was sensitively dependent on the surface condition and the content of nanoparticles. In particular, the uniform distribution of nanoparticles and alloy phases in β-In3Sn phase was critical for the control of mechanical strength of excessively ductile In-48Sn solder. As a result, the addition of 6 wt% Ag-encapsulated Cu nanoparticles improved the shear strength from 18.7 to 49.5 MPa and their electrical resistance from 0.23 to 0.1 mΩ. Flexible LED signage interconnected using these solder bumps performed very well after 100 000 cycles of bending test.

High Strain Rate Properties of Various Forms of Ti6Al4V(ELI) Produced by Direct Metal Laser Sintering

For analysis of engineering structural materials to withstand harsh environmental conditions, accurate knowledge of properties such as flow stress and failure over conditions of high strain rate and temperature plays an essential role. Such properties of additively manufactured Ti6Al4V(ELI) are not adequately studied. This paper documents an investigation of the high strain rate and temperature properties of different forms of heat-treated Ti6Al4V(ELI) samples produced by the direct metal laser sintering (DMLS). The microstructure and texture of the heat-treated samples were analysed using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction detector for electron backscatter diffraction (EBSD) analysis. The split Hopkinson pressure bar (SHPB) equipment was used to carry out tests at strain rates of 750, 1500 and 2450 s−1, and temperatures of 25, 200 and 500 °C. The heat-treated samples of DMLS Ti6Al4V(ELI) alloys tested here were found to be sensitive to strain rate and temperature. At most strain rates and temperatures, the samples with finer microstructure exhibited higher dynamic strength and lower strain, while the dynamic strength and strain were lower and higher, respectively, for samples with coarse microstructure. The cut surfaces of the samples tested were characterised by a network of well-formed adiabatic shear bands (ASBs) with cracks propagating along them. The thickness of these ASBs varied with the strain rate, temperature, and various alloy forms.

A strategy for high performance of energy storage and transparency in KNN-based ferroelectric ceramics

Lead-free transparent ferroelectric ceramics with superior energy storage properties are highly desirable for pulsed power technologies and the increased optical transparency demand. However, the transparency and energy storage density of lead-free bulk ceramics cannot meet the requirements of their wide applications due to the coarse microstructure and relatively low breakdown strength. In this study, a design strategy is proposed to optimize the energy storage characteristics and transparency of ceramics by introducing nanodomains, increasing the band gap energy and reducing the grain size. The results showed that submicron grain (0.21 μm) and large band gap energy (Eg = 3.21 eV) were achieved through the introduction of the second component Sr(Sc0.5Nb0.5)O3 (SSN). An excellent transparency of up to 84.5% in the near-infrared region (1378 nm), a high energy density (W) of ~4.42 J·cm−3 and an extremely small strain of ~0.022% were simultaneously achieved in the 0.825(K0.5Na0.5)NbO3-0.175Sr(Sc0.5Nb0.5)O3 (0.825KNN-0.175SSN) ceramics. These results revealed the potential applications of (K0.5Na0.5)NbO3-based ceramics for energy storage and provide a feasible approach of domain engineering to develop new lead-free energy storagetransparency materials.

Factors that govern the electric permittivity of carbon materials in the graphite allotrope family

Through comparing carbons in the graphite allotrope family, namely isotropic graphite (IG), exfoliated-graphite-based flexible graphite (FG), graphite-flake thick film (not sintered), carbon fibers (PAN, pitch) and highly-oriented pyrolytic graphite (HOPG), all differing in the microstructure, this work addresses for the first time the factors (including the structure-property relationship) that govern the electric permittivity (kHz frequency) of carbons. The permittivity is more sensitive to the microstructural heterogeneities than the resistivity, which only slightly influences the permittivity. Smaller crystallite dimensions promote the permittivity. PAN-based carbon fibers exhibit higher permittivity than pitch-based carbon fibers, IG, FG or HOPG. The crystallographic preferred orientation is not influential to the permittivity, though it influences the resistivity. Interfaces (e.g., the grain boundaries in IG and the interface between the carbon layer ribbons in a fiber) promote the permittivity, being sites for the carrier-atom interaction that enables the polarization. A large number of sites is more important than a large site size for promoting the permittivity. The fiber diameter is not influential to both permittivity and resistivity. The thick film exhibits the highest relative permittivity (5 × 105), due to its low electrical connectivity. IG (grain size 12 μm) gives the lowest relative permittivity (5 × 102), due to its coarse microstructure.

Synthesis, microstructure and mechanical properties of high-entropy (VNbTaMoW)C5 ceramics

High-entropy ceramics (HEC) with a fixed composition of (VNbTaMoW)C5 were prepared by spark plasma sintering (SPS) from 1500 °C to 2200 °C. XRD, TEM, HRTEM, SAED and EDX were used to investigate effects of the sintering temperatures on compositional homogeneity, constituent phases and microstructure of the HECs. The results showed that single-phase HEC formed at a temperature as low as 1600 °C while ultimate elemental distribution homogeneity could be obtained at 2200 °C. Elemental distribution homogenization was accompanied by microstructural coarsening and oxide impurities aggregating at grain boundaries as temperature increased. SPS at 1900 °C for 12 min could yield uniform HECs (VNbTaMoW)C5 with Vickers hardness, nanohardness, fracture toughness and Young’s modulus reaching 19.6 GPa, 29.7 GPa, 5.4 MPa m1/2 and 551 GPa, respectively. The resultant HECs showed excellent wear resistance when coupled with WC at room temperature.

Microstructure changes occurring in heat-resistant steels during severe plastic deformation and subsequent creep

This work investigates creep and microstructure characteristics in P92 martensitic and 304L austenitic stainless steel after the imposing of large value of plastic strain. The as-received coarse-grained steels were strongly deformed at room temperature (RT) before tensile creep testing. The flat specimens were tested at 873 K and 923 K under different applied stresses. Microstructures were analysed by means of scanning electron microscope with EBSD (electron back-scatter diffraction) camera and transmission electron microscope. It was observed that application of severe plastic deformation (SPD) techniques at RT significantly reduces grain size and changes phase structure of 304L austenitic steel. The microstructure investigations revealed that the growth of new phases is enhanced in plastically deformed P92 martensitic and 304L austenitic stainless steels compared to their undeformed states. Creep results showed that the mean grain size of SPD-processed steels is still near ultrafine-grained region even after long-term creep testing and the coarsening of microstructure is predominantly influenced by creep time and strain.

Effect of La2O3 particle size on the microstructure and properties of Al Si alloys deposited via wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) of AlSi alloy deposits exhibits the advantages of a high deposition rate and easy operability, while being suitable for fabricating large-sized metal parts. However, the alloy deposits suffer from various problems such as a large number of pores, coarse and uneven microstructures, and poor mechanical properties. Herein, we added La2O3 powders with different particles sizes (nano-sized, micron-sized, and ordinary-sized particles) to AlSi alloy deposits for optimizing the properties of the specimens. The results show that the surface roughness of the specimens increased with the addition of micron-sized or ordinary-sized particles. However, the addition of nano-sized La2O3 powder improved the forming quality of the alloy deposit, eliminated the slender columnar grains near the fusion line and the central region of the coating, and refined the grains. The addition of La2O3 powder also reduced the maximum texture intensity in the specimen, for which adding nano-sized La2O3 powder had the most obvious effects. In addition, the microhardness and tensile strength of the specimens with nano-sized La2O3 powder were improved, while the average microhardness increased by 13% from 55.2 HV to 62.4 HV. The average tensile strength in the horizontal direction increased by 28% from 172 MPa to 221 MPa, and that in the vertical direction increased by 13% from 135 to 152 MPa. The addition of La2O3 powder was effective in improving the properties of the AlSi alloy deposit. The nano-sized La2O3 powder was the most suitable powder for the WAAM of AlSi alloy.

Optimization of SOFC Electrode Microstructures with Long Term Performance Modeling and Machine Learning

Performance degradation over time is a major barrier to the commercialization of solid oxide fuel cells (SOFCs). A major driver of this degradation is grain coarsening in the 3D microstructure of the porous, composite electrodes. The rate and extent of this microstructural degradation depends on the initial microstructure itself, raising the question of whether an optimal electrode microstructure can be determined for long-term performance. The high dimensionality of parameters that determine the initial microstructure makes parametric optimization challenging. In this work, we present a fully resolved 4D model for simulating microstructural and performance changes over 1,000 hours of cell operation by combining a phase-field coarsening model with spatially resolved microstructural analysis and continuum-level multiphysics performance modeling. The integrated model framework was run in a highly parallelized fashion to simulate the long-term performance of hundreds of LSM-YSZ cathodes and Ni-YSZ anodes. These electrode microstructures were synthetically generated to intentionally span 11 independent initial microstructural parameters. The results of the long-term performance model included electrochemical performance every 100 hours throughout the entire 1,000 hours of operation. Together with a novel figure of merit that accounts for both the initial and long-term performance, this high-dimensional bank of results was used to train a machine learning regression model. The machine learning model was trained to predict the long term performance based on the 11 independent initial microstructural parameters, allowing interpolation to values not included in the fully resolved models, as well as a more robust analysis of each parameter’s influence in this highly dimensional parameter space. A SHAP analysis [1] was performed on the trained machine learning model to determine the relative impact of each of the 11 independent initial microstructural parameters. This demonstrated that, for example, the LSM/YSZ ratio was the most impactful parameter in the cathode microstructure, with lower LSM/YSZ ratios generally leading to more energy produced over the cell’s lifetime. The fully trained model and SHAP analysis are also able to make specific recommendations for changes that would improve existing electrodes, providing a valuable tool for SOFC electrode developers and manufacturers.[1] S. M. Lundberg, S-I. Lee. A Unified Approach to Interpreting Model Predictions. NIPS 2017. Figure 1

Microstructure stability during high temperature deformation of CoCrFeNiTa eutectic high entropy alloy through nano-scale precipitation

Dual-phase microstructures arising out of eutectic reactions offer several advantages: ease of casting, composite properties and tunable characteristic length scale of the phases. Eutectic high entropy alloys (EHEA) with a multi-component solution phase and a hard intermetallic phase are candidate materials to identify alternate high-temperature materials. Alloying elements can be chosen to improve the resistance to lamellar microstructure degradation and coarsening. In this work, we present the CALPHAD guided design of a hypo-eutectic high entropy alloy CoCrFeNiTa0.395 with primary dendritic FCC phase and fine eutectic (FCC solution + Laves phase) microstructure possessing good high-temperature mechanical properties. The primary FCC phase fraction is 0.42 ± 0.02. The interlamellar spacing of the eutectic is 0.69 ± 0.12 μm. The alloy exhibited a balanced yield strength of 1303 ± 18 MPa, a fracture strength of 2237 ± 23 MPa and a fracture strain of 0.3 under room temperature compression testing. The mechanism for change in the lamellar morphology during deformation at temperatures beyond 0.8 TE is explained schematically. Strain field distribution obtained by FEM simulation correlates well with the observed microstructure gradients in the deformed samples. The formation of nano-scale precipitates in the low strain rate deformation is attributed to thermal effects. High-temperature precipitation of two types of precipitates (L12 ordered and Ni3Ta type) in the FCC-solution phase extended the useable temperature range of this alloy.

Numerical modeling and experimental investigation of brass wire forming by friction stir back extrusion

Friction stir back extrusion (FSBE) is used to produce brass wires, and then numerical modeling is developed to model the process by the coupled Eulerian-Lagrangian (CEL) technique and verified by experiments. Next, the effects of FSBE parameters, including tool rotational and plunging speed, on the strain and temperature distributions, microstructure, and material flow patterns are studied. The results show that the highest temperature and strain occur near the tool/workpiece interface but at a further distance from the tool axis. Additionally, in the cross section of an FSBE wire, the microstructure is more refined in the sample’s periphery. A higher rotational speed or a lower plunging speed results in a coarser microstructure. The material flow pattern is a conical helix and does not change meaningfully by the process parameters. The points at the further distance from the tool axis, along with an upward movement, experience an inward spiral motion which is amplified by higher rotational speed. However, the materials near the tool axis almost take an upward movement and endure a significantly lower strain.

Anodic polarization creates an electrocatalytically active Ni anode/electrolyte interface and mitigates the coarsening of Ni phase in SOFC

An intimate Ni anode/electrolyte interface is vital for the performance and operating stability of solid oxide fuel cells, while the interface is greatly influenced under anodic polarization conditions. Here for the first time Ni anodes are directly assembled on 8 mol% Y2O3-stabilized ZrO2 (YSZ) electrolyte without high temperature sintering process, and the interface formation between Ni anode and YSZ electrolyte is studied at varied anodic polarization currents at 800 °C. The results show that an intimate anode/electrolyte interface can be created by applying anodic polarization current, leading to remarkably enhanced electrocatalytic activity and operating stability. The major reason of enhanced stability is ascribed to the mitigated microstructure coarsening of Ni anodes due to the strong interfacial bonding induced by the anodic polarization. The present findings shed lights on rational design of Ni based anode/electrolyte interface by tuning the anodic polarization.