Observation Of Microstructural Evolution Research Articles

Achieving high anti-sintering performance of plasma-sprayed YSZ thermal barrier coatings through pore structure design

Sintering is one of the critical factors leading to thermal and mechanical degradation of plasma-sprayed thermal barrier coatings (TBCs) in service under high temperature. The sintering resistance of the coating is expected to be improved through pore structure design. In this study, through a strategically improved powder feeding process, 8YSZ powders with fine-sized and coarse-sized pores were injected from the tail of plasma flame during plasma spraying to design porous coatings containing two differently flattened particles with different pore structures. The microstructural and property evolutions during sintering of the coatings were systematically studied compared with traditionally structured coatings. The effect of pore morphology on its sintering behavior was analyzed by quasi-in-situ observation of microstructure evolution. Results show that the anti-sintering performance of plasma-sprayed coating can be improved by adjusting the pore structure of coating with porous unmelted particles. In particular, the introduction of unmelted particles with coarse-sized pores can ensure that the coating has excellent thermal and mechanical properties for a long time. The results of this study provide guidance for structural design of anti-sintering plasma-sprayed coatings.

Effect of welding thermal treatment on the microstructure and mechanical properties of nickel-based superalloy fabricated by selective laser melting

Strengthening the heat-affected zone in actual weld joint of Inconel 625 alloys was crucial for engineering applications in aerospace, petroleum, nuclear energy, and marine industries. In this study, Gleeble thermal simulations were employed to investigate the influence of welding heat on grain morphology, carbide evolution, and mechanical properties of Inconel 625 alloys manufactured by selective laser melting (SLM). Typical elongated columnar grains along the building direction and parallel columnar array with obvious layer boundaries along the scanning direction were observed in SLM-processed Inconel 625 alloys with hot isostatic processing treatment. After Gleeble thermal simulations, the prominent equiaxed grains appeared in the entire gauge region that was directly exposed to the simulated welding heat owing to the recrystallization process. Meanwhile, the size of equiaxed grains significantly increased with the increasing peak temperature. Particularly, only MC-type carbides were observed in the γ-Ni matrix before and after the thermal simulation owing to the fairly rapid heating (100 °C/s) and cooling (30 °C/s) process. Simulated welding heat produced a negative effect on the mechanical properties of the Inconel 625 alloys, especially for the peak temperature of 1350 °C. Then, the respective contributions of the four conventional strengthening mechanisms were successfully quantified for understanding the effect of the simulated peak temperature on the yield strength of the Inconel 625 alloys. Finally, the observations of microstructural evolution indicated that the cross-slip of dislocations was the dominant deformation mechanism to account for the large strain-hardening rate of the Inconel 625 alloys.

Observation of microstructure evolution during inertia friction welding using in-situ synchrotron X-ray diffraction.

The widespread use and development of inertia friction welding is currently restricted by an incomplete understanding of the deformation mechanisms and microstructure evolution during the process. Understanding phase transformations and lattice strains during inertia friction welding is essential for the development of robust numerical models capable of determining optimized process parameters and reducing the requirement for costly experimental trials. A unique compact rig has been designed and used in-situ with a high-speed synchrotron X-ray diffraction instrument to investigate the microstructure evolution during inertia friction welding of a high-carbon steel (BS1407). At the contact interface, the transformation from ferrite to austenite was captured in great detail, allowing for analysis of the phase fractions during the process. Measurement of the thermal response of the weld reveals that the transformation to austenite occurs 230 °C below the equilibrium start temperature of 725 °C. It is concluded that the localization of large strains around the contact interface produced as the specimens deform assists this non-equilibrium phase transformation.

Hot deformation behaviour and interfacial characteristics of bimetal composite at elevated temperatures

The hot deformation behaviour and interfacial microstructure evolution have a critical influence on the hot workability and mechanical properties of bimetal composite. This study investigated the flow performance and interfacial characteristics of 2205 duplex stainless steel/AH36 low carbon steel bimetal composite (2205/AH36 BC) at elevated temperatures, based on the hot tensile tests over the temperature range of 950–1250 °C and strain rate range of 0.01–1 s−1. The results indicate that the deformation behaviour of 2205/AH36 BC is similar to that of AH36 low carbon steel at high temperatures, and the softening mechanism of bimetal composite significantly depends on the imposed working temperatures and strain rates, which has been verified by the electron backscatter diffraction (EBSD) observation of microstructure evolution. Extra geometrically necessary dislocations (GNDs) were observed to accumulate in AH36 carbon steel layer adjacent to the interface after the hot working test at relatively low strain rates, resulting from the different thermal expansions and hot ductility between the 2205 stainless steel and AH36 carbon steel, and some existing defects on the interface from the previous hot rolling process. These observations can be employed as a reference to make the medium-thick bimetal composite be fabricated to the final products with hot working process in practice.

Metallographic Observation for Evaluating Microstructural Evolution on Various Cross-Sections of Forged Part upon Air-Cooling from Finishing Temperature

Utility properties of forgings, particularly the mechanical ones, are among the primary aspects of interest to the customers of forge shops. These properties arise from internal structure whose evolution depends predominantly on the combination of parameters of deformation processes applied during forging, on the temperature profile during cooling and on the shape of the forged part. As microstructural evolution depends on the shape of the particular cross section of the forged part, an appreciable inhomogeneity of mechanical properties occurs in forgings. This article deals with observation of microstructural evolution in a chosen forged part, depending on cooling profiles of its various cross sections. The experimental programme of mechanical working and treatment of the forged part was based on the material-technological modelling approach. Microstructural evolution was studied using light and electron microscopic methods. Results of this analysis provided a basis for outlining optimization steps for mechanical working and treatment of the forged part.

In-situ observation of dark phase precipitation during heating and soaking process of a high nickel steel

In-situ observation of microstructural evolution during heating and soaking process was carried out for a high nickel steel using HTCLSM. Dark phases were observed when soaking at 900 °C. Results showed that the number of the dark phases culminated in about 50 s during soaking at 900 °C. With the increase of soaking time the area proportion of the dark phases increased and reached the maximum value in about 3 min. When temperature rose from 900 °C, the dark phases remained steady initially, but started to dissolve into the matrix at about 1 060 °C and completely disappeared at 1 132 °C. When the specimen soaked at 900 °C was cooled down to room temperature (RT), the dark phases kept stable. Energy spectrum analysis results showed that the dark phases contained much more Cr and Mn elements than the matrix and were also rich in V. Tensile test results showed that the dark phase strengthened the steel with the maximum tensile strength obtained after soaking at 900 °C for 3 minutes.

The Determination of Solution Temperature for NCu30-4-2-1 Alloy

NCu30-4-2-1 alloy was treated by the two methods of in situ observation experiment at high temperature and common heat treatment to determine solution temperature. High temperature metallurgical microscope was used to in situ observation of microstructure evolution of NCu30-4-2-1 alloy during heating process. The microstructures of the samples were observed by optical microscope. The results show that the as-cast microstructure of NCu30-4-2-1 alloy is composed by dendritic α-Ni-based solid solution and net-like (α+β) eutectics. The morphology of strengthening phase β-Ni3Si is graininess distributing on the grain boundary. The optimum solution temperature is both determined as 950°C by the two methods. The two methods of in situ observation experiment at high temperature and common heat treatment have comparability.

An Experimental Study on Local Thinning and Pinhole Formation Phenomena in PEM Fuel Cell Membranes

Membrane degradation is a key lifetime limiting factor in polymer electrolyte membrane fuel cells (PEMFCs). Observation of microstructure evolution in aged membrane electrode assemblies using scanning and transmission electron microscopy has shown a non-uniform degradation across the membrane surface [1]. It is known that under typical automotive operating conditions of PEMFCs, the membrane functional properties and stability decrease due to chemical and mechanical degradation mechanisms [2-3]. However, it is largely unknown why some distinct regions of the membrane experience remarkably more severe degradation, resulting in local thinning and pinhole formation and ultimately fuel cell failure due to significant hydrogen leaks. On this account, several studies have been performed on performance and failure analysis of PEMFCs due to pinhole formation [1, 4-5]; however, very limited knowledge is available on possible causes and initiators of this local phenomenon. In the present work, three new hypotheses are introduced, followed by an extensive experimental validation and analysis, with the aim of understanding the fundamental mechanism of this localized process.A comprehensive review of historical membrane failure analysis data from field operated Ballard MEAs was conducted to determine possible causes of localized membrane degradation and correlations with the MEA structure. Based on this review, it was proposed that catalyst layer delamination and cracks may play a significant role in accelerating the membrane degradation in PEMFCs. Therefore, customized MEAs were designed and fabricated in order to artificially create these two catalyst layer related defects. MEAs were tested under two different in-situ accelerated stress test conditions and extensive post mortem analysis was done on the end-of-life samples with the aim of developing an improved understanding of the relationships between pre-existing catalyst layer delamination and cracks and the localized membrane degradation process. Two possible cases of catalyst layer delamination were simulated, with and without a thin coating of Pt particles on the membrane surface. Figure 1 shows a cross sectional SEM image of the customized MEA. Delamination was simulated by precisely placing a small, thin, highly porous and hydrophilic polycarbonate film between the membrane and the catalyst layer. Also, Figure 2 shows the manipulated MEA with cracks in the cathode catalyst layer.The third hypotheses considered in this study was pinhole formation generated by local sources of Fenton’s reagents. In order to investigate this hypothesis, Iron oxide particles (Iron II and III oxides) were inserted at the membrane-catalyst layer interface. Figure 3 shows a cross sectional SEM image of this setup.The observations suggested a significant accelerating effect for iron contamination on the chemical membrane degradation process in a global nature, leading to remarkably shorter lifetimes, but dismissed the local traces of iron oxide as the local initiators or accelerators of this phenomenon. Studying the potential effects of catalyst layer delamination revealed that having this defect on the anode side can lead to an increasingly thinned membrane, while the same anomaly, if placed at the cathode catalyst-membrane interface, has a negligible effect on the rate of membrane thinning under identical operating conditions. Moreover, a substantial mitigating effect for platinum remainders on the site of delamination was observed in both tests. This was in agreement with comparable observations made by the same group [6]. In the case of artificial catalyst layer cracks, it was verified that anode and cathode cracks had no significant impact on local membrane degradation phenomena. In sum, anode delamination was found to be the most significant MEA feature resulting in accelerated local membrane thinning, while Fenton’s reagents were shown to accelerate global membrane thinning. The anode catalyst layer and its interaction with the membrane may warrant further research to elucidate the complex, local membrane degradation phenomena. Acknowledgements This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant. Special thanks and appreciations also go to Dr. Lida Ghassemzadeh, for her persistent and generous support during this project.

Observation of Microstructural Evolution in Li Battery Cathode Oxide Particles by In Situ Electron Microscopy

AbstractWe developed a simple approach to carry out in situ electron microscopy of single Li‐ion battery cathode particles during electrochemical cycling. We focused on Li(Ni0.8Co0.15Al0.05)O2‐based cathode materials because life‐cycle tests suggest a strong contribution of the cathode material to changes in cell impedance. In situ scanning electron microscopy was carried out operando during cycling and at various stages by interrupted cycling. Our work revealed several important aspects of cathode oxide particle dynamics: significant separations develop between grains even during the very first charge (oxide delithiation) and electrolyte penetration through that crack network all the way into the particle interior. Comparing these results to post‐test microstructural characterization of oxide particles subjected to extensive cycling confirms the occurrence of these processes in practical cells and suggests that the physical separation and isolation of grains may contribute to performance degradation of lithium‐ion cells.

Study by electron microscopy of proton exchange membrane fuel cell membrane-electrode assembly degradation mechanisms: Influence of local conditions

Observations of microstructural evolution by scanning and transmission electron microscopy in different regions of aged membrane/electrode assemblies (MEA) have shown that degradation is not always uniform through the MEA surface; after load cycling operation, the degradation is more severe in the region located near the air inlet compared to air outlet. After constant load operation the degradation appears more uniform. Two types of microstructural evolution have been observed. The first one consists in the modification of the cathode leading to nanoparticles larger than initially but still well dispersed. The second type of evolution ends up with big agglomerates formed by large Pt particles in the cathode, with also a noticeable degradation of the carbon support, both phenomena being always coupled with the precipitation of Pt particles inside the membrane. The first type of evolution results from the electrochemical Ostwald ripening mechanism and appears when the cathode potential remains low. In contrast, the second one appears when the cathode is locally exposed to a high interfacial potential resulting from the reverse-current mechanism. Hydrogen starvation induced by the load cycles and oxygen crossover that increases with membrane damage, are the two main factors responsible for this severe degradation mechanism.

Influence of microstructure and its evolution on the mechanical behavior of modified MAR-M247 fine-grain superalloys at 871°C

Abstract This study added Re to Mar-M247 fine-grain superalloy and investigated the influence of microstructure and its evolution on the mechanical behavior at 871 °C. Mar-M247 and modified Mar-M247 alloys consisted of dual precipitation of primary and secondary γ ′ particles in the γ matrix. An increase in Re content led to a reduction in the primary γ ′ phase with the γ ′ phase becoming finer; however, the addition of Re did not have an obvious influence the secondary γ ′ phase. Tensile and creep tests (871 °C/379 MPa) showed that both tensile and creep strength increased with an increase in Re content up to a maximum of 3 wt%. In the early creep stage, the addition of Re improved creep resistance by increasing the strength of the γ matrix and decreasing the inter-particle spacing. In the later creep stage, the linear fraction of the γ ′ raft in stress axial direction increased following the addition of Re, resulting in improved creep resistance. Observation of microstructure evolution indicated that only secondary γ ′ phase coarsened directionally into a rafting structure and the primary γ ′ phase was unaffected by creep. An interrupted creep tests verified that the γ ′ rafting structure initiated in the primary creep stage and completed in the secondary creep stage. The addition of excessive quantities of Re, such as 5 wt%, resulted in the formation of needle-like P phase, which damaged the tensile and creep properties. During tensile and creep tests, cracks initiated and propagated along grain boundary (GB) in alloys containing 0–3 wt% Re; and propagated along both the GB and P/γ interface in an alloy containing 5 wt% Re. In conclusion, optimal results were obtained for fine-grain Mar-M247 following the addition of 3 wt% Re.

Disclination dipoles observation and nanocrystallization mechanism in ball milled Cu–Nb powders

The object of this work is to investigate the structure of disclination dipoles and the nanocrystallization mechanism in ball-milled Cu–Nb powders. The structure of the partial disclination dipole in the nano-sized, face-centered cubic Cu crystals is directly revealed to be constituted by individual dislocations. It is found that such disclination defects can produce large lattice rotations at the nanometer scale in crystals during milling. The observation of microstructural evolution and analysis of the nanocrystallization mechanism indicate that the grain refinement induced by severe plastic deformation dose not only depend on the actions of dislocations and twins, but also on the disclination defects, especially for the further refinement of nano-size crystals.

Application of a Genetic Algorithm to Optimize Purification in the Zone Refining Process

Zone refining has been extensively used as a process of final purification of materials. Extensive theoretical and experimental efforts have been made to improve the purification efficiency of zone refining by optimizing parameters such as the length and the number of passes of the liquid zone. In the present study, a genetic algorithm (GA) integrated to a mass transfer numerical model has been used in the search of optimized operational parameters of the zone refining process. Experiments were carried out with naphthalene as the reference material having rhodamine as the impurity. Transparent organic substances, such as naphthalene, have been widely used as metals analogues materials because they allow in situ observations of microstructure evolution. Numerical predictions of the impurity profile along the refined naphthalene ingot is also ascertained by the present experimental data. Zone refining experiments were carried out by varying the zone length in each zone pass in order to permit the optimized data to be compared. It was found that the use of a GA integrated to the numerical mass transfer model has permitted the planning of operational conditions which are able to increase the purification efficiency up to 40% if compared with that attained when using conventional simulation approaches.

In-Situ Scanning Electron Microscopy/Electron Backscattered Diffraction Observation of Microstructural Evolution during α → γ Phase Transformation in Deformed Fe-Ni Alloy

This article presents in-situ observation of ferrite (α)/austenite (γ) phase transformation in an Fe-8.5 at. pct Ni alloy deformed by rolling using an automated scanning electron microscopy/energy backscattered diffraction (SEM/EBSD) system. During heating, recrystallization in α phase and α → γ phase transformation independently occurred. The γ grains nucleated in unrecrystallized α grains were most probably incorporated into the grain interior of recrystallized α grains. They did not have any specific orientation relation (OR) with recrystallized α grains and grew in an isotropic manner. On the other hand, the intragranular γ grains nucleated in recrystallized α grains had a Kurdjumov–Sachs (K-S) OR with the α grains and grew in a considerably anisotropic manner. They preferentially grew along the common direction of surface traces of {110}α/{111}γ. Approximately half of grain boundary (GB) allotriomorphs had either the K-S OR or the Nishiyama–Wasserman (N-W) OR with the parent α grains. The γ allotriomorphs predominantly grew into the α grain having the special OR with themselves. The GB character distribution of γ phase at high temperatures was measured. The fraction of CSL boundaries was as high as 63 pct, particularly that of Σ3 grain boundaries (GBs) was 54 pct.

Time resolved X-ray diffraction observations of phase transformations in transient arc welds

In situ X-ray diffraction methods have been developed at Lawrence Livermore National Laboratory for direct observation of microstructural evolution under quasi-steady state and transient welding conditions. Using intense highly collimated synchrotron radiation, the crystal structures in the weld heat affected and fusion zones are probed in real time to monitor solidification and solid state phase transformations during welding. Here the authors review recent work on the development and use of the time resolved X-ray diffraction (TRXRD) technique during transient welding and illustrate its unique capabilities to: directly observe the solidification mode; discover, in real time, the definitive sequence of phase transformations that lead to the final microstructure; and provide quantitative kinetic data of phase transformations through synthesis of TRXRD data with the temperature history obtained through heat transfer modelling. The TRXRD technique has been used to investigate welding induced phase transformations in titanium alloys, low alloy steels, and stainless steel alloys. The results show some of the first real time observations of the weld solidification mode and the evolution of equilibrium and non-equilibrium phases during rapid heating and cooling. When combined with numerical modelling, quantitative phase transformation kinetic data are obtained, allowing for its use in a wide variety of isothermal and non-isothermal processing. The potential for future applications of this and similar techniques is also addressed.

Glass formation and microstructure evolution in Al–Ni–RE (RE = La, Ce, Pr, Nd and misch metal) ternary systems

Glass formation has been systematically studied in melt-spun Al-rich Al–Ni–RE (RE = La, Ce, Pr, Nd and Mm; Mm = misch metal) alloys by navigating the compositions following the observation of microstructure evolution in the resulting ribbons. The optimum glass-forming regions are similarly located around Al85Ni10RE5 and found in the centre of the composites with primary phase α-Al, Al11RE3 and Al3Ni. The similarities in the critical cross-section below which a component is fully amorphous and the optimum compositions in these Al–Ni–RE systems are interpreted in terms of their competing crystalline phases and thermodynamic properties. Interestingly, it is indicated that Ni content is markedly higher than RE content in the best glass-forming alloys, which may be associated with strong interaction between Ni–Al atom pairs and the dense packing due to Ni-centred clusters.

放射光X線マイクロCTによるSn-Pbはんだ接合部における微細組織変化の観察

放射光X線マイクロCTによるSn-Pbはんだ接合部における微細組織変化の観察

Formation of Ultra Fine Ferrite and Determination of Critical Strain in the Strain Induced Dynamic Transformation for 0.22C-Mn Steel

The low carbon steel of 0.22wt% carbon was tested to estimate the dynamic phase transformation behavior from austenite to ferrite. The samples were deformed at just above Ar3 temperature by hot torsion at condition of strain rate (0.5/sec) and strain (5.0). The flow curve obtained at just above Ar3 significantly differed from others due to dynamic transformation. Based on the analysis of flow stress curve and observation of micro-structure evolution, the initiation and finish points of strain induced dynamic transformation (SIDT) could be determined. An inflection point observed at early deformation range (0.2–0.3) from the work-hardening rate and stress plot meant that new ferrite grains were nucleated in austenite matrix and these nuclei could be also confirmed by optical microscope. Subsequently in strain range of 0.7-1.0, the flow stress had the maximum value and new fine ferrite grains were dynamically generated inside untransformed austenite grains as well as prior austenite grains. The dynamic phase transformation induced by deformation made eventually fine ferrite grains under 3 ㎛ and decreased stress level with a fixed gradient.

In situ Transmission Electron Microscopy Investigation of Radiation Effects

In situ observation is of great value in the study of radiation damage utilizing electron or ion irradiation. We summarize the facilities and give examples of work found around the world. In situ observations of irradiation behavior have fallen into two broad classes. One class consists of long-term irradiation, with observations of microstructural evolution as a function of the radiation dose in which the advantage of in situ observation has been the maintenance of specimen position, orientation, and temperature. A second class has involved the recording of individual damage events in situations in which subsequent evolution would render the correct interpretation of ex situ observations impossible. In this review, examples of the first class of observation include ion-beam amorphization, damage accumulation, plastic flow, implant precipitation, precipitate evolution under irradiation, and damage recovery by thermal annealing. Examples of the second class of observation include single isolated ion impacts that produce defects in the form of dislocation loops, amorphous zones, or surface craters, and single ion impact-sputtering events. Experiments in both classes of observations attempt to reveal the kinetics underlying damage production, accumulation, and evolution.

New Understanding of Abnormal Grain Growth Approached by Solid-State Wetting along Grain Boundary or Triple Junction

Although it has been generally believed that the advantage of the grain boundary mobility induces abnormal grain growth (AGG), it is suggested that the advantage of the low grain boundary energy, which favors the growth by solid-state wetting, induces AGG. Analyses based on Monte Carlo (MC) simulation show that the approach by solid-state wetting could explain AGG much better than that by grain boundary mobility. AGG by solid-state wetting is supported not only by MC simulations but also by the experimental observation of microstructure evolution near or at the growth front of abnormally growing grain. The microstructure shows island grains and solid-state wetting along grain boundary and triple junction.