Dynamic plasticity and failure of high-purity alumina under shock loading

Most high-performance ceramics subjected to shock loading can withstand high failure strength and exhibit significant inelastic strain that cannot be achieved under conventional loading conditions. The transition point from elastic to inelastic response prior to failure during shock loading, known as the Hugoniot elastic limit (HEL), has been widely used as an important parameter in the characterization of the dynamic mechanical properties of ceramics. Nevertheless, the underlying micromechanisms that control HEL have been debated for many years. Here we show high-resolution electron microscopy of high-purity alumina, soft-recovered from shock-loading experiments. The change of deformation behaviour from dislocation activity in the vicinity of grain boundaries to deformation twinning has been observed as the impact pressures increase from below, to above HEL. The evolution of deformation modes leads to the conversion of material failure from an intergranular mode to transgranular cleavage, in which twinning interfaces serve as the preferred cleavage planes.     

Effect of stress state and microstructural parameters on impact damage of alumina-based ceramics

Alumina discs of two grain sizes (4 and 24m), and three compositions (99.4% purity, 85% purity, alumina + partially stabilized zirconia) were subjected to planar normal impact in a gas gun at a nominal pressure of 4.6GPa. The alumina discs were confined in copper and aluminium capsules, which provided solely compressive and compressive plus tensile pulses in the ceramic, respectively. These experiments were conducted at different pulse durations (controlled by the thickness of the flyer plates). The surface area of cracks per unit volume was measured in order to estimate the impact damage. Compression followed by tension produced significantly more damage than compression alone. The small grain-sized discs exhibited more damage than the large grain-sized discs. The amount of damage increased with the duration of the tensile stress pulse. The addition of partially stabilized zirconia ( 14%) did not enhance the resistance to fragmentation of the discs; X-ray diffraction did not reveal an impact-induced phase transformation. Although the pressures generated were below the Hugoniot elastic limit of alumina, considerable fracturing of the specimens took place. Scanning electron microscopy revealed that the fracture was intercrystalline in regions away from the spall plane. In the spall plane energy was sufficient to comminute the grains, producing considerable grain debris and transgranular fracture. Transmission electron microscopy revealed the onset of damage to the structure, in the form of dislocations (present in only a small fraction of grains), microcracks nucleating at voids, and intergranular microcracks.     

A comparison study of the agglomeration mechanism of nano- and micrometer aluminum particles

The agglomeration mechanism of micro- and nanosize aluminum particles with a primary mean particle diameter of 4.5 μm and 75 nm, respectively, was comparatively investigated under an incident shock wave. The morphology, particle size, and agglomeration process of micro- and nanometer alumina particles were comprehensibly compared by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. Images of X-ray diffraction reveal that a varied of phases of alumina (γ-, δ-, ε-, and α-Al₂O₃) were simultaneously found in the nanosize alumina products, which may give some detail information of the wide variety of reacting temperature of aluminum nanoparticles, while Al₄C₃ was detected in micrometer alumina products, which also gives some dynamic information of aluminum to alumina, i.e., aluminas have actually reacted with the free active carbon atoms to produce their intermediates. The microstructure of aluminas induced by the incident shock waves was detected and analyzed by using transmission electron microscopy combined with X-ray photoelectron spectroscopy spectrum. These results are an additive evidence to support that the initial stage sintering of the alumina nanosize powders is dominated by grain boundary diffusion, while the volume diffusion is the main character for the initial stage sintering of the micrometer alumina powders.     

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract

Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina.

Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.     

Detection of impact damage in CFRP laminates

The initiation and propagation of damage in carbon fibre composites subjected to impact loading has been investigated. High velocity impact tests were conducted on a variety of stacking configurations using a nitrogen operated gas gun. The damage processes were characterised using X-radiography, ultrasonic C-scanning, optical microscopy and the deply technique. The merits and weaknesses of applying such damage detection techniques to the monitoring of impact damage in composites are discussed. The consequence of the various fracture mechanisms on residual tensile strength is also considered.     

Study on growth of hollow nanoparticles of alumina

This article addresses the growth of hollow nanocrystalline particles of γ-alumina by the post-oxidation of nano-aluminium particles in air. The nanoparticles of aluminium were synthesized in a DC-transferred arc thermal plasma reactor. The as-synthesized nano-aluminium particles were oxidized, in air, at different temperatures. The as-synthesized parent nano aluminium and their daughter nanoparticles of aluminium oxide were thoroughly characterized with the help of X-ray diffraction analysis, high resolution transmission electron microscopy and thermogravimetric analysis. Two-step oxidation behaviours, unique in nanoparticles, are found to be the main driving force behind the formation of hollow spherical structures. The entire phenomenon is compared with the oxidation behaviour of coarse grain aluminium. The content of γ-alumina, identified by X-ray diffraction, relative to that of unreacted aluminium, has increased almost exponentially with the oxidation temperature in the case of nano aluminium. Similar behaviour is not observed in the case of coarse grain aluminium. The crystalline features of alumina, forming the walls of the hollow sphere, were confirmed by high resolution transmission electron microscopy.     

Influence of Ultrafine‐Grained Layer on Gaseous Nitriding of Large‐Sized Titanium Plate

In this paper, mechanical shot blasting on a large sized titanium plate is conducted to induce severe plastic deformation, which generates an ultrafine‐grained surface layer. The effect of an ultrafine‐grained layer on nitriding is evaluated at nitriding temperatures from 600 to 850 °C. The structural phases and mechanical property improvements are investigated and compared to those of a coarse‐grained specimen by using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and mechanical property measurements. The results indicate that an ultrafine‐grained layer enhances the nitriding kinetics and produces a thicker nitrided layer than that of a coarse‐grained plate at the same gaseous nitriding temperatures. The improved kinetics are attributed to a greater number of grain boundaries and defects introduced into the titanium plate surface by the mechanical shot blasting treatment. Meanwhile, the surface and cross‐sectional hardness values improve compared to the coarse‐grained plate due to the thicker nitrided layer resulting from deeper nitrogen diffusion.     

Alumina scale growth at zirconia-MCrAlY interface: a microstructural study

High-temperature oxide scale growth at the ceramic-metal interface is a major contributor to the thermomechanical resistance of thermal barrier coatings for hot stages of gas turbines. In order to better understand this phenomenon, microstructural observations of the alumina scales formed at 1100 and 1200 °C under air, between low-pressure plasma-sprayed NiCrAlY and air plasma-sprayed ZrO₂-8.5 wt % Y₂O₃, have been performed by classical and analytical transmission electron microscopy on transverse thin foil specimens. The evolution of the oxide grain morphology from the metal-oxide to the oxide-oxide interface suggests that the scale growth principally takes place at the metal-oxide interface. Segregation of yttrium at oxide grain boundaries has been detected as well as significant quantities of zirconium inside the alumina grains. The oxide growth seems to be dominated by a classical grain-boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains also suggests that Al₂O₃ partially forms by chemical reduction of ZrO₂ by AI. The comparison between the microstructures observed and that of alumina scales grown under similar conditions on bare MCrAlY alloys gives some insight into how the ceramic top-coat modifies NiCrAlY high-temperature oxidation mechanisms.     

喷枪扫描速率对常压冷等离子喷涂Cu薄膜的影响

研究了喷枪扫描速率对常压冷等离子喷涂Cu薄膜过程的影响。采用N_2和NH_3的混合气体作为等离子体气源产生常压冷等离子体,将Cu(NO_3)_2溶液雾化后通入等离子体射流的下游,雾化驱动气体是流量为4L/min的N_2,保护气体是流量为12L/min的Ar,利用射流型常压冷等离子体喷涂Cu薄膜。通过X射线光电子能谱仪分析制备的铜薄膜中铜元素化学状态的变化,用扫描电子显微镜观察制备的铜薄膜的微观形貌,讨论了制备薄膜过程中喷枪扫描速率的作用和影响。以N_2和NH_3的混合气体作为等离子体气源喷涂Cu薄膜时,在实验范围内,随着喷枪扫描速率的增大,制备的薄膜样品中铜元素的化学状态从Cu~(2+)变为Cu~+,再变为Cu,且薄膜的晶粒尺寸逐渐减小。在喷涂过程中,Cu(NO_3)_2在等离子体中会发生分解反应,并产生中间产物,NH_3在等离子体中产生的活性粒子会与中间产物反应沉积得到Cu薄膜。     

Fabrication of low specific resistance ceramic carbon composites by colloidal processing using glucose as soluble carbon source

Ceramic carbon composites were fabricated by colloidal processing using glucose as soluble carbon source. Glucose is converted into conducting carbon at high temperature treatment. Ceramic carbon composites were sintered in flowing argon at \(1400{^{\circ }}\hbox {C}\) showing resistor behaviour even at low carbon content (1 wt%). In this work, ceramic carbon composites were fabricated using alumina–clay slurries with addition of glucose as a soluble carbon source. Morphology of the sintered samples was characterized by field emission gun (FEG) electron microscopy. Phase analysis was done by X-ray diffraction (XRD). Electrical properties of ceramic carbon composites were measured by broad band dielectric spectrometer. Carbon produced from glucose at high temperature was characterized independently by BET surface area, dynamic light scattering, field emission gun scanning electron microscopy, field emission gun transmission electron microscopy, XRD and Raman spectroscopy.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

HIGH TEMPERATURE FATIGUE BEHAVIOUR OF A CAST COBALT-BASE SUPERALLOY

Abstract The initiation and the propagation of fatigue cracks and the low cycle fatigue life of a cast cobalt base superalloy was studied at 293K, 973K and 1173K by optical and scanning electron microscopy. A substantial decrease in fatigue life occurred at 973 and 1173K when compared to room temperature life. A time-dependent bulk damage was evidenced at 1173K which was determined by quantitative microscopy for two plastic strain levels. High strain fatigue crack propagation experiments were carried out at room temperature and at 1173K. From these experiments the decrease of the overall fatigue life at high temperature was shown to result from a considerable reduction of the initiation period due to oxidation and also from a significant acceleration of the crack propagation rate in the presence of oxidation and bulk damage.     

The effect of cristallographic orientation on damage in MgO due to spherical particle impact

A single stage gas gun has been used to fire spherical chrome steel and WC particles at variously oriented and prepared MgO single crystals at velocities of up to 350 m sec^–1. The resultant damage has been studied by optical and scanning electron microscopy, dislocation etching and surface profilometry, and mass losses have been determined gravimetrically. The measured crater dimensions and the mass loss data have been compared with the predictions of a simple analytical model of the impact event.     

An impedance spectroscopy study of high-temperature oxidation of thermal barrier coatings

Oxidation of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) was carried out in air at 950 °C and was investigated using impedance spectroscopy coupled with scanning electron microscopy and X-ray diffraction. After oxidation for between 500 and 3000 h, a continuous alumina scale was formed at the bond coat/top coat interface in the TBCs, and was evaluated using impedance spectroscopy. The impedance spectra of the oxidised TBCs showed that there were four relaxation processes, which were attributed to the yttria-stabilised zirconia (YSZ) bulk of the TBCs, the thermally grown alumina scale, the YSZ grain boundary, and the metal electrode effect. Impedance analysis showed that the resistivity of the alumina scale increased with increasing oxidation time, demonstrating the thermally grown oxide (TGO) growth. The resistance of the YSZ grain boundaries also increased after oxidation for 2000 h, suggesting the composition change at grain boundaries after a long-term oxidation.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Microscale characterization of granular deformation near a crack tip

This paper presents a study of microscale plastic deformation at the crack tip and the effect of microstructure feature on the local deformation of aluminum specimen during fracture test. Three-point bending test of aluminum specimen was conducted inside a scanning electron microscopy (SEM) imaging system. The crack tip deformation was measured in situ utilizing SEM imaging capabilities and the digital image correlation (DIC) full-field deformation measurement technique. The microstructure feature at the crack tip was examined to understand its effect on the local deformation fields. Microscale pattern that was suitable for the DIC technique was generated on the specimen surface using sputter coating through a copper mesh before the fracture test. A series of SEM images of the specimen surface were acquired using in situ backscattered electronic imaging (BEI) mode during the test. The DIC technique was then applied to these SEM images to calculate the full-field deformation around the crack tip. The grain orientation map at the same location was obtained from electron backscattered diffraction (EBSD), which was superimposed on a DIC strain map to study the relationship between the microstructure feature and the evolution of plastic deformation at the crack tip. This approach enables to track the initiation and evolution of plastic deformation in grains adjacent to the crack tip. Furthermore, bifurcation of the crack due to intragranular and intergranular crack growth was observed. There was also localization of strain along a grain boundary ahead of and parallel to the crack after the maximum load was reached, which was a characteristic of Dugdale–Barenblatt strip-yield zone. Thus, it appears that there is a mixture of effects in the fracture process zone at the crack tip where the weaker aspects of the grain boundary controls the growth of the crack and the more ductile aspects of the grains themselves dissipate the energy and the corresponding strain level available for these processes through plastic work.     

Nanopore domain growth behavior by nanopore changes near domain boundaries in porous anodic alumina

We investigate the nanopore changes near domain boundaries during the nanopore growth in porous anodic alumina (PAA) to understand the domain growth behavior with the anodization time. In order to observe the pore changes with the time, we analyze cleavage planes of PAA according to the nanopore length using a field emission scanning electron microscopy. The domain growth can be explained with three kinds of nanopore changes observed near domain boundaries: a change of pore diameter, a pore-branching, and a pore-movement.     

Electron backscattering diffraction study of coalesced bainite in high strength steel weld metals

Abstract

Coalesced bainite is a coarse constituent recently found to develop along with the classical martensite, lower and upper bainite in steel weld metals. Its crystallography has been characterised using electron backscattering diffraction in combination with field emission gun scanning electron microscopy. It is confirmed that coalesced bainite grains are crystallographically homogeneous but do contain orientation gradients. The misorientations across different grains of coalesced bainite and relative to conventional bainite have also been studied. The observations are discussed in the context of the mechanism by which coalesced bainite evolves.     

Observation and Modeling of Brittle Fracture Initiation in a Micro-Heterogeneous Material

Observations and characterization of brittle fracture initiation in a micro-heterogeneous material (sandstone) are conducted using the standard indirect tensile strength test. Acoustic emissions, optical microscopy and scanning electron microscopy (SEM) are employed for monitoring and characterizing the discrete micro-mechanical events preceding macroscopical fracture. The observations suggest that brittle fracture initiation is the end result of a microscopic damage accumulation process. A simple statistical model of micro damage accumulation leading to brittle fracture in a micro-heterogeneous material is also proposed. The model is calibrated by matching the coefficient of variation of measured ultimate stress with that resulting from the proposed model.     

Fatigue damage behaviors of carbon fiber-reinforced epoxy composites containing nanoclay

The effects of nanoclay inclusion on cyclic fatigue behavior and residual properties of carbon fiber-reinforced composites (CFRPs) after fatigue have been studied. The tension–tension cyclic fatigue tests are conducted at various load levels to establish the S-N curve. The residual strength and modulus are measured at different stages of fatigue cycles. The scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM) are employed to characterize the underlying fatigue damage mechanisms and progressive damage growth. The incorporation of nanoclay into CFRP composites not only improves the mechanical properties of the composite in static loading, but also the fatigue life for a given cyclic load level and the residual mechanical properties after a given period of cyclic fatigue. The corresponding fatigue damage area is significantly reduced due to nanoclay. Nanoclay serves to suppress and delay delamination damage growth and eventual failure by improving the fiber/matrix interfacial bond and through the formation of nanoclay-induced dimples.