Variation In Microstructural Features Research Articles

Variation in microstructural features of melt-pool structure in laser powder bed fused Al–Fe–Cu alloy at elevated temperatures

The present study was undertaken to understand the effect of annealing on the microstructural features of the melt-pool structure and the associated multiscale mechanical properties of the Al–2.5%Fe–2%Cu alloy manufactured by laser powder bed fusion (LPBF). Microstructural characterizations and tensile tests were conducted for the LPBF-built specimen and those subsequently annealed at various temperatures ranging from 200 to 500 °C. Nanoindentation hardness mapping was used to evaluate the mechanical inhomogeneity of the melt-pool structure and its changes by annealing at different temperatures. The LPBF-manufactured sample exhibited a melt-pool structure containing numerous particles of the Al23CuFe4 phase (oC28, orthorhombic structure) formed because of local melting and rapid solidification during the LPBF process. The relatively coarsened cellular structure localized along the melt-pool boundary resulted in local soft regions. The local vulnerability contributed to the direction dependence of the tensile ductility. A slight variation was observed in the inhomogeneous microstructure of the samples annealed at 200 or 300 °C. The formation of numerous Al23CuFe4 nanoprecipitates in the α-Al supersaturated solid solution prevented strength loss after post-heat treatments. In addition, considerable coarsening of the intermetallic phase after annealing at 500 °C eliminated the melt-pool structure. The tensile performance of the specimens demonstrated a ductile fracture mode, wherein ductile fracture occurred in the α-Al matrix with low hardness while the harder θ-Al13Fe4 stable phase was embedded within it. The anisotropy in the mechanical properties was less pronounced owing to the significant microstructural changes.

The changes in microstructural features and physical properties of ceramic matrix composite bonding tools under ultrasonic service

Tape automated bonding (TAB) process plays a key role in the production of disk drives, microchips and other microelectronics components. Such tools should sustain their structures and properties throughout ultrasonic operation and are expected to last until reaching the targeted numbers of TAB cycles. This project aims to evaluate the changes in microstructures and physical properties of the bonding tools under ultrasonic service. Three commercial waffle-type tools from different suppliers were focused, namely Tool A, Tool B and Tool C. They provide different levels of bonding efficiency up to 50k cycles under relatively similar ultrasonic practice. Non-destructive and destructive testing methods were examined by the means of X-Ray and mass spectroscopy techniques, respectively. The waffle-end tips of the selected tool were observed using a confocal microscope. The changes in grain size and grain size distribution before and after ultrasonic service were quantitatively analyzed via electron micrographs. Also, their bulk density, hardness and toughness were determined. The experimental work revealed the variation in microstructural features and properties among the three, leading to the difference in ultrasonic efficiency, even they all contained similar phase composition. The relationship between microstructures, properties and ultrasonic efficiency was also reported and discussed.

Electrochemical Corrosion Resistance of Mg Alloy ZK60 in Different Planes with Respect to Extrusion Direction

The electrochemical corrosion resistance of a Mg-Zn-Zr alloy, ZK60, in different planes with respect to the extrusion direction was investigated in 3.5 wt% NaCl. The motivation of this study lies in the influence of extrusion on the grain size, texture and precipitation characteristics of magnesium alloys, and the profound role of these characteristics in the corrosion resistance of the alloys. Corrosion resistance was found to be considerably superior in the plane transverse to the extrusion direction (TD) than in the extrusion direction (ED) or normal to the extrusion direction (ND). The difference in the corrosion resistance was attributed to the variations in microstructural features in the TD, ED and ND directions.

Microstructural characterization of cement in the presence of alkanolamines

In order to understand the potential issues regarding the cement performance over time in the presence of alkanolamines, such as triethanolamine (TEA) and triisopropanolamine (TIPA), this investigation provided a series of quantitative phase characterization to monitor the hydration and microstructure of cement pastes. The addition of 0.02 % alkanolamines to cement promoted the strength development and altered the hydration kinetics at early ages. The aluminate reaction was greatly promoted, which resulted in the formation of hydrosulfoaluminates with a high monosulfoaluminate-to-ettringite ratio due to the facilitated dissolution of the intermediate phase. The morphology and chemical composition of the hydrates, as well as the porosity of the pastes, were also influenced by the altered hydration kinetics. Particularly, TIPA markedly reduced Si/Ca ratio (from 0.61 to 0.51) and increased Fe/Ca ratio (from 0.016 to 0.036) in the C-S-H gel at the early stage of hydration. Based on the variations in microstructural features subjected to the alkanolamines, the durability and long-term performance of the cement, including the phase variation, resistance to sulfate or chloride ingression and volume stability, were discussed.

Three-dimensional rendering of trabecular bone microarchitecture using a probabilistic approach

In the past, researchers have attempted to model trabecular bone using computational techniques. However, only a few of these models are visually similar, but not representative of the microstructural characteristics of real trabecular bones. In this study, we hypothesized that probabilistic modeling approaches could be used to generate representative digital models that capture the microstructural features of real trabecular bones. To test this hypothesis, we proposed a novel mathematical framework to build the digital models and compared the digital models to real bone specimens. First, an initial three-dimensional cellular structure was generated using Voronoi tessellation, with the faces and edges of the Voronoi cells considered as a pool of potential trabecular plates and rods, respectively. Then, inverse Monte Carlo simulations were performed to select, delete, or reassign plates and rods until the underlying size, orientation, and spatial distributions of the plates and rods converged to the target distributions obtained from real trabecular bone microstructures. Next, digital graphics techniques were used to define the thickness of trabecular plates and the diameter of trabecular rods, followed by writing the model into a Standard Tessellation Language file and then smoothing the model surfaces for a more natural appearance. To verify the efficacy of the digital model in capturing the microstructural features of real trabecular bones, forty-six digital models with a large variation in microstructural features were generated based on the target distributions obtained from trabecular bone specimens of twelve human cadaveric femurs. Then, the histomorphological parameters of the digital models were compared with those of the real trabecular bone specimens. The results indicate that the digital models are capable of capturing major microstructural features of the trabecular bone samples, thus proving the hypothesis that the proposed probabilistic modeling approach could render real trabecular bone microstructures.

Microstructure and Mechanical Properties of Medium Carbon Steel Deposits Obtained via Wire and Arc Additive Manufacturing Using Metal-Cored Wire

Wire and arc additive manufacturing (WAAM) is a 3D metal printing technique based on the arc welding process. WAAM is considered to be suitable to produce large-scale metallic components by combining high deposition rate and low cost. WAAM uses conventional welding consumable wires as feedstock. In some applications of steel components, one-off spare parts need to be made on demand from steel grades that do not exist as commercial welding wire. In this research, a specifically produced medium carbon steel (Grade XC-45), metal-cored wire, equivalent to a composition of XC-45 forged material, was deposited with WAAM to produce a thin wall. The specific composition was chosen because it is of particular interest for the on-demand production of heavily loaded aerospace components. The microstructure, hardness, and tensile strength of the deposited part were studied. Fractography studies were conducted on the tested specimens. Due to the multiple thermal cycles during the building process, local variations in microstructural features were evident. Nevertheless, the hardness of the part was relatively uniform from the top to the bottom of the construct. The mean yield/ultimate tensile strength was 620 MPa/817 MPa in the horizontal (deposition) direction and 580 MPa/615 MPa in the vertical (build) direction, respectively. The elongation in both directions showed a significant difference, i.e., 6.4% in the horizontal direction and 11% in the vertical direction. Finally, from the dimple-like structures observed in the fractography study, a ductile fracture mode was determined. Furthermore, a comparison of mechanical properties between WAAM and traditionally processed XC-45, such as casting, forging, and cold rolling was conducted. The results show a more uniform hardness distribution and higher tensile strength of the WAAM deposit using the designed metal-cored wires.

Variation of microstructural features on the tensile property and corrosion resistance of Zr-Sn-Nb-Fe-Cu alloy

Zr-Sn-Nb-Fe-Cu alloy was processed by two kinds of heat treatment and different microstructures were obtained. We focus on the effect of microstructure variation on the tensile properties and corrosion resistance. Results found that all specimens annealed at 550 °C/5 h (group A) could induce complete recrystallized structure, while dislocation cells and sub-grains formed after β-quenching and subsequent annealing at 480 °C/5 h (group B). Meanwhile, the average size and density of second phase particles (SPPs) for group A were slightly higher than that for group B. The tensile test results indicated that the yield phenomenon with distinct upper and lower yield points in tensile curves is observed due to the complete recrystallized structure with little dislocations in group A. For group B, however, the direct work hardening behavior was obtained at the initial deformation due to the interaction of existing dislocations with GBs and SPPs. The samples in group B showed a significant improvement in yield strength (YS), with a medium loss in plasticity compared with that for group A. The multiple strengthening contributions to YS mainly originate from the combination of grain refinement and dislocation. Theoretical analysis results found that the dislocation strengthening exhibited a significant effect on the tensile properties. Autoclave corrosion test indicated the specimens for group B possessed a relatively worse corrosion resistance. It was attributed to the higher O ion diffusion rate in defects and more vacancies or dislocations produced in the oxide of group B.

Uncertainty analysis of microsegregation during laser powder bed fusion

Quality control in additive manufacturing can be achieved through variation control of the quantity of interest (QoI). We choose in this work the microstructural microsegregation to be our QoI. Microsegregation results from the spatial redistribution of a solute element across the solid–liquid interface that forms during solidification of an alloy melt pool during the laser powder bed fusion process. Since the process as well as the alloy parameters contribute to the statistical variation in microstructural features, uncertainty analysis of the QoI is essential. High-throughput phase-field simulations estimate the solid–liquid interfaces that grow for the melt pool solidification conditions that were estimated from finite element simulations. Microsegregation was determined from the simulated interfaces for different process and alloy parameters. Correlation, regression, and surrogate model analyses were used to quantify the contribution of different sources of uncertainty to the QoI variability. We found negligible contributions of thermal gradient and Gibbs–Thomson coefficient and considerable contributions of solidification velocity, liquid diffusivity, and segregation coefficient on the QoI. Cumulative distribution functions and probability density functions were used to analyze the distribution of the QoI during solidification. Our approach, for the first time, identifies the uncertainty sources and frequency densities of the QoI in the solidification regime relevant to additive manufacturing.

Finite Interface Dissipation Phase Field Modeling of Ni-Nb Under Additive Manufacturing Conditions

During the laser powder bed fusion (L-PBF) process, the built part undergoes multiple rapid heating-cooling cycles, leading to complex micro structures with nonuniform properties. In the present work, a computational framework, which couples a finite element thermal model to a non-equilibrium phase field model was developed to investigate the rapid solidification micro structure of a Ni-Nb alloy during L-PBF. The framework is utilized to predict the spatial variation of the morphology and size of micro structure as well as the micro segregation in single-track melt pool micro structures obtained under different process conditions. A solidification map demonstrating the variation of micro structural features as a function of the temperature gradient and growth rate is presented. A planar to cellular transition is predicted in the majority of keyhole mode melt pools, while a planar interface is predominant in conduction mode melt pools. The predicted morphology and size of the solidification features agrees well with experimental measurements.

Achieving exceptional creep resistance in rare-earth-free Mg-base alloys by engineering the shape, size and fraction of eutectic, particles and precipitates

Creep characterization of five rare-earth-free Mg-base alloys having distinct variations in microstructural features is investigated. Mg-base alloys exhibiting double minimum creep is observed for the first time. Lamellar eutectic microconstituent is responsible for this unique creep behaviour. Dislocation substructures at minimum creep rate observed to be similar in all the five alloys. Majority of the dislocations are of 〈a〉 type lying on basal plane. Presence of fine precipitates of Mg2Ca and particles of CaMgSn along with thick lamellar eutectic enhanced the load-bearing capacity resulting in higher creep resistance. Thick lamellar eutectic decreases the damage nucleation by arresting the cracks.

Microstructural and Mechanical Characterization of Al-0.80Mg-0.85Si-0.3Zr Alloy

Abstract In this study, Al-0.80Mg-0.85Si alloy was modified with the addition of 0.3 wt.-% zirconium and the variation of microstructural features and mechanical properties were investigated. In order to produce the billets, vertical direct chill casting method was used and billets were homogenized at 580 °C for 6 h. Homogenized billets were subjected to aging practice following three stages: (i) solution annealing at 550 °C for 3 h, (ii) quenching in water, (iii) aging at 180 °C between 0 and 20 h. The hardness measurements were performed for the alloys following the aging process. It was observed that peak hardness value of Al-0.80Mg-0.85Si alloy increased with the addition of zirconium. This finding was very useful to obtain aging parameters for the extruded hollow profiles which are commonly used in automotive industry. Standard tensile tests were applied to aged profiles at room temperature and the results showed that modified alloy had higher mechanical properties compared to the non-modified alloy.

Influence of SLM scan-speed on microstructure, precipitation of Al3Sc particles and mechanical properties in Sc- and Zr-modified Al-Mg alloys

Additive processing via Selective Laser Melting (SLM) of Sc- and Zr-modified Al-Mg alloys (commercially known as Scalmalloy®) provides significant advantages over traditional 4xxx casting alloys. This is due to the high strength and ductility at very low mechanical anisotropy, which is a result of the alloys' very fine-grained microstructure combined with weak texture in the build-up direction. Next to their advantageous mechanical properties, the reliability of alloys processed by additive manufacturing is of great importance. Variations in microstructural features due to the use of different in processing parameters (e.g. laser scan speed) are well known for traditional alloys. This study analyses the influence of varying laser scan speeds on the static mechanical properties of SLM-processed Scalmalloy®, and discusses the evolving microstructure and precipitation of nm-sized Al3Sc particles at the correspondingly different energy inputs. It is found that by about doubling the laser scan speed the peak grain sizes in the fine-grained regions decrease from 1.1μm to 600nm, whereas in the coarse grained region almost no influence is observed. Al3Sc particles are only precipitated during processing at low scan speeds due to the corresponding high laser energy input, or by the intrinsic heat treatment from subsequent layers being built.

A Method for Quantitative 3D Mesoscale Analysis of Solid Oxide Fuel Cell Microstructures Using Xe-plasma Focused Ion Beam (PFIB) Coupled with SEM

Three-dimensional (3D) distributions of microstructural features in electrodes of solid oxide fuel cells (SOFCs) directly influence their electrochemical performance. Commercial grade SOFCs produced in cost-constrained environments can be expected to exhibit variations on the mesoscale (≈ hundreds of µm) of the microscale features (≈ tens of µm) that determine the local electrochemical activity. We are interested in quantitatively determining the mesoscale distributions of microscale properties in SOFC electrodes, especially quantifying the tails of distributions to establish correlations between such outliers and long-term performance. In this talk, we describe a new method that allows for the fast reconstruction of incredibly large volumes while maintaining a resolution on the order of tens of nm, allowing for fine-scale features to be accurately quantified. We use a Xe plasma focused ion beam (FIB) for slicing and a scanning electron microscope (SEM) for imaging. The pFIB allows for an increase in material removal rates by a factor of ≈ 100 compared to Ga ion FIBs, while the SEM resolution remains very high (herein ≈ 50nm), comparable to nano-CTs. In the same time as required for collecting Ga-FIB or nano-CT data sets, we collected a pFIB volume that is ≈ 70-80 times larger than attainable with Ga-FIB-SEM and ≈15-20 times larger than with nano-CT. From this data, we quantified the mesoscale distributions, containing both the mean and the variance, of key microscale features including volume fraction, average particle size, tortuosity, and triple phase boundary density. The commercial SOFC electrodes investigated were found to exhibit significant variations in the mesoscale distributions of their local microstructural features, which will be discussed. Importantly, these mesoscsale variations in microstructural features are expected to result in significant variations in electrochemical activity throughout the electrode. Using an effective medium theory model of polarization resistance, we quantified the distribution of microscale polarization resistance and mapped it spatially. Though the average value of polarization resistance is within range of expectation, the measoscale distribution of the microscale/local value of polarization resistance around the average indicates that several local areas are well outside the average, and potentially operating in regions expected to impact long-term cell performance.

Microstructural characterisation of thermally degraded 2·25Cr–1Mo steel using ultrasonic measurement and its correlation with mechanical properties

Ultrasonic testing is a sensitive tool not only for defect detection and evaluation of components and structures but also to characterise microstructural features which affect mechanical properties. In this study, four different thermally degraded specimens of 2·25Cr–1Mo steel have been prepared by aging at 873 K for 10 h, 923 K for 20 h, 973 K for 40 h and 973K for 80 h to obtain variation in microstructural features. Mechanical properties of each specimen having different microstructures were investigated using tensile and hardness tests. Ultrasonic parameters of each specimen having different microstructural features were studied by pulse echo technique using 5 MHz longitudinal probe. It has been observed that ultrasonic velocity and attenuation both increased with increase in the size of carbide precipitates obtained due to increased time and temperature of aging. Our results indicated that these ultrasonic parameters could be used as a tool to assess the degraded microstructure of operating components in service.

Challenges in Manufacturing Aluminium Based Metal Matrix Nanocomposites via Stir Casting Route

The influence of material processing conditions for preparing aluminium based metal matrix nanocomposites through stir casting route is reviewed. The role of particle size with respect to Brownian motion, Stokes settling velocity and strengthening mechanism is assessed from theoretical understandings. Variation of microstructural features and mechanical properties of the nanocomposites are predicted from theoretical concepts and related mathematical models. Experiments conducted to validate the theoretical predictions show that both Orowan and grain refinement strengthening mechanisms remain operative which is the key to the improved strength property of the nanocomposites.

Modeling fatigue small-crack growth with confidence – A multistage approach

Measurement of fatigue small-crack growth on the surface of smooth or notch specimens has evolved into a mature experimental technique. However, post data processing and model correlation for fatigue small-crack growth has not progressed in a parallel manner to the measurement technique. A new physics-based, mathematically precise fatigue small-crack growth modeling method is proposed for the specifics of surface small-crack growth in heterogeneous metals and rigorous statistical analysis. A multistage fatigue modeling philosophy was implemented and the fatigue damage incubation, small-crack growth, and long-crack growth regimes were identified. The uncertainty in measurement errors and the effects of random variation of microstructural features on the small-crack growth were explored using Monte Carlo (MC) simulations. The reliability of life prediction is directly correlated to the uncertainty-sources of measurements and the random nature of material microstructures through uncertainty propagation in the model correlation process. Fatigue damage incubation life is proven to be an essential quantity, both mathematically and physically, and it is introduced into the modeling process when correlating multiple experiments under the same loading condition. The proposed model is demonstrated using fatigue surface crack growth measurements of a dual-phase Ti-alloy.

Microstructure of Equal-Channel Angular Pressed Cu and Cu-Zr Samples Studied by Different Methods

Polycrystalline samples of technical-purity Cu (99.95 wt pct) and Cu with 0.18 wt pct Zr have been processed at room temperature by equal-channel angular pressing (ECAP). The microstructure evolution and its fragmentation after ECAP were investigated by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), positron annihilation spectroscopy (PAS), and by X-ray diffraction (XRD) line-profile analysis. The first two techniques revealed an increase in the fraction of high-angle grain boundaries (HAGBs), with increasing strain reaching the value of 90 pct after eight ECAP passes. The increase was more pronounced for pure Cu samples. The following two kinds of defects were identified in ECAP specimens by PAS: (1) dislocations that represent the dominant kind of defects and (2) small vacancy clusters (so-called microvoids). A detailed XRD line-profile analysis was performed by the analysis of individual peaks and by total profile fitting. A slight increase in the dislocation density with the number of ECAP passes agreed with the PAS results. Variations in microstructural features obtained by TEM and EBSD can be related to the changes in the XRD line-broadening anisotropy and dislocation-correlation parameter.

Structural and dielectric properties of ferroelectric Sr 1− xBa xBi 2(Nb 0.5Ta 0.5) 2O 9 and Sr 0.5Ba 0.5Bi 2(Nb 1− yTa y) 2O 9 ceramics

Ferroelectric Sr 1− x Ba x Bi 2(Nb 0.5Ta 0.5) 2O 9 and Sr 0.5Ba 0.5Bi 2(Nb 1− y Ta y ) 2O 9 were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions ( x= y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature ( T c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba 2+ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba 2+ substitution on Sr 2+ site induces diffused phase transition and diffuseness increases with increasing Ba 2+ concentration.

Microstructural, Dielectric, Pyroelectric and Ferroelectric Studies on Partially Grain-Oriented SrBi2Ta2O9 Ceramics

Partially grain-oriented (48%) ceramics of strontium bismuth tantalate (SrBi2Ta2O9) have been fabricated via conventional sintering. The grain-orientation factor of the ceramics was determined, as a function of both the sintering temperature and duration of sintering using X-ray powder diffraction (XRD) techniques. Variations in microstructural features (from acircular to plate like morphology) as a function of sintering temperature of the pellets were monitored by Scanning Electron Microscopy (SEM). The dielectric constant and loss measurements as functions of both frequency and temperature have been carried out along the directions parallel and perpendicular to the pressing axis. The anisotropy (ern/erp) associated was found to be 2.21. The effective dielectric constant of the samples with varying porosity was predicted using different dielectric mixture formulae. The grain boundary and grain interior contributions to the dielectric properties were rationalized using the impedance spectroscopy. The pyroelectric coefficient for strontium bismuth tantalate ceramic was determined along the parallel and perpendicular directions to the pressing axis and found to be −23 μC/m2K and −71 μC/m2K, respectively at 300 K. The ferroelectric properties of these partially grain-oriented ceramics are superior in the direction perpendicular to the pressing axis to that in the parallel direction.

Effect of Particulate Size on the Microstructural Evolution, Aging Behavior and Mechanical Properties of Al-4.5 Cu/SiC Composites

In the present study, an aluminum based metallic matrix (Al-4.5wt.% Cu) was successfully reinforced with SiC particulates of two different sizes using an innovative disintegrated melt deposition technique. The results of microstructural characterization studies revealed that smaller SiC particulates (8 μm) lead to a more equiaxed matrix grain morphology when compared to the larger SiC particulates (34.4 μm). The matrix grain morphology results were rationalized in terms of particulate-size influenced movement and destabilization of the solid-liquid interface. The results of the aging studies revealed an accelerated aging kinetics in the case of composite samples containing 8 μm SiC particulates when compared to the composite samples with 34.4 μm SiC particulates and unreinforced samples. The accelerated aging kinetics which is unusual in case of aluminum matrices containing copper in excess of 4 weight percent was rationalized in terms of particulate size associated variation in microstructural features. Results of ambient temperature mechanical tests revealed an increase in 0.2% yield strength and decrease in ultimate tensile strength and ductility in the case of composite samples when compared to the unreinforced samples in the as-processed condition. The mechanical properties characterization results were found to reveal direct correlation with the variation in microstructural features of the metallic matrix such as grain size and dislocation density originating due to the presence and different sizes of SiC particulates.